Abstract:
A method and system for adjusting a HIFU device compensates for shifts in transducer impedance so that the acoustic output from a HIFU transducer remains at a desired level. In accordance with a first aspect, the disclosure includes dynamically adjusting the tuning of a tuning network that causes the transducer/system to maintain an optimal power transfer to the acoustic output. In accordance with a second aspect, the disclosure monitors the acoustic output of the HIFU device and adjusts the electrical signal provided to the HIFU transducer to maintain a desired acoustic output.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/799,589, filed Mar. 15, 2013, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    The power levels output by a high intensity focused ultrasound (HIFU) device can cause a shift in the impedance of the transducer of the HIFU device and/or a shift in the transfer characteristics of the electrical power output stage (pulser) of the HIFU device. 
         [0003]    Ceramic transducers are predominantly capacitive and are typically tuned to appear resistive at a resonant frequency that allows the energy transfer to be maximized. Tuning of transducers normally takes place in a factory during device manufacturing, utilizing low voltage measurement techniques to measure transducer impedance. The impedance of the piezoceramic used for ultrasound transducers can be highly temperature and voltage dependent. In addition, ceramic transducers can change or degrade over time, causing shifts in impedance during use. 
       SUMMARY 
       [0004]    The following summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0005]    The present disclosure recognizes a determined need to dynamically adjust HIFU devices to compensate for shifts in transducer impedance so that the acoustic output remains at an intended level. A first aspect of the disclosure dynamically adjusts the tuning of the transducer/system to maintain optimal power transfer. A second aspect of the disclosure monitors the acoustic output of the device and adjusts the device electrical output to maintain a constant acoustic output. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0007]      FIG. 1  illustrates a typical transducer impedance plot; 
           [0008]      FIG. 2  illustrates a preferred embodiment of a block diagram for controlling transducer tuning; 
           [0009]      FIG. 3  illustrates a transducer tuning control feedback with sensors at an output of a power amplifier; 
           [0010]      FIG. 4  illustrates a transducer tuning control with acoustic feedback; 
           [0011]      FIG. 5A  illustrates a typical passive transducer tuning circuit; 
           [0012]      FIG. 5B  illustrates an enhanced tuning circuit with variable capacitance; 
           [0013]      FIG. 5C  illustrates an enhanced tuning circuit with both variable inductance and variable capacitance; 
           [0014]      FIG. 5D  illustrates an enhanced tuning circuit adding a series capacitance variable; 
           [0015]      FIG. 6  illustrates a feed forward configuration for controlling transducer tuning; 
           [0016]      FIG. 7  illustrates a block diagram for acoustic feedback with digital processing; 
           [0017]      FIG. 8  illustrates a block diagram for acoustic feedback with analog processing; 
           [0018]      FIG. 9A  illustrates an acoustic receiver laminated to the front of a HIFU bowl transducer; 
           [0019]      FIG. 9B  illustrates an acoustic receiver laminated to the back of a HIFU bowl transducer as shown in  FIG. 9A ; 
           [0020]      FIG. 9C  illustrates a flat acoustic receiver transducer offset in front of a HIFU bowl transducer as shown in  FIG. 9A ; 
           [0021]      FIG. 9D  illustrates a flat acoustic receiver transducer offset from the back of a HIFU bowl transducer as shown in  FIG. 9A ; 
           [0022]      FIG. 9E  illustrates small element receiver transducers on the front surface of a HIFU bowl transducer as shown in  FIG. 9A ; 
           [0023]      FIG. 9F  illustrates small element receiver transducers offset in front of a HIFU bowl transducer as shown in  FIG. 9A ; 
           [0024]      FIG. 9G  illustrates small element receiver transducers on the back surface of a HIFU bowl transducer as shown in  FIG. 9A ; and 
           [0025]      FIG. 9H  illustrates small element receiver transducers that are offset from the back surface of a HIFU bowl transducer as shown in  FIG. 9A . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    In a first aspect, the following specification describes an automatic impedance compensation method and mechanism to auto tune and track the resonance of a transducer in real time and thereby allow a HIFU device to adapt to variances in transducer impedance. 
         [0027]      FIG. 1  shows a typical transducer impedance plot exhibiting both impedance  5  and phase  7 . As indicated, the slope of the curve is fairly steep at the frequency of interest, so a slight shift in characteristics can cause a large shift in impedance. 
