WITH OFFSET CAVITATION SENSE ELEMENT LOCATIONS

This invention relates to medical diagnostic ultrasound systems and, in particular, to ultrasonic transducers which are used for controlled heating of body tissues by high intensity focused ultrasound, known as HIFU.

Ultrasonically delivered elevated temperature treatments are used for a variety of therapeutic purposes In HIFU treatment, ultrasonic energy is focused to a small spot within the body so as to heat the tissues to a temperature sufficient to create a desired therapeutic effect. The technique is similar to lithotripsy, where focused energy is high enough to break up kidney stones, but with considerably less energy that is delivered over an extended time rather than a sudden pulse. The HIFU technique can be used to selectively destroy unwanted tissue within the body. For example, tumors or other pathological tissues can be destroyed by applying focused

ultrasonic energy so as to heat the cells to a temperature sufficient to kill the tissue, generally about 60 to about 80 degrees C, without destroying adjacent normal tissues. Other elevated-temperature treatments include selectively heating tissues so as to selectively activate a drug or to promote some other physiological change in a selected portion of the subject's body.

HIFU transducers are often formed as spherical or parabolic dishes with a radius of curvature that gives the transducer a geometric focal point. See, for example, the HIFU transducer described in

international patent application IB2010/054985, filed November 3, 2010. The transducer described in this application is formed of a small number of composite ceramic piezoelectric tiles. The tiles are curved in two dimensions so that they will fit together to form the desired spherical transmitting surface of a desired geometric focus. Each tile can be

individually fabricated and tested before assembly, assuring that the complete transducer will be fully function as specified after assembly. Such composite ceramic piezoelectric tiles can exhibit an energy conversion efficiency of 80-85% during transmission.

When destroying tissue cells by thermal effects, it is generally desirable to avoid the development of higher ultrasonic energy levels that will produce more deleterious effects such as cavitation.

Consequently HIFU transducers often include a

cavitation sensor which is used to monitor for evidence of cavitation. Stable and inertial

cavitation can be detected by the appearance of certain noise and harmonic signal levels as described in US provisional patent application no. 61/392,067 entitled "MONITORING AND CONTROL OF MICROBUBBLE

CAVITATION IN THERAPEUTIC ULTRASOUND" (Vignon et al . ) See also US Pat. 5,827,204 (Grandia et al . ) which uses a hydrophone to detect acoustic evidence of cavitation. The cavitation sensor is generally located at the center of the HIFU transducer.

However, when the HIFU transducer is shaped

spherically or parabolically for focusing, it has been found that the center of the dish shape can receive energy and heat reflected back from the interface of the transducer' s fluid bath and acoustic window. This heat and energy can focus at the center of the HIFU transducer and damage the cavitation sensor. A damaged cavitation sensor, whether from reflected energy or damage during the manufacturing process, can render the entire transducer

unacceptable, even if the HIFU elements themselves are still fully functional. Hence it is desirable to prevent damage to the cavitation sensor and scrapping of an otherwise functional HIFU transducer.

In accordance with the principles of the present invention, a spherical HIFU transducer is described with a cavitation sensor which is located off-center from the center of the transducer. Locating the cavitation sensor in this manner places the sensor at a point where it is not receiving the maximum amount of focused reflected energy. In accordance with a further aspect of the present invention, a plurality of such locations are located around the center of the transducer with a cavitation sensor located at one of the locations. If the cavitation sensor becomes damaged, another cavitation sensor can be installed at one of the other off-center locations and used in place of the damaged sensor.

In the drawings:

FIGURE 1 illustrates in perspective a spherical transducer matching layer separately formed for a HIFU transducer of the present invention.

FIGURE 2a illustrates an end view of a sheet of ceramic piezoelectric material which has been diced to form a composite transducer array for a HIFU transducer of the present invention.

FIGURE 2b illustrates a composite transducer array with a nonmagnetic via constructed in

accordance with the principles of the present

invention .

FIGURE 3 illustrates a composite transducer array with emitting elements and nonmagnetic vias constructed in accordance with the principles of the present invention. FIGURE 4 illustrates a composite piezoelectric tile prior to spherical shaping for a HIFU transducer of the present invention.

FIGURE 5 illustrates in cross-section the placement of composite piezoelectric tiles on the matching layer for a HIFU transducer of the present invention .

