CA2236255A1 - Ultrasonic energy delivery system and method - Google Patents

Abstract

An energy delivery system and method control the frequency of the power driving an ultrasonic device (24) to achieve more efficient power delivery. During operation of the ultrasonic device to deliver power to a patient site (16), the system and method automatically sweep the drive power through a frequency range, locate the series and parallel resonance frequencies, calculate the average of those frequencies and lock the power generator at that average frequency to drive the crystal. This frequency sweep procedure occurs automatically when the ultrasonic crystal is located at the patient site and the power generator operator presses the power-on switch to apply power. The method of tuning the power generator thus occurs when the crystal is at the site temperature and is transparent to the operator. The application of an external bio-layer to the crystal increases its bandwidth and its robustness. Mounting a temperature sensor (28) or sensors at the crystal permits monitoring of the crystal temperature and allows drive level control over the power generator to control the temperature at the crystal.

Description

CA 022362~ 1998-04-29 ULTRASONIC ENERGY DELIVERY SYSTEM AND METHOD

The application is a continl~tion-in-part application of serial number 08/434,004, filed May 2, 1995.
BACKGROUND
The invention relates generally to power control, and more parri~ rly, to a system and method for the more ~ iPnt transfer of energy from an ultrasonic power delivery system to biological tissue.
Improper growth of or damage to the conductive tissue in the heart can interfere10 with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as cardiac arrhylhmia.
Arrhythmia can be controlled in many cases by ablating the errant heart tissue.
Once the origin~ti~n point for the arrhythmia has been located in the tissue, the 15 physician may use an ablation procedure to destroy the tissue c~ ing the arrhythmia in an atternpt to remove the electrical signal irregularities and restore normal heart beat - or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythrnia initiation site usually termin~tes the arrhythmia or at least moderates the heart rhythm to acceptable levels.
Electrophysiological ("EP") ablation is a procedure more often employed in termin~ting cardiac arrhythmia. This procedure typically involves applying s~ ntenergy to the interfering conductive tissue to ablate that tissue, thereby removing the irregular signal pa~Lw~y.
The distal end of an EP r~th~t~r may in~ mapping electrodes for locating the 25 arrhythmia iniri~tion site as well as an ablation device for perforrning the ablation procedure on the interfering conductive tissue. In another case, different c~rheters may be used for mapping and for ablation. One type of device locatable at the distal end of a cath~ot.or for delivering ablation energy to biological tissue is an ultrasonic device, such as a piezoelectric tr~n~ lcer or crystal. The piezoelectric tr~nc~.cer, ~rcite~l by electrical 30 energy to oscillate at ultrasonic freqllPncies, imparts acoustic energy to the target tissue thereby c~lsing ablation. However, an ukrasonic device is effectively an electrical to ultrasonic tr~nc~lllcer and must be driven properly by electrical energy for effective WO 98/11826 PCTIUS9711~87Q

power transfer. Ultrasonic devices incorporating piezoelectric elements are in effect complex circuits that are a combination of resistance, cap~it~nce, and in~ll.ct?nce In such systems, there are one or more freqll~ncies where the total irnpedance of the circuit will appear purely resistive. These freq~lPnries are referred to as resonant freq~lPn~i~s.
For an electro-me~h~nif~l transducer such as apiezoelectric tr~nc~ er comprisingcomplex circuitry, it has been found that resonant freqllen~i~s occur in pairs of relatively closely-spaced freqll~on~ies where the irnpedance is resistive and the phase angle is zero.
The first of these freqllenc;tos is the so-called series resonant impedance frequency, and the second is the so-called parallel resonant irnpedance frequency. Typically, the most 10 efficient energy transfer occurs at a resonant frequency. Less energy is lost in the conversion of electrical to acoustic energy and less energy is lost as heat during radiation of the acoustic energy. However, it has been found that typically with ceramic piezoelectric tr~ncfltlct-rs, energy transfer is optimal at a frequency midway between these closely-spaced resonant frequencies and the power factor approaches, or is, one.15 Depending on the frequency sensitivity, or "Q", of the particular tr~n~llcer, energy transfer may be greatly reduced when the frequency of the electrical energy applied to the tr~n~ c~ r varies from the optimum frequency.
Resonant freqllenciPs can vary subst~nti~lly between different transducers depending on their physical structure. Additional~y, the resonant freqll~-nfies of a 20 particular tr~ncrl~lcer can vary substantially depending on the loading on the transducer at the time. The environm~n~l loading, such as the temperature to which the tr~n~ cer is exposed, can cause a shift in the resonant freqllPnl ieS as can changes in the tissue or contact loading on the tr?nc~ cPr. -It has been noted that a signifi~?nt change in the optimal frequency can occur in 25 a tr~n~ lcPr when it is first exposed to room temperature (20~ C) and is then introducedto the patient at normal hllm~n temperature (37~ C). Calibrating the tr~nc~ ctor for the optimal frequency at room temperature thus may result in a frequency far different from the optimal frequency when the tr~nc~ cPr is at patient body temperature. This may result in a much lower power factor, less Pffi~itont transfer of power to the body tissue, 30 and the need for longer ablation times or increased input power to achieve the desired ablation. Longer ablation tirnes and increased input power are both lln<l~cirable. The former is undesirable due to the increased trauma to the patient and the latter due to the CA 022362~ 1998-04-29 increased risk of exposing the patient to higher power levels. Thus it would be desirable - to be able to calibrate the transducer at the actual temperature of the site.
During an EP surgical procedure, both the environm~ nt~l and contact loading on an ablation transducer can vary widely. When the ablation device contacts body tissue, 5 such as interfering conductive tissue in the heart, the loading is increased. The loading varies as the tissue is ablated or otherwise modified during the procedure and the temperature rises. These variations in loa&g can cause corresponding variations in the transducer's resonant freqtt~nries, thereby rall~ing variations in the Pffiriency of power transfer. Therefore, it is preferable to have a lower "Q" tr~n~llc~r so that changes in 10 the loading of the tr~n~f~llcPr during an EP procedure do not cause an unacceptable lowering of the power factor.
A comrnon method of determining resonant frequency is to apply an alternating current to the tr~n~ cer and compare the resulting phase angle between the voltage applied to the trtns~l~cer and the current drawn by the transducer. The phase angle 15 equals ninety degrees for purely inductive circuits and minus ninety degrees for purely capacitive circuits. The phase angle equals zero for purely resistive circuits, with the voltage and current being in phase with each other. A phase angle of zero also inflir~tes a resonant frequency of the tr~nc~llrPr. H~w~ver, this method of determining theresonance fre~uency is undesirable because the instrttmPnrs required to determine the 20 phase angle belween voltage and current are relatively expensive and are less effective at higher freq~l~nriec~ such as at 10 mE~z. Thus a more practical, but accurate, apparatus and method for determining the optimal operating frequency of a tr~n~ cer while in VtVO iS desirable. Such apparatus and method should be accurate at higher freql~ent-it s as well as relatively inexpensive and simple to manufacture.
Another conslderation in the design of ablation devices is the size of the device used. The device must be small enough to be introduced percutaneously into a patient while at the sarne time, must be large enough to be mounted on a c~theter shaft that has room within for the passage of electrical wires and fluid htmPns, depending on the application. Making the ultrasonic crystal device too small and too thin results in a 30 fragile device. Such crystals are inherently extremely hard due to their crystalline structure and many times will be darnaged by rough tre~tm~nt In some cases, those h~n~ling a c~theter with an ablation crystal mounted on its distal end may drop the CA 022362~ 1998-04-29 distal end of the catheter subjecting the crystal to a sharp shock. If the crystal is too - thin, it may crack thereby rendering it unusable. If the cr,vstal is too thick, it will be difficult to introduce it into a patient.
Thus, a need exists to make the ultrasonic crystal sm~ r but less fragile and at5 the same time, keep the crystal biologica~ly compatible. Also, the ablation device should have a reasonable co~ffici.ont of thermal con~lllctivity so that heat r~hing the device during an ablacion procedure will be con~llctel~ rapidly by the device. Controlling the temperature at the ablation device is important so that blood boiling and tissuecoagulation on the device do not occur. Coagulated tissue on the ablation device can 10 cause an over-loading condition and the crystal may actually cease its vibrations if such loading exceeds the device's limit. It would be desirable to put a temperature sensor in the ablation device to monitor the temperature of the device and control the energy provided to the device to hold the temperature within limits. Howc ver, if the ablation device does not conduct thermal energy at a rapid rate or uniformly, the device may be 15 hotter in one area than in another. If the temperature sensor is placed in a lower temperature area, higher temperatures c~llcing blood boiling and coagulation at another part of the device may not be ~etecte~ early enough. Thus, providing a crystal with relatively rapid thermal conductivity, sPncing the temperature at the crystal, and controlling the energy supplied to the crystal for ablation are desirable.
Hence, those skilled in the art have recognized a need for an energy delivery system and method that can provide improved energy delivery to biological tissue.
Additionally, those skilled in the art have recognized a need for an energy delivery system and method that can determine the optimal operation frequency of a transducer in-vivo relatively inexpensively and sirnply. Also, those skilled in the art have 25 recognized the need for a system and method that is relatively in~en~;t;ve to resonant frequency changes in the tr~n~ cer caused by loading changes during an EP procedure.
Furthermore, those skilled in the art have recognized the need for an irnproved ~r~nc~ c~r that is less susceptible to breakage due to physical shocks, yet is small enough to be introduced into a patient and which is large enough to house a temperature sensor 30 or sensors. The present invention fulfills these needs and others.

