WO1998039669A1

WO1998039669A1 - A method for carrying out a medical procedure using a three-dimensional tracking and imaging system

Info

Publication number: WO1998039669A1
Application number: PCT/IB1998/000534
Authority: WO
Inventors: Ivan Vesely; Wayne Smith
Original assignee: Sonometrics Corporation
Priority date: 1997-03-07
Filing date: 1998-03-06
Publication date: 1998-09-11
Also published as: AU6514298A; US5797849A

Abstract

A method for carrying out a medical procedure using a 3-D tracking and imaging system (1600). A surgical instrument, such as a catheter, probe, sensor, needle or the like is inserted into a living being, and the position of the surgical instrument is tracked as it moves through a medium in a bodily structure. The location of the surgical instrument relative to its immediate surroundings is displayed to improve a physician's ability to precisely position the surgical instrument. The medical procedures include removal of an obstruction from the circulatory system, a biopsy, amniocentesis, brain surgery, measurement of cervical dilation, evaluation of knee stability, assessment of myocardial contractibility, eye surgery, prostate surgery and trans-myocardial revascularization (TMR). In addition, the method of the present invention also finds use in connection with the generation of 2-D echo planes.

Description

A METHOD FOR CARRYING OUT A MEDICAL

PROCEDURE USING A THREE-DIMENSIONAL

TRACKING AND IMAGING SYSTEM

Related Applications

The present application is a continuation-in-part (CIP) of co-pending

International Application No. PCT/CA96/00194, filed March 24, 1996, which is a

continuation-in-part (CIP) of U.S. Application Serial No. 08/411,959 filed March 28,

1995, now U.S. Patent No. 5,515,853.

Field of the Invention

This invention relates in general to a method for carrying out medical

procedures, and more particularly to a method for carrying out medical procedures

using a 3-D locating and imaging system.

Background of the Invention

Using the time-of- flight principle of high frequency sound waves, it is possible

to accurately measure distances within an aqueous medium, such as inside the body of

a living being during a surgical procedure. High frequency sound, or ultrasound, is

defined as vibrational energy that ranges in frequency from 100 kHz to 10 MHz. The

device used to obtain three-dimensional measurements using sound waves is known

as a sonomicrometer. Typically, a sonomicrometer consists of a pair of piezoelectric

transducers (i.e., one transducer acts as a transmitter while the other transducer acts as a receiver). The transducers are implanted into a medium, and connected to electronic

circuitry. To measure the distance between the transducers, the transmitter is

electrically energized to produce ultrasound. The resulting sound wave then

propagates through the medium until it is detected by the receiver.

The transmitter typically takes the form of a piezoelectric crystal that is

energized by a high voltage spike, or impulse function lasting under a microsecond.

This causes the piezoelectric crystal to oscillate at its own characteristic resonant

frequency. The envelope of the transmitter signal decays rapidly with time, usually

producing a train of six or more cycles that propagate away from the transmitter

through the aqueous medium. The sound energy also attenuates with every interface

that it encounters.

The receiver also typically takes the form of a piezoelectric crystal (with

similar characteristics to the transmitter piezoelectric crystal), that detects the sound

energy produced by the transmitter and begins to vibrate in response thereto. This

vibration produces an electronic signal in the order of millivolts, that can be amplified

by appropriate receiver circuitry.

The propagation velocity of ultrasound in an aqueous medium is well

documented. The distance traveled by a pulse of ultrasound can therefore be

measured simply by recording the time delay between the instant the sound is

transmitted and when it is received.

Prior art ultrasound tracking systems suffer from a number of shortcomings

which limit their utility. Firstly, conventional sonomicrometers use analog circuitry to transmit and receive signals (e.g., phase capacitative charging circuits). The

voltage representing the measured distance is then output to a strip chart recorder in

analog form. This data must then be digitized for computer analysis.

Secondly, conventional ultrasound tracking systems use analog potentiometers

to adjust the inhibit time and the threshold voltage that triggers the receiver circuits.

This often requires the use of an oscilloscope. Each time the tracking system is used,

these settings must be manually set and adjusted in order to tune the system. This can

be time consuming and annoying. As a whole, the function of the tracking system

cannot be changed. The repetition frequency is fixed, regardless of the number of

channels used, and the tracking system is therefore very limited in terms both of the

distances that can be measured, and the temporal precision with which the tracking

system operates.

Thirdly, conventional ultrasound tracking systems feature pairs of transmitter

and receiver crystals that are energized sequentially at fixed repetition rates. As such,

prior art tracking systems lack experimental flexibility. For example, before a pair of

crystals is implanted in the medium (e.g., a bodily structure, such as a human organ),

the user must decide the function of each crystal; similarly, the user must determine

which distances are to be measured by which crystal pair. This can be awkward

because surgery often necessitates changes during the procedure. If either of the

receiver or transmitter crystals malfunctions, the distance between them cannot be

measured. Critical measurements can therefore be lost after a significant amount of

effort is put into setting up the surgery. Fourthly, conventional ultrasound tracking systems measure only a straight

line distance between any isolated pair of crystals. Three-dimensional information is

therefore impossible to acquire. Even if multiple combinations of distances could

somehow be linked together, the inherently analog nature of the data would

necessitate the use of additional, complex hardware.

Finally, conventional ultrasound tracking systems use discrete elements, such

as threshold capacitors and potentiometers requiring large plug-in units to increase the

number of channels. The systems are very large, usually two feet wide by 18" deep,

and up to 12" high. Additional hardware such as strip chart recorders must be used,

for visualization and subsequent processing. This can be very awkward given the

space constraints at busy research institutes and hospitals.

The foregoing drawbacks to prior art systems limited their utility, and hence

limit the practicality of using the systems to perform various types of medical

procedures.

Summary of the Invention

According to the present invention there are provided a variety for methods for

carrying out a medical procedure using a three-dimensional tracking and imaging

system. The 3-D tracking and imaging system provides enhanced functionality for

diverse clinical and medical research applications.

The 3-D tracking and imaging system of the present invention uses modern

day digital electronics in conjunction with an integrated personal computer. External

A/D converters are not required, as the data is acquired digitally, directly from the sensors. Due to the speed of the controlling computer, the tracking system of this

invention is capable of detecting distance increments as small as 19 μm. The acquired

data can be displayed on the computer screen as it is being obtained, and can be saved

to the computer's storage media with a simple key stroke. After an experiment or

surgical procedure, the saved data can be examined and manipulated according to the

user's specifications.

According to a preferred embodiment of the present invention, virtually every

function of the 3-D tracking and imaging system is digitally controlled, and therefore

very flexible. To begin, a set-up menu is generated which allows the user to select

which transducers are active as well as the function of each channel. Next, a data

display program permits the parameters of the transducer to be customized for specific

applications. For example, if very few channels are being used, the repetition

frequency can be increased so that data can be acquired at several Khz. On the other

hand, if the system is being used in vitro, where persistent echoes from a container

vessel may present a problem, the repetition frequency can be reduced to allow the

echoes to attenuate between successive measurements.

The duration of the power delivered to the transducers can be reduced for

precision work or increased if greater distances are required to be measured. The

duration of the delay required to overcome electromagnetic interference between

transducer leads is adjustable by means of a variable inhibit feature. Additionally, the

number of samples displayed and stored in any given data save is variable according

to the length of time that a user's protocol demands. Finally, the resolution of the displayed information is variable in conjunction with the degree of motion of the

measured specimen. All of these functions are controlled digitally by means of

custom designed digital cards or modules discussed in greater detail below, which, in

turn, are software controlled.

Additional customized software is included in the 3-D tracking and imaging

system of the present invention for post processing and visualizing the acquired data.

In these routines, stray data points can be easily removed, three point filters can be

applied for smoothing, level shifts can remove areas of discontinuity, channels can be

derived, beat analyses can be performed, and automatic minimum/maximum level

sensing can be applied. Finally, routines can be provided that allow animated data

points in a Cartesian coordinate system while providing volumetric and position

information.

The 3-D tracking and imaging system of the present invention overcomes the

limitation of prior art transducer pairs. The present system can work with many

individual transducers that can be energized sequentially at very high repetition rates,

thereby giving the impression that several distances are being measured

instantaneously. In reality, the distances are measured in sequence, but since the

delay time between successive measurements is in the order of 100 microseconds, the

measurements occur virtually simultaneously for most biological applications.

Additionally, the 3-D tracking and imaging system of the present invention

provides the option of combining the transmitter and receiver circuitry into one

transceiver. This provides a researcher with the freedom to affix an array of transducers to a test object (e.g., catheter, needle, probe, etc,) and then decide which

transducers are to function as transmitters and which are to function as receivers.

Moreover, this type of configuration does not need to be limited strictly to transmitter-

receiver pairs. By using transceivers, the duty cycle between implanted transducers

can automatically alternate between transmit and receive modes, so that every

possible combination of distances between a group of transducers can be determined.

This type of application is particularly useful for studies which require redundancy of

measurement, as well as for establishing in vivo reference frames from which to base

three-dimensional tracking.

The 3-D tracking and imaging system of the present invention is configurable

into a true 3-D mode. In this configuration four or more transceivers are implanted

within an object (i.e., specimen) in which distances are to be measured, thereby

serving as a mobile reference frame. Multiple transmitters are then attached to the

specimen at various locations. Since any three transceivers can send and receive

signals, they essentially create an x,y plane. The fourth transceiver is then used to

determine the z coordinate of the surrounding transducers by determining if the active

transmitter lies above or below the reference plane.

Because the 3-D tracking and imaging system of the present invention uses

modern day integrated circuitry and custom programmed logic chips, it is physically

much smaller than prior art units. A large part of the system of the present invention

is implemented within the user PC (personal computer). The entire unit is composed

of three digital computer cards that plug directly into a standard AT computer mother board. A single cable connection connects the controlling computer and the discrete

peripheral transmitter/receiver/transceiver unit. This convenient set-up drastically

reduces the amount of experimental space required over prior art conventional units.

Moreover, the 3-D tracking and imaging system allows the position of a

device being tracked to be displayed in relation to the surrounding environment using

a 3-D template.

The present invention provides a variety of medical procedures which utilizes

the unique features of the 3-D locating and imaging system.

Brief Description of the Drawings

A detailed description of the preferred embodiment is provided herein below

with reference to the following drawings, in which:

Figure 1 is a schematic representation of four transducers in three-dimensional

space, for tracking and triangulating the three-dimensional positions of each

transducer, in accordance with the present invention;

Figure 2, comprising Figures 2A, 2B, 2C and 2D, is a schematic diagram of a

computer interface architecture used on all digital cards or modules of the preferred

embodiment;

Figure 3, comprising Figures 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 31, 3J, 3K, 3L,

3M, 3N, 30, 3P and 3Q is a schematic diagram of a controller card architecture

according to the preferred embodiment; Figure 4, comprising Figures 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, 41, 4J, 4K, 4L,

4M, 4N, 40 and 4P, is a schematic diagram of a counter card architecture according to

the preferred embodiment;

Figure 5, comprising Figures 5A, 5B, 5C, 5D, 5E, 5F, 5G, 5H, 51, 5J, 5K, 5L,

5M and 5N is schematic diagram of an A/D card architecture according to the

preferred embodiment;

Figure 6, comprising Figures 6A, 6B, 6C and 6D, is a schematic diagram of a

transmitter/receiver/transceiver architecture according to the preferred embodiment;

Figure 7, comprising Figures 7A and 7B, is a timing diagram showing

operation of the counter module according to the preferred embodiment;

Figure 8, comprising Figures 8A and 8B, is a timing diagram showing

operation of the A/D module according to the preferred embodiment;

Figure 9 is a schematic illustration of a catheter guidance system according to

a specific implementation of the present invention;

Figure 10 is a schematic diagram of a multiple transducer catheter according to

the preferred embodiment;

Figure 11 is a flow chart of a 3-D visualization algorithm which uses the

tracking system of the present invention;

Figure 12 is a perspective view of a cylindrical or ring-shaped transducer

according to a first alternative;

Figure 13 is a perspective view of a ring-shaped array of crystals, according to

a second alternative embodiment; Figure 14 is a perspective view of a composite transducer, according to a third

alternative embodiment;

Figure 15 is a schematic illustration of the external reference frame of the

catheter guidance system according to the implementation of Figure 9;

Figure 16 is a block diagram of the 3-D tracking and imaging system

according to a preferred embodiment of the present invention;

Figure 17A is a perspective view of an ultrasound imaging head with a

tracking clip attached thereto;

Figure 17B is an exploded view of the ultrasound imaging head with tracking

clip; and,

Figure 18 is a 3-D scene showing a reference frame, location and direction of a

surgical instrument and an ultrasound sector image.