FIGURE 6 illustrates in perspective the back of a nine-tile HIFU transducer of the present invention.

FIGURES 7a and 7b illustrate the front and back surfaces of a curved printed circuit board with extended compliant contacts for a HIFU transducer of the present invention.

FIGURE 8 illustrates in perspective the back of a HIFU transducer of the present invention with a support frame attached for the printed circuit boards of FIGURES 7a and 7b.

FIGURE 9 is a rear view of a spherical HIFU transducer with three off-center locations for cavitation sensors.

FIGURE 10 is an enlarged view of the central region of the HIFU transducer of FIGURE 9 showing three off-center cavitation sensors.

FIGURES 11 and 12 are cross-sectional views of a HIFU transducer with an off-center located modular cavitation sensor.

Construction of a HIFU transducer of the present invention may begin with fabrication of a spherical or dish-shaped matching layer. The matching layer (s) of a transducer provide at least a partial matching of the acoustic properties of the piezoelectric transducer to the acoustic properties of the

patient's body or the medium between the transducer and the patient. The properties matched may include acoustic impedance, velocity of sound, and material density. In the conventional construction of an ultrasound transducer the matching layer is generally formed on the transducer stack and is formed over the reference electrodes on the emitting surface of the piezoelectric material. For the HIFU transducer described in this disclosure a spherical matching layer is formed by itself, separate from the rest of the transducer. There are several ways to form the spherical matching layer, including casting, molding, thermoforming, or machining. The spherical matching layer of the HIFU transducer described herein is made of a loaded epoxy which is loaded with particles which provide the matching layer with its desired acoustic properties as is known in the art.

Preferably the particles are non-magnetic. In casting or molding the spherical matching layer, the loaded epoxy is poured into a concave fixture of the desired spherical shape. A convex fixture is closed over the concave fixture, forcing the liquid epoxy to fill the spherical space between the two fixtures.

The epoxy is cured and removed from the fixtures, then peripherally machined to its final form. In a thermoform process a planar sheet of the desired thickness is formed of the loaded epoxy, then

partially cured. The sheet is then placed over a heated convex or concave fixture of the desired curvature which warms the sheet so that it becomes pliant and conforms to the curvature of the fixture. When the sheet has attained its desired spherical shape it is cured and finished. In a machining process a disk of loaded epoxy is cast or molded and cured. The disk is then machined on one side to form a convex surface. The disk is then put on a concave fixture and the other side of the disk is machined to form the concave side of the spherical matching layer. In a constructed embodiment the finished spherical matching layer from any of these processes is 0.5mm thick, has a diameter of 140mm, and a spherical radius of 140mm, the size and shape of the finished HIFU transducer. FIGURE 1 illustrates such a spherical matching layer 10. The concave surface 12 is the emitting surface of the finished transducer which faces the patient and the convex surface 14 is sputtered to produce a redundant signal return electrode, then covered with composite piezoelectric tiles. The rigid matching layer thus provides a form of the desired curvature for assembly of the

piezoelectric tile layer. Since the matching layer 10 in front of the tiles is a continuously formed surface, it provides the desired electrical and environmental isolation of the rest of the HIFU transducer from the patient and the external

surroundings in front of the HIFU transducer.

Construction of the composite piezoelectric transducer array begins with a sheet 30 of ceramic piezoelectric material as shown in FIGURES 2a and 2b. In a constructed transducer the sheet 30 is 1.2mm thick (T) . First, a number of holes are drilled through the sheet 30 where it is desired to have electrical connections from the back to the front

(emitting side) of the transducer. The holes are then filled with silver-filled epoxy to form vias 32 through the sheet. The silver filling provides electrical conductivity and is non-magnetic for operation in a magnetic field of an MRI system.

Other non-magnetic conductive material may be used for the conductive filling. The silver epoxy is cured. The sheet is then diced part-way through the thickness with parallel cuts 16 in one direction as shown in the view of the edge of the sheet 30 in FIGURE 2a. Then the sheet is diced part-way through with parallel cuts in the orthogonal direction, leaving a plurality of upward projecting

piezoelectric posts 18 and vias 32. The dicing cuts are then filled with non-conducting epoxy and

partially cured. The top and bottom surfaces of the sheet are then machined flat to the depths indicated by dashed lines 34 in FIGURE 2a. This will result in a finished sheet of a matrix of piezoelectric posts 18 and conductive vias 32 in epoxy 36 as shown in

FIGURE 2b. The finished sheet comprises a 1:3 matrix of piezoelectric posts, each of which has its

dominant vibrational mode in its longitudinal

direction through the thickness of the sheet, and which transmits ultrasound predominately in a

direction toward the front (patient facing) side of the transducer. This predominant vibrational mode of the composite material reduces unwanted lateral transmission across the array to other active areas of the array.