CA 022362~ 1998-04-29 SUMMARY OF THE IN~VENTION
- - Briefly and in general terms, the present invention provides a system and a method for applying acoustic energy to biological tissue, comprising a catheter having distal and proximal ends, an ultrasonic transducer that tranC~ ces electrical energy into 5 acoustic energy, said tr~ncrlllcer having first and second resonance freqll~ncies, said tr~nC~l~c~r mounted at the distal end of the c~th~otPr, a temperature sensor mounted at the distal end of the catheter that senses temperature and provides a temperature sensing signal, a tuning system connecte~ to the ultrasonic transducer providing electrical energy to the tr~nc~lcer and monitoring the tr~nc~cer's response thereto to determine the first 10 and second resonance freqllencies of the transducer, said tuning system providing first and second resonance signals as a result of the determin~tion, and a power supply that provides electrical energy to the ultrasonic tr~nc~llcer at a frequency that is halfway between the first and second resonance freqllPnries at a drive level dependent on the temperature sensed by the temperature sensor.
In another aspect, the invention provides processing means for c~lcnl~ting a center frequency as an average of the first and second resonant freqll.onri~oc. In yet more clet~ile~l aspects, there is provided a ~iologically-compatible, non-met~ layer mounted on the outside of the ultrasonic tranC~lcer that lowers the frequency sensitivity of the tr~n~ lcer, ~n a further aspect related to temperature control, the processor decreases the power drive level when the temperature; signal represents a temperature above a predeterrnined first threshold temperature. Additionally, the ultrasonic tr~nc~l.cer is cylindrically shaped and the temperature sensor is mounted in the ultrasonic tr~nc~lllc~or.
In accordance with another aspect of the inv.ontion, the power supply comprises 25 a power application switch wherein the power supply autom~tir~lly applies test power to the ultrasonic tranc~ er to determine the first and seCon~3 resonant frequencies and then applies full power as s~lectef~ at the halfway frequency. Furthermore, the power supply antom~ti~lly sweeps through a predetermined range of frequencies to determine ~ the first and second resonant freqll~ n~iec In yet another aspect, the power supply system holds the frequency constant while varying the power level to m~int~in the temperature within a predeterminedrange.

Other aspects and advantages of the invention will become apparent from the - following ~let~ile~l description and accompanying drawings, illustrating by way of example the features of the invention.
BRIEF DESCRIPTIO N OF T HE DR~ GS
FIG. 1 ls a dlagr~mm~tlc vlew of a hl~m~n heart m partlal cross secuon showmg an electrophysiological catheter disposed internally and located so that one side of a "side-fire" energy tr~ns~ fer mounted at the catheter's distal end is against the endocardium for performing an electrophysiological procedure;
FIG.2is an enlarged, partially broken, cross-sectional side view of the distal end 10 of the EP ablation catheter shown in FIG.l including a piezo-electric side-fire transducer having a plur~lity of temperature sensing devices;
FIG.3is an enlarged sectional side view of one of the temperature sensing devices in FIG.2 showing its mounting configuration in the catheter;
FIG. 4 is an exploded perspective view of the distal end of the EP catheter shown 15 in FIG.2;
FIG. 5 is an enlarged cross-sectional view of the distal end of another embor~ nt of an EP catheter in which an "end-fire" ultrasonic tr~nC~lr~r is formed with a convex-shaped tip and also in~ es a temperature sensing device disposed in the tr~ns~llc~r;
FIG. 6 is a side view of the c~theter of FIG. 2 showing its distal end disposed 20 parallel to and in contact with an ablation site for "side-fire" use;
FIG. 7is a side view of the c~tht~ter of FIG. 5 showing its distal end disposed perp~n~i~ll~rly to and in contact with an ablation site for "end-fire" use;
FIG.8is a diagrarn showing frequency versus power transfer for three ultrasonic tr~ns~ cers having difrer~ frequency sensitivities;
FIG. 9 is a diagrarn showing shifted frequency versus power transfer curves for the three ultrasonic transducers of FIG. 8 that have undergone a temperature change;
and FIG.10is a s~hem~ti~ diagr~m illustrating an ultrasonic energy delivery system for controlling the drive level of an ablation device in accordance with the principles of 30 the present invention; and FIG. llis a flow chart illustrating a method for controlling the drive level in accordance wi~h the present invention.

WO 98/11826 PCT/US97/1~;870 DETAII ED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- In the following description, like reference numerals will be used to refer to like or corresponding elements in the different figures of the drawings. Referring now to FIG. 1 in more detail, an electrophysiology ("EP") type c~theter 10 is shown inserted 5 into the right ventricle 12 of a human heart 14 for localized f3i~nosiC or tre~tm~nt of the endocardial tissue 16 thereof. The catheter includes, in this case, an elongated catheter tube or body 18 having a distal end 20 with an electrode 22 mounted at the distal tip, a cylindrical ultrasonic tr~nC~llcer 24, in this case a piezoelectric device, mounted proximal to the tip electrode, and a band electrode 26 mounted proximal to 10 the piezoelectric tr~nc~lcer 24. The electrodes 22 and 26 and the piezoelectric tr~nC~llc~r 24 may be individually or simultaneously ~ct..~te~ to perform various electrophysiological procedures. In FIG. 1, the distal end of the cath~ t~or is shown parallel to and in contact with the endocardium for performing a "side-fire~ EP ablation procedure with the piezoelectric tr~nc~t.cer 24.
As used herein, a "side-fire" device is one that is mounted such that it conducts energy sideways in relation to the cathe{~r shaft. This would inrl~ the tr~ncmicsion of energy in the radial direction. An "end-fire~ device is one that is mounted such that is conducts energy at the distal end of the c~thet~ r in relation to the c~thet.or shaft. This would in~ e the tr~ncmicsion of energy in the axial direction.
The distal end 20 of the elongated catheter body 18 is steerable and has sllffi~ient torsional and axial rigidity for maneuvering the distal end through the vascular system and to selected sites within the heart chamber. The c~thet.or body 18 is of s..ffiri~nt length, for inct~n~e to allow for a tr~ncll.min~l percutaneous brachial approach to the heart of an adult patient and/or a transluminal percutaneous femoral approach.
Referring now in more detail to FIG. 2, the distal tip electrode 22 may be a mapping type electrode used to send or receive electrical signals from ~ c~nt endocardial tissue for locating aberrant conductive tissues. Suitable materials for the tip electrode 22 inrln(le pure platinum, a platinum iridium alloy such as "pl~tinl~m 10 iridium" (90% pl~tinllm 10% iridium), a gold alloy, pure tit~nillm~ and/or.pure tungsten.
30 The band electrode 26, located proximal to the piezoelectric tr~nc~.ct~r 24, may also be used either individually or simultaneously with the tip electrode 22 to perform EP
mapp~ng procedures.

