CA1238405A

CA1238405A - Curvilinear array ultrasonic scanner

Info

Publication number: CA1238405A
Application number: CA000480743A
Authority: CA
Inventors: Darwin P. Adams
Original assignee: Picker International Inc
Current assignee: Philips Medical Systems Cleveland Inc
Priority date: 1984-06-28
Filing date: 1985-05-03
Publication date: 1988-06-21
Also published as: US4582065A; DE3522757A1; JPS6120540A

Abstract

Abstract An electronically step scanned real time ultrasonic imaging system and method is disclosed. The system in-cludes a transducer assembly having an unequally spaced array of elements, each with an axis of transmission along which it transmits its main ultrasonic energy when electrically stimulated. The elements are disposed in a curvilinear array, wherein their axes of transmission are approximately coplanar, but divergent in the common plane. This configuration provides a relatively large scanned area without need for either electronic or mech-anical sector scanning techniques to steer the incident ultrasonic energy over a large angle.

Description

Description CURVILINEAR ARRAY ULTRASONIC SCANNER

Technical Field This invention relates to the field of ultrasonic imaging equipment, and more particularly to an ultrasonic medical diagnostic system employing an improved trays-dicer assembly and examination method.
Background Art In recent years, the field of diagnostic ultrasound has seen the emergence of a so called "real time" ultra-sonic B scanning examination system. The term "real -time" means that the systems produce successive images at a rapid enough rate so that images are generated faster than the retention rate of the human eye so that moving objects appear in continuous motion. Thus, in real time operation, the course of the study can be influenced by the operator contemporaneously with the actual study, based on his observation of the rapidly produced image succession This real time feature is considered an improvement over previous forms of ultrasonic examine-lion, in which only a single image is developed slowly and gradually during the course of a study by moving a single transducer about the patient's skin. In addition to allowing the operator to influence the course of the study, real time systems allow for "stop action" images of moving body parts, and also for motion studies.
Real time ultrasonic examination systems have mainly fallen into two general types, i.e., linear scanning and sector scanning. Electronic linear scanning systems utilize a transducer assembly including a large linear array of individual piezoelectric ultrasonic transducer elements. Imaging circuitry fires a succession of dill-event groups of elements in accordance with a predator mined repeated sequence. This produces a succession of resultant ultrasonic beams propagated along respective parallel paths extending outwardly from the transducer assembly. The assembly is held stationary against the patient's body during image generation.
This technique, in conjunction with known forms of imaging circuitry and display apparatus, produces from received ultrasonic echoes information defining a two dimensional rectangular image ox the internal body struck lure of the patient taken in a common planet or "slice"
through part of the body near the transducer array.
One coordinate ox each point on the image plane is deter-mined by the amount of time required for incident ultrasonic energy to be reflected back to the transducers from a tissue interface within the body.
The other coordinate is determined by the location along the transducer array, of the axis of the resultant ultrasonic beam which caused the reflected energy.
By operating this system to repeatedly step the incident beam origin along the linear transducer array at, for example, thirty repetitions per second, the rapid sequence of ultrasonically produced image frames which result can show motion of a moving body part.
Alternately, a single frame of image data can be held for display, in order to stop rapid motion of such a body part.
The display area scanned by such linear step scan-ens is rectangular and suitable for presentation on a two dimensional display system, such as a CRT. The electronics required for such a system are relatively inexpensive and simple, since all the beams are parallel and stepped over uniform increments. Moreover, linear stepped scanning systems exhibit substantially uniform field of view throughout their display area.