The flat composite piezoelectric sheet 30 is machined to a trapezoidal shape as shown by the peripheral shape of the composite piezoelectric tile 40 of FIGURE 4. In a constructed HIFU transducer the tiles have the trapezoidal shape of FIGURE 4 to allow for a circular spherical center tile as described below. Alternatively, each tile may be machined in the shape of a slice of pie, so that the tiles will cover the matching layer without need for a center tile. The tiles could also take on other geometric shapes arranged to cover the spherical surface including but not limited to pentagons mixed with hexagons as demonstrated by the panels of a soccer ball. The flat trapezoidal tile of FIGURE 4 is then given its desired spherical curvature. Since the composite transducer is formed of a matrix in epoxy, the tile can be heated to soften the epoxy so that the tile can be conformed to the desired curvature. This can be done by placing the tile 40 on a heated concave or convex fixture, then pressing the tile into conformance with the convex or concave shape. While the tile is held in the desired curvature, the fixture is cooled and the epoxy is allowed to fully cure. The result is a spherical-shaped composite piezoelectric tile for a spherical HIFU transducer.

After the tile has been curved the top and bottom surfaces 38 are metallized by sputtering a conductive material onto the surfaces of the sheet as shown for the sheet 30 of FIGURE 3. Preferably the conductive material is non-magnetic such as gold or titanium/gold . The metallized surfaces are

electrically connected by the conductive vias 32, providing electrical connection from the back surface of the composite sheet to the front. Active

(transmitting and receiving) areas of the composite piezoelectric sheet are then isolated by diamond core drilling, laser drilling, or ultrasonic machining around desired active areas from the back (convex) surface of the tile. Several such defined active areas 44 are shown in FIGURES 3 and 4. The cuts 42 which define the active areas cut through the

metallization of the surface of the sheet to

electrically isolate the areas and preferably extend over half-way through the composite sheet so as to acoustically isolate the active area from the

surrounding areas of the sheet and other active areas. Alternatively, the active areas can be electrically and acoustically isolated after the tiles are bonded to the matching layer.

In a constructed tile the active areas 44 are not symmetrically arranged in rows or columns or circles or other regular patterns but are irregularly or randomly arranged as shown in FIGURE 4. The random pattern prevents any significant additive combining of the acoustic sidelobes of the active areas which would diminish the effective energy delivered by the HIFU transducer.

Eight of the spherical trapezoidal tiles 40 are then thin bonded adjacent to each other around the convex surface 14 of the matching layer 10, which thereby provides a form for assembly of the tiles. If the spherical tiles 40 are pie-shaped as described above, the tiles will completely cover the convex side of the matching layer 10. When the spherical tiles are trapezoidal as shown in FIGURE 4, they will cover the convex side of the matching layer except for the center of the matching layer. This circular spherical space can be left open. Alternatively it can be covered with a circular spherical thermal conductor such as aluminum for cooling. Returning acoustic energy will tend to be focused in the center of the HIFU transducer by virtue of its spherical geometric shape. Locating a thermal conductor here can aid in cooling the HIFU transducer.

Alternatively, a circular spherical composite

piezoelectric tile 48 can fill this space. For example, the circular sheet of FIGURE 3, with its own active areas, can be formed into a spherical shape and located here, providing full composite

piezoelectric coverage of the matching layer 10 as shown by the cross-sectional view of the trapezoidal and circular tiles on the matching layer 10 in FIGURE 5. In a constructed transducer of this full coverage design, the nine tiles provide the HIFU transducer with 265 active areas, 256 for transmit and nine for receive .