Detailed Description of the Preferred Embodiment

As discussed above, the ultrasonic tracking system of the present invention

utilizes a plurality of transceivers, each of which can be programmed to operate as a

transmitter or a receiver. By utilizing four or more transceivers, full three-

dimensional measurement capability is provided, as shown in Figure 1. Any three

transceivers (TRxl , TRx2 and TRx3) lay in a plane (i.e., the x,y plane). The fourth

transceiver (TRx4) may then be used to determine the z coordinates of the

surrounding transducers (i.e., multiple crystals Tx) by determining if an active one of the transmitter transducers lies above or below the reference plane established by

transceivers TRxl, TRx2 and TRx3. Each of the many transmitters (Tx) attached to

the specimens are sequentially fired, while all reference transceivers record the

receiver signals. Since the distance from each transmitter to the reference plane

created by the transceivers is known, the relative x, y, z, coordinates of the transmitters

can be determined. This is done in real time on a personal computer (PC) with the use

of triangulation. This method of networking the transducers is unique to the tracking

system of the present invention, and permits the user to trace the three-dimensional

motion of an object under investigation. Obviously, the greater the number of

transmitters, the better is the reconstruction.

Specific applications of the ultrasonic tracking system which utilize three-

dimensional tracking and triangulation, are discussed in greater detail below.

As indicated above, the ultrasonic 3-D tracking system according to the

present invention is preferably fully integrated into the standard AT-style computer

motherboard found in modern PCs. The three digital cards which comprise the

majority of the hardware for the present invention, perform specific, modular

functions in the overall operation of the unit. As such, each card is provided with a

proper system interface structure in order to be compatible with the ISA architecture

of the controlling processor.

Figure 2 is a block diagram of the computer interface and addressing scheme

common to all three digital cards. It should be noted that the system is classified as an I/O mapping device as opposed to memory mapping device. Consequently, dedicated

I/O registers within the controlling processor are responsible for all data throughput.

As illustrated in Figure 2, the system computer interface architecture features a

full two byte data transfer (DO-D15), as well as partial address decoding (A1-A13).

Full two byte address decoding is not required. All signals sent to, or taken from the

AT bus are buffered using octal buffers (dl & d2) for both address and control lines,

and transceivers (d3 & d4) for the data lines. In terms of decoding, each board

features an eight position dip switch (d7) or equivalent for address selection. Address

lines A6-A13 are used for this function, thus providing 256 distinct addressing

locations, each with a resolution of 40 (hex) (i.e., 26). It should be noted that AO is

not used for address decoding.

An 8-bit magnitude comparator (d5) is used to equate the manually set dip

switch with address lines polled by the computer mother board. When a match is

found, a signal is generated which gates demultiplexes d8 and d9, each of which is a

l-of-8 demultiplexes. The lower three address lines (A1-A3) are used as inputs to

both of these Read and Write demultiplexes. To distinguish their functionality, the

buffered IOR signal is sent to opposite polarity enables on each demultiplexer. Thus

if IOR is in a high state, the system computer interface is in a Write mode. To avoid

Reading and Writing from the I/O address ports, A4 is also used as an opposite

polarity input to do and do. This has the effect of offsetting the Reads from the

Writes by precisely 10 (hex) (i.e., 24). The result of this is two controllable ranges of

eight data bits used for gating "reads" from the digital boards, and "writes" to the digital boards. A single PLD (d6) serves to handle the glue logic between the other

components of the decoder circuitry.

Due to the architecture of the x86 family of microprocessors, there are only a

finite amount of I/O registers. These registers can be partitioned into either 65535 8-

bit registers, or 32767 16-bit registers. Due to the nature of the data transfers to and

from the boards, and by selection of an active low signal to the I/O CS16 input of the

AT bus, only 16-bit data transfers are employed by the system.

The only remaining control line extending to the digital circuit card is the

Address Enable (EN). This signal is used in conjunction with the I/O Read and I O

Write signals to gate the magnitude comparator (d5). By doing so, Direct Memory

Access (IQMA) conflicts are avoided between the tracking system and other internal

computer modules of the PC.

The first functional module in the ultrasonic 3-D tracking system of the

present invention is the controller card. A functional diagram is provided in Figure 3,

which comprises Figures 3 A, 3B, 3C, 3D, 3E, 3P, 3G, 3H, 31, 3J, 3K, 3L, 3M, 3N, 30,

3P and 3Q. The controller card employs the identical bus decoding scheme described

above with reference to Figure 2, to govern and pace the functionality of the overall

system. As with all of the digital cards, the controller is preferably a four layer

Printed Circuit Board, (PCB), with the embedded layers being the power and the

ground planes, respectively.

The operation of the card is as follows: A single Programmable Logic Device

(PLD), cl, is programmed to cycle through a full two byte count at 32 MHz. The output registers of cl are always active, so that the counter is constantly outputting a

count value between 065535. These outputs are used for both comparative and timing

purposes throughout the system. For this reason, a highly reliable, fast-response PLD

is required. Functional blocks C.-C. latch predetermined values from the decoding

circuitry, and compare them to the output of cl. Thus, upon system start-up, specific

values are written to the registers of C.-C, and once those values are matched by the

output of cl, respective signals are generated to govern such features as Pulse Length

(6-bit), Cycle Length (8-bit), and Inhibit (15-bit). As illustrated, the "equating"

outputs of the low data byte comparison (c2 & c5) require an edge triggering flip-flop

(cl 1) to hold their equated state. The Output of the high data byte comparator (c4) is

of sufficient duration to feed directly to clO and cl2. Using a 80 MHz clock, the

Pulse Length signal is variable between Oμs and 2.00μs at 31.25ns increments, the

Inhibit signal between Oμs and 2.048ms and 62.5ns increments, and the Sub-Cycle

Length signal is variable between Oμs and 2.048ms at 16μs increments. Typical

values are loaded into the registers of c2-c5 to best suit a given application, as

discussed in greater detail below.

A second function of the cl counter is to generate signals to a resetting l-of-8

demultiplexes (clO) which in turn generates signals for application to cl and cl 1 for

resetting important system parameters. As can be seen in Figure 3, one of these

parameters is the Mode function which governs the direction of data flow in the octal

transceivers located on the remaining system cards discussed in greater detail below. Four cl outputs are also used to cycle through the RCVR lines of the system, thereby

providing a default of 16 receiver modules.

A second major role of the controller card is to manage the performance of the

transmitter activation bits. Using a transmitter PLD (c6) as a preloadable up counter,

a value indicative of the start transmitter is latched to its input registers. Using an

output of the clO multiplexer as a clocking signal, c6 increments the six transmitter

bits and outputs them both to a transparent buffer (cl3), and to a 6-bit comparator

(c9). Since the transmitter bits are sent to all three digital boards, as well as to the

computer peripheral, the transparent buffer is required to avoid capacitive loading.

The ending transmit value is sent to the second side of the 6-bit comparator

after it has been latched by c7. The octal latch (c8) is used simply to read the status of

the transmitter bits by the controlling software. Once the 6-bit comparison is made

and equated, a value is sent out to the local bus to clock the address incrementors on

the remaining two digital cards. Although 6-bits are used for equating the transmitter

increment bits, the default system allows for a 4-bit transmit value, corresponding to

16 possible transmitter channels. However, higher tier models of the ultrasonic

tracking system of the present invention may employ up to 32 transmit cycles,

corresponding to a 5-bit transmit value.

An 8-bit latch (cl4) is also used by the system to generate and gate signals

used to control address counters, interrupt controls, and trigger toggles.

Before most of the signals reach the local bus connecting the digital cards,

they pass through cl2, which is a simple "glue logic" PLD that ensures correct timing and signal polarity. This circuit module is also responsible for generating such

parameters as the external system trigger for pacing and gating additional laboratory

equipment.

Unlike the controller card which generates signals, the counter card (Figure 4)

receives signals to consolidate the ultrasonic distance information. The counter card

features an external db25 connection to the transmitter/receiver/transceiver peripheral

unit (Figure 6). This twenty-four conductor, individually shielded connection between

the counter card and the peripheral transmit/receive unit carries the 4-bit transmitter

increment signals (TX BITS), the transmitter Pulse Length signals (CSI and CS2) as

well as the sixteen default receive lines accommodating 1 transmitter channels

(upgradable to 32). Again it should be noted that not all embodiments of the

ultrasonic 3-D tracking systems according to the present invention, employ the full

range of sixteen receivers. Therefore, where a receive line is unused, it is grounded so

as to avoid interfering with the desired signals.

A functional diagram of the counter card or module is provided in Figure 4.

The functionality of the counter module is best described in two stages, data writing

and data reading. Examining the data writing stage, at precisely the moment when a

valid signal is sent out by the external peripheral unit (Figure 6) to activate a

transmitting transducer, the expandable bank of receiver PLDs (sl0-sl3) are reset, to

zero. These counters then count up from a zero value in accordance with respective

internal 32MHz clocks. Each PLD (sl0-sl3) is connected to an individual receive

transducer (Figure 6). As the 15-bit digital count in each PLD (sl0-sl3) is incremented past a predetermined value, an internal register within the PLD is

activated which permits the reception of a receive signal. This predetermined value is

used to implement the inhibit feature of the system and is designed to block out the

electromagnetic interference caused by activating a transmit transducer. Once the

mechanical vibration of the transmitted ultrasound is detected by a receive transducer

it is converted to an electrical signal, amplified, filtered, and sent back to the

appropriate counter PLD. This has the effect of stopping the digital count within the

chip.

Next, a l-of-16 multiplexer (si 4) is activated for causing the output enable

feature of the counters to be sequentially activated. The captured digital, value

corresponding to the separation distance between the active transmitter and each

connected receiver is then output in two bytes to the on-board RAM modules (s8 &

s9) for temporary storage. Each time the RAM modules are activated, a default of

sixteen locations are written to, according to the sixteen default receive signals. This

cycle is then repeated for the next transmitter in the system. The incrementing of the

RAM addresses is handled by s5, an octal buffer that outputs the 8-bit quantity

representing the receiver/transmitter value at any time. Once all the transmitters in the

system have been sequentially activated and recorded, the master cycle signal from

the controller module triggers si, the counter address incrementor PLD. This module

then increments the RAM addresses to the next major block for the next

transmit/receive master cycle. Typically, the on-board RAM modules s8 & s9 are 8-bit by 131,072. Thus, in

the-default configuration of sixteen transmitters and sixteen receivers, the RAM is

cycled through 512 times before reaching its capacity. Options exist for upgrading the

on-board RAM to 8-bit by 524,288, so as to allow for 2048 complete

transmitter/receive cycles. It should be noted that for most biological investigations, a

repetition frequency of 200 Hz is demanded. Thus, even with 256kB of storage

capacity (128kX2), the on-board RAM can be completely filled in as little as 2.56

seconds. Consequently, the system of the present invention includes software

functionality for downloading the stored information. This process is described in

greater detail below.