WO 98/11826 PCT/US97/1~870 The cylindrical piezoelectric tr~nc~ er 24 directs ultrasonic acoustic energy ina radial outward direction for "side-fire~ operation. When tr~ncmitting ultrasonic energy radially outward, tissue located ~jaclont the transducer will be ablated.
During an ablation procedure, the piezo-electric tr~ns~lcer or crystal 24 may be5 subject to overhe~t;ng if precautions are not taken to control heat buildup. Heat buildup can be prevented by m~lrimi7ing heat transfer away from the crystal 24. In one embo~im.om silver is desired as the outer coating on the crystal. Silver is one of the most thermally conductive materials available, so heat created by ablating the patient is rapidly concil.cterl into the blood flow. However, silver is not as bio-compatible as 10 other materials.
The silver can be coated with gold, which is bio-compatible, but the crystal would be rendered more fragile. In the embodiment shown, the transducer has an exterior coating comprising a biologically-compatible, non-met~llic layer 27 mounted on the outside of the ultrasonic transducer. The non-met~ layer 27 serves to protect and 15 strengthen the crystal as well as lowering the frequency sensitivity of the ultrasonic tr~nC~llc.or. The thirl~necc of the bio-layer in one embo~limPnt was selected so that the section of catheter having the bio-layer had the sarne outer ~i~meter as the section of the c~thetPr proximal to the bio-layer.
As shown in FIG. 2, a pair of temperature sencing devices Z8 are mounted in the 20 wall of the cylindrical piezoelectric tranC~.lcPr 24. For purposes of illustration, two sensing devices are shown; however, more or fewer sencing devices may be mounted in the tr~ncc~lc~r. In one particular embo~limPnt three temperature SPncing devices are mounted in the tr~nc~ er wall and are spaced equi-angularly apart (120 degrees) in a common transverse plane. As is ~icn~sse~ below, having a greater number of 25 temperature sensing devices in the tr~ncrl~lcPr may be more desirable to obtain an accurate temperature in~lic~tion in a side-fire application The cylindrical trancrl~lcPr 24 has inner 30 and outer surfaces 32 and sensor bore holes 34 are formed completely through those surfaces and the wall 33 of the tr~nc~lllcPr.
Each of the sensing devices 28 is in the form of a point sensor mounted within the 30 respective sensor bore hole.
The bore holes 34 may be formed through the wall of the cylindrical piezoelectric tr~nc-inf Pr by a non-me~h~nir~l contact, ultrasonic m~hining process available CA 022362~ 1998-04-29 commercially. The bore holes, in one embodiment were 0.1778 mm (0.007 in.) in met~r. It is desirable that the temperature slon~ing devices 28 be as small as possible so that when the devices are mounted in the sensor bore holes of the piezoelectric tr~n~ c~r 24, the tr~n~ fer's ultrasonic performance is min;m~lly affected and the 5 temperature response times are minimi7e~
Referring now to FIG. 3, thermocouples 33 used as the temperature sensing device 28 are shown. The thermocouple includes an elongated electrical sensing lead pair 35 comprising individually insulated flexible electrical temperature s~nsing leads 36 and 38. The electrical leads include respective electrically conductive wires, 40 and 42, 10 formed of ~ imil~r materials. The distal portion of each wire is stripped of its insulation 44 and is coupled with the stripped distal portiorl of the other lead to form the thermocouple. In one embodiment, one wire 40 is formed of copper and the other wire 42 is formed of constantan (~T" type). Alternatively, the thermocouple 33 may be constructed of other ~i~cimil~r m~t~llic materials.
The distal portions of the ~i~cimil~r wires 40 and 42 may be joined such as by welding or bonding together, for instance by co~ ctive solder 46, to form the thermocouple junction along the length of the solder joint. The electrical temperature sensing leads 36 and 38 are formed from a forty-four gauge (AWG) bifilar wire. Abifilar wire of this sort is available from Hudson International of Trenton, Georgia and 20 when connecte~l as shown, it forms a T-type thermocouple 33. Alternatively, the r~ic~imil~r met~ wires may be joined by TIG or laser welding to form an enlargedweld bead f~.ofining the thermocouple 3unction.
In both configurations, electrical current may thus pass through the thermocouple junction to create the thermocouple effect. The opposite ends of the respective sensing 25 leads 36 and 38 may be connectec~ to a connector (not shown) mounted on a manipulation handle at the proximal end of the catheter. The sencing leads carr,v the s~n~ing signals responsive to the temperature sensed at the thermocouple. Those s~ n~ing signals may be used by monitoring e~uipment to derive temperature in~ir~tions.
In the embodiment shown in FIG. 3, the sensor bore hole 34 is of a-uniform 30 f~i~meter along its length. In this emboc3imrnt a flexible elastomeric tubular sheath 48 is provided for receipt within the sensor bore hole, the sheath having an inner bore 50 therethrough. Preferably, the tubular sheath is composed of an elastomeric polyamide WO 98/11826 PCTrUS97115870 h~aving an inner bore 50 diarneter sized for snug receipt of the pair of electric sensor leads 36 and 38 and having an outer ~i~m.oter sized for a snug fit within the sensor bore hole 34.
To assemble the temperature sensing device 28 to the transducer 24 in the 5 embodiment shown in FIG. 3, the sheath 48 is pulled into the sensor bore hole 34 from the outside surface 32 of the tr~nc~ cer. The length of the sheath is greater than the thirknesc of the wall of the transducer and the non-metallic layer 27 so that an excess length protrudes inwardly from the inner surface 30 of the transducer. The outersurface of the sheath is cut or otherwise positioned such that it is fLush with the outer 10 surface of the non-metallic layer 27. An ~nn~ r bead 52 of an adhesive, such as cyanoacrylate, is applied around the periphery of the sheath along the inner surface of the tr~nc~llcer to secure the sheath thereto. Not only does this bead 52 anchor the sheath in the bore, but it also provides an inner fluid seal to further prevent the entry of body fluids into the interior of the cath~ tor.
The prox~nal ends of the bifilar sensor leads 36 and 38 are then received through the inner bore ~0 of the sheath 48 from the outside of the transducer such that the distal extremity of the thermocouple 33 is positioned substantially at the same level or flush with the outer surface of the non-m~t~llic layer 27 as shown in FIG. 3. Because the sheath toxt~n~lc inward~y past the edge of the bore 34, it provides a strain relief for the 20 sensing thermocouple leads 36 and 38 as well as protecting them from the possible loss of their insulation layers 44 should they scrape against the tr~nC~l~r~r 24. Movement of the tr~nc~l.fer 24 occurs due to the nature of a piezoelectric tr~nc~ cer and such movement can be detrim~nr~l to sensor leads.
Thereafter, an adhesive 51 is applied to the thermocouple, she~th, and non-25 met~ r coating 27 to seal and anchor the assembly as one as shown in FIG. 3. A crownallows the thermocouple to be mounted flush with the outer surface of the non-metallic coating 27. The non-mtqt~llic coating 27 provides a better acoustical impedance match between the piezoelectric crystal and heart tissue for more effici~nt energy transfer as discussed below in more detail. The coating 27 also provides a bio-layer and adds 30 m~o~h~ni~l strength to the tr~nc~..ce~.
As mentioned above, it has been found that piezoelectric transducers will effecta pumping action of fluid through an associated orifice or opening due to the movement CA 022362~ 1998-04-29 .