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Linear systems, however, do have some disadvan-taxes. For example, the transducer assembly must of necessity be rather long, and therefore clumsy to use, since the length of one side of the rectangular display equals the length of the transducer array. Since all the ultrasonic beams produced by the linear scanner are propagated along parallel lines, the linear scanner is not generally capable of imaging portions of the patient's body which are hidden behind other nearer portions, such as an organ which may be located behind a rib.
A known type of electrically stepped linear array ultrasonic system is described in the following public cation, Hovels, J. F., et at, "Medical Ultrasonic Imaging: An Overview of Principles and Instrument station", Proceedings of the IEEE, Vol. 67, No. 4, April, 1979, pp. 620-641.
Another type of known real time electronic ultra-sonic scanner is the electronic sector scanner. In such devices, a linear array of transducer elements is employed as in the case of linear step scanning. The length of the array, however, is considerably shorter than in the case of the step scanned linear device described above.
In using the electronic sector scanner, the trays-dicer assembly is held stationary near the portion of the patient's body to be examined. All elements are repeatedly fired in a single group. Phase delay air-quoter is associated with imaging circuitry which is utilized to control ultrasonic beam emission and recap-lion by the transducer elements. By proper phase delay of respective elements, the ultrasonic beam repeatedly produced by the transducer array is "steered" at differ-en angles to the face of the transducer assembly. The angle of the ultrasonic beams produced by successive firings of all the elements of the transducer array is repeatedly scanned in increments from one side to another, such that the successive ultrasonic beams collectively sweep through the patient's body at different angles in a common plane.
Several advantages over the linear stepped scanner are achieved by use of the electronic sector scanner.
First, the transducer assembly is significantly more compact than in the case of the stepped scanner, and can thus be used at almost any location on the patient's body. Since the ultrasonic beams are directed into the subject at different angles, the electronic sector scanner can image portions of the body which might be hidden from view of the linear stepped scanner because of their location behind other more opaque portions of the body, such as bone.
Electronic sector scanning, however, does have its own inherent disadvantages. One such disadvantage is that these scanners have a narrow field of view in regions of the body close to the transducer assembly. This is because the field of view of the sector scanner resembles a sector of a circle and, close to the transducer assembly, the excursion of the sweep of the ultrasonic beam is quite small.
Another disadvantage of the electronic sector scanner is the relatively high cost of such units, due in large measure to the complexity of the electronics necessary to achieve the delay scheme employed to effect beam steering. While a typical linear transducer step scanner costs in the neighborhood of $15,000 to $30,000, the corresponding range of cost for electronic sector scanners is about $65,000 to $100,000 each.
Mechanically steered real time linear and sector scanners, using oscillating or rotating single crystal transducers, have also been proposed. Such systems, however, suffer from relatively large physical size, and problems associated with reliability of the motion-teal drive. They also usually require the transducer to be immersed in a fluid.
Known proposals for electronic and mechanical sector scanners are described in the above referenced Hovels, et at, publication.
Another system, a variant of ultrasonic step scan-in, (Bushman "New Equipment and Transducers for Ophthal-mix Diagnosis", Ultrasonics, Vol. 3, pages 18 et sex, January-March, 1965/) has been proposed relating to ultrasonic examination of the eye. It is suggested to utilize ten transducers arranged in an arc such that the ultrasonic beams emitted by each of the transducers mutually converge near the center of the eye ball. Puts-in circuitry is applied to separately wire each of the transducers in a sequence.
A disadvantage of this type of examination stems from the fact that tissue interface points within the patient's body which generate ultrasonic echoes may be struck by primary incident energy from more than one transducer. Each such point could thereby lack unique-news of location on the image display, causing blurring.
This lack of unique location of multiply-struck interface points would be caused by in homogeneity in the patient's body. Acoustic velocity differs among tissue types. If the time required for an ultrasonic echo from one transducer to return to that transducer from the subject point is different from the correspond-in return time with respect to another transducer whose energy also strikes the point/ the subject point will show up at slightly different spots on the display.
It is an object of this invention to provide an economical ultrasonic scanning system having the flex-ability, compactness and swept beam characteristics of an electronic sector scanner without the sector scanner's limited close up field of view and high price, while preserving the uniqueness of display location for each imaged point, all for roughly the cost of a simple linear step scanner.
Disclosure of Invention The ultrasonic scanning system of this invention overcomes or reduces the disadvantages of the stepped linear scanner as well as these of the electronic and mechanical sector scanners, while combining advantages of both.
A system embodying this invention includes an ultra-sonic transducer element array, and imaging electronics coupled to actuate the transducer array for emitting incident ultrasonic energy and to convert received echoes to electrical signals. The system also includes appear-private display apparatus to convert the electrical sign nets to a visual image describing internal structure of the patient's body.
The transducer array has a curvilinear arrangement of its elements. The transducer elements are disposed with their axes of primary transmission being divergent within a common plane. This feature enables the system to direct ultrasonic energy beams into a patient's body at different angles depending on which elements are fired. This facilitates the obtaining of ultrasonic echoes from body tissue interfaces located behind body parts which would obscure such interfaces if the ultra-sonic beams were parallel. The divergent beams also provide a larger imaged area than would exist with a linear scanner employing the same length array.
In accordance with a more specific aspect of the invention, imaging electronics is provided for repeatedly firing the transducer elements in a sequence of groups to effect step scanning of the ultrasonic energy along I

the array for real time ultrasonic imaging. This feat lure enables the system to produce a series of resultant ultrasonic beams which sweep across the subject repeat-edgy and in a succession of varying angles, similar to the beam sweeping operation of an electronic sector scanner. Due to the fact that the sweeping, or angle changes, is effected by firing the elements in steps along the length of the curvilinear array similarly to a linear scanner, rather than by means of complex delay timing techniques, the beam sweeping motion of the pro-I sent system is obtained with electronics far simple Rand less expensive than in the electronic sector scanner.
Moreover, since the resultant ultrasonic beams are emitted from spaced points along the curvilinear array, rather than being generated from a common origin, as in the case of the electronic sector scanner, field of view close to the array is improved over that of the sector scanner.
The advantages of these features and the operation of this scanner are obtained with a transducer array which is sufficiently compact to permit applicability for use on nearly any part of the human body. This obviates the previous difficulty associated with linear step scanners generating parallel beams, wherein the actual array length must correspond to a dimension of the imaged area.
In accordance with other more specific features of this invention, the system can be equipped with delay control circuitry for focusing the transmitted ultra-sonic energy at a predetermined distance from the trays-dicer array face. Alternately, or in addition, the delay circuitry may be operated in the receive mode, in order to enhance detection of reflections from paretic-ular reception focal zones at a predetermined distance from the transducer assembly.

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additionally the reception focusing circuitry may be of the dynamic focusing variety, wherein the recap-lion focal zone is changed with time to recede outwardly into the subject to track the propagation of ultrasonic energy into the body.
According to another specific inventive feature, the transducer assembly is configured as a curvilinear convex array, wherein the assembly face defines an arc of a circle, and is approximately 5 centimeters in length, having about 76 ultrasonic transducer elements equally spaced along that length. This specific configuration is believed to be effective in medical diagnostic work.
Another specific feature of this invention, involve in the use of a curvilinear array of ultrasonic trays-dupers r relates to the configuration of the array and to its particular adaptability to an especially effi-client means of processing ultrasonically derived informal lion into a visible image.
In accordance with this specific feature, the ultra-sonic transducers are distributed at unequal intervals along the curvilinear path. More specifically, the transducers are distributed at such intervals that the respective tangents of each angle of ultrasonic beam divergence from the ultrasonic axis of the center trays dicer differ from one another by equal increments. Thus, where the axes of ultrasonic propagation of a series of ultrasonic transducers diverges from that of the center transducer by angles lo ~2...~nr the angles lo I
on are chosen such that their respective tangents differ from one another by integral multiples of a constant.
Such an unequally spaced array of ultrasonic trays-dupers facilitates processing of ultrasonically derived information into a visual image by means of particularly efficient scan conversion. In such an embodiment, the scan converter has a memory with each address being ~L~38~