It is seen in FIGURE 3 that the vias 32 are located so as to connect the metallized area around the active areas on the back surface to the

metallized surface on the front (patient-facing) side of the tile. In a constructed HIFU transducer the metallized area around the active areas 44 is

electrically coupled to a reference potential. The vias 32 couple this reference potential to the metallized surface on the other side of the tile, the side not visible in FIGURE 3. The vias are thus used to apply a reference potential to the patient-facing side of the composite piezoelectric tiles, and also to the metallization on the patient-facing side of the active areas 44. Since the patient-facing side of the tiles 40 are bonded to the matching layer 10 and are thus inaccessible for electrical connections, the vias provide the needed electrical connection through the piezoelectric sheet to the front side of the tile.

Next, a plastic support frame 50 is attached to the back of the assembled tiles by bonding, snap fit, or fasteners as shown in FIGURE 6. In a constructed transducer each of the nine tiles 40,48 is accessible between the ribs of the support frame. The support frame is used to mount eight trapezoidal and one circular printed circuit boards 52 in a spaced relation above the back surfaces of the composite piezoelectric tiles 40. FIGURES 7a and 7b illustrate the front and back (54) surfaces of the trapezoidal printed circuit boards 52. Located on the back surface 54 are printed circuit connections 56 from a connector 57 which are connected by plated through- holes 59 through the board to active areas of the HIFU transducer. On the front surface of the printed circuit boards are compliant metallic contacts 60 which span the space between a printed circuit board and its tile and electrically connect the printed circuit connections to the active areas 44 and vias 32 of the opposing composite piezoelectric tile 40.

Located at one edge of the printed circuit board 52 which is at the periphery of the HIFU transducer are cooling notches 58.

A printed circuit board 52 is bonded to the support frame 50 above each tile such as tile 40 shown in FIGURE 6. When a printed circuit board is assembled in this manner it appears as shown by printed circuit board 52 in FIGURE 8. Before this assembly, the extended ends of the compliant metallic contacts 60 are coated with conductive epoxy. When the printed circuit board is assembled on the frame, the ends of the contacts 60 will contact metallized areas of the opposing tile and become bonded in electrical connection with the metallized areas when the conductive epoxy cures. The contacts 60 thus provide electrical communication between the printed circuit boards and active and reference potential areas of the piezoelectric tiles.

While the printed circuit boards can be

fabricated as conventional planar printed circuit boards, the printed circuit board 52 of FIGURES 7a and 7b preferably have a spherical curvature, matching that of the opposing composite piezoelectric tiles 40 to which they are connected by the contacts 60. The printed circuit boards can be curved on just the side facing the tile as shown in FIGURE 7a, or on both sides. The printed circuit boards can be formed as curved boards in several ways. One is to start with a thick planar sheet of glass epoxy board material and machine or grind the surface of the board to the desired curvature. The other technique is to use thermoforming to heat the board material and soften the epoxy, then form the curvature by compressing the sheet against a fixture of the desired curvature. The circuit boards can be double- clad with photo-imaged and chemically-etched

conductive lines on the top and bottom surfaces interconnected by plated through-holes formed in the boards. The circuit boards can also be multilayer boards with three or more layers of conductive lines formed on the surfaces and within layers of the board for more complex, higher density circuit

configurations. The rigid boards 52 are also capable of securely mounting other electrical components such as the connector 57.

In accordance with the principles of the present invention, a dish-shaped HIFU transducer such as the spherical shaped HIFU transducer previously described includes a cavitation sensor which receives acoustic signals which may be indicative of the onset of cavitation in the treatment region of the body.

FIGURE 9 is a rear view of a spherical HIFU

transducer 120. In this view the printed circuit boards 52 are removed and the rear of the tiles 40 in the frame 50 are visible. A circular printed circuit board in the center of the frame 50 for making connections to the cavitation sensor is also removed so that the off-center locations for the cavitation sensor are visible. A cavitation sensor 90 is shown which is bonded to one of the three off-center locations. Since cavitation sensor 90 is located at a position offset from the center of the spherical HIFU transducer, energy and heat from therapeutic transmission which is reflected back to the center of the transducer is not pinpointed at the offset sensor, thereby reducing the possibility of damage from these events. An enlarged view of the center of the HIFU transducer is shown in FIGURE 10.