W O9~/~1826 PCT~US97/15870 of the transducer. Fluid entry into the interior of an EP c~thetor is lln~ecirable because - it may .5ignific~ntly dampen the piezoelectric tr~ns~llrPr's performance rendering the catht-ter effectively useless. The approach described above and illustrated in the accompanying figures prevents fluid entry. In the embo-liment shown in FIG. 3, the t 5 use of a resilient sheath 48 compressed in the bore through the piezoelectric tr~ns~llcer 24 provides a first, main defense against leakage. By applying adhesive/sealant about the sheath on the outside and the inside provides further protection against leakage as well as performing other functions described above.
With reference now to FIGS. 2 and 4, the construction of the distal end 20 of the 10 catheter will be described inclu&g the assembly of the distal end electrode 22, the cylindrical piezoelectric transducer 24, and the band electrode 26 onto the distal end of the c~thet~r. In general, the distal tip electrode 22, piezoelectric transducer 24, and barid electrode 26 are mounted to a mounting member or base 58 and the base is mounted to the distal end of the catheter body 18.
The mounting base 58 is generally an elongated cylinrlri(-~l body having a longit l~in~l axial bore 60 therethrough. The distal end of the base has a head 62 formed with an ~nn~ r O-ring retention groove 64. The base is formed with a sm~llPr ~i~m~ter neck 66 ~rten~ing in the proximal direction from the head 62 to a larger diameter flange 68 also formed with an annu~ar O-ring retention groove 70. A radial sensor lead bore 20 72 is formed through the wall of the neck 66, generally at the medial portion thereo~
Formed proxim~lly from the flange 68 is a larger ~ m~ot~r shoulder 74 having a flat surface 76 along one side. The flat surface 76 provides room for a welding joint of an electrical lead to the electrode, as ~ lcse~ below. An electrode lead bore 78 is formed from the flat surface radially inwardly to the axial bore 60. The shoulder 74, from the 25 proximal end thereof, further expands radially to a larger r3;~metPr abutment plate 80 and is formed therefrom with a smaller diameter elongated mounting stem 82 (shown - partially broken in FIG. 2). The base 58 is electrically insulative and may be formed of VLTEM for example.
The tip electrode 22, in the embo~;m~nt shown in FIGS. 2 and 4, is generally 30 bullet-nose in shape but may take other shapes. The proximal end of the tip electrode 22 is formed with a small rli~m~ot~r~ axially projecting mounting post 84 in this embo~limPnt The proximal end of the post is formed with an axial electrode connector WO 98/11826 PCT/US97/lS870 bore 86 for conn.oction to an elongated electrical conductor wire 88 having an insulative jacket 90. The insulative jacket at the distal end of conductor wire 88 is stripped away and the distal tip of the wire conductor 88 is received within the connector bore 86.
The conductor is affixed to the tip electrode by crimping the post 84 about the S co~lrtor or by soldering the conductor thereto, or by other means. The length of the conductor wire 88 is s~lecte~ such that the conductor may extend and attach to an electrical connector at a manipulation handle (not shown) at the proximal end of the catheter 10. In the preferred configuration, the conductor wire is formed of a high strength copper beryllium that conducts electrical signals from the tip electrode to the 10 connector at the proximal end of the r~rheter. In addition, the conrlllctor wire acts as a safety chain to ensure that the tip assembly remains on the catheter shaft while being used in an EP procedure.
The piezoelectric tr~nc~llcer 24 has an outer ~;~met~r that is just slightly less than the catheter body 18 and the tip electrode 22 so that the ~ m~ter is the same when the 15 non-m.ot~llic coating 27 is applied. The tr~ncf~llcer 24 may be composed of a ceramic crystalline material. The outer 32 and inner 30 surfaces of the tr~ncc~llc~or have a thin film, electrically conductive m~t~llic coating (not shown) such as gold, silver, or nickel disposed thereon to provide trancducer ~cit~tion electrodes. A first electrically conductive transducer wire 92 is soldered to the met~llic coating disposed on the outer 20 surface 32 of the tr~nc.lllc~r at the proximal end thereof. A second electrically conductive tr~ncrll~cer wire 94 is soldered to the mPt~llic coating disposed on the inner surface 30 of the tr~ncr~lct~r at the distal end thereof. Each of the tr~n~llc~or wires has an electrically insulative jacket (not shown) that insulates the respective wires along their lengths to prevent short circuiting.
The band electrode 26 is generally a thin walled ring having an outer ~ metf~r substantially the same diameter as the c~thPt~r body 18, and the tip electrode 22. The inner ~i~m~ t~ of the band is sized for mounting over the shoulder 74 of the base 58.
The electrode band is electrically conductive and may be formed of pl~rinl~m or gold or other materials. An electrical sensor lead 96 is provided and has its distal end bonded 30 to the inrler surface of the band, for instance by soldering or wel~m~ont. The electrode sensor lead 96 has an electrically insulative jacket (not shown) that inslll~tes the lead along itS length.