To successfully realize the data reading stage, the counter card or module

monitors the addresses that are automatically incremented to the RAM, and writes

values to those addresses. This task is carried out by the octal transceivers (s2 & s3).

Using the Mode function generated by the controller card, the addressing data shifts

from a reading to a writing state in accordance with the system timing. This gives the

software the ability to activate any address in the RAM by simply writing out a 16-bit

value to s2 and s3. Since the incrementing of the transmitter and receiver bits is

automatic, there is no need to monitor their value. Thus, s4 can be simply an octal D-

type flip-flop rather than an octal transceiver.

Once an address is written to the RAM for data output, the octal buffers s6 and

s7 are opened to permit the PLD distance data to be passed along the low and high

byte data paths into the I/O registers of the motherboard processor, then to the computer RAM, and finally to the hard disk for permanent storage. As can be seen in

the system timing diagrams (Figures 7 & 8), the system is in a data output mode for

the majority of each system cycle. Data input to the RAM occurs regularly, but only

for 8μs intervals.

A second major function of the counter module or card is to provide an analog

signal output. Despite the fact that digital data acquisition is superior in many ways to

conventional analog circuitry, many users are required to work with analog signals.

The Digital-to- Analog (DAC) converter (si 7) is thereby provided as an option on the

standard tracking units of the preferred embodiment. The DAC of the present

invention operates as follows. Successive 8-bit values are latched into one side of the

one of four magnitude comparator (si 5b, d, f & h). These values are selectable

through the software to permit any combination of transmitter/receiver output signals

to be transferred to the four analog outputs. The opposite side of each comparator

(si 5b, d, f & h), is directly connected to the constantly cycling transmitter and

receiver bits. When the value applied to both sides of a comparator are equal, the

output is passed to a 4-to-2 line encoder (si 6), before being passed to a DAC (si 7).

Under this configuration, four distinct, 12-bit analog channels can be connected to an

output port from the computer.

Finally the counter card or module also employ a "glue logic" PLD (si 8) to

coordinate the timing of the output enable signals, as well as the handling of thirty-

two versus sixteen transmit channel capability. It should be noted that the foregoing counter card is suitably replaced by other

types of well known timer modules which are configured to measure transit time.

The final digital card or module in the ultrasonic 3-D tracking system of the

present invention is a synchronized Analog to Digital (A D) converter card or module.

During typical experiments, a user may wish to acquire more than the networked

distance measurements. For example, in a cardiac investigation, analog signals such

as pressure, ECG, and blood flow are also important. For this reason, an expandable

A/D card is integrated into the tracking system of the preferred embodiment. The

basic system is perfectly provided with four A D channels. However, up to sixteen

independent, 12-bit channels may also be provided ranging from ±IOV.

As illustrated in Figure 5, the A/D module functions in virtually the same

fashion as the counter card. Analog channels are fed in via a db25 cable connection

(RGB174U coax connectors) to al-a4. During the data input mode, all analog

channels are internally converted fed into two 8-bit by 131,072 RAM modules (a6 &

a7). The RAM is automatically incremented using the four gated receiver bits (al3).

An incrementing address PLD (al4), which receives the same clock as the counter

address incrementor, is used to provide the remaining thirteen address lines to the

RAM. Thus, every time a complete transmit receive cycle is performed, both the A/D

RAM and the counter RAM registers are increased. During the write, or data output

mode, an address is written to the respective octal D-type flip-flop (al2) and

transceivers (alO & al 1) to access the proper RAM location. The octal buffers a8 and

a9 are opened allowing the converted analog information to be transmitted along the high and low byte data buses to the computer storage device. Finally, a controlling

PLD (a5) is used to coordinate the timing signals on the A/D module. By

congruously activating the A/D and counter information, it is possible to synchronize

the digital distance information with the converted analog data.

A second function of the A/D card is to provide for direct digital inputs. Thus,

up to four digital input channels may be received via latch al5 and monitored via

octal buffer a8 during an experiment in the same fashion as the analog data.

The final hardware component in the ultrasonic 3-D tracking system of the

present invention is the peripheral transmitter/receiver/transceiver unit, shown in

Figure 6. Each peripheral board of the preferred embodiment possesses the capacity to

support sixteen transmitters with eight receivers, or eight transceivers. These

components are mounted onto a two-layer printed circuit board and connected to the

host computer system by means of the twenty- four conductor, individually shielded

computer cable discussed above. The external peripheral unit receives its transmit

voltage level and biasing voltages from an independent power supply (t5). The unit

also possesses a two color LED to indicate whether the unit is in active or standby

mode.

The peripheral unit works as follows. The digital signals from the computer to

the unit are passed through pull up resistors to a CMS l-of-16 decoder (try). The

decoded signals are then transmitted to selectable transmitters or transceivers. The

variable duration Pulse Length signal is sent via filtering and biasing elements to the

gate of an N-Channel Enhancement Mode VMS transistor (Q). The gate signal bridges the transmit voltage level to ground. This signal is then passed through a

step-up isolation transformer (TO) and out of the peripheral unit via a coated, 32

gauge, multi stranded wire (t2) to the transducer (xl).

The transducer (xl) is preferably a cylindrical piezoelectric ceramic crystal,

encapsulated with an electrically insulating sealant.

Using a network of similar receivers, the mechanical vibration from a

transmitter crystal is detected and converted to an electrical signal. Each individual

receiver circuit consists of step-up isolation transformer (Tl), a two stage amplifier

(Al) collectively providing a 48dB gain, a linear operational amplifier (tr3), a half-

wave rectifier (DI) and a TTL level inverter (tr4A and tr4B). The digital waveform

output from the TTL inverter is further isolated using an RF choke (t9) before it is

transmitted back through the shielded cable to the appropriate LLDS.

According to the best mode of implementing the receiver, the single-ended

amplifiers Al may be replaced by a differential amplifier.

For a further understanding of the operation of the 3-D tracking system

according to the present invention, a set of timing diagrams are provided in Figures 7

and 8. These figures illustrate the operation of the counter module (Figure 4) and the

A/D module (Figure 5), respectively, during both the read and the write phases of

operation. By default, the counter module actively acquires data for sixteen receivers

during every Sub-Cycle Length. Conversely, the A D data acquisition occurs only

once during the same time interval, or once every Master Cycle Length. For

simplicity, both timing diagrams are based on a transition from a transmitter "x" to a transmitter "x+1". Despite the apparent equal time-sharing between read and write

cycles, in actual fact, the read cycle is significantly longer. More particularly, in the

preferred embodiment the write cycle is limited to a 12μs window per sub-cycle.

Referring to Figure 7, the counter module (Figure 4) operates as follows. At

the beginning of the read cycle, an impulse signal is sent out to the VMS transistor (to

in Figure 6) to activate a transmit crystal (xl). At precisely the same time, the

associated counter PLD (slOa-d, sl3a-d) is released from its count of zero and begins

counting up at a clock speed of 32MHz. As discussed above, assertion of the

CountPLD Inhibit signal prohibits electromagnetic interference between crystal leads

by remaining at a logic low level. After a user-adjustable delay, the CountPLD signal

changes state, thereby permitting the reception of a valid signal on the associated

CountPLD RCVR line (RCVRO-3).

Once the first valid ultrasonic signal is detected and processed, the digital

counter value is held on the PLD's output registers. The period of time for this

distance count to occur is also variable in duration according to the user's

specification. During this time, the transceivers which govern the read/write state of

the system permit the downloading of the previously acquired digital distance values

from the system RAM (s8,s9) (CountADD OE in a high state). By constantly

monitoring the RAM addressing values using s2-s4 (Figure 4) the computer is able to

keep track of the RAM status. As the RAM (s8, s9, Figure 4) approaches its capacity,

a downloading is carried out during this read window. The write window of operating the counter module is delimited by the 12μs

active high Sub-cycle length signal. At the moment this signal is asserted, the

following conditions occur: the CountADD OE signal changes state, indicating that

the automatic addressing mode has been invoked, the CountBUS DIR signal changes

states to allow the opposite flow of data through the transceivers, the CountBUS OE

signal is invoked to activate the output registers of the addressing PLD (si) the

CountRAM OE signal is disabled to prepare the RAM (s8, s9) for data storage, the

CountPLD OE signal enables cycling through each of the sixteen individual counters,

and the CountRAM WE signal toggles to store each digital count value in RAM

(s8,s9). The signals used to control these functions are generated by various Boolean

combinations of the control module counter (C). As the default 4-bit receiver values

are cycled through to produce the automatic RAM addressing, the CountBUS MODE

signal is toggled to sample the current addressing value generated by the addressing

PLD (si, Figure 4). This value is stored in memory for proper downloading of data

during the next write window. These functions are carried out during the first 8μs of

the 12μs sub-cycle window.

Once all sixteen receivers (Figure 6) have downloaded their distance data to

the RAM (s8, s9), the Master Cycle length value is incremented to indicate the next

major cycle. At the same moment, the CountRAM WE signal is disabled along with

the polling of the receiver distance values. Finally the remaining 4μs expire putting the counter module back into its read

mode, while resetting the receiver chips (COUNTPLD RST), and each of the

incrementing counter bits from the controller card (Figure 3).

Using Figure 8 as a guide, the A/D module of the ultrasonic 3-D tracking

system works in an identical fashion as the counter module, with one major exception.

Write modes occur only during transition of the Master Cycle Length signal. When

such occur, the default sixteen converted analog channels are cycled through and

written to their respective RAM locations. The same A/D BUS MODE sampling

occurs to ensure individual RAM chips are provided in banks of four channels, each

chip is given a 2μs window in which the A/D CHIP SELECT signal is toggled low for

data throughput. At the end of 8μs, the A/D parameters are reset to their write state

while sampling of the analog channels begins once again, once the transition has

occurred to activate the next array of transmitters, the AD INTERRUPT signal drops

to a logic low value to indicate that the conversions of the active channels are

complete.

The machine language codes that carry proper collection and processing of

data acquired by the peripheral unit (Figure 6) are all preferably based around a x86

processor. The transfer of information through the system is both quick and seamless.

Given a typical system with sixteen transmitters and sixteen receivers, or sixteen

transceivers, 256 2-byte distance data saves are carried out every cycle of the Master

Cycle length signal. Since the on-board RAM (s8, s9) in a typical unit is 128kB, the

RAM has the capacity to save 512 Master Cycles before overwriting occurs. Since most clinical experiments typically demand a 200Hz data saving rate to sufficiently

track biological motion, only 2.56 seconds of data saving can be correctly obtained.

Since this is clearly unsatisfactory for a typical data run, software routines

have been written for the system of the present invention to periodically download the

RAM modules during the read cycles of the system.

The transfer of information out of the system is as follows: each time the

digital boards (Figures 3-5) are accessed, a total of 1024 bytes of data are secured.

This lkB is written to a dedicated 64kB buffer in the mother board RAM of the

resident PC. Provided that the computer is not responsible for carrying out any

additional tasks, the machine language code implemented thereon, also shunts this

information to the display. This function can be performed 64 times before the RAM

buffer of the mother board RAM is full. Once this happens, the system software

performs a binary save of the data held by the 64kB buffer. At this stage, a standard

disk-cache such as DOS's smartdrv.exe is activated to accept all of the 64kB binary

files and commit them to the hard disk drive of the PC at the end of a data save

command. Under this scenario, the only limit to the duration of a data save is the

capacity of the disk cache. In this manner, the ultrasonic 3- D tracking system of the

present invention can be tailored to meet the specific needs of customers simply by

providing additional memory to the base PC computer.