The enlarged view of FIGURE 10 illustrates the placement of three cavitation sensors 90 in the central region of the HIFU transducer but each offset from the geometric center of the spherical transducer where the damages hazard is greatest. The three offset locations are labeled as 130, 130', and 130". Each cavitation sensor 90 comprises a piezoelectric receive element. The piezoelectric receive element may be formed from solid piezoelectric ceramic, a composite ceramic formed by dicing a piezoelectric ceramic disk at right angles and filling the dicing cuts with epoxy filler, or an element made of

piezoelectric PVDF material. In either case, the piezoelectric element is ground or lapped to a thickness which achieves the desired receive

frequency. The outer surfaces of the piezoelectric element are metalized to provide signal and return contacts. A circular isolation cut 104 is formed in the rear surface of the piezoelectric element to separate the metallization on the rear surface into two contact electrodes, a circular contact electrode 98 in the center of the rear surface and an annular peripheral contact electrode 96 which is contiguous with the metallization on the front of the element. The element is then electrically poled. A matching layer 100 is bonded to the front surface of the piezoelectric element 90. The cavitation sensors are then bonded into place in apertures at the offset locations in a mounting plate 140 located in the center of the frame 50. Bonding may be done with an epoxy that forms a liquid-tight seal around the periphery to the sensor, thereby preventing ingress of the coupling/cooling fluid in front of the HIFU transducer. A circular printed circuit board behind the cavitation sensors has electrical contacts which make spring contact with the contact electrodes 96, 98 of the cavitation sensors, coupling the electrical signals received by a cavitation sensor to the printed circuit board for subsequent distribution and processing for indicia of cavitation events.

While FIGURE 10 illustrates the use of three cavitation sensors, it will be appreciated that only one may be used initially, to be replaced by another sensor at another offset location should the initial sensor become damages or inoperative. There is no need to remove the initial sensor which, while inoperative, remains bonded in place. In such case the apertures at the other two locations will be initially plugged against fluid ingress until they are needed for installation of a new offset sensor.

FIGURES 11 and 12 illustrate an implementation of an offset cavitation sensor which is installed in a removable modular housing. When the cavitation sensor is removable instead of permanently bonded in place, an unlimited number of replacement sensors can be installed, prolonging the life of the HIFU

transducer in the presence of repeated damage to the cavitation sensor. FIGURES 11 and 12 are cross- sectional views of a modularly housed cavitation sensor installed in the HIFU transducer 120, with FIGURE 12 showing the assembly in an enlarged view. The modular cavitation sensor housing 92 contains a piezoelectric sensor element 90 bonded in the housing 92 and is faced with a matching layer 100. An 0 ring 102 is placed around the modular housing 92 before the threaded exterior of the housing is screwed into the matching threads of the mounting plate 140, which in this implementation extends cylindrically rearward to support a printed circuit board 110. When the modular housing 92 is fully screwed in place, the 0 ring 92 is compressed between the modular housing 98 and the mounting plate 140 to form a liquid tight seal around the housing. When the housing 92 is seated in this manner, electrical contacts 112, 114 extending from printed circuit board 110 make

electrical contact with the metalized contact

electrodes 98, 96 of the piezoelectric element.

These contacts couple piezoelectric signals received by the element 90 to circuitry on the printed circuit board 110, from which the received signals, which may be indicative of cavitation, are coupled to

electrical circuitry and processed.

Should the cavitation sensor become damaged during manufacturing or during use, the damaged cavitation sensor can be replaced by unscrewing the modular housing 92 and sensor element 90 from the transducer 120. In a constructed embodiment multiple holes are formed in the patient-facing side of the modular housing 98 for engagement of a spanner wrench to thread and unthread the housing from the mounting plate 140. After the damaged sensor element is unthreaded and removed, a new modular housing and sensor element are threaded back into the aperture until the 0 ring 102 is compressed again to form the fluid seal. The resilient spring contacts 112, 114 of the printed circuit board 110 make contact with the electrodes 98, 96 on the new sensor and the HIFU transducer with its new sensor is then ready to be put back into service.

When only one modular cavitation sensor is used with the HIFU transducer, at aperture location 130 for instance, the other two apertures 130' and 130" may be plugged with plugs that have the same outer size and threads as the modular housing 92. These plugs with sealing 0 rings are screwed into the two unused apertures to plug them against fluid ingress until such time as they may be replaced with active modular cavitation sensors. If the cavitation sensor at offset location 130 experiences repeated failures, for instance, the damaged sensor at that location may be left in place, inactive, and a new sensor

installed in one of the other locations 130' or 130" which may be less susceptible to damage.