Patent Application
20080039724
The detailed description of the drawings particularly refers to the accompanying figures in which:
Referring to
As discussed in more detail below, the second surface 16 of the crystal 12 is formed to include a recessed portion 18 therein. The recessed portion 18 is illustratively circular in shape and located in substantially a central portion of the crystal 12. It is understood that the recessed portion 18 may extend to the opposite side edges 20 and 22, if desired.
Ultrasound transducer 10 further includes a matching layer 30 which is applied to the first, generally concave front surface 14 of crystal 12. In one embodiment, matching layer 30 is an epoxy mixture applied to the surface 14. In another embodiment, matching layer 30 is a polymer. In the illustrated embodiment, the matching layer 30 is optimized for a therapy function of the transducer 10 as described in detail in U.S. application Ser. No. 11/175,947 which is incorporated herein by reference. A therapy element or electrode 40 is coupled to second surface 16 and substantially surrounds the recessed portion 18 formed in second surface 16 of crystal 12. Therapy electrode 40 may comprise multiple separate electrodes. An imaging electrode 44 is configured to be located within the recessed portion 18 formed in the second surface 16 of crystal 12. The imaging electrode 44 is electrically isolated from the therapy electrode 40. A common ground electrode 45 is located between the crystal 12 and the matching layer 30 as shown in
Transducer electrodes 40 and 44 are each individually drivable by a control system 50. In one embodiment, therapy electrode 40 is used in therapy applications and is driven by a therapy driver 52 to provide therapy, such as HIFU therapy, to portions of a surrounding environment 56 (see
In one embodiment, therapy driver 52 is configured to provide HIFU therapy. Exemplary HIFU therapy includes the generation of a continuous wave at a desired frequency for a desired time duration. In one example, the continuous wave is sustained for a period of time sufficient to ablate a target tissue at the desired location, such as a treatment site 59 or treatment zones within a prostate 58 or other tissue such as the kidney, liver, or other targeted area. The location of treatment site 59 generally corresponds to the focus of transducer 10 which generally corresponds to the center of curvature of the crystal 12.
In one embodiment, control system 50 is configured to generate with therapy driver 52 a sinusoidal continuous wave having a frequency in the range of about 500 kHz to about 6 MHz, a duration in the range of about 1 second to about 10 seconds, with a total acoustic power at the focus in the range of about 5 Watts to about 100 Watts. In one example, the continuous wave is sinusoidal with a frequency of about 4.0 MHz and a duration of about 3 seconds. In another example, the continuous wave is sinusoidal with a frequency of about 4.0 MHz and a duration of about 3 seconds with a total acoustic power of about 37 Watts at the focus. This time period can be increased or decreased depending on the desired lesion size or the desired thermal dose.
Imaging driver and receiver 54 is configured to drive imaging electrode 44 to oscillate crystal 12 and emit an imaging signal. Electrode 44 and receiver 54 also receive echo acoustic energy that is reflected from features in the surrounding environment 56, such as, for example, prostate 58. The received signals are used to generate one or more two-dimensional ultrasound images, three-dimensional ultrasound images, and/or models of components within the surrounding environment 56 in a conventional manner. In addition, control system 50 may be further configured to utilize imaging electrode 44 for Doppler imaging of moving components within surrounding environment 56, such as blood flow. Exemplary imaging techniques including Doppler imaging are disclosed in PCT Patent Application Serial No. US2005/015648, filed May 5, 2005, which is expressly incorporated herein by reference.
As discussed in the '947 application, matching layer 30 is altered such that transducer 10 is optimized for a transducer for use in a therapy application at a desired frequency for most efficient power transfer. Referring to
Providing both imaging and therapy functions on the same crystal 12 maintains focus alignment between the image focus and the therapy focus. As discussed above, the matching layer thickness 32 is optimized for a therapy function of the transducer 10. However, the desired frequency of operation for therapy typically is not the same as the desired frequency of operation for imaging. The desired imaging frequency is typically higher than the desired therapy frequency. Furthermore, while therapy operation is typically performed with a single frequency (narrow band operation, such as 4 MHz), better imaging performance is achieved using a wide band of frequencies (wide band operation).