dedicated to a particular Y and tan I. In reproducing the image in a set of ZOO coordinates on a CRT monitor, the Y displacement of each event is read directly from the memory. The x displacement of the corresponding event is obtained by merely multiplying the Y displace-mint by the other value associated with the memory toga-lion from which the event data is sampled, namely Y-tan .
These and other features of this invention will be understood in greater measure by reference to the follow-in detailed description, and to the drawings, in which:
Description of the Drawings Figure 1 is a generalized block diagram illustrate in an ultrasonic examination system incorporating the present invention;
Figure 2 is more detailed block diagram illustrate in the system more generally exhibited in Figure l;
Figures 3-4 are schematic drawings illustrating portions of the system shown in block form in Figure 2;
Figure 5 is a graphical drawing illustrating matte-matinal parameters of components of the system shown in Figure 2;
Figure 6 is a block diagram illustrating an alterna-live embodiment portion of the system of Figure 2.
Figures 7-9 are partially graphical, partially block diagrams of ultrasonic systems incorporating a specific embodiment of the present invention.
Figure 10 is a plan view of a portion of the ultra-sonic system illustrated in Figure 9.
Best Mode for_Carryinq Oath Invention Figure 1 illustrates in general form a system S
incorporating the present invention. The system S directs ultrasonic energy into a subject, such as a patient's body, and in response to echoes produced by the incident energy, produces an image representing internal structure or condition of the body.
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Tune system S includes a curvilinear transducer as-symbol 10 for producing incident ultrasonic energy and for receiving echoes thereby caused. Imaging circuitry 12 actuates the transducer assembly to produce the inch-dent energy, and receives electrical signals from the transducer produced in response to the echoes. Data from the imaging circuitry, in the form of these electric eel signals, is directed to a display apparatus 14 which produces the image. Display format adaptor circuitry 16 provides format generating signals to the display apparatus 14, defining an array of image lines constitute in the image, in response to data and timing control signals received from the imaging circuitry 12.
The transducer assembly to preferably includes 76 individual transducer elements, such as indicated by tune reference character 18. Each transducer element comprises an individual piezoelectric ultrasonic trays-dicer of known type, having a particular axis along which ultrasonic energy from the element is primarily directed. The ultrasonic transducer elements 18 are arranged in a curvilinear disposition along a circular arc. The axes of transmission of the elevens such as indicated by the dotted lines 20 in Figure 1, diverge radially from the imaginary center of the circle defined by the arc along which the transducer elements are arranged In this preferred embodiment, the radius of curve-lure of the arc along which the transducer elements are disposed is approximately 10 centimeters (cm). The arcuate length of the transducer element array is approxi-mutely 5 centimeters.
The imaging circuitry actuates the transducer eye-mints to produce short bursts of ultrasonic energy, each burst having a frequency of approximately 3.5 mesa-hertz (MHz.). The imaging circuitry actuates a sequence of groups of transducer elements 18 such that resultant ultrasonic beams transmitted from the transducer assembly lo scan the subject body in a sequence of different angles relative to the transducer assembly. This mode ox scanning is of known type and is sometimes referred to as "real time stepped ultrasonic scanning".
Echoes returning from tissue interfaces within the patient's body cause the transducer elements to produce electrical signals representing characteristics of those echoes. These electrical signals are received and prick eased by the imaging circuitry, which then directs them lo as data signals to the display apparatus 14, which may preferably comprise a cathode ray tube (CRT) display apparatus.
Preferably, the imaging circuitry 12 actuates or "fires" successive groups of 16 transducer elements each. The imaging circuitry 12 fires each group of transducers in a phased delay fasilion, such that inch-dent ultrasonic energy produced by the transducer assembly lo is focused at a distance of approximately Senate meters from the transducer assembly. Additionally, the receiving periods of the members of each group of trays-dupers are delayed in varying amounts in order to focus the zone from which echoes are received most readily at a distance of approximately 6 centimeters from the trays dicer assembly These focusing delay characteristics are described in more detail below.
Display format adaptor circuitry 16 receives data and timing signals from the imaging circuitry 12, and produces format generating signals for causing the disk play apparatus 14 to produce a display comprising a number of divergent image lines collectively arranged in the form of radii of a truncated annuls. The arcuate length of the inner portion ox the annuls display area skin level) is approximately 5 centimeters, and the corresponding distance, or width, at the outer edge of I