In addition to data saving and display software, the units according to the

present invention preferably also utilize post-processing software routines to

manipulate and visualize the saved binary data files. A three-dimensional (3-D) tracking and imaging system applicable for use in

connection with a variety of procedures, including those described in detail below,

will now be described with reference to Figure 16. 3-D tracking and imaging system

1600 is generally comprised of a computer system 1620, mobile transducers 1672,

reference transducers 1674, an instrument 1670 and an optional robotics subsystem

1690.

Computer system 1620 is generally comprised of a 3-D tracking system 1622,

an imaging modality system 1624, an image registration system 1626, an image

warping and geometry transformation system 1628 ("warp system"), a user interface

1650 and a display 1660. It should be appreciated that 3-D tracking system 1622 may

take the form of a sound-based system or an electromagnetic-based system. Both time

of flight and phase relationships may be used to determine distance.

Instrument 1670 may take the form of a catheter (e.g., see Figure 10), a probe,

a sensor, a needle, a scalpel, a forcep or other device used in a surgical or diagnostic

procedure. Mobile transducers 1672 and reference transducers 1674 may take the

form of an ultrasonic transducer or an electronic transducer. However, for purpose of

illustrating a preferred embodiment of the present invention, transducers 1672 and

1674 will take the form of ultrasonic transducers (i.e., piezoelectric crystals) described

above.

A plurality of mobile transducers 1672 are fitted to instrument 1670. One or

more reference transducers 1674 provide a reference position relative to mobile

transducers 1672. In this respect, reference transducers 1674 may be located to provide an internal reference frame inside a patient's body or on the surface of a

patient body to provide an external reference frame.

As indicated above, reference transducers 1674 may be transmitters,

transceivers or receivers that can generate ultrasound or electromagnetic radiation,

that can be detected by mobile transducers 1672.

For the purpose of illustrating a preferred embodiment of the present

invention, 3-D tracking system 1622 will take the form of the ultrasonic 3-D tracking

system described in detail above. 3-D tracking system 1622 transforms the multiple

distance measurements between all of the transducers 1672, 1674 into XYZ

coordinates relative to a referenced axis, as described in detail above. It should be

appreciated that the reference frame provided by reference transducers 1674 must be

self-determining, that is, if the reference frame becomes distorted, this distortion

needs to be detected by reference transducers 1674. Detection is typically done by

using transceivers that can determine the distance between any combination of two

transducers, and hence their relative spacial coordinates in 3-D space. In this regard,

the position of the transducers is obtained in 3-D from the images acquired of the

bodily structure (e.g., tissue/organ) that show "dots" where the transducers are

located, and also from the transducers themselves when they are in the bodily

structure. If there is some discrepancy in the distances between all combinations of

transducers, then the bodily structure must have deformed ( i.e., "warped") after the

images were acquired. A mathematical coordinate transformation can be used to

specify exactly how to correct the image set and account for the warping. The distance between any combination of two transducers is determined by having each

transducer send a signal to all other transducers. In this way, all the distances between

the transducers is known. From these distances, XYZ coordinates can be calculated,

in reference to some transducer as the origin.

Imaging modality system 1624 acquires 2-D, 3-D or 4-D image data sets from

an imaging source, such as fluoroscopy, an MRI (magnetic resonance imaging), CT

(computerized tomography) or 2-D or 3-D ultrasound device, to provide a "template"

through or against which the shape, position and movement of instrument 1670 being

tracked can be displayed. The template typically takes the form of an image of the

environment surrounding the instrument (e.g., a bodily structure). It should be noted

that if multiple (3-D) volumes are acquired at different time intervals, a 4-D image is

obtained (i.e., 3-D image changing over time).

Image registration system 1626 registers the position of instrument 1570

within the spatial coordinates of the image data set provided by imaging modality

system 1624. The position of instrument 1670 is provided by the 3-D tracking system

1622. Image registration system 1626 will provide a display of instrument 1670 at its

proper 3-D location inside the bodily structure and orientation relative to the bodily

structure itself. It should be appreciated that registration system 1626 may be user

assisted, or completely automated if image processing algorithms are implemented to

automatically detect the spacial locations of the transducers (typically the reference

transducers) in the image data set. Warp system 1628 is a software-based system that transforms or "warps" the

image data sets by the appropriate values to correspond to a deformation that has

occurred in the reference frame between the time that the image data set were acquired

and the time that the procedure is to be implemented during surgery. Accordingly,

warp system 1628 is typically comprised of a matrix transformation routine that maps

the deformed geometry onto the original image data set, and distorts it appropriately.

User interface 1650 enables a user to interact with computer system 1620,

including programming computer system 1620 to perform a desired function. For

example, a particular view for display can be selected. Instruments 1670 (e.g., probes

or catheters) can be activated using user interface 1650. Display 1660 displays to the

user registered images provided by image registration system 1626.

Optional robotics system 1690 is generally comprised of a robotics control

system 1692 and a robotic manipulator system 1694. Robotics control system 1692

controls robotic manipulator system 1694 to follow a programmed path that can be

appropriately changed, based on shifting, warping or changes in the shape of a bodily

structure at the time of surgery. Robotic manipulator system 1694 physically moves

instrument 1670 as instructed by robotic control system 1692.

As discussed above, 3-D tracking and imaging system 1600 can

display existing or user acquired image data sets as a template through which, or

against which the position, shape or motion of an instrument can be referenced inside

the body or organ. The algorithm for carrying out this feature will now be described

with reference to Figure 11. It should be appreciated that portions of the "Path 1" algorithm can run both on the PC that houses the circuit boards embodying Figures 2-

6 ("PC") , and/or in a separate computer (not shown) or workstation ("WS") with

additional processing power and 3-D visualization capability.

The process begins with the PC that houses the digital circuit boards. The PC

completes a data acquisition cycle and has many numbers in memory, each

corresponding to a time that the ultrasound pulse took to travel the distance between

all combinations of transducers within the measuring volume (module 1100). Within

this volume, there exist a number of mobile transducers mounted on the instruments

being tracked(see Figure 9), as well as reference transducers located on the patient in

strategic reference locations (see Figure 15). The reference transducers may be

mounted internal to the patient to provide an internal reference frame, or mounted

external to provide an external reference frame. This propagation delay measure, or

"signal", can be corrupted with noise, accordingly some signal processing may be

need to be performed to recover the likely values of the original signal (module 1102).

This can be done by testing for the range of the signal, and by smoothing or predictive

fitting to previous trajectories of the data signal.

Following signal processing, the improved "signal" is converted in the PC,

according to the methodology discussed in detail above with reference to Figures 2-8,

into "data" that correspond to real measurements of distance between pairs of

transducers. This is done by converting the propagation delay into a distance

measurement by taking into account the speed of sound in the particular medium.

This conversion can be a simple linear process, or can be scaled non-linearly, depending on the likely medium through which the sound is propagating. The output

of this conversion is distance measurement "data" (module 1104).

It should be appreciated that the distance measurement data may be corrupted

due to signal dropouts resulting from poor signal propagation throughout the

measurement volume. However, there usually are more than enough individual

distance measurements available to reconstruct the 3-D location of the transducers,

since many extra distances between transducer pairs are obtained. A process of "data

filling" can be performed to fill in the missing data, based on the many combinations

of other distance measurements that are available. "Data filling" can be done using a

'multidimensional scaling algorithm, or variants of it. "Data filling" is an iterative

process and is typically done on the computer workstation ("WS"). The output of the

"data filling" preprocessing step is more complete data.

The data output from module 1106 is then converted (in a well known manner

using geometric algorithms) into 3-D coordinates of the points that are being tracked

(module 1108). These 3-D coordinates are passed to a 3-D scene relationship and

evaluation module that takes the 3-D coordinates, and based on previously obtained

information from user input or a library database, arranges the coordinates in the

correct sequence to construct 3-D structures (module 1110). For example, it would be

known in advance that, for example, transducers numbered 3, 5, 6 and 9 are mounted

on a predetermined one of the instruments (e.g., a catheter), so the coordinates of the

transducers mounted to the instrument would be connected together. The scene

relationship and evaluation module would then construct a 3-D image that would represent the position, size and shape of the instrument, based on the 3-D coordinates

of the individual transducers mounted to the instrument body.

In a similar manner, the transducers mounted to the instrument can be located

in such a way as to build up a 3-D surface patch image of a bodily structure, such as

an organ. For example, transducers mounted to a catheter can be located in such a

way as to build up a 3-D surface patch image of the inside of a beating ventricle, by

simply dragging the catheter along the wall of the ventricle in the area of interest.

The output of module 1110 is a '3-D scene' that contains many of the

elements being processed, some of which represent the instrument and the individual

transducers affixed to the patient (Figure 15). The 3-D scene is then rendered by a 3-

D graphics subsystem rendering/display (module 1112) and output to a display.

If the instrument is stationary, the 3-D scene does not need to be re-rendered

or updated in any way. Therefore, a module 1114 is provided that detects any

changes in the stream of incoming data. If there are changes, this module signals

another module 1116 that determines whether the new 3-D coordinates that have been

acquired and processed by the WS have changed significantly from the previously

rendered objects or scene. If they have, then this updated information is incorporated

into the existing model of the 2-D scene and passed onto the rendering/display module

1112.

The display of the instruments is only one component of the scene relationship

and visualization module 1110. In this regard, the instruments need to be displayed in

reference to some recognizable features, such as a 2-D or 3-D image showing the environment surrounding the instrument. The algorithm for carrying out external

image acquisition is shown schematically in Figure 11 as "Path 2", and begins with

the input of an image from an external image modality (module 1118). As discussed

above, system 1600 includes an imagining modality system 1624 providing externally

acquired image data sets in 2-D or 3-D form. It should be appreciated that these 2-D

or 3-D images may already be in digital form, or may be analog data input directly

from a live video source using a frame grabber.

The acquired image data sets must first be converted into a format that is

suitable for processing and manipulation inside the WS (module 1120). Accordingly,

any analog data is converted to digit data. Therefore, the image data sets that are

output from module 1120 are "digital images" that can be manipulated further inside

the WS.

The image data sets may need to be preprocessed in some way to make them

fit appropriately into the 3-D scene. For instance, if they are to be shown along with

the instruments, the image data sets may need to be scaled appropriately. If the

images are to be moving, they will need to be updated or reformatted in memory so

that they can be output to the 3-D scene rendering/display module 1112 in the correct

sequence. Any such manipulation is performed by the preprocessing module 1122.

Moreover, for video information, an appropriate sync signal is required for

appropriate sequencing (module 1124).

One of the most critical aspects of the 3-D scene relationship and evaluation

module 1110 is the placement of the 3-D image of the instrument in the correct spatial relationship with the underlying images showing the environment surrounding the

instrument. This is done by registering features in the images, such as the reference

transducers, with their position in the measuring coordinate system. This process uses

standard coordinate transformation operations, and only requires for input information

as to which feature in the image space corresponds to the same feature (i.e.,

transducer) in the measurement space. This information can be input by the user

during initial set up (module 1126), or can be automatically detected using image

processing algorithms. Once the instrument image is registered with the underlying

images, the information describing the image set that is to be displayed at a given

instant is sent to the 3-D scene relationship evaluator module 1110. Additionally, to

test whether new image information has arrived and needs to be used, an appropriate

signal is sent to module 1114 that detects changes and instructs the system to update

the scene.

For moving image sets, such as 2-D video loops, or 3-D ultrasound loops of

the heart, the motion of the image data sets need to be output at a rate that continually

matches that of the patient heart beat (see Path 3 in Figure 11). If the image data set

that is played back is not synchronized with the current state of the heart, then the 3-D

scene will not be displayed in a recognizable format and abnormal motion of the

catheters relative to the images, will result.