         [0028]    By monitoring the impedance or either the instantaneous or average voltage and current, in either the time or frequency domain, the optimum impedance can be calculated or iterated towards. In various embodiments, impedance compensation is achieved by varying the elements of an impedance network residing between the transmitter and the ceramic transducer of the HIFU device. With impedance compensation, the phase difference can be minimized and power transfer maximized via the following equation: 
         [0000]        P=V*I* cosθ
       where       
 
         [0030]    P=electrical power, V=voltage, I=current, and Θ=phase difference between the voltage and current. 
         [0031]    Although the equation above is for electrical power (e.g., at the input of the transducer), it can be shown that one can also close the loop by monitoring the waveform of the acoustic output via a secondary acoustic transducer in the acoustic field. 
         [0032]    The present disclosure allows a transducer to be kept tuned and achieve a longer useful lifecycle. In addition, it allows a transducer and/or cable to be changed out on a system without requiring the tuning networks in the system to be replaced. 
         [0033]      FIG. 2  shows an embodiment of the present disclosure in which voltage and current are monitored at the input of the transducer. A HIFU controller  110  sets output levels and waveform timing of a signal to be used for driving a HIFU transducer  160 . As controlled by the HIFU controller  110 , a power amplifier  120  generates the waveform for driving the transducer. A tuning network  140  is used for “matching” the impedance of the transducer to the power amplifier  120 , as described herein. A voltage and current monitor  150  senses and monitors the output waveform voltage and current. HIFU transducer  160  converts the output electrical waveform (energy) into an acoustic waveform (energy). An optional acoustic monitor  170  (e.g., as shown in  FIG. 4 ) uses an acoustic sensor for detecting and monitoring the acoustic waveform in the acoustic field. A power factor controller  180  receives monitored data and calculates a compensation to improve the impedance matching. Based on the calculated compensation, a tuning control  190  communicates with the tuning network  140  for adjusting tuning component values in the tuning network  140 . 
         [0034]      FIG. 3  shows an alternate embodiment of the present disclosure in which an optional voltage and current monitor  130  monitors the waveform voltage and current prior to the tuning of the transducer  160  by the tuning network  140 . 
         [0035]      FIG. 4  shows yet another embodiment of the present disclosure that relies on monitoring of the acoustic waveform for a predetermined optimal characteristic, and then adjusting the tuning of the transducer  160  by the tuning network  140  for optimal acoustic output by the HIFU transducer  160 . 
         [0036]      FIG. 5A  depicts a tuning network  140  that can be used in any of the foregoing depicted embodiments. The tuning network  140  comprises an LC network with component parts  305 ,  310 ,  320 ,  330 . The component parts  305 ,  310 ,  320 ,  330  are designed and values are chosen for the inductor and capacitor elements to tune the overall phase and impedance of the driving waveform to match the load of the transducer  160 . 
         [0037]    In another embodiment of the tuning network  140  that can be used in any of the embodiments depicted in  FIGS. 2-4 , the capacitors of the LC network are replaced with varicap diodes. Varicap diodes have the property such that the component capacitance changes in a specified manner with applied voltage. Although this property exists in other diodes, it is an explicitly defined parameter for these devices. This approach is sometimes limited in voltage due to the nature of the parts, so other methods are needed at higher voltages. 
         [0038]    In yet another embodiment shown in  FIG. 5B , the tuning network  140  may include multiple capacitors with field effect transistors (FET) or pin diodes as switches  340 ,  345 , that are used to vary the effective capacitance  306 ,  331 . In this embodiment, the FET or diode capacitance has to be taken into account. 
         [0039]    In cases where the capacitance of the device dominates the capacitance of the capacitor being inserted, a relay could be used to insert the capacitor into the circuit. As would be understood by one skilled in the art, the FET switches  340 ,  345  could be replaced with many currently available devices, such as bipolar transistors, mechanical relays, pin diodes, etc. 
         [0040]    In yet another embodiment shown in  FIG. 5C , the inductance presented by the inductive element  320  can be varied using inductor  321  in addition to or instead of varying the capacitance of the tuning network  140 . In this embodiment, the inductor  321  has several taps on its core, which are brought out to FETs, relays, diodes, or other switches  350  such that they are connected or disconnected to vary the inductance. 
         [0041]    It should be obvious to one skilled in the art that although only one switch  340 ,  345  is shown on each capacitor node  306 ,  331  and one switch  350  is shown across the inductor  321 , there may be multiple capacitor/switch elements and multiple indictor/switch elements to affect the desired granularity and range of the variable capacitance and inductance of the tuning network  140 . In addition, the ground connection need only be an AC ground, which could also include a bias rail or other intermediate voltage. 