the annuls (corresponding to the maximum range of about 20 cm.) is approximately 15 centimeters. Where the in-tenor edge of the truncated annuls is located at the patient's skin line, the range of system operations is approximately 20 centimeters into the body. The included angle of the truncated annuls is approximately 30 degrees.
Figure 2 illustrates in more detail an embodiment of on ultrasonic examination system incorporating the present invention. The imaging circuitry 12 includes timing and control circuitry 22 which sequences the operation of the remainder of the system So The timing circuitry 22 actuates purser circuitry 24 to fire the appropriate groups of transducer elements 18. Electric eel signals from the pursers 24 are transmitted along respective parallel signal channels to actuate the transducer elements 18 by way of delay control circuitry 26 and switching circuitry 23.
The switching circuitry 28 is controlled by the timing circuitry 22 to close appropriate members of the switching circuitry in order to govern the sequence of actuation of the transducer elements 18. Likewise, the focusing delay circuitry 26 is controlled by the timing and control circuitry 22 to impose delays on the various channels into which the purser produces the actuation signals.
When echoes return to the respective transducer elements 18 which have been fired, the transducer eye-mints convert the echoes to respective electrical signals.
These received signals are transmitted back over each of the respective channels by way ox the switching air-quoter 28 and focus delay circuitry 26.
The focusing delay circuitry is controlled in the receive mode by the timing and control circuitry 22 to impose receiving delays upon the received signals. These receiving delays focus the receiving zone of the trueness dicer elements 18 in phase delay fashion to enhance sensitivity of the system to echoes generated in a par-titular reception zone relative to the transducer post-lion.
The received and delayed signals are passed through a summing circuit 30 and directed to receiver circuitry 320 The receiver circuitry 32 transmits the summed no-ceiled signals to the "Z", or intensity control, input of the display apparatus 14, which preferably is embodied by a cathode ray tube device.
Delay modification circuitry 34, described in more detail below, is provided between the timing and control circuitry 22 and the delay focusing circuitry 26. The delay modification circuitry controls the delays inter-posed by the various delay elements in each channel during both the transmit and receive modes, in order to impose the proper focusing delays on the various signals, taking into account the curvature of the transducer assembly 10.
The purser, receiver, and summing circuitry, as well as the focusing delay circuitry, switching circuitry and timing and control circuitry are exemplified for example in the analogous circuitry of an ultrasonic examination system, Model LS1000, sold by wicker Corpora anion, North ford, Connecticut U.S.A.
The display format adaptor circuitry 19 includes a pair of ramp generators 36, 38 and ramp control circuitry 400 The outputs of the ramp generators 36, 38 are coupled to the Y axis and X axis inputs, respectively, of the display apparatus 14D By proper adjustment by the ramp control circuit of the starting times, initial values, and slopes of the ramp signals produced by the ramp generators, an array of divergent radii having a common center can be generated on the screen of the display apparatus. As shown in Figure 2, this array of lines I
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provides a display in the format of a truncated annuls.
Each of the divergent radii on the display corresponds in location to a respective one of the divergent ultra-sonic beams generated in sequence by the transducer as-symbol 10.
Thus, the system produces a display in the form of a truncated annuls whose interior edge represents the patients skin surface, at the face of the transducer assembly and whose outer edge represents the maximum range of the system field of view. The use of the curvy-linear transducer assembly, with its corresponding trunk acted annular display, provides a much larger field of view than was previously obtainable by the use of a linear transducer assembly having the same length of that of the novel curvilinear transducer assembly. This larger field of view is obtainable without the aid of electronic delay circuitry for changing the incident angle of the produced ultrasonic energy. The larger field is likewise obtainable without the use of motion-teal sector scanning techniques which can be expensive and cumbersome.
In operation, the two ramp signals defining the slope of each line component of the display format are initiated in response to a signal appearing on the lead indicated "ramp start". The ramp start signal is pro-duped by the timing and control circuitry 22, and is timed to be synchronized relative to the firing of the transducer elements by the purser circuitry 24. The ramp control circuitry 40 is controlled by a signal from the timing and control circuitry 22 appearing on the line "number" lead which identifies the particular radial line component of the image to be generated in response to information derived from the current firing of the purser circuitry 24 Preferably, the transducers are fired in groups of 16, and the pleasure and delay circuitry correspondingly define 16 electrical channels. The system is operated to produce real time images at approximately 30 frames per second. Each image preferably comprises 120 lines.
A 120 line image can be obtained, if desired, from a 76 element transducer assembly by the employment of known fractional stepping techniques, such as described in the following publication which ire Essay nabber Yoshikawa, Y. et. at., "Scanning Methods in Electro-Scanning Ultrasonic Drag-Gnostic Equipment".
As noted above, the incident ultrasonic energy produced in the transmit mode is focused by phase delay technique at 4 centimeters prom the transducer array.
The delay program for accomplishing this focusing, taking into account transducer array curvature, is defined in Table I:

Transducer Delay Group Elements (Nanoseconds) 1 and 16 0

2 and 15 113

3 and 14 210

4 and 13 290

5 and 12 355

6 and 11 403

7 and 10 437 3 and 9 453 Similarly, the reception focal zone is focused at approximately 6 centimeters from the transducer array.
The delay program for accomplishing this delay in the receive mode is defined by the following Table II:

Transducer Delay Group Elements (Nanoseconds) 1 and 16 347 2 and 15 261 3 and 14 187 4 and 13 125 5 and 12 75 6 and 11 38 7 and 10 13

8 and 9 0 Figures 3 and 4 illustrate in schematic form the circuitry embodying the ramp generators 36, 38 and the ramp control circuitry control 40.
Figure 3 shows the schematic diagram of a ramp generator circuit. The ramp generator circuit of Figure 3 corresponds to either of the ramp generator circuits - 36, 38, their circuitry being identical. For purposes of simplicity, only one such ramp generator circuit is illustrated in detail.
The ramp generator circuit produces a ramp output voltage signal at a lead 1~0 which is the output of operational amplifier 102. Control over the ramp char-acteristics is influenced by the ARC circuit 104 coupled between input and output of the amplifier 102. Closure of a switch 106 in the circuit 104 initiates production of the ramp signal. The switch 106 is closed by way of a signal appearing on "ramp start" input 108, generated by timing control circuitry 22.
Other signals, from the ramp control circuitry 40, govern aspects of the ramp signals generated at the lead 100. More specifically, a signal on a lead 112 defines the slope of the ramp signal generated. another signal from the ramp control circuitry 40, appearing at a lead 114, governs the initial value of the ramp at its starting time.
Signals on the leads 112, 114 are input to the operational amplifier 102 by way of a two-position switch 110. The condition of the signal on the lead 108 con-trots the position of the switch 110. Prior to the initiation of the ramp signal output, the switch 110 is in its lower position, such that it defines the initial .