The first step in synchronizing "video loops" with a patient's heart beat is to

input a raw ECG signal into the processing computer (module 1128). The ECG signal

is converted into digital data using a standard A/D converter (module 1130). The digital data is then fed into sync generator module 1124, which includes an algorithm

that produces a timing signal that corresponds to the current activity of the heart. For

example, the sync generator module 1124 can activate a memory location or an input

port, or generate an interrupt, at the precise time that a QRS complex is identified.

The sync generator module 1124 does this by following the input signal and testing

for large rates of change, combined with zero crossing and other information relevant

to the expected morphology of the signal. The sync generator module 1124 can run in

the PC, the WS, or an external device designed to identify QRS complexes and output

a sync signal to the WS.

Control information is provided by the user interface (module 1126), discussed

above. The user interface checks for user input from a keyboard and/or mouse and

sends appropriate control information to the 3-D scene generator (module 1110), and

to other modules that can be affected by the user input. Typically, user input would

involve the modification of the type of information that is to be displayed on the

display screen, and not the way the signals are processed. The user can also assist in

registering the catheter location of the instrument with the underlying image set.

The system also has a provision for the merging of other auxiliary data

information, such as the display of electric potential over any 3-D structures that are

displayed (module 1132). This information is peripheral to this main system, and is

assembled in a way that can be readily incorporated into the 3-D scene generator

(module 1110). As indicated above, the transducer of the present invention may take many

forms. According to a first alternative embodiment, a cylindrical or ring shaped

ultrasonic transducer is provided, as shown in Figure 12, for attachment to an

instrument (e.g., a catheter or other probe), for the purpose of tracking its position in

three dimensions inside the body or organ. The transducers can be either rigid or

flexible. If they are rigid, they are typically constructed from PZT material, and cast

or milled into an appropriate shape. If the transducers are made flexible, they are

typically constructed from PVDF material that is laminated onto the surface of an

instrument. This material is flexible and can be applied to rounded surfaces. As a

result of the relatively low transmit efficiency of PVDF material, it is likely to be used

for a transducer used in receive mode only. The geometry of the crystal preferably

has principal mode of vibration in the 100 kilohertz to 4 megahertz range, or in the

range referred to as "low frequency ultrasound". The transducers are polarized along

a principal axis (i.e., either through the wall thickness, or along the cylindrical axis)

and the appropriate surfaces are coated with a conductive material to facilitate

connection to leads. If the material is poled through the wall thickness, then the inner

and outer surfaces of the cylinder are plated and conductors are affixed thereto. While

the size of the transducer will depend on the application of the tracking technology,

the inner diameter of the cylinder is typically 5 millimeters or less for catheters and 5

millimeters or more for larger endoscopic probes. It should also be appreciated that

several sections of a cylinder may be placed around the instrument of interest, thus making each individual transducer small for ease of manufacture, mounting or to

control resonant frequency.

As shown in Figure 12, the cylindrical crystal or transducer may incorporate a

lossy backing 1200 on which the piezoelectric material 1210 is disposed. The lossy

backing prevents excessive ringing of the PZT material. As the crystal is energized,

an ultrasound wave propagates both forward and backward. When the ultrasound

wave reaches the interface between the crystal and the outside medium (e.g., water or

air) it meets an impedance mismatch and most of the wave bounces back into the

crystal. This is why the crystal rings for many cycles. The lossy backing enables the

backwards traveling wave to exit the crystal (i.e., it has similar impedance) and

dissipate with minimal reflection. The backing material is typically-epoxy with

tungsten powder mixed in. Ideally, the backing material should be many times thicker

than the crystal itself.

The piezoelectric material 1210 may be coated with a 1/4 wavelength

matching layer of ultrasound conductive material 1220 (e.g., polymer material).

Electrically conductive wires (not shown) are connected to the piezoelectric material.

As discussed above, the forward propagating wave of ultrasound typically bounces off

of the crystal/water interface, unless some impedance matching material is provided.

The purpose of this material is to provide an intermediate impedance between water

and PZT so that at each material interface there is less mismatch, and more of the

ultrasound wave propagates forward, rather than reflecting backward. Typically one

or two layers are deposited on the crystal with intermediate impedances. The thickness of the layers must be 1/4 of the wavelength of the ultrasound wave so that

destructive interface occurs between the reflected waves, thus reducing the ringing of

the crystal.

If PVDF is used for the piezoelectric material 1210, then the film or material

can be wrapped around the instrument, or could be molded or cast directly upon it,

essentially becoming a component of the instrument. It is also contemplated that an

existing instrument (.e.g., catheter) can be retrofitted with PVDF material in

accordance with the embodiment of Figure 12, to facilitate tracking thereof inside the

body. It is also contemplated that the piezoelectric film (e.g., PVDF) can be wrapped,

cast or deposited over the instrument in several locations.

According to a second alternative embodiment for the ultrasonic transducer, a

ring-shaped array of crystals, or a segmented single crystal can be provided, as shown

in Figure 13 A, with a configuration that enables the ultrasound energy to radiate at a

large angle away from perpendicular to the axis of the cylinder, such that the crystal

array functions as a line source of ultrasound energy, or as a collection of point

sources, each radiating ultrasound energy in a fan substantially away from the plane of

the cylinder, as shown in Figures 13B and 13C.

The crystal is provided with a plurality of facets 1300, each being in the order

of a millimeter in size, so as to resonate individually at a resonant frequency dictated

by the size of the facet, rather than the size of the entire ring. The ring is plated with a

conductor 1310 on both sides, as depicted in Figure 13, rather than on the inner and

outer surfaces thereof. According to a third embodiment of the transducer, a composite ultrasonic

transducer is provided comprising a PZT substrate 1400 on a lossy backing 1410. A

PVDF film 1420 is bonded to the PZT substrate 1400. This embodiment offers the

advantages of high transmitting efficiency of PZT (i.e., conversion of electrical

energy into acoustical energy) and the high receiving efficiency of PVDF (i.e.,

conversion of acoustical energy into electrical energy). It is contemplated that the

PVDF and PZT films 1420 and 1400 can be directly connected (as shown), or

electrically isolated with appropriate layers of insulator or conductor therebetween. It

is also contemplated that the PVDF or PZT structure can be in the form of a slab, as

shown in Figure 14, or can be cylindrical, as in the embodiments of Figures 9, 10, 12

or 13.

Figure 15 illustrates the manner in which the external reference transducers are

placed. The purpose of the external reference transducer is to provide an external

reference frame (i.e., outside the body) to monitor the accuracy and movement of the

transducers mounted on the instrument. As can be seen, the transducers are placed in

a harness-type apparatus that is worn around the chest by the patient during a surgical

procedure. A number of radio-opaque transducers are fastened to the harness in

locations suitable for optimal signal reception through the chest cavity. Alternatively,

the external reference transducer may be affixed directly to the patient at strategic

locations, using self adhesive mounting film or adhesive tape.

Under the disclosed configuration, it is possible to monitor the position and

direction of the instruments that are introduced into the body, (e.g., catheters introduced into the human circulatory system). This methodology significantly

reduces both the risk and the procedural time associated with current

electrophysiology and angiology operations, while providing improved positioning

accuracy.

A detailed description of various exemplary medical procedures using the 3-D

tracking and imaging system described above, as set forth below,

i) TRACKING OF CATHETERS THROUGH THE HUMAN CIRCULATORY

SYSTEM

Catheters are devices that are inserted into the veins or arteries of humans as

part of a procedure in which qualified hospital personnel, remove blockages and

obstructions from the circulatory system, or correct other related problems. The 3-D

tracking and imaging system of the present invention may be configured to operate as

a catheter guidance system (CGS) that can be used to track various types of

instruments, including catheters, probes and needles.

The current method of tracking catheters involves frequent exposure of the

patient to an x-ray source. Each successive x-ray provides information on the

movement of the catheters within the patient. In addition, contrast agents are

frequently injected into patients during catheter procedures. These injections can

provide further information on the actual location of the catheter and help physicians

to plan subsequent catheter movements. X-ray radiation and contrast agent injections are each potentially harmful to

the health of the patient. Further, these methods of tracking are also time consuming,

often introducing additional stress and patient complications.

Three primary advantages result from the present invention when used to track

catheters:

1) The need for using harmful x-rays and contrast agents are virtually

eliminated while determining the location of catheters) within the

patient;

2) Procedure times are substantially reduced with benefits in both safety

and cost; and

3) Extremely exact positioning of the catheter is obtained as a result of

the theoretical resolution of 19μm, according to the present

embodiment of the system.

The basic principle of the catheter guidance system (CGS) of the present

invention involves the establishment of an internal reference frame and an (optional)

external reference frame in three dimensions from which the catheter can be tracked.

Using the transceiver hardware and the triangulation algorithm discussed above, the

crystal positioning data can be captured and processed to resolve the location of the

catheter of interest.

To further facilitate visualization of the catheter location by the administering

hospital staff, the transducer position information may be overlaid onto a recorded

video loop of the region of interest. This video loop can be generated from an imaging modality such as x-ray or scanning ultrasound and is meant to illustrate the

natural movement of the biological structures during one or more cardiac cycles. In

addition to this, the video loop can also depict the position of the opaque piezoelectric

crystals (XI) used by the CGS to track the catheters. These piezoelectric crystals

serve as "landmarks" (whether they are internal or external). By identifying these

"landmarks" in the video, the positions of the guiding piezoelectric crystals can be

correlated with the captured video information. In this fashion, the imaging process

and the ultrasound positioning process can be linked for one or more complete cardiac

cycles. Once the imaging modalities are linked, the graphic video loop can be

substituted for the potentially harmful imaging (and contrast agent injections)

throughout the rest of the procedure.

Typically, the catheters used in these procedures are introduced into the body

through the femoral vein or artery. From the point of entry, the catheters are pushed

and steered, using internal guide wires to the region of interest, usually the human

heart. Physically, the catheters are constructed with a biocompatible plastic and

feature such options as electrode sensors and actuators for detecting the cardiac

activity in electrophysical operations to inflatable balloons for arterial expansion in

angiology procedures.

A concept that is of importance in implementing the CGS application of the

present invention is the merging of piezoelectric crystals and the imaged catheters.

Since the design of catheters used for these procedures are well established, consideration has been given to the design of the ultrasonic sensor, including the

following aspects:

1. The type of piezoelectric material used.

2. The encapsulation procedure.

3. The shape of the transducer.

4. The operating frequency.

5. The activation procedure.

The material selected for use in both the internal and external reference frames

must possess superior transmission and reception characteristics in order to properly

communicate with each other. Since operating temperatures inside the human body

are not a major concern, a higher dielectric material with lower Curie temperature can

be employed. Essentially, this provides for an increased ultrasonic output per input

volt. The preferred material for this purpose is PZT (lead zirconate titanate).

Since these materials are non-biocompatible, an appropriate encapsulation

material is used. The encapsulant must not only be biocompatible, but must also

possess an acoustic impedance that does not tinder the ultrasonic wave propagation.

This is of key importance for the internal reference frame transducers.

The external reference transducer crystals require an acoustic coupling gel

similar to that used for standard B-type ultrasound scans. Omni-directional

ultrasound transmission, cylindrical crystals (XI) are used for the internal reference

frame. The cylindrical crystals maintain omni-directional radiation patterns while demonstrating excellent transmission and reception characteristics. Externally, larger

disk-type or hemispherical crystals are employed for the reference transducers.

Due to the variable software controls of the 3-D tracking and imaging system

according to the present invention, the activation frequency can be optimized for

maximum performance and efficiency. In the case of the internal reference frame,

smaller distances are monitored, therefore higher activation cycle frequencies can be

used. The opposite is true of the external reference frame.

For both reference frames, the method of transducer activation is identical.