CA 022362~ l998-04-29 WO 98/11826 PCTrUS97/15870 The catheter body 18 is formed with a longitudinal inner lumen 97 that e7~ten-~cthe entire length of the body to its proximal end. The distal extremity of the catheter body is formed with an ~nn~ r mounting hole 99 having an inner diameter sized for receipt of the mounting stem 84 of the base 58.
When the distal end of the r~th~ t~r 10 is assembled, the proximal end of the electrode sensor lead 96 of the electrode band 26 is passed inwardly through theelectrode lead bore 78 of the base 58 and ~rt~n~e~ in a proximal direction out through the inner bore 60 thereof. The electrode band is thereafter assembled over the shoulder 74 of the ba e into contact with the abl-tnn~nt plate 80. The band is adhesively bonded 10 to the shoulder, for instance using epoxy, to securely affix the band electrode to the base.
A pair of elastomeric O-rings 98 and 100 are provided to center the piezoelectric transducer 24 on the mounting base 58 at the catheter distal end and fix it in position.
They also vibrationally isolate the piezoelectric tr~nc~ cer 24 from the other 15 components of the c~theter 10. The first O-ring 98 is positioned in the second retainer groove 70 of the flange 68 at the proximal end of the neck 66. The second O-ring 100 is disposed within the firct reta ner groove 64 at the head 62 of the base.
The C~-rings 98 and 100 may be composed of a low durometer material such as a silicone based polymer that provides s~ nt high frequency vibration isolation 20 characteristics while providing s~ nt hardness such that the ultrasonic vibrations generated from the piezoelectric tr~nc~..c.or 24 are not unduly damped.
The piezoelectric transducer 24, having the temperature sensing devices 28 mounted thereto, is then mounted to the base 58. The first transducer wire 92 at the distal end of the transducer is received inwardly through the electrode lead bore 78 25 beneath the electrode band 26 and directed in a proximal direction through the inner bore 60 of the base. The second tr~n~l..cer wire 94 at the distal inner end of the transducer, along with the respective temperature sensor lead pairs 35 of the respective temperature sensing devices 28 are guided inwardly into the sensor lead bores 71 and 72 of the neck 66 of the base 58. The cylindrical tr~nC~lcer 24 iS then mounted over 30 the O-rings 98 and 100 and the neck of the base. The proximal end of the transducer is spaced a short ~i~t~nce from the electrode band 26 and an electrically insulative spacer bead 102 ~IG. 2) of adhesive/sealant is applied between the band 26, base 58, and tr~n.~ r~r 24 to seal the space between the transducer 24 and the electrode band 26 and affix the transducer 24 in position. The adhesive/sealant is of a low durometer, bio-compatible adhesive/sealant and may be composed of a silicone-based polymer having sllffiriPnt vi~rational isolating and electrical insulating characteristics.
The proxima} end of the tip conductor 88 of the tip electrode 22 iS then received within the distal end of the inner axial bore 60 of the base 58 and the mounting post 84 of the tip electrode pressed into the distal end of the inner bore 60. An adhesive, such as epoxy, bonds the mounting post within the inner bore 60 of the base. The proximal surface of the tip electrode is spaced a short distance from the distal end of the 10 piezoelectric tr~nC~llcf~r 24 and a second electrically insulative spacer bead 104 of adhesive/sealant is applied between the tip electrode 22, base 58, and tr~n~llcer 24 to seal the space between the tr~n~ c~r and the tip electrode ~ffi5ring the transducer in position. This adhesive/sealant is also a low durometer, bio-compatible material having sllffiritont vibrational isolating and electrical inslll~rirlg characteristics. The combin~tion 15 of the two internal O-rings 98 and 100 and the two adhesive spacing and sealing beads 102 and 104 at either end of the piezoelectric tr~nc~uc~r optimize the transfer of acoustic energy from the tran~ rer to the tissue.
To complete the assembly of the distal end 20 of the c~theter 10, the proxirnal ends of the temperature sensor lead pairs 35, electrode sensing lead 96, tr~nc~lllrer wires 20 93, 94 and the tip electrode conductor wire 88 are gathered together and directed into the distal end of the ir~er lumen 97 of the catheter body 18. The mounting stem 82 of the base 58 is pressed into the mounting hole 99 of the c~th~ter body and fixedly secureIy thereto, for in~t~nre by an epoxy adhesive The proximal ends of the sencing lead pairs, tip electrode conductor wire, electrode srn~ing lead, and tr~n~7~cer wires 92 25 and 94 are conn~cte~l to an electrical connector of a manipulation handle (not shown) at the proximal end of the r~thet~r body. They may be used for operative conn~ction to temperature signal processing, mapping, and ultrasonic ablation operating systems.
The adhesive/sealant beads 102 and 104 provide a liquid seal that prevents bloodand other fluids from re~ching the underside of the piezoelectric transducer 24 and 30 enter;ng the inner lumen 97 of the c~theter body 18. This also protects the various electrically conductive leads and wires conr~in~cl within the c~rh~otor from short circuit CA 022362~ 1998-04-29 by body fluids. Additionally, the adhesive rings electrically insulate the electrode band 26, tr~nc~llcer 24 and tip electrode 22 from each other to prevent short circuiting.
In the embo~imPnr shown in FIGS. 1, 2, 3, and 4, the transducer 24 is a piezoelectric tr~nc~lllc~r having a generally cylindrical shape. The cylindrical5 piezoelectric tr~nC~7llcer 24 directs ultrasonic acoustic energy in a radial outward direction for side-fire operation. However, the tr~ncrl~lcer 24 may take other forms, such as the end-fire configuration shown in FIG. 5. In an alternative embo-Jiment shown in FIG. 5, a piezoelectric tr~nc~tlc~r 108 is formed in a different configuration. Some reentrant pathways may be located in positions where a side-fire catheter cannot reach.
10 In such cases, an end-fire catheter configuration may be successfully used. Additionally, an end-fire c~th~tPr may also be used for more precise ablation procedures where thin but deep lesions are preferred.
In this embo~liment, the transducer 108 has a hollow cylindrical portion ~efining a mounting flange 110 and has an integral, genera~ly hollow, convex-shaped tip to result 15 in a bullet-shaped appearance. The convex tip may be a paraboloid or a hemisphere or other shapes. By having the integral mounting flange 110 as shown, the transducer 108 is easier to mount to the tip of the c~thetPr in comparison to previous ~i~osignc having only a hemispherical-shaped tr~nC~llcer. ~ itio~lly, it can be more accurately and easily mounted on the distal tip because of the larger surface area of the mounting flange 20 and the use of an O-ring 143. Thic larger surface area for mounting also provides a larger surface area for the application of an adhesive to attach the tip to the c~the~er shaft. Irnproved sealing of the inner volume of the tr~ncrlltcer should result.
The outer diameter of the flange 110 of the piezoelectric transducer 108 is sized to conform to the outer diameter of the catheter body 18. As shown, the distal-most 25 end of the hernispherical tip 112 of the tr~nc~ cer 108 is formed with an axially ~ligne~l sensor bore hole 114. The sensor holes are formed by the ultrasonic m~rhining technique described above.
The inner 116 and outer 118 surfaces of the transducer 108 are plated or coated with an electrically conductive coating ~not shown), such as gold, silver, or nickel to 30 provide tr~ncclllc-~r e~rrit~tion electrodes.
A first tr~ncrlllc~r wire 120 is bonded to the metallic coating disposed on the outer surface 118 of the transducer at the proxirnal end of the mounting flange 110. ~

WO 98111826 P-_l/U~9711S870 second tr~nc~llcer wire 122 is bonded to the m~t~llir coating disposed on tke inner surface 116 of the tip 112 of the tr~nC~l-cer. The wires are bonded to the respective surfaces by electrically conductive solder or other means to provide electrical continuity.
Each of the transducer wires has an electrically insulative jacket ~not shown) that S inc~ res the respective wire along its length to prevent short circl.iting.
The piezoelectric tr~nC~I~.cer 108, in this embodiment is constructed for mounting to the distal end 20 of the c~theter body 18 through the use of a generally cylindrical mounting member or base 124. In this emborlim~n~, the base has a distally projecting cylindrical mounting neck 126 formed at the proximal end thereof with a larger ~ met~r 10 ablltmt nt plate 128. The mounting neck is of smaller ~i~m~ter than the inner ~ meter of the tr~nc-l~-cer flange 110. The outer ~ meter of the ablltm~nt plate 128 is sized to conform to the outer ~i~meter of the catheter body 18. The proximal end of the abnrmt nt plate is formed with an axially projecting mounting stem 130 sized for snug receipt within the mounting hole 99 at the distal end of the c~tht ter body 18. The base 124 is also formed with an axial through bore 132 sized subsr~nti~lly the same ~ mto~er as the central inner lumen 97 of the cath~t.or body 18. A transducer wire bore hole 134 is formed from the outer surface of the mounting neck to the through bore 132 for receipt of the first tr~nc.lllcer wire 120 therein. The base may be composed of an electrically insulative material such as VLTEM.
To assemble the convex piezoelectric tr~nc~lc~r 108 onto the distal end 20 of the catheter body 18, a thermocouple sensor 136 such as that shown in FIG. 3 is first formed as described above. The temperature sensor lead pair 138 is directed through the inner lumen of the sheath (not shown) disposed within the axial sensor bore hole 114 of the convex tr~ncr~lrer so that the lead pair is disposed within the inner volume of the 25 tr~nccll7c.or. The thermocouple end of the lead pair is bonded within the axial sensor bore hole so that the thermocouple is disposed generally flush with the outer surface 118 of the tr~ncr3llcer. The thermocouple is bonded within the sensor bore hole using an appropriate adhesive sealant. The adhesive is shaped into a raised mound having a rounded crown 140 slightly above the outer surface of the transducer, the periphery of 30 the crown having a ~i~meter greater that the fli~m~t~or of the sensor bore hole 1~4. The adhesive is cured to securely affix the thermocouple 136 in position in relation to the outer surface and the sensor bore hole of the tr~ncc~nfer.

CA 022362~ l998-04-29 .

WO 98/11826 PCTrUS97/1~870 Once the adhesive is cured, the crown 140 protects the temperature sensing device - 136 from damage and prevents the thermocouple thereof and sensor lead 138 from being pulled inwardly through the sheath after assembly. Furthermore, the adhesive crown 140 permits mounting the thermocouple flush with the outer surface of the transducer 5 and provides a liquid seal that pr~vell~s blood and other fluids that may come into contact with the distal end of the c~thtoter from re~hing the underside of the piezoelectric tr~n~ cer 108 through the tr~nc~tl~r bore hole 114.
To further assemble the hemispheric piezoelectric tr~nC~ cer 108 onto the distalend 20 of the ~th~ter, the first transducer wire 120 is directed inwardly through the 10 radial tr~nC~llcer wire bore 134 and the second tr~n~ cer wire 122 and the temperature sensor lead 138 are gathered together and directed through the axial bore 132 of the mounting base 124. An O ring 143 is mounted on the base 124 to center and support the tr~n~lcer 108 when it is mounted on the base.
The proximal end of the mounting flange 110 of the transducer 108 iS then 15 disposed over the mounting neck 126 of the base 124. The O ring assists in disposing the tr~n~ lr~r in concentric ~lignm~nt with the neck 126. Because the inner r3i~meter of the mounting flange is larger than the outer ~i~m~tl~r of the mounting neck, adhesive/sealant 144 is applied between the two as well as in the space 146 between the ablltmPnt plate 128 of the base and the mounting flange 110. The adhesive/sealant 20 conforrns with the outer ~t~meter of the flange 110. The tr~n~ c~r wire bore 13~ may also be filled with the adhesive/sealant 144.
The adhesive sealant 144 is of a low durometer, biocompatible polymer that securely affixes the transducer to the mounting neck 126 of the base and has s~ nt vibrational isolating and electrical insulating characteristics. The adhesive sealant seals 25 the interior of the catheter body 18 from the entry of bodily fluids that may cause undesirable tr~n~ cPr damping or possible short circuiting.
The respective tr~n~lcPr wires 120 and 122 and the temperature sensor lead 138 are then directed through the inner lumen 97 of the cathetPr body 18 to the proximal end of the catheter. The mounting stem 130 of the base is then pressed into the 30 mounting hole 99 of the distal end 20 of the catheter body and affixed therein using an appropriate epoxy, for inct~nCc, The proximal ends of the sensing leads 138 and transducer wires 120 and 122 are connected to an electrical connector of a manipulation CA 022362~ 1998-04-29 WO 98/11826 PC~IUS97115870 handle connector (not shown) for operative connection to a temperature measurement processing system and tr~nc~l~cer operating system.
When the convex transducer 108 iS in operation, the adhesive 144 between the mounting flange 110 and the mounting neck 126 provides minim~l darnping at the 5 hemispherical tip 112 of the tr~nc~llrer because no adhesive is in contact with the irmer surface thereof to cause such damping.
Referring to FIG. 6, a side-fire application of the r~theter 10 illustrated in FIGS.