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ramp signal value. Upon initiation of ramp signal pro diction, the switch 110 is moved to its upper position, such that the ramp slope information input on the lead 112 is then applied to the operational amplifier 102, to control ramp slope The ramp generators 36, 38, conjunctival define the ZOO location of each radial image display line gent crated on the display screen. The ramp generators govern both the starting and ending position of each display image line, and its slope or path on the screen. The ramp generators perform this function by application of the ramp signals to the X and Y deflection plates, no-spectively, of the display CRT.
The slope of the actual display image line is a function of the ratio of the slopes of the respective ramp signals produced by the generators 36, 38. Thus the slope of the image line displayed is distinct from, but a function of, the slopes of the individual ramp signals produced by the generators 36, 38.
The initial position of the trace of the image display line is determined by the initial values of the ramp signals produced by the two generators. Each in-trial position, in known fashion, provides the Wry goof-donate location of the beginning point of the cores-pounding image display line.
Figure 4 illustrates in schematic form a prefer-able embodiment of the ramp control circuitry 40. The ramp control circuitry produces four outputs, two out-puts directed to each of the ramp generator circuits 36, 38. The ramp control circuit outputs to each ramp generator an analog signal indicating the initial value of the ramp to be generated and the slope of that ramp.
These signals are produced in response to a digital signal from the timing and control circuitry 22 indicate in by number the particular image display fine which is to be generated by the next ramp signals produced.

More specifically, signals appearing at the out-puts 114, 112 indicate the initial value and slope, respectively, of the ramp signals to be produced by the ramp generator 38 for the image display line under con side ration. Similarly, signals at the leads 114 7 and 112' define the analogous parameters for the Y axis ramp signal to be generated by the generator circuit 36.
The outputs on leads 114, 114' 112, 112' are produced by the operational amplifiers 120, 122, 124 and 126 as indicated in Figure 4.
These operational amplifiers are fed input signals from the output of digital to analog converters, 130, 132, 134, 136, respectively. The inputs to the digital to analog converters are supplied as digital outputs from a series of six PROM programmable read only mom-ones) 140, 142, 144, 146, 148, 150. The function of the PROMS circuits is to receive a digital input identi-lying the line number of the individual display line to be produced in response to the immediately subsequent action of the ramp control circuitry 40. In response to each line number input to the PROMS, each PROM pro-dupes a preprogrammed unique digital signal, The PROMS are programmed such that their digital signal outputs, as they are clocked by the "line number"
digital signal, establish the proper initial conditions, ramp slopes and ramp timing to generate on the display the appropriate corresponding image line.
It is believed that those of ordinary skill in the art relevant to the subject matter discussed here would be able, by the use of ordinary trigonometry to provide appropriate programming for the PROMS by analyze in the geometry of each desired image display line individually. However, for those not intimately familiar with this art, Figure 5 is provided, illustrating the mathematical consideration involved in programming the 40r~

PROM to generate appropriate initial conditions and slopes for each respective display line. In the embody-mint described in Figure 5, the display is configured as a truncated annuls having several individual display lines. The angle varies in increments equal to the total angular excursion of the display area divided by the number of lines. The equations for programming each output for the PROMS corresponding to each indivi-dual display image line, are set forth near the bottom of Figure S. The initial conditions and slopes for both X and Y are determinable by substituting for each individual angle of each display image line which is desired to be produced. The embodiment of the disk play format adaptor circuitry 16 described above coy proses analog circuitry. As a matter of choice, how-ever, those of ordinary skill in the art may embody the display format adaptor circuitry 16 in a digital form.
More specifically, such an embodiment could suit-ably comprise a sector form digital scan coveter. A
suggested embodiment for such a digital scan converter is illustrated in Figure ho The scan converter of Figure 6 comprises an analog to digital converter 151, a random access memory 15~, address counter circuitry 154, 156 and address counter control circuitry 158.
In operation, the "Z" signal from the receiver circuitry, appearing upon a lead 160, is converted to digital form by the converter 151 and presented to the random access memory (RAM). The address counters and counter control circuitry determine the address in the RAM at which the incoming digitized æ signal is to be placed. The address counters 154, 156 are used to write the RAM in polar coordinates. The counters are operated by variable address clock rate signals from the counter control circuitry 158~ The counter control circuitry 158 operates in response to signals from the timing and control circuitry 22 appearing on the leads 164, 166.
The signal on the lead 166 indicates the particular line of the composite image to be currently displayed.
The signal on the lead 166 is a synchronizing signal to synchronize the production of the displayed line rota-live to the firing of the transducers.
Conversion to polar coordinates R, from X, Y
coordinates is in accordance with the relation Y - R
coy 3. This conversion is achieved in known form by controlling clocking rates, in each of the embodiments that are described below.
When a digital representation of an image frame has been accumulated in the RAM by steering the incoming digitized Z axis signals among the appropriate RAM ad-dresses, the RAM contents are read out in ZOO television format, and presented as inputs to a CRT video monitor display apparatus 14.
There are several ways in which ultrasonically derived data from the transducer array of this invention can be stored, processed and read out to form a visual display on a CRT monitor.
One system uses a so-called I memory format, wherein each pixel, or image portion, on the display has a corresponding memory location, expressed in X and Y coordinates.
Figure 7 illustrates a curvilinear array 200 of ultrasonic transducers for directing ultrasonic energy upward, as shown in this Figure, into a field of view denoted as 202. Figure 7 illustrates two lines 204, 206 of ultrasonic propagation, and illustrates the manner in which data from those two lines are written into the memory and subsequently processed to form an image in a CRT screen.
The line 204 emanates centered with the central one of the ultrasonic transducers of the array 200, and ~23~ 5 its angle of propagation is arbitrarily chosen as =