This process in discussed in detail above with reference to Figure 6. An insulated

conducting wire is used to carry the activation impulse from the control unit to the

transducers. In the case of the transducers mounted to the catheter, the signal wires

are internally routed through the same sheath as the steering guide wires. Finally,

placement of the transducers is contingent upon which reference frame is employed.

Figure 9 illustrates the placement of the cylindrical transducers with respect to the

catheter tip, according to the proposed catheter guidance application of the present

invention. As can be seen, two ultrasonic crystals (XI, X2) are used on each catheter.

This permits the transducers to communicate with each other, as well as to every other

internally placed transducer in the region, and also the external reference transducers.

By using the information from two concentric transducers mounted on a catheter,

vector data can be acquired to illustrate not only the position of the tip, but also the

direction. By using three or more transducers, the curvature and 3-D shape of the

catheter can be reconstructed. As can be seen, the two (or more) crystals (XI, X2) are permanently

positioned concentrically along the axis of the catheter (C) at an appropriate

separation distance for indicating catheter location, orientation and curvature. The

piezoelectric material can bet affixed to the catheter with a variety of means, such as a

pressfit, bonding, costing or vapor deposition.

One embodiment of the transducer arrangement of Figure 9, is illustrated in

cross-section in Figure 10. A multi-lumen catheter 100 (or any other suitable probe)

is inserted into the body, such that the 3-D shape or extent of the device can be

measured or represented, as discussed in greater detail below. As an alternative to

using piezoelectric crystals 110, film patches may be used, such as PVDF

(polyvinyldifluoride). PVDF is not a crystalline material, but a polymer. It is

therefore made in sheets or strips and can be affixed to the catheter as a thin,

rectangular patch of film. Its principle of operation is similar to that of PZT. PVDF is

essentially a piezoelectric material that can be easily molded into different shapes and

configurations.

The catheter 100 can be fabricated from any suitable polymer. A wire or wires

(not shown) can pass through one of the lumens of catheter 100, or can be

incorporated into the polymer during manufacture. The piezoelectric crystals 110 can

be partially or completely embedded into the wall of the catheter 100 or can be affixed

to the surface thereof. The crystals are preferably mounted on a suitable lossy backing

130 to which electrical conductors 140 are connected. The crystals 110 can also be

provided with a dome-shaped polymer lens 150 affixed thereto. ii) TRACKING OF INTRAVASCULAR ULTRASOUND IMAGING

CATHETERS, BALLOON CATHETERS, AND STENT DEPLOYMENT CATHETERS THROUGH CORONARY ARTERIES AND THROUGH PERIPHERAL VASCULATURE

The tracking of catheters can be extended into the field of intravascular

ultrasound. Intravascular ultrasound imaging is gaining increased acceptance as a

means of diagnosing the severity and spatial distribution of atherosclerotic plaque.

Intravascular ultrasound imaging involves the placement of ultrasound imaging

transducers (e.g. PZT transducers) on the tip of a catheter. The ultrasound imaging

transducers rotate to provide a 2-dimensional circular picture of a cross-section of a

coronary artery and the local pathology at the level of the artery where the tip of the

catheter is located. The ultrasound frequency of commercially available intravascular

ultrasound imaging systems is typically 30MHz. An intravascular ultrasound imaging

system can detect whether the plaque is calcified or just fibrosed, as well as the

overall shape of the blockage and the remaining lumen of the coronary artery. In this

regard, calcified plaque procues a shadow behind it, and it can be identified by a

trained physician, and sometimes through image processing. A fibrous plaque is often

a bump in the wall of the vessels and both proximal and distal edges are visible on an

ultrasound image, as the plaque is somewhat transparent to ultrasound, just like the

vessel itself. This information is often used as a diagnostic technique to plan

interventions such as angioplasty (a balloon dilation of the artery) or intravascular

stenting a metallic tube that keeps the artery distended and patent.

It has recently become clear that simple 2-D imaging of the artery generated

by the ultrasound imaging transducers is not sufficient. Very often, contrast angiography under continuous fluoroscopy is used to determine where the catheter is

sitting relative to the branches of the coronary arteries. Often, it is important to

determine if the blockage and lesion is near a bend or not, and also what is the 3-D

topography of the lesion. Often, if the lesion is near a bend, a different therapeutic

plan may be implemented. For example, if the lesion is at the bend, an intravascular

stent would not be deployed in that region. It should be noted that a lesion is visible

on across-section of the artery using intervascular ultrasound imaging, and during

contrast angiography, is also visible a narrowing of the artery. It should be

appreciated that the precise shape of a vent of an artery may determine the way stents

are placed.

Three-dimensional reconstruction of the intravascular ultrasound images adds

diagnostic value to the procedure. Typically, the intravascular ultrasound images

generated by the ultrasound imaging transducers are saved continuously as the

ultrasound imaging catheter is slowly pulled back along the coronary artery. These

images are then stacked beside each other, and displayed as a 3-D reconstruction of

the artery. The problem is that as the ultrasound imaging catheter is pulled back to

acquire a series of slices, only the position along the length of the vessel can be

measured. The result is that a vessel is always reconstructed as if it were a straight

tube. Even a highly tortuous vessel will be reconstructed as a straight tube, since the

pull-back distance is measured at the point of entry into the patient, not at the imaging

site. Furthermore, the pull-back position is not very indicative of the true distance

along the artery. Between the entry point and the imaging tip of the ultrasound imaging catheter, there are considerable bends. A small pull-back of the ultrasound

imaging catheter at the entry point is seldom reflected by a similar motion at the

imaging tip. Very often the imaging tip moves as a series of jumps, rather than a

smooth continuous motion.

The position of the intravascular ultrasound imaging catheter can be easily

tracked by mounting a low frequency transmitter near the imaging tip of the

ultrasound imaging catheter. By having a dual display showing the view inside the

vessel with the ultrasound, and the position of the imaging area relative to the gross

moφhology of the vessel on the angiogram, the angiologist can better treat the lesions

and reduce the procedural risks to the patient.

The procedure for tracking the ultrasound imaging catheter will now be

described in detail. By mounting PZT or PVDF transducers on the imaging tip of the

ultrasound imaging catheter and mounting additional reference transducers internal or

external to the patient to provide one or more reference frames, the catheter guidance

system (CGS) can be used to track the 3-D position of the imaging tip, and the spatial

location of the multiple slices can be recorded. The spacing of the slices can be

adjusted to reflect non-uniform motion at the imaging tip, and the angulation of slices

is also determined from the orientation of the imaging tip. It should be appreciated

that the transducers should be placed close enough to the imaging tip of the ultrasound

imaging catheter such that its motion can be referenced to the transducers. If the

imaging tip is rigid, then two transducers can be placed along the rigid section to

obtain a vector peφendicular to the imaging plane. Typically, the imaging transducers are not located at the imaging tip, but a little further down, since the

imaging tip often has a little rubber flexible section to enable easier insertion into an

artery.

Tracking the imaging tip enables the reconstruction of the true 3-D shape of

the artery with all its twists and bends. Such information is very valuable for the

planning of dilation or coronary stenting procedures. During such a procedure, the

computer system will typically record the 3-D position of the imaging tip, as well as

its angular orientation, and save the data to a file as the same instant that the

ultrasound image is saved. During image processing and 3-D reconstruction, this

information will be used to reformat and spatially locate the sequential images, such

that an accurate shape of the reconstructed artery can be obtained. It should be

appreciated that the x,y,z, coordinates of the catheter imaging tip, referenced to time

or image number, are saved to enable the reformatting and reconstruction of a 3-D

image data set. The same 3-D data set can be used to advance the therapeutic

catheters to the appropriate position, using the 3-D tracking procedures described

above,

iii) TRACKING OF BIOPSY CATHETERS OR BIOPSY NEEDLES

Biopsies are typically performed to diagnose organ diseases, (e.g., cancer) or

organ rejection. The tracking of biopsy catheters is of particular importance, because

occasionally the biopsy "bites" are taken from the wrong part of the heart, liver, breast

or other tissue or organ to be sampled. This is particularly significant in biopsies that

are supposed to be diagnostic of cancer. In breast, liver and other internal organs, the tissues are highly deformable and catheter-based or stereotactic biopsies are typically

not successful. In the case of the heart, sometimes a piece of the coronary artery is cut

off, or the cardiac valve is damaged, with obvious complications to the patient. By

following the path of the biopsy device, using single or multiple angiograms, x-ray

images, or ultrasound image sets and real time overlay of the tracked biopsy catheter,

the biopsy procedure itself can be made more precise and safe.

Biopsy needles can also be tracked with ultrasound, such as when cannulating

the carotid artery or the femoral artery. Prior art systems rely on having the needle

cast a faint shadow in the B-mode ultrasound image. This shadow is not readily

visible to the untrained eye, and has obvious limitations in precision. A true 3-D

tracking of the needle under real time ultrasound using the principles of the present

invention greatly simplifies such procedures.

These procedures for tracking biopsy catheters and biopsy needles can be

improved significantly by using an internal reference frame. In this regard, reference

transducers are inserted into the surrounding area (internal reference frame) using

large gauge needles, or affixed to the surface of the patient (external reference frame),

such as in the case of the breast procedures. The transducers mounted to the needle

are preferably thin PVDF film, or very small slivers of PVT, embedded in the metal

shaft of the needle. Importantly, the transducers must be located such that they

surround the area of interest. The bodily structure (e.g., organ or tissue) is imaged as

usual, the 3-D image set is registered with the reference transducers, and the

procedure performed as described above. The visualization system may provide 3-D images or a collection of 2-D images, in which the progression of the biopsy catheter

or needle can be viewed in real time,

iv) TRACKING OF AMNIOCENTESIS NEEDLES

Another application of the real time tracking system of the present invention in

the tracking of needles for use in the procedure of amniocentesis. A 3-D or 2-D image

set of the fetus with the motion of the needle displayed, can increase the precision and

speed of the procedure and can prevent injury to the fetus. In the case of

amniocentesis, the reference transducers are placed on the sides and top of the

patient's belly.

v) GUIDING OF PROBES DURING STEREOTACTIC SURGERY

During some delicate surgeries, particularly in the brain, it is important to

know precisely the 3-D position of the probe inserted into the head. Typical brain

conditions that require the guidance of a probe during stereotactic surgery are removal

of brain tumors, the surgical correction of aneurysms, and electrical abnormalities like

epilepsy (ablation, destruction of neural tissue).

The conventional method for guiding probes involves rigidly fastening the

patient's head to a stereotactic frame by placing screws and pins into the patient's

skull. The patient, with the frame attached, is then imaged using MRI or CAT, and a

3-D reconstruction of the patient's head is created. Pathologic tissue or lesions, such

as tumors, are then precisely located relative to the frame. The patient is then taken to

the operating room and the required instruments, such as electrodes or ablators, are

affixed to guides that allow the instruments to be moved along the specific paths into the patient's head. Once the surgical instrument is in place, the lesion can be

corrected, destroyed or treated in some way. The foregoing approach is tedious,

costly and subject to measurement error in translating the 3-D coordinates from the

images to the actual position of the probes within the stereotactic frame.

An alternative approach involves the use of a 3-D wand. This instrument

consists of an articulating metallic arm that is rigidly affixed to a surgical table. Each

of the joints in the arm has an angular position sensor so that the 3-D coordinates of

the probe tip can be calculated from the joint sensors. By matching visual landmarks

on the patient's head to the same landmarks on the 3-D image using the probe, the

head and the 3-D image can be registered with each other. The probe is then used

during surgery to hold instruments and guide them into the brain in a manner similar

to the stereotactic frame. The advantage of the wand is that it has many more degrees

of freedom, and can be held by the surgeon. The disadvantage is that it is very

expensive, and very bulky. Also, the position of the probe tip is always only as

precise as the original calibration against the patient's head. The patient's head must

remain rigidly affixed to the table to which the articulating arm is fixed.