2, 3, and 4 is shown. The distal tip 22 and band electrode 26 may be used for mapping purposes to locate an aberrant endocardial tissue site on the endocardial wall 16 of the 10 heart charnber. Once the site 16 has been targeted, the distal end 20 of the catheter 1û
is positioned against the targeted endocardial site 16 in a parallel orientation as shown in FIG. 6 to perform an ablation procedure. For optirnum ablation effectiveness, the distal end of the catheter is oriented such that one longinl~in~l side of the cylindrical piezoelectric tr~nC~l.c~r 24 contacts the target tissue site. In this orientation, the 15 rliniri~n may activate the piezoelectric tr~nc~tlcer to ablate the target endocardial tissue adjacent the tr~nc~llcer.
When activated for ablation, the cylindrical tr~nc~.lrPr 2~ radiates ultrasonic energy at a st lecterl frequency radially outwardly to the endocardial wall 16 to ablate the target tissue. Because there are a plurality of temperature sensors 28 arld they are 20 located substantially at the outer surface of the cylindrical transducer, the temperature sensors are able to sense the temperature of the ablated tissue, ~ rent flowing blood and the surface temperature of the tr~nc~ r~r itself very rapidly. By monitoring sensor outputs or by processing them in other ways, the temperature of the ablation site may be determined.
Referring now to FIG. 7 for an end-fire applic~tion, the c~thPt~r having the hemispherical tr~n~lllcer 108 at its distal tip is illustrated. The hernispherical tip transrl~cPr 108 configuration may be useful in certain applir~tions where the aberrant target tissue is located at a position of the heart chamber not conducive to use of the c~theter having the cylindrical transducer 24 of FIG. 6. Due tO the contours of the 30 heart chamber, the cylindrical tr~ncrillr~r may be too large to fully contact the tissue along its longitll~in~l side and may therefore iimit the cylindrical tr~n~ lcPr's eLre~;Liv~ lless.

CA 022362~ 1998-04-29 -W O 98/11826 PCTrUS97/15870 In the perp~n~lic~ r ori~nt~tisn shown in F~G; 7, the tip portion 112 of the convex tr~ns~llCer 108 and the distal tip temperature sensing device 136 are in contact with the target tissue of the endocardial wall 16. In this configuration, the tip transducer 108 is powered to ablate the target tissue. The convex piezoelectric 5 transducer 108 shown may result in a relatively thin but deep lesion. Because the temperature senCing device 136 is in direct contact with the ablated tissue site, a direct in~ic~tion of the temperature thereof is provided and thus the operation of the tr~n~ cer may be more precisely and accurately controlled to maintain the temperature . .
wlthm llrrllts.
In either of the catheter configurations shown in the figures, having the temperaNre sensing devices 28, 136 disposed in sensor bore holes 34, 114 formed in the piezoelectric transducer 24, 108 itself provides a desirable temperature sensing configuration. Because the thermocouple of the sensing device is disposed at theperiphery of the transducer, a more accurate and faster temperature sensing response is 15 provided. In addition, the ch~nct-s of having a sensor in close proximity to the endocardial ablation site provides the ~lini~i~n with a greater ability to control tissue and blood heating during the ablation procedure minimi7ing adverse effects to the patient.
The temperaNre s~n~ing devices 28 offer a more rapid temperature s~n~ing response in~ic~tion of the endocardial ablation site and the flowing blood ~ cPnt the ablation 20 electrode so that ablation procedures can be more accurately and positively controlled.
Furthermore, the mounting means and adhesive/sealant configurations between the distal end of the catheter body and the particular piezoelectric transducer provide a secure mounting arrangement while preventing lln~l~cirable lea~age of bodily fluids into the catheter body as well as reduced damping of the tr~nctll.c~r.
During the ablation procedure, the target tissue is he~te~ and the resulting heat buildup in the transducer can impair tr~nc~ cer operation. Because of its mass, the - target tissue has a greater thermal inertia than the tr~ncr~llc~r itself. Thus, when power is discontinl~e~ to the tr~nc~ cer, its temperature will decrease faster than the temperature of the ~arget tissue. Additionally, at least a part of the tr~n~t11lc~r is located 30 in the cooler flowirlg blood that will carry away some of the heat of the transducer.
Thus the power to the transducer can be reduced or even shut off, the transducer can be allowed to cool, and the power can be increased or resumed to the transducer before CA 022362~ 1998-04-29 WO 98/11826 PCT/US97/lS870 the temperature of the target tissue decreases to any sllkst~n~i~l degree. These steps can be accomplished by controlling the drive level of the power generator supplying ablation power to the transducer, as is discussed below in more detail.
FIG. 8 is a representative diagram showing frequency versus power transfer for three ultrasonic transducers Rl, R2, and R3. Each transducer has a characteristic frequency or frequencies. The first transducer Rl has a relatively high frequency sensitivity (Q). The series resonant frequency fs and parallel resonant frequency fp, both of which are characteristic frequencies of this transducer, occur where the power transfer is m~rimum in this case. The power transfer drops off rapidly to either side of the 10 resonant freq~lPnfies as well as in between the resonant freq~l. nc~ies. At the center frequency fc, the power transfer is greatly decreased from both that of the series and parallel resonance frequencies.
~ ;or a tr~nc~cer having a lower frequency sensitivity, such as that shown by R2, there are also two characteristic frequencies, the series resonant frequency fs and parallel 15 resonant frequency fp and they still occur where the power transfer is m~imllm, but the power transfer does not vary as widely at freqllPn~iPs between fs and fp. Power transfer still drops off sharply in the region below the series resonant frequency fs and in the region above the parallel resonant frequency fp. However, in the area between the series and parallel resonant freq~len~iPs fs and fp inc~ ing the center frequency fc~ the power 20 transfer remains relatively high with only a slight dip at fc.
A tr~n~lcer having a bio-compatible outer layer that greatly reduces frequency sensitivity ~ower Q) is represented by the line R3. For R3, the power transfer is m~imllm at an operating frequency g betw-een the series resonant frequency fs and parallel resonant frequency fp and is acwally greater than at freqllen~ies fs and fp, with 25 the power transfer tapering off beyond the region between the series and parallel resonant freqll.on~i~s In this case, power transfer is greatest at the frequency fc rnidway between the series and parallel resonance freqll~nçiPs.
During the operation of the tr~nc~lc~r when subjected to fh~nging temperature and /-h~nging loading conditions, the series resonant frequency f5 and parallel resonant 30 frequency fp will typically drift to some extent, depending on the load applied to the transducer and heat experienced. A shift in the resonant freq~l.on~ies results in a change in the power transfer curve. For exarnple, the downward shift in the resonan~