         [0042]      FIG. 5D  illustrates yet another embodiment of an enhanced tuning network  140  adding a series capacitance variable  351 . In yet another embodiment, tuning may be accomplished with the use of a transformer (auto, isolation, etc.). In this embodiment, the transformer windings may be “tapped” through the use of the aforementioned variety of switches to effectively vary the winding ratio of the respective transformer. 
         [0043]    It should also be noted that one could use a feed forward technique where the transfer function (measured value OUT with respect to both the programmed value IN and TIME) is characterized for a given HIFU device prior to the HIFU device being used for treatment. Current calibration techniques for ultrasound devices are performed at a single time value or averaged over a period of time. This technique generates a time dependent calibration table that is used to compensate for component variation due to heating, power supply droop, etc. The measured OUT value(s) may be measured in either the electrical (voltage and/or current) or acoustic (pressure) domains. 
         [0044]    As illustrated in  FIG. 6 , external test equipment  200  may be used to capture electrical data  230  and/or acoustic data  220 . This data is read into an external computer  210  along with a programmed value N from the HIFU controller  110  that is used in connection with a look up table  215  to set the output of the HIFU controller  110 . The devices  200 ,  210 ,  220  shown with dotted line connections are in place for calibration and are not necessary during runtime. The computer  210  generates a table of values that correlates a desired output level to a programmed value N representing the transfer function of output acoustic power as a function of both the programmed value N and time, and then programs this table into the look up table  215  or into another part of the device/software to be written into the look up table  215  at runtime. The HIFU device uses the time dependent data in the look up table  215  to vary the programmed values of the power amplifier  120  at runtime to compensate for the aforementioned component heating affects, power supply droop, etc. The HIFU device may be characterized on an element-by-element basis or as an aggregate of all channel/transducer elements, with the corresponding compensation applied during runtime. 
         [0045]    In addition to varying the system tuning (with tuning network  140 ) to match the impedance of the transducer  160 , the HIFU device may use acoustic feedback to close the loop and compensate the amplitude of the power amplifier output for cases where the transducer or system output varies over time. Causes for these variations may be due to normal heating of the devices during use, aging of the devices over time, ambient conditions, etc.  FIG. 7  shows one embodiment of a HIFU system where an acoustic receiver  40  is placed in the acoustic field to sense/monitor the relative amplitude of the transmitted HIFU field. The system comprises a power supply  10  coupled to an amplifier/pulser  20  that provides an output signal to a HIFU transducer  30  for outputting acoustic energy. The acoustic receiver  40  (e.g., one or more transducers made of ceramic, PVDF, etc.) receives a portion of the acoustic field  35  produced by the HIFU transducer  30  and delivers a corresponding signal to an amplifier  50 ; A computer/processor  60  is coupled to the output of the amplifier  50  to process the received signal for feedback and compensation of the output of the power supply  10 . 
         [0046]    During operation, the computer/processor  60  sets the power supply  10  to a setting associated with the desired output power. The computer/processor  60  then sends an appropriate waveform to the amplifier/pulser  20  where the amplifier/pulser  20  drives the HIFU transducer  30  to output the desired acoustic waveform. The acoustic receiver  40  transforms a portion of the output waveform into an electrical waveform and transmits it to the amplifier  50  and on to the computer/processor  60 . The computer/processor  60  compares the received waveform to a predetermined expected value. In one embodiment, the computer/processor compares the measured power in the received waveform to an expected power. If the output power of the power supply  10  and the amplifier/pulser  20  do not cause the transducer  30  to produce the target acoustic power, the computer/processor  60  reprograms the power supply  10  to a new value, resulting in a waveform output from the power supply  10  and amplifier/pulser  20  with an output power closer to the target value. This process is repeated (active feedback) in order to keep the value of the output power very close to the target value. In one embodiment, the feedback process is repeated continually for dynamic and continuous control of the output. In another embodiment, the feedback process is performed prior to treatment output. The transfer function for this configuration (acoustic pressure sensed/acoustic power out) can be characterized in the factory. The transfer function can also be recharacterized or verified at a customer site. Where  FIG. 7  indicates digitization and digital processing of the acoustic field measurement data  35  for feedback and control, appropriate feedback and control can be achieved in the analog domain using analog signal processing  70 , as shown in  FIG. 8 . 
         [0047]      FIGS. 9A-9H  show a variety of configurations of a HIFU transducer and one or more sensing transducers for sensing/monitoring the acoustic field generated by the HIFU transducer.  FIGS. 9A and 9B  show configurations in which receiver transducers  410   a,    410   b  are bonded or otherwise coupled directly to the surface of the HIFU transducer  400 . These receiver transducers  410   a,    410   b  receive acoustic energy from all sectors of the HIFU transducer  400  in cases where the HIFU transducer  400  is comprised of multiple radial sectors. In the case of  FIG. 9A , the receive transducer  410   a  would need to allow most of the transmitted acoustic energy from the HIFU transducer  400  to pass with minimal losses, to avoid device damage and/or poor efficiency. In the case of  FIG. 9B , the receive transducer  410   b  may still require very low losses and absorption when considering heat and efficiency. 