An existing alternative to the 3-D wand is an air-based 3-D locator system,

which is the subject of U.S. Patent No. 5,517,990. In this system, the position of a

freely held wand is determined using triangulation of audible sound pulses in air. One

major disadvantage to this approach is that the air-based 3-D locator system requires

microphones to be placed at fixed sites in the operating scene. The patient's head must still be rigidly affixed to the table and the surgeons must take care not to get in

the way of the sound path. This severely limits the freedom to perform the surgery.

A further application of the 3-D tracking and imaging system according to the

present invention involves affixing reference transducers anywhere on the patient's

head, and several transducers on the tip and shaft of the probe. The transducers can be

conventional PZT material, or PVDF. Typically, a minimum of four reference

transducers would be placed against the patient's skull, in order to generate an

external reference frame. Other reference transducers would be inserted into the brain

to provide an internal reference frame.

As the probe is inserted into the head, its movement relative to the reference

transducers can be tracked in real time 3-D. The probe will be inserted into the brain,

toward the lesion which is visible on a 3-D data set that was previously acquired

through CT or MRI. Accordingly, the path of the probe is followed on a computer

screen toward the lesion, rather than actually looking at the brain. The lesion itself is

located by the position using MRI or CT imaging. The reference transducers affixed

to the patient's head can be imaged along with the patient, simplifying the registration

process, and since they are affixed to the head, movements of the head relative to the

operating table do not pose a problem with respect to tracking. Since the sound path

is inside the patient's head, surgeons have complete freedom to move about the

patient. As in the catheter guidance system (CGS) described above, the location of

the probes is tracked with respect to a reference system. For the head and brain, the

most appropriate reference system are 3-D MRI images. Patients with electrical disturbances of the brain, such as epilepsy, need to

have the location of the epilepsy mapped properly prior to surgical intervention. This

is done by placing surface electrodes subdurally over the brain. These electrodes are

pushed along the brain through small access holes drilled into the skull. However, the

location of the electrodes is often difficult to know precisely. Transmitter or receiver

transducers are arranged on an electrode pad and on complementary electrode pad

located on the outside of the skull. Accordingly, the motion of the electrodes can be

tracked in real time, or can be verified with images of the brain taken previously. This

greatly simplifies the mapping of brain wave activity anomalies.

The system for carrying out the foregoing procedure would preferably be

comprised of the 3-D locating and imaging system described in detail above. Several

transducers (PZT or PVDF) are mounted on the surgical probe to be inserted into the

brain. Reference transducers are mounted either on the outside of the skull, or slid

inside the head, between the brain and the skull. The reference transducers placed on

the outside of the patient's head may be taped on or include adhesive backing tape.

The reference transducers inserted into the brain will be anchored by sutures, hooks,

or simply by friction alone. The patient may be imaged using MRI, CT, or any other

multidimensional imaging modality, with the reference transducers in place.

Accordingly, a volume reconstruction of the patient's head would be obtained. The

reference transducer may be fitted with components that would enable them to be

clearly seen on the 3-D image sets. A physician could then locate the lesion, or plan

the surgical approach with respect to these referenced transducers. For example, the physician could draw a path in 3-D space that would be optimal to follow during the

procedures. This path, as well as the intended location of the end point of the

procedure, would be recorded by the computer system. The physician could also

mark the location of the multiple transducers on the 3-D images, so that the computer

system could register the physical location of the transducers to a location in the 3-D

image space. It should be appreciated that depending on where the lesion is located in

the brain, the surgical probe will be inserted from a different direction. Moreover, it is

often necessary to go around critical areas so that minimal damage is done to the brain

during insertion of the surgical probe.

The patient would be taken to the operating room with the reference

transducers still affixed to the head, and connected to the 3-D locating and imaging

system. The surgical probe would then be inserted into the brain, and its location in

3-D space would be instantly displayed within the 3-D volume of the brain, properly

registered with the location of the reference transducers. By watching the progression

of the surgical probe through the brain, the surgeon would be able to follow a

predetermined path towards an intended target.

If the reference transducers are placed on the brain, or implanted within the

brain, the path towards a lesion could be tracked even more precisely, since some

deformation of the brain is expected upon manipulation of the patient and the

insertion of the surgical probes into the brain. Therefore, if the reference transducers

detect that their mutual spatial relationship has changed significantly, a coordinate

transformation may be implemented, such that the 3-D image set of the brain is waφed to the new geometry, as provided by the reference transducers. The required

path towards the lesion can be altered according to this spatial transformation and the

surgeon can be confident that the surgical probe is actually progressing towards its

intended position. It is because of the expected motion or deformation of the

reference transducers, that the internal reference frame is of significant benefit. If a

rigid, external reference frame is used, deformation of the brain during surgery could

not be detected and the stereotactic probe could very well miss its intended target.

It should be appreciated that the foregoing procedure can be easily integrated

with robotic surgery. Therefore, rather than relying on a surgeon to slowly insert the

probate, a much more precise robotic arm may be used. Again, the presence of the

internal reference frame, and its ability to detect and correct for internal deformations

can ensure that the probe is moving along its intended path through the brain, even

though the brain tissue in its path may deform.

It should be appreciated that robotic surgery is particularly advantageous

where a"steady hand" is required. In this regard, a robotic arm can precisely

maneuver along a predetermined path determined by a physician in a 3-D image of the

brain. In order to monitor the progress of the robotic arm, a feedback signal may be

sent to a control system to make sure that the robotic arm is actually following the

intended path.

vi) EMBOLIZATION OF LEAKING BRAIN VESSELS

The 3-D tracking and imaging system described above is also useful during

catheter based brain surgery to seal leaking cerebral arteries. Presently, when a patient has a burst cerebral artery, the vessel needs to be sealed up surgically through

open brain surgery, or by plugging it up internally with microcatheters.

Microcatheters are generally .3 mm I.D., .5 mm O.D., with a guide wire inside.

Typically microcatheters take the form of very small plastic tubes that are flexible in

bending, but rigid in torsion so that they can be twisted and manipulated. The

microcatheters are threaded into the brain under fluoroscopy until they are near the

site of the lesion. The catheter is then used to inject a sealant, such as glue (e.g., n-

butyl-cyanoacrylate) mixed with a contrast agent, that hardens and embolizes, and

then plugs up the micro artery down stream. Other sealants that may be deposited are

PVAC sponge (100 - 300 mm dia.), and platinum micro coils (0.010" dia.). The

problem with the foregoing procedure is that physicians do not know exactly where

the released materials go, and can only track their path using multiple views on

fluoroscopy.

The 3-D tracking and imaging system described above dramatically improves

the precision for the foregoing procedures. As in the case of brain surgery, a 3-D

image set of the cerebral vasculature is obtained and the progression through the

vasculature may be monitored by projecting the position of the catheter within the 3-D

scene of the vasculature. This may be done by mounting appropriate PZT or PVDF

transducers on the catheters and tracking them as described above. The 3-D scene of

the cerebral vasculature may be obtained through a CT scan with contrast agent

injected into the patient, or with MRI tuned to show moving blood. Similar to the

brain surgery procedure described above, transducers are arranged on the catheter and reference transducers are arranged on the patient's head. It should be noted that the

placement of the external reference transducers is dictated by the location of

"ultrasound" windows that enable signals to be sent into the skull. Accordingly, the

preferred position for the reference transducers are the base of the head near the back,

the eye sockets, or under the chin.

The catheters are inserted into the brain initially through an artery near the

head, such as the carotid, and maneuvered into the brain. Leaking vessels are located

in the brain using contrast and angiography. A contrast agent is injected into the

blood stream, and the patient's head is imaged under x-ray fluoroscopy. The presence

of the contrast agent makes the arteries into the head visible and the leaks in the brain

appear as fuzzy areas of fluid leakage. The 3-D scene of the vasculature is registered

with the position of the reference transducers attached to the patient's head. The

location and shape of the microcatheter will then be displayed on a computer screen.

Accordingly, the physician can interactively steer and advance the catheter through

the many branches until it reaches the intended site. The specific sealant used in the

procedure can then be released at just the right spot. The principal advantage of the

foregoing approach is that the potentially harmful, long-term exposure to X-rays

required when using fluoroscopy, can be eliminated,

vii) MEASUREMENT OF CERVICAL DILATION

The onset of labor can be a well controlled process. During the first set of

contractions, nurses periodically track the dilation of the cervix. At present this is

done by checking the width of the cervix manually. More specifically, one or two fingers are inserted to feel for the head of the fetus, and estimate the degree of cervical

dilation. These dilation measurements are done at regular intervals and a time/dilation

curve may be plotted. As a result, the obstetrician can plan the delivery, since the

major contractions will come once the rate of cervical dilation increases.

The plotting of such dilation curves can be automated and managed for many

mothers in the delivery room by measuring the dilation of the cervix with ultrasonic

transducers according to the principles of the present invention. Therefore, a

maternity ward can be networked so that progress of many mothers going through

labor can be monitored remotely by a few nurses at a central station. The obstetrician

is thus able to predict which patient is due to deliver at what time, and can plan his or

her activities more precisely,

viii) EVALUATION OF KNEE STABILITY

In some orthopaedic procedures, the stability of the knee needs to be evaluated

quantitatively during walking. Knee stability can be assessed through manual

manipulation; however, only by using a complex imaging technique can the motion of

the knee during walking be mapped. By implanting the transducers of the present

invention in the knee, the relative motion of the joints can be measured quantitatively

during normal gait, and any surgery to augment ligaments can be better planned,

ix) ASSESSMENT OF MYOCARDIAL CONTRACTILITY FOLLOWING

SURGERY

Following open heart surgery to repair the myocardium or the coronary

arteries, the patient has to be monitored to adjust the levels of drugs that are administered. This is referred to as "titration" of drugs. The myocardial contractility

is measured with a Swan-Ganz catheter and the drug level adjusted to obtain optimal

cardiac function. Unfortunately, the Swan-Ganz catheter measures pressure, which is

an indirect measure of contractility and can produce inadequate data.

However, a pair of transducer according to the present invention provide direct

measure of myocardial contractility if attached to the beating ventricle. The

transducers can be attached to the myocardium during open chest surgery and can

measure the contractility of the heart directly while the chest is open. The leads can

then be strung out through the chest wall, and monitoring of myocardial contractility

can continue for a few hours or days post operatively. This approach replaces the less

precise Swan-Ganz catheter, and can be used to titrate the drugs given to the patient.

If the transducers are properly positioned, they can be removed post operatively by

pulling on them, in much the same way that pacing electrodes are removed,

x) EYE SURGERY

Recently, ultrasonic microscopic imaging has made tremendous progress.

Accordingly, it is now possible to obtain 3-D images of the eye using high resolution

3-D ultrasound. For highly precise eye surgery, stereotactic tracking of surgical

probes can be done using the 3-D tracking and imaging system of the present

invention. Like the brain surgery procedures described above, the movement of the

probes can be tracked in real time, and their position displayed within a 3-D image of

the organ. In the case of the eye, a high quality 3-D image of the eye may be used to provide the template through which the probes are to be moved. It should be

appreciated that this application is highly suitable for robotic microsurgery,

xi) PROSTATE SURGERY

Prostate surgery may be required when a prostate has become enlarged. This

condition can cause such problems as incontinence and impotence. In many cases, the

enlarged prostate is caused by a tumor. The tumor will need to be destroyed or

removed, otherwise it may spread and kill the patient.