A memory 210 it provided having an array of memory address locations which can be characterized graphically as a two dimensional pattern of dots 211. In the memory 210, each column of dots, as shown in Figure 7, is deli-acted to a particular value of the X coordinate of the image pixel. Each row of elements is dedicated to a particular value for the Y coordinate of the pixel.
Thus, each memory address stores an image amplitude value for an image region about a particular ZOO toga-lion.
Interposed between the curvilinear array 200 and the memory 210 is address calculator circuitry 20~ whose function is described in more detail below.
Since the X coordinate of each point on line 204 = Y tan I, and = 0 for line 204, the X coordinate of each point on line 204 = 0. It is thus quite simple to represent in the memory 210 each image pixel defined by the line 204, since X = 0 for each point on the line The line 204 can be collectively represented by each of the memory addresses lying along the line 204 as defined in the portion lo Figure 7 describing the memory 210.
Line 206, however, diverges from line 204 by an angle I Since not every point on the line 206 cores-ponds precisely to an address represented by one of the memory locations 211 of the memory 210, the scan converter hardware must choose which memory addresses are to be written into by information from the ultrasonic energy propagated along the line 206, and which are to be left unwritten. This necessitates the use of a fairly coup-ligated hardware system comprising the address calculi-ion circuitry 208 to make these decisions and to avoid generation of digital artifacts in the displayed image.
The address calculator circuitry, in responding to data I

derived from ultrasonic energy propagated along the line 206, must often write each data point into the memory address most closely approximating the actual location of the structure which caused the generation of the data.
A description of this problem and its solution is provided by the publication Larsen, H., et at, "An Image Display Algorithm For Use In Real Time Sector Scanners With Digital Scan Converters", 1980 IEEE, Ultrasonics Symposium Proceedings, pp. 763-767.
In the system as illustrated in Figure 7, data thus stored in the memory 210 can read out directly in ZOO
format onto a CRT monitor to produce a visual display of an image corresponding to the information developed in response to ultrasonic energy emanating from the array 200.
Figure 8 illustrates another mode of scan con-version adaptable for use with the curvilinear array of this invention. Figure 8 shows a curvilinear array 220 of ultrasonic transducers, three of which, for example, propagate ultrasonic energy into a field of view along lines 22~, 226, 228. As in the case of the Figure 7 embodiment, energy propagated along the line 224 is arbitrarily assigned an angle 0 = 0. Energy propagated along the line 226 diverges from the energy of line 224 by an angle 1~ while energy propagated along the line 228 diverges from that of the line 224 by an angle I
The embodiment of Figure 8 employs a memory 230 having a structure similar to that of the memory 210 in Figure 7, but with a different format of geometrical correspondence between the memory address locations 232 and the geometry of the field of view 222. Instead of being formatted in rectangular coordinates, the memory 230 is formatted in Yo-yo coordinates. In memory 230, I

each column of address locations is dedicated to a part-ocular angle I, while each row of address locations is dedicated to a particular value of the coordinate Y.
In Figure 8, information from the memory 230 is read out through a calculation circuit 234 which sub-sequently transmits data to a CRT display 236, which produces a visual display corresponding to the informal lion developed by propagation and reflection of the ultrasonic energy.
The system of Figure 8 thus performs angle convert soon between the memory and the display. This memory format is known as a "Yo-yo" format. In the Memory there is a direct correspondence between the angle of divergence of the ultrasonic energy from each transducer element and memory location.
ash angle to which each column of memory address locations 232 is dedicated corresponds to one of the angles By, By at which ultrasonic energy emanating from a particular ultrasonic element diverges from the angle = 0.
In the embodiment of Figure 8, the required convert soon of data to the display is performed as the data is read from the memory In Figure 8, each of the ultrasonic transducer elements is aimed at equally spaced angles on with respect to = 0, which is the orientation of the central element. When reading data from the memory into the display for producing the image, the address calculator distinguishes a particular Y and value for the data from each memory location. In order to generate the image on the display in a sector scanning format, each point in the memory is sampled and displayed on the CRT screen in a pattern described by the following relations: The Y coordinate on the display screen is simply the value for Y associated with the particular I