CA 02236255 l998-04-29 frequencies causes the frequency v. power transfer curves of FIG. 8 to shift to the left, as shown by FIG. 9.
Depending on the particulàr transducer's frequency sensi~iv;~y, even a very small shift in the resonant freqll~nries (and hence in the power transfer curve) can cause a 5 large change in power transfer. Referring now to FIG. 9, curves for tr~ncr3.1c~rs Rl, R2, and R3 subjected to different loading conditions than in FIG. 8 are once again plotted on a graph of power transfer versus frequency. In this graph, the original series and parallel resonance freql1.onri.os f5 and fp that are shown in FIG. 8 are now shown in dashed lines. The new series and parallel resonance freqtlencies f5' and fp' are shown and 10 are at lower freqllenries FIG. 9 shows that tr~nc~llclor Rl, which has a high frequency sensitivity, experiences a substantial change in power transfer when the resonance frequencies shift. If the frequency of energy driving the tr~nc~llcPr were m~inr~ine~ at either of the initial resonant freq~ nries f5 or fp, the power transfer would substantially decrease when the resonance freqll~nri~c shift downward to f5' and fp'. Transducer R2 15 also experiences a change in power transfer, although the change is not as great as the change experienced by tr~n~llctor Rl.
For the tr~ncr31~cer R3 having an external bio-layer with resulting reduced frequency sensitivity, shifts in the series and parallel resonant freq~lPnri~s to fs' and fp.
cause smaller variations in the power transfer curve for an operational frequency at fc 20 and other freq~lenries lying between the series and parallel resonant freqll.onries. By setting and m~int~ining the operational frequency fc at a point midway between the original series fs and parallel fp freq~l~ncies, a small to moderate shift in the series fs and parallel fp resonant frequencies will result in a relatively minor change in the power transfer. The power transfer can be held relatively stable as the loading on the25 tr~ns~ cer changes.
Accordingly, the bio-layer coating 27 is added to the exterior of the ultrasoniccrystal to control the frequency sensitivity and achieve results similar to the transducer R3 shown in FIG. 9. In one embo~im~nt the bio-layer was s~lectef~ to be 0.03 mrn(0.0012 in.) thick. It was found that with a bio-layer of this thirkness, an incignific~nt 30 power transfer change occurred when the operational frequency fc of the ukrasonic crystal was determined with the ultrasonic crystal at 37~ C and then the crystal was elevated in temperature to 85~ C during an ablation procedure. This bio-layer effectively broadens the bandwidth of the R3 crystal by m~king it operational over a wider temperature range. As is seen in FIG. 9, the frequency range at which the response varies by an incignific~nt amount is much broader for R3 than for Rl or R2.
The responses for Rl and R2 have dropped much more than for R3.
It was also noted that the bio-layer formed on the external surface of the ultrasonic tr~ncllvcer makes the tr~nc~ cer more durable and less sub)ect to breakage when subjected to shocks during h~n~ling. In one embo~imtont, the bio-layer was formed of 353 ND epoxy made by Epoxy Technologies. The m~h~nic~l strength of the crystal is ~nh~nce~ by the bio-layer with the crystal now being much more robust 10 and can withct~n~ larger impacts without shattering. It was also found that the mentioned epoxy had an acceptable thermal co~ffici~nt and transferred heat along the layer at an acceptable rate.
Referring now to FIG. 10, a block diagram of an embo~limtont of a power generator 170 is presented. The power output line 172 carries the power output from 15 the power control 174 to the tr~nc~t.c~r 24 at the distal end of the c~th~t~r 10 to provide the energy needed for ablation. The temperature input line 176 carries temperature sensor signals ~rom the temperature sensors located at the distal end of the cathet~r.
These temperature sensor signals are processed in the temperature sensor processor 178 which Oll~uLS a temperature signal represen~ing the temperature sensed to the control 20 processor 182 through an analog-to-digital ("A/D") converter 180.
The power generator 170 preferably generates a power output having a sine wave.
The power controller preferably incl~ s a frequency synthesizer 184 that controls the frequency of the power output. The control processor 182 controls the frequency syntheci7~r 184 as part of its control of the operation of the power control 174. The 25 power control 174 may in~ a power amplifier 186 in order to produce the power output 172 nee~e~l to be delivered to the catheter 10 for ablation.
The power supply 188 provides power to the various toTtqmtontc of the power generator 170. A power transfer sensor, such as the voltage and current monitor 190, monitors the voltage and current of the ablation energy to determine the impedance 30 pres~nte~l by the ultrasonic crystal 24. The voltage and current monitor 190 provides signals repres~nring the voltage and current values through an A/D converter 192 to the control processor 182. The control processor 182 controls the frequency syntheci?lor to CA 022362~ 1998-04-29 alter the frequency driving the ultrasonic crystal 24 until the series and parallel resonant ilnpedance freqtlen~ies have been found.
In one embodiment, the control processor 182 autom~tif~lly causes the frequency syntheci7er 184 to sweep through a predetermined frequency range to identify the5 resonance freq--Pncies of the ultrasonic tr~nc~ cer 24 in response to acnlation of the power-on or power application switch 185. At these freqll~n~i~s, the impedance will be the lowest inrli~ting that m~im~lm power transfer is occurring. The control processor 182 then calculates the average of the series and parallel resonance frequencies and controls the frequency synthf ci7er 184 to operate at the c~lclll~te~ average frequency.
Spe~ifi~lly in one embo~im-ont, the voltage is held constant as the frequency isswept. The current monitor 190 monitors the current flow to the ultrasonic transducer 24 and provides a current level signal to the processor 182 through the A/D converter 192. The highest current flow inrlir~tes the lowest impedance of the ultrasonic tr~nC~ c~r. The frequency at which the highest current flow occurs is the series15 resonance frequency. The lowest current flow inf~ir~tes the highest impedance of the ultrasonic tr~nC~ c~or. The frequency at which the lowest current flow occurs is the parallel resonance frequency. As states above, the processor 182 averages the series and parallel resonance freq1~n~ies to arrive at the operating frequency.
In one embodiment, this frequency sweep and operating frequency lock-on 20 procedure was completed in less than one second. A high impedance of 200 Q was observed with some crystals and a low impedance of 40-50 Q was observed. An average operating impedance was observed to be 70-80 ~. The control processor 182 controls the power control 174 to apply power to the tr~nc~ Pr 24 at a constant frequencywhile altering the drive level to m~int~in the temperature at the tr~nc~lllc.or within a 25 predetermined range or level.
FIG. 11 is a flow chart illustrating the operation of an energy delivery method in accordance with aspects of the present invention. The cath.oter having an ultrasonic tr~nc-l~lc.or is introduced into the patient and located at the site where energy is to be delivered. Such may occur for an ablation procedure. During this introduction 30 procedure, the ultrasonic crystal may have stabili7ed at patient temperature, such as 37~
C. The lead for the ultrason}c crystal is plugged into the power generator 170 ~FIG. 10) and the physician then causes the power generator to be placed in the ~power onn mode CA 022362~ 1998-04-29 WO 98/11826 PCT/US97/1~870 2~
200. The control processor causes the frequency synthPci7~or to sweep through a range - of frequencies to locate the series resonant fre~uency and the parallel resonant frequency autom~ti~lly 202. The control processor then averages the two freq~ltoncies to determine a center frequency 204. The operational frequency of the instrument is set 5 at the center frequency 206 at which time a predetermined level of power is autom~ti~lly applied to the tr~n~ cor for performing the energy delivery process.
The above process is transparent to the user of the system. Pressing the power-on switch causes the system to autom~ti~lly tune the power to the particular ultrasonic crystal in use at the particular patient temperature to which it has stabilized and then 10 once the tuning is completed, autom~tir~lly apply power to perform the energy delivery to the patient site. The method and system therefore accurately tune the drivingfrequency for each crystal and for each energ,v delivery process. As mentioned above, in one case, the ~.ltom~tic tuning process took less than one second.
A temperature sensor, such as the temperature sensor 28 shown in FIG. 3, is used15 to sense the temperature at the distal end of the c~theter, and the sensed temperature is monitored at step 208 to control the drive level. When the temperature is determined to be above a first selected temperature, such as 85~ C 210, the drive level of tke power outp-ut is decreased 212 to allow the transducer to cool down. If the temperature is determined to be below a second sl~lecte~l temperature 214, the drive level is increased 20 216 to a higher level to apply more power to the ablation site. The first and second temperatures may be i~entic~l or the second temperature may be below the first by some amount, depending on the desired temperature sensitivity.
During the procedure shown in PIG. 11, the operating frequency is maint~in~
constant at the center frequency established in step 106. Controlling the drive level as 25 shown in FIG. 11 permits control over the temperature so that tissue charr.ng and blood boiling can be avoided.
The above method spe~ lly mentions sweeping and tuning the drive frequency at only one time, after intro~llf~ion to the patient and when at the patient energy delivery site. However, the automatic tuning process can be con~ll.c~e~l at other lirnes 30 also. For e~r~mrle, it may be autom~ti~lly conducted while energy is being delivered.
The control processor may autom~ti~lly and periodically sweep the freqll~ncit s to deterrmine the optimum operating for the crystal at any tirne, including when the .