         [0048]      FIGS. 9C and 9D  show configurations where the receive transducers  410   c ,  410   d  are still receiving acoustic energy from all sectors of a radially sectored HIFU transducer  400  or a representative annular ring of a single element HIFU transducer  400 . For this configuration, the flat receive transducers  410   c,    410   d  are of a simpler construction when compared to the transducers  410   a,    410   b  shown in  FIGS. 9A and 9B . However, flat offset receive transducers, such as  410   c,    410   d  shown in  FIGS. 9C and 9D  are challenged with receiving acoustic signals from multiple paths, that is from different parts of the HIFU transducer surface  400 , when compared to the receive transducers  410   a,    410   b  that are directly coupled to the HIFU transducer  400 . The receive transducer  410   c  may be mounted to the transducer assembly or another part of a HIFU applicator such as a patient interface membrane. In such case, a PVDF technology may be more appropriate than a ceramic device for the transducer. Receive transducer  410   d  may be constructed of a variety of materials and technologies, since it is not in the direct path of the treatment acoustic energy. 
         [0049]      FIGS. 9E and 9F  show one or more small receive transducers  410   e,    410   f  offset from the surface of the HIFU transducer  400 . In cases where the HIFU transducer is comprised of  2  or more radial sectors, the number of receive transducers  410   e,    410   f  could equal the number of sectors the HIFU transducer  400  such that the acoustic energy is sensed from each of the HIFU transducer sectors. The size of the receive transducers  410   e,    410   f  may be small relative to the transmitted wavelength so as to minimize the cancellation of the acoustic field across the receive transducer  410   e,    410   f.    
         [0050]      FIGS. 9G and 9H  show small receive transducers  410   g    410   h  laminated or otherwise directly coupled to the HIFU transducer  400 . This configuration may be preferred over other configurations since the receivers  410   g,    410   h  are directly coupled to the transmit transducer, thereby eliminating multipath cancellations across the receive transducers  410   g,    410   h.  The configurations shown in  FIGS. 9G and 9H  also minimize the effect on the transmitted acoustic field due to their small size. In addition, the manufacturability of these devices may be less challenging than the larger ring transducers  410   a,    410   b.    
         [0051]    One challenge of the configuration shown in  FIGS. 9G and 9H  may be related to the lower sensitivity of the small transducers  410   g,    410   h  when compared to the potentially larger surface areas of transducers  410   a,    410   b.  Data may be received from a configuration where the receive transducers are positioned to receive acoustic signals from specific elements of a HIFU transducer array  400 . It should be obvious to one skilled in the art, that if a HIFU transducer  400  were constructed of annular rings, then the receiver transducers  410   e,    410   f,    410   g,    410   h  could be positioned radially to receive the acoustic power from the individual annular rings of the HIFU transducer  400 . 
         [0052]    In some embodiments, such as the configurations with the receive transducer are positioned behind the HIFU transducer  400 , the receive transducers may be constructed of any acoustic-to-electrical transfer device, such as a piezoceramic transducer or Polyvinylidene Fluoride (PVDF) transducer. In cases where the receive transducers are in the HIFU field (e.g., configurations with the receive transducer is positioned in front of the HIFU transducer), an acoustically “transparent” material such as a PVDF transducer would be more appropriate. 
         [0053]    In addition, one could use a mechanical property such as heating of a sensor within the acoustic field. Although a heat transfer configuration may not be a preferred embodiment, a heat transfer characteristic can be determined in the factory, relating the output of a thermoelectric transducer embedded in the HIFU transducer assembly. The thermoelectric transducer may be a thermistor embedded in the backing of the HIFU transducer with a characterized heat transfer path. 
         [0054]    While embodiments of systems and methods have been illustrated and described in the foregoing description, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the present disclosure. In addition, computer-executable instructions that cause one or more computing devices to perform processes as described herein may be stored in a non-transitory, computer-readable medium accessible to one or more computing devices. It should also be understood that rearrangement of structure or steps in the devices or processes described herein that yield similar results are considered within the scope of the present disclosure. Accordingly, the scope of the present disclosure is not constrained by the precise forms that are illustrated for purposes of exemplifying embodiments of the disclosed subject matter.