I

address location being sampled. The X coordinate is equal to the product Y x tan I.
This value, Y tan I, is provided by the address calculating circuitry 234. This circuitry is required to first calculate the tangent of the angle represented by the currently sampled memory location. The calculator circuitry then must produce a signal indicating the product of the tan times the Y value.
This information is then applied to the display 236 to produce an indication of the X and Y coordinates of the image point represented by the Y, value of the particular currently sampled memory address location 232 of the memory 230.
The coordinate conversion implemented by the embodiment illustrated in Figure 8 can be computed to a high degree of accuracy by proper digital hardware design.
As mentioned above, only two mathematical operations need be performed, i.e., a multiply function and a tan-gent function, This Yo-yo technique reduces expensive memory costs and provides images which are essentially free of digital artifacts.
While the embodiments of Figures 6-9 are described in terms of only a single transducer element causing each ultrasonic energy line, this is done for simplicity end is not to be construed as limiting. Rather, each ultrasonic line can be a resultant line caused by phased or simultaneous firing of a different group of elements as described above Dynamic focusing can also be used.
A third type of scan conversion technique is illustrated generally in Figure 9. This technique even further simplifies the required hardware for producing the visual image, while providing high quality displays.
This method uses a Stan memory format.
In the Yo-yo memory system, as explained in connect lion with Figure 8, data is acquired from the trays-dupers at equal angle increments and stored in the memory under their correct coordinate. However, in the Stan system, data is acquired at unequal angles I, the angles having, however, equal tan increments.
Figure 9 illustrates a system incorporating the Stan memory format. A curvilinear array 240 of ultra-sonic transducers directs ultrasonic energy into a field of view 242, such as along lines 244, 246, 248. As in the instance of the system described in connection with Figure 8, the line 244 of Figure 9, being centrally located, is arbitrarily assigned an angle = 0. Lines 246, 248, diverge from line 244 by angles I and 2~ !
respectively.
An important aspect of this format is that the angles of divergence between adjacent ultrasonic propaga-lion axes, such as I By are not equal. Rather, the angles By, etc., are chosen such that the tangents of each of the respective adjacent angles differ by a constant increment across the field of view 242.
Data from the transducer array 240 is directed to a memory 250 having a plurality of memory locations graphically indicated by dots 252. Each of the columns of address locations in the memory 250 is dedicated to a particular value of tan corresponding to that tan value of one of the lines of ultrasonic propagation from the curvilinear array 240. Each of the rows of memory address locations is dedicated to a particular value of Y.
A calculator 254 samples data from each of the memory locations and develops ZOO coordinates for input to the CRT display 256. It can be seen from the fore- ;
going that the only function the calculation circuitry must perform is the multiplication of the Y value times the tan value associated with each sampled memory address.

- I

The Y coordinate of each displayed pixel is directly derived from the Y value to which the sampled address location is dedicated. To obtain the x value corresponding to that same location, the calculation circuitry need only multiply the Y value, already pro-sent in the memory, with the tan value, which is likewise already present. Thus, only a multiplication calculation must be made.
A scan converter employing the Stan memory format is identified as a model 672, manufactured by Hughes Aircraft of Carlsbad, California, U.S.A.
In the case of a sector format probe such as a mechanical sector scanner, having capability for prop-grating ultrasonic energy along only one axis at a time, the probe is directed, not to equal increments of angle I, but to increments of angle such that each function tan differs by equal increments from the tan of each of its adjacent angular positions. Under this format, the axes of ultrasonic transmission near the edges of the scan are spaced more closely in angle increments than they are near the center of the scan i.e., where = 0.
Thus, the Stan memory format can minimize hardware costs, while at the same time providing high quality image displays.
When using a Stan B memory format with a convex curvilinear array such as described above, one must design that array to scan the ultrasonic energy at unequal intervals. A way to do this is to space the array elements unequally across the face of the transducer array. See Figure 10, reference character 260. The amount of such spacing varies across the array, depend-in upon the angle I. The element spacing is designed such that the resultant ultrasonic axes correspond to the following mathematical relationships:

I.
x = y tan = tan~lx Y

do do do y dry 1 + (Zoo Or, expressing in I, since y = tan do dry 1 tan I

Some examples of the ratio of spacing are as `
follows: `

Space no 0 1.0 + 7.5 0.~83 + 15 0.933 + 30 0.750 For small angles B, the spacing changes very little across the array, as at 262. For larger angles, such as a 60 total scan angle (+ or - 30) such as at the elements referred to at 264 a more significant change occurs with elements closely spaced at the ends of the array. If the spacing changes significantly with one selected group of elements used to generate one ultra- !
sonic resultant line, compensation may be required in the electronic focusing circuitry to provide a well focused beam. A typical array might comprise a 5 Senate meter (cm) array line with 15 curvature and 80 elements, using 15 elements at a time to generate each resultant line. This provides an effective aperture of 0.94 cm.
When the scan is at the end of the array (7.5) the spacing at this end will be 0.983, while at 15 elements inside of this point the spacing would be 00993. Since each element is spaced from its neighbor by approximately one wavelength, the spacing error will be only in the order of one-tenth of a wavelength. This error can be easily accommodated.
It should be kept in mind that the foregoing disclosure is intended to be illustrative, rather than exhaustive, of the invention. Those of ordinary skill in the pertinent art may be able to make additions, deletions, or modifications to the preferred embodiment described above without departing from the spirit ox -scope of the invention, as defined in the appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINES AS FOLLOWS:

1. A medical diagnostic system for producing an image of a patient's internal body structure by the use of ultrasound, said system comprising:
a) a transducer assembly comprising a curvi-linear array of ultrasonic transducer elements, said elements being unequally spaced along said array;
b) imaging electronics for causing the trans-ducer elements to emit incident ultrasonic energy into the patient and to convert received echoes caused by the incident energy to electrical signals representing internal body structure of the patient, and c) a display system responsive to the elec-trical signals for producing an image of the patient's internal body structure.

2. The system of claim 1, further comprising:
said curvilinear array being convex.

3. The system of claim 1, further comprising:
said ultrasonic transducer elements having respective mutually divergent principal axes of ultra-sonic emission, said axes lying substantially in a single plane.

4. An ultrasonic transducer assembly for use in a medical diagnostic system, said assembly comprising:
a) a plurality of individual ultrasonic transducer elements: and b) mounting structure for disposing said ultrasonic transducer elements in a convex curvilinear array with mutually divergent primary axes of ultra-sonic transmission, said ultrasonic transducer elements being unequally spaced along the transducer array.