CA 022362~ l998-04-29 application of power by the ultrasonic crystal has raised the site temperature to 85~ C
or higher. This operation was not selected in the previously~iccllcse~ embodiment because it was found that with the bio-layer, even though the optimum operating frequency changed with remperature, there was so little loss of power transfer that it was 5 deemed to be not necessary to re-tune or recalibrate the crystal. However, such periodic re-tuning or recalibration would be possible with the system and method preslonte~l above.
Additionally, the above embodiment has spefific~lly mentioned the use of the average of the series and parallel freq~l~nries as the operating freguency. However, a 10 different operating frequency may be selected based on these detected freql.en~ies or another detected freguency or frequencies. For example, where a higher Q crystal is nee~ for a particular application, the tuning system may be used to locate either the series or parallel frequency and operate at one of those freq~l~n~ies or at a frequency that may be determined by reference to them. This frequency location and selection may 15 be automatic as described above.
Although preferred and alternative embo~iments of the invention have been described and illustrated, the invention is susceptible to modifications and adaptations within the ability of those sl~illed in the art and without the exercise of inventive faculty. Thus, it should be understood that various changes in form, detail, and usage 20 of the present invention may be made without departing from the spirit and scope of the invention. Accordingly, it is not int~nf~e~l that the invention be limitefl, except as by the appended claims.

Claims (32)

26What is claimed is.

1. A system for delivering energy to biological tissue, comprising:
a catheter having distal and proximal ends;
an ultrasonic transducer adapted to transduce electrical energy into acoustic energy, the transducer having a characteristic frequency, the transducer mounted at the distal end of the catheter;
a power supply that provides electrical energy to the ultrasonic transducer at aselectable frequency;
a power transfer sensor coupled to the ultrasonic transducer that measures the response of the ultrasonic transducer to the power provided to it by the power supply, the sensor providing a power transfer signal;
a processor adapted to automatically control the power supply to vary the frequency of electrical energy applied to the transducer and to monitor the power transfer signal in response to the frequency variation to determine the characteristic frequency of the transducer.

2. The system of claim 1 wherein the processor automatically controls the power supply to sweep through a predetermined range of frequencies while monitoring the power transfer signal to determine the characteristic frequency.

3. The system of claim 2 wherein the processor automatically controls the power supply to operate at the determined characteristic frequency.

4. The system of claim 2 wherein the processor is adapted to automatically process the determined characteristic frequency to derive an operation frequencytherefrom and automatically control the power supply to operate at the operationfrequency.

5. The system of claim 1 wherein the tuning system automatically controls the power supply to sweep through a predetermined range of frequencies while monitoring the power transfer signal to determine first and second resonance frequencies of the ultrasonic transducer.

6. The system of claim 5 wherein the processor automatically controls the power supply to operate at or near one of the determined resonance frequencies.

7. The system of claim 5 wherein the tuning system automatically processes the determined resonance frequencies to derive an operation frequency therefrom and automatically controls the power supply to operate at the operation frequency.

8. The system of claim 7 wherein the processor automatically averages the first and second resonance frequencies and automatically controls the power supply to operate at the average frequency.

9. The system of claim 8 further comprising a power application switch;
wherein the processor is responsive to actuation of the power application switchto automatically control the power supply to apply power to the ultrasonic transducer while varying the frequency to determine the first and second resonant frequencies while monitoring the power transfer signal and to then apply a predetermined level of power as selected at the operation frequency.

10. The system of claim 1 further comprising a biologically-compatible layer formed on the outside of the ultrasonic transducer that lowers the frequency sensitivity of the transducer.

11. The system of claim 10 wherein the biologically-compatible layer is non-metallic and has a relatively high coefficient of thermal conductivity.

12. The system of claim 1 further comprising a temperature sensor mounted at the distal end of the catheter that senses temperature and provides a temperature sensing signal.

13. The system of claim 12 wherein the processor receives the temperature sensing signal, compares it to a predetermined first threshold temperature and controls the power supply to decrease the power drive level when the temperature signal represents a temperature above the first threshold.

14. The system of claim 13 wherein the processor automatically controls the power supply to hold the frequency constant while varying the power level to maintain the temperature within a predetermined range.

15. The system of claim 12 wherein the temperature sensor is mounted in the ultrasonic transducer.

16. The system of claim 15 wherein the ultrasonic transducer is cylindrically shaped.

17. A method of delivering energy to a biological site, comprising the steps of:applying electrical power to an ultrasonic transducer that is adapted to transduce electrical energy into acoustic energy, the transducer having a characteristic frequency;
measuring the response of the ultrasonic transducer to the power provided to it,the sensor providing a power transfer signal;
automatically varying the frequency of electrical energy applied to the transducer and monitoring the power transfer signal in response to the frequency variation to determine the characteristic frequency of the transducer.

18. The method of claim 17 wherein the step of varying the frequency comprises the step of automatically sweeping through a predetermined range of frequencies while monitoring the power transfer signal to determine the characteristic frequency.

19. The method of claim 18 comprising the further step of automatically applying power to the transducer to operate at the determined characteristic frequency.

20. The method of claim 18 comprising the further step of automatically processing the determined characteristic frequency to derive an operation frequency therefrom and automatically applying power to the transducer at the derived operation frequency.

21. The method of claim 17 further comprising the step of automatically sweeping through a predetermined range of frequencies while monitoring the powertransfer signal to determine first and second resonance frequencies of the ultrasonic transducer.

22. The method of claim 21 comprising the further step of automatically applying power to the transducer at or near one of the determined resonance frequencies.

23. The method of claim 21 comprising the further step of automatically derivingan operation frequency from the determined resonance frequencies and automatically applying power to the transducer to operate at the operation frequency.

24. The method of claim 23 comprising the step of averaging the first and secondresonance frequencies and automatically applying power to the transducer to operate at the average frequency.

25. The method of claim 24 further comprising the steps of:
actuating a power application switch;
automatically applying power to the ultrasonic transducer while varying the frequency in response to actuation of the power application switch to determine the first and second resonant frequencies while monitoring the power transfer signal; and applying a predetermined level of power to the transducer at the operation frequency.

26. The method of claim 17 further comprising the step of forming a biologically-compatible layer on the outside of the ultrasonic transducer that lowers the frequency sensitivity of the transducer.

27. The method of claim 26 wherein the step of forming the biologically-compatible layer comprising forming the layer of a non-metallic material that has a relatively high coefficient of thermal conductivity.

28. The method of claim 17 further comprising the steps of sensing temperature at the distal end of the catheter and providing a temperature sensing signal.

29. The method of claim 28 further comprising the steps of comparing the temperature sensing signal to a predetermined first threshold temperature;
decreasing the power drive level to the transducer when the temperature signal represents a temperature above the first threshold.

30. The method of claim 28 further comprising the step os automatically controlling the frequency to be constant while varying the power level to maintain the temperature within a predetermined range.

31. The method of claim 28 further comprising the step of mounting the temperature sensor in the ultrasonic transducer.

32. The method of claim 31 comprising the step of forming the ultrasonic transducer to be cylindrical in shape.