As in the case of eye surgery, 3-D ultrasound is useful in the diagnosis of

prostate tumors. 3-D ultrasound may be used to visualize the size and shape of the

prostate and may also be used to aid in stereotactic surgery of the prostate. However,

it should be noted that 3-D ultrasound typically gives poor quality images and does

not provide real time position feedback. Accordingly, there is a need to track the

position of the surgical probes in real time using some other modality. The approach

used in connection with the prostate and other internal organs would be essentially the

same as the approach used in connection with the eye. In this respect, the organ of

interest would be imaged using 3-D ultrasound, and the probes visualized with the 3-

D tracking and imaging system of the present invention, as the probes are manipulated

through the organ. The region to be treated is initially determined using an x-ray or

ultrasound.

The probes are inserted through the urethra, or through skin below the penis.

Since it takes several minutes to obtain a new 3-D image, the 3-D scene can be

periodically updated to show the progress of the surgery. Transducers are placed at several sections along the length of the probe, to provide accurate tracking of the

probe location. Reference transducers are placed on the patient's body at locations

that enable them to communicate with the transducers mounted to the probe.

Therefore, the reference transducers need to be in locations that surround the area of

interest, and also have appropriate ultrasound "windows" through which the sound

can pass. Accordingly, bones and air pockets may not be in the way.

xii) TRACKING OF TMR CATHETERS

TMR is an acronym for Trans-Myocardial Revascularization. This procedure

involves the "drilling" of holes through the myocardium wall to improve circulation

through the tissue. When coronary arteries become blocked with atherosclerotic

plaque, the heart muscle downstream receive less blood than necessary and becomes

ischemic. The traditional surgical approach to this problem is coronary artery bypass.

A novel approach to revascularizing ischemic myocardium, without the lengthy and

potentially complicated coronary bypass procedure, is to use TMR. The conventional

approach to TMR is to expose the heart through a chest incision and drill holes

through the muscle from the outside in, all the way into the ventricular chamber. This

is done using a fiber optic light guide, (about 1 to 2 millimeters in diameter) that

carries laser light energy. The fiber guide is placed against the heart, peφendicular to

the surface and pushed into the heart as the laser is fired. The laser drills a channel

ahead of it as the light guide is pushed through the muscle. Once the hole has been

drilled, blood very briefly squirts out through the hole, but the outer portion of the

hole quickly seals with clot and physically contracts as the heart muscle beats. During a typical procedure, the surgeon would drill a dozen or more holes through the

myocardium in the ischemic area. The benefit to the patient is almost immediate, and

continues for many months afterwards.

While it is not clear exactly how and why the procedure works, it is speculated

that in the short term, the holes created allow blood to percolate through the muscle in

some way, while in the long term, the injury created stimulates angiogenesis, or the

growing of new collateral capillaries and arterioles.

The obvious disadvantage of the conventional approach to TMR is the need to

expose the heart. A preferred method would be to use a catheter based approach that

would drill the holes from the inside out. Such a procedure, however, requires proper

guidance and visualization of the catheter within the heart. The 3-D tracking and

imaging system of the present invention is well suited to this medical procedure. The

TMR catheters may be fitted with ultrasonic transducers in much the same way as

electrophysiology catheters.

Since the laser energy must be applied to the tissue directly, and cannot be

passed through the blood, the light guide must be in contact with the tissue before the

laser is fired. The required TMR catheter can therefore be fitted with a solid state

(MEMS) force transducers that can measure the applied tip force against the catheter

in 3 axes. The tip of the catheter may also be fitted with an ultrasonic transducer to

look forward and determine to what depth the catheter has been passed through the

muscle, and if it is about to come out the other side of the ventricle,

xiii) A 3-D ENVIRONMENT WITH 2-D ECHO PLANES PRESENTED WITHIN A large number of diagnostic and surgical procedures are performed with the

assistance of conventional 2-D echo imaging techniques. In this respect, the physician

manipulates a surgical instrument (e.g., a probe or catheter) inside the patient's body,

and at the same time tilts the imaging head of the echo machine until the ultrasound

beam transects the instrument and the instrument becomes visible on a display

monitor. Very often, the instruments are located after considerable "hunting", and the

process itself is quite cumbersome. This procedure is often used in the case of

amniocentesis and biopsies. In each case, needle or "biting" tools are inserted under

ultrasound imaging guidance. In the case of amniocentesis, a physician inserts a

needle through the abdomen into the uterus, while at the same time an assistant holds

the ultrasound probe steady, making sure that it transects the amniocentesis needle, so

that the needle can be visualized.

It would be advantageous to use a virtual 3-D environment that can be

visualized on the display monitor of a suitable 3-D graphics computer, according to

the present invention. In this regard, several reference transducers are placed on the

back and abdomen of the patient, and a simple coordinate system is generated on the

computer monitor, showing the direction towards the head and feet, the left and right

sides of the patient, and the front and back. Initially, this would appear as an empty

box with simple graphics or arrows, as well as the surface transducers shown

graphically. When an amniocentesis needle is inserted into the abdomen, one or more

ultrasonic transducers mounted along the needle shaft are tracked and displayed

within the 3-D environment as simple graphical elements. This 3-D scene can be visualized from any viewpoint by simply moving the mouse and rotating the visual

scene on the computer.

A preferred embodiment of an echo imaging system according to the present

invention will now be described with reference to Figs. 17 A, 17B and 18. A typical

echo machine ultrasound imaging head 1700 has a plastic hand held component with a

cable 1710 that connects to a main unit (not shown). Imaging heading 1700 has a

window through which ultrasound is transmitted and received by an image transducer

1702. The ultrasound imaging head 1700 is fitted with a transducer housing 1704

which clips to imaging head 1700. An exploded view is shown in Fig. 17B.

Transducer housing 1704 holds three or more position transducers 1706 that

form a plane peφendicular to the imaging beam. Thus, position transducers 1706

reside between the imaging head 1700 and the skin that imaging head 1700 contacts.

It should be appreciated that while four position transducers 1706 are shown in Figs.

17A and 17B, only three position transducers 1706 are need to measure all angles.

Reference transducers (not shown) are mounted to the patient's skin (e.g., back and

abdomen). As the imaging head 1700 is tilted and angulated while pressed against the

abdomen, the coordinates of position transducers 1706 define a plane that is

peφendicular to the ultrasound imaging beam. It should be noted that transducer

housing 1704 makes contact with the abdomen. Once the position and orientation of

the imaging plane is known in 3-D space relative to the coordinate system of the

patient, the typical pie-shaped sector scan produced by the ultrasound imaging head

can be inserted into the 3-D scene of the patient. The 3-D scene will therefore contain a perspective rendering of the patient frame of reference, the location and direction of

the surgical instrument (e.g., amniocentesis needle), and the pie-shaped, ultrasound

sector image, properly oriented within this scene, as shown in Fig. 18.

The ultrasound image may be shown in real time in perspective by texture-

mapping the video signal onto a pie-shaped polygon drawn in the 3-D scene. Current

generation graphics computers enable this type of real-time image transformation. It

should be noted that the location of all of the transducers with respect to each other

can be determined, in the manner described above. Accordingly, one of the

transducer locations is chosen as the origin, another as the x axis, and a third as the y

axis, and a fourth as the z axis. The coordinates system may be defined by the user.

The orientation of the imaging plane is calculated from the angle of the four imaging

head transducers 1706, and the coordinate system defined by the reference transducers

mounted to the patient's body.

By visualizing the location of the imaging plane relative to the inserted

surgical instrument, the imaging head 1700 can be more quickly manipulated and

angled until it transects the surgical instrument (e.g., amniocentesis needle). The

shadow of the surgical instrument then becomes visible in the texture mapped, real

time ultrasound image, and the surgical instrument graphic can be shown piercing the

ultrasound image where its shadow is visible. Accordingly, a physician can

immediately determine in which direction to angulate imaging head 1700 or move the

surgical instrument to get proper orientation within the visual scene. Accordingly, the

foregoing procedure is thus safer, faster and more precise than prior art procedures. The foregoing description provides specific embodiments of the present

invention. It should be appreciated that these embodiments are described for puφoses

of illustration only, and that numerous alterations and modifications may be practiced

by those skilled in the art without departing from the spirit and scope of the invention.

For instance, it should be appreciated that the transducers may use the time of flight,

or phase differences as a means of determining position. Moreover, the transducers

may take the form of ultrasonic transducers or electromagnetic transducers. It is

intended that all such modifications and alterations be included insofar as they come

within the scope of the invention as claimed or the equivalents thereof.

Claims

The invention claimed is:

1. A method of performing an in vivo medical procedure on an associated body using a 3-D tracking and imaging system, said method comprising:

mounting a plurality of mobile transducer means to an instrument

means;

mounting one or more reference transducers means to locations on

the body, the reference transducer means having a position fixed relative to the

body; generating three-dimensional coordinates of the mobile transducer

means relative to a reference frame established by the reference transducer means;

generating image data of the environment surrounding the instrument

means to provide an image template; and

registering the three-dimensional coordinates with the image data to

form a 3-D image scene showing the position of the mobile transducer means

relative to the image template.

2. The method as defined in claim 1, wherein the body is a human

body.

3. The method as defined in claim 2, wherein the medical procedure is the

removal of an obstruction from the circulatory system, and said instrument is one of the following: a standard catheter, an intravascular ultrasound imaging catheter, a

balloon catheter and a stent deployment catheter.

4. The method as defined in claim 2, wherein the medical procedure is a

biopsy, and said instrument is at least one of a biopsy catheter and a biopsy needle."

5. The method as defined in claim 2, wherein the medical procedure is

amniocentesis, and said instrument is an amniocentesis needle.

6. The method as defined in claim 2, wherein said medical procedure is

stereotactic brain surgery, and said instrument is a surgical probe.

7. The method as defined in claim 6, wherein said method further

comprises: wa╧åing said image data in response to the changes in the position of said

reference transducer means.

8. The method as defined in claim 7, wherein said method further

comprises: guiding a robotic arm in accordance with a path established in the 3-D

image scene.

9. The method as defined in claim 2, wherein said medical procedure is

brain surgery to seal leaking cerebral arteries, and said instrument is a catheter.

10. The method as defined in claim 2, wherein said medical procedure is

measurement of cervical dilation.

11. The method as defined in claim 2, wherein said medical procedure is

evaluation of knee stability.

12. The method as defined in claim 2, wherein said medical procedure is

assessment of myocardial contractibility, said mobile transducer means mounted to

the myocardium.

13. The method as defined in claim 2, wherein said medical procedure is

eye surgery.

14. The method as defined in claim 2, wherein said medical procedure is

prostate surgery, and said instrument is a medical probe moved through the prostate.

15. The method as defined in claim 2, wherein said medical procedure is

trans-myocardial revascularization (TMR), and said instrument is a TMR catheter

moved through the heart.

16. A method for carrying out a medical procedure by displaying a 3-D

image scene having 2-D echo planes presented therein, the method comprising:

mounting a plurality of mobile transducer means to an instrument

having an ultrasound image transducer means for generating a 2-dimensional echo

image plane;

mounting a first reference transducer means to a first fixed location;

mounting a second reference transducer means to a second fixed

location;

generating three-dimensional coordinates of the mobile transducer

means relative to a reference frame established by the first and second reference

transducer means;

registering the 2-dimensional echo image plane with the three-

dimensional coordinates; and,

displaying the 2-dimensional echo image plane at the three-

dimensional coordinates, in relation to the reference frame established by the first and

second reference transducer means.

17. The method as defined in claim 16, wherein said first fixed location is

a patient's back, and said second fixed location is a patient's abdomen.

18. A method according to claim 1, wherein the locations on the body for

mounting the reference transducer means are internal to the body.

19. A method according to claim 1, wherein said method further comprises the step of:

ablating a portion of the bodily structure when the instrument means has been positioned at a location in a bodily structure where ablation is desired.

20. A method as defined in claim 19, wherein the instrument means is a

catheter.