5. An electrically scanning real time ultrasonic diagnostic system comprising:
a) a probe having ultrasonic beam emitting surfaces, which probe is pressed to contact the surface of a body to be examined and in which plural ultrasonic wave transducers are convexly arranged with unequal spacing, said probe emitting detector scanning ultrasonic beams into the body, and b) means for energizing to drive selected ones of the ultrasonic transducers according to a pre-determined program.

6. The system of claim 5, further comprising:
a) said probe comprising plural ultrasonic wave transducers arranged in an arcuate shape at unequal distances along the arc, and b) a transmitting and receiving wave control circuit actuating said ultrasonic transducer plural groups each combining two or more transducers and repeat-edly performing ultrasonic beam transmitting and receiv-ing action with a program of ultrasonic directivities respectfully associated with each group.

7. The system of claim 1, wherein:
said curvilinear transducer assembly configura-tion describes substantially an arc of a circle approxi-mately 5 centimeters in length.

8. The system of claim 7, wherein:
said circle corresponding to said arc has a radius of about 10 centimeters.

9. The system of claim 1, wherein:
said transducer array comprises about 80 sep-arate transducer elements.

10. An ultrasonic transducer assembly comprising:
a) a plurality of unequally spaced ultrasonic transducer elements each having a transmission axis along which ultrasonic energy is primarily propagated when the element is appropriately electrically stimulated, and b) structure for holding the transducer elements disposed in an unequally spaced curvilinear array such that their respective transmission axes are substantially coplanar and divergent at unequal angles.

11. The system of claim 1, wherein said imaging electronics comprises:
circuitry for focusing the reception characteris-tics of a set of said transducer elements to relatively enhance reception of echoes from a particular reception focal zone within the subject.

12. The system of claim 11, further comprising:
circuitry for dynamically focusing groups of said transducer elements for changing with time the location of the reception focal zone.

13. A medical diagnostic system for producing an image of internal body structure of a patient by the use of ultrasound, said system comprising:
a) a transducer assembly comprising a curvilin-ear array of ultrasonic transducer elements having coplanar divergent axes of ultrasonic emission;
b) imaging electronics for causing the trans-ducer elements to emit incident ultrasonic energy into the patient and including a scan converter having a Y,.theta.
memory format to convert received echoes caused by the incident energy to electrical signals representing an image of the internal body structure of the patient, and c) a display system responsive to the elec-trical signals for producing an image of the patient's internal body structure.

14. The system of claim 1, wherein:
a) said transducer elements define axes of primary ultrasonic transmission, and b) adjacent ones of said axes of ultrasonic transmission diverge from one another at angles whose tangents differ by equal increments.

15. The system of claim 1, wherein said imaging electronics comprises:
a scan converter having a Y,tan .theta. memory for-mat.

16. An electronically scanned ultrasonic imaging system comprising:
a) a transducer assembly comprising a ]
ear array of ultrasonic transducer elements defining divergent axes of ultrasonic emission defining a plane, said adjacent ones of said axes diverging from one another by angles whose tangents differ by an equal amount;
b) imaging electronics for causing the trans-ducer elements to emit ultrasonic energy into a subject and to convert received echoes caused by the incident energy to electrical signals representing an image of internal structure of the subject, said imaging electro-nics including a scan converter having a Y,tan .theta. memory address format, circuitry for sampling Y,tan .theta. values stored at each memory address, and calculator circuitry for producing an x-coordinate signal corresponding to information at the sampled location by calculating the product of the Y and tan .theta.values stored at the memory address, and c) display hardware responsive to said sampled Y values and to the calculated product of Y and tan .theta.to produce an image of internal subject structure whose points comprise the respectively developed X and Y coor-dinates corresponding to information from each sampled memory address.

17. A method for examining a subject by the use of ultrasound, said method comprising the steps of:
a) transmitting ultrasonic energy into a subject along paths which define substantially a single plane and diverge from one another at unequal angles, the angles of divergence of each path from its adjacent paths having tangents which differ by equal amounts from the tangent of the angle of said each path;
b) converting echoes caused by the incident ultrasonic energy into electrical signals;
c) storing said electrical signals in the memory of a scan converter having a Y,tan .theta. memory address format such that information stored at each memory address comprises an indication of the Y and tan .theta. values corresponding to polar coordinates of the loca-tion of the echo which caused generation of the informa-tion;
d) sampling the contents of each memory address;
e) producing an indication of the Y and Y-tan .theta. values corresponding to the information at the sampled memory address;
f) transmitting the Y and Y-tan .theta. indica-tions to a display apparatus, and g) causing the display apparatus to display the sampled information in X,Y coordinates, the Y coordin-ate of each image point corresponding to the Y indica-tion, and the X coordinate of each image point corres-ponding to the Y,tan .theta. value, of the sampled memory address.

18. A method for examining a subject by the use of ultrasound, said method comprising the steps of:
a) transmitting ultrasonic energy into the subject along paths which define substantially a single plane and diverge from one another at unequal angles;
b) converting echoes caused by the incident ultrasonic energy into electrical signals representing internal subject structure, and c) producing a visual display of internal subject structure by the use of the electrical signals.

19. A method for examining a subject by the use of ultrasound, said method comprising the steps of:
a) transmitting ultrasonic energy into the subject along paths which define substantially a single plane and which diverge from one another;
b) converting echoes caused by the incident ultrasonic energy into position indicating electrical signals representing points of an image of internal subject structure;
c) storing the electrical signals in the memory of a scan converter having a Y,.theta. format thus storing the location information in accordance with a set of polar coordinates;
d) converting the information stored in polar coordinates to signals representing XY coordinates of corresponding locations, and e) applying the X,Y coordinate signals to a display to produce an image of internal subject structure in X,Y coordinates.