In order to increase the imaging frequency, portions of the crystal 12 are selectively removed from the second surface 16 of crystal 12 to form the recessed portion 18. Typically, thinner crystals have a higher frequency of oscillation. Therefore, the natural frequency of the crystal in the area of the thinner recessed portion 18 is increased. Accordingly, the transducer 10 operates at two different frequencies when driven by electrodes 40, 44. The first frequency (or vibration mode) is mainly defined by the crystal thickness. The second frequency (or vibration mode) is mainly defined by the thickness 32 of matching layer 30. As long a the separation between the imaging and therapy operating frequencies is not too large, the frequency spectrums combine to form a wider frequency band system with an overall higher center operating frequency and larger bandwidth with a negligible loss of overall sensitivity.
The imaging ability of transducer 10 may be further improved to compensate for the overall/global therapy optimization of matching layer 30 by placing a thicker/heavier backing 46 on the imaging electrode 44. In one embodiment, backing 46 is about 1 mm to about 2 mm thick and is made of 4538 epoxy. The density of the epoxy may be further increased, for example, by adding tungsten powder of various mesh sizes to achieve a higher density. The heavier the backing is the more damping provided by the backing 46. The heaviness of backing 46 may be increased by either increasing the thickness of backing 46 and/or increasing the density of backing 46.
Preferably, the depth of recessed portion 18 is controlled to set the imaging frequency at a frequency less than or equal to twice the therapy frequency. Thicknesses are measured with a micrometer for accuracy. In other words, if the therapy frequency is about 4 MHz, the imaging frequency should be less than or equal to about 8 MHz, otherwise, the separation between both peaks will be too large, degrading the imaging performance of such a transducer. In an illustrated embodiment, the depth of recessed portion 18 is controlled to set the imaging frequency at about 7 MHz. Therefore, the depth of recessed portion 18 is illustratively about ⅕ to about ½ the overall thickness 34 of crystal 12. It is understood that these ratios may vary outside the illustrative ranges.
In summary, for therapy, the crystal 12 is designed to operate at a particular frequency (about 4 MHz) due to the material thickness 34 of crystal 12, and the composition (thickness 32, etc.) of matching layer 30, that is also optimized for this same frequency (about 4 MHz). For imaging, the crystal 12 is designed to operate at a higher imaging frequency (about 7 MHz, for example, vibrating in its natural mode or thickness mode) due to its reduced material thickness in the area of recessed portion 18. However, the crystal is partially forced to work at a different frequency, being imposed on the system by the matching layer 30 that is not ideal for its natural frequency. The end effect is a system that works at neither frequency/mode, but somewhere in between, but which has overall better imaging performance due to a higher center frequency and a wider bandwidth compared to the transducer 110 of
The illustrated embodiments therefore improve the imaging characteristics of such a transducer (frequency and bandwidth) while maintaining a smooth outer surface 31 of the matching layer 30. In other words, the outer surface 31 of matching layer 30 is a continuous, generally even or regular surface, free from projections or indentations. Creating a recessed portion in the matching layer 30 is difficult and costly to machine, less pleasing to the eye, and increases the likelihood of contaminants getting trapped in the recessed portion of the matching layer 30 making such a transducer more difficult to clean than the transducer of
The imaging performance of the higher-frequency, wider-bandwidth imaging transducer may be further customized by adding (selectable) electrical matching circuitry 55 between the imaging transducer electrode 46 and the driver 54 as shown in
Additional image enhancements may be generated by exciting the imaging transducer at a lower frequency to obtain greater penetration depth at a given power level and receiving the echo at a higher frequency to obtain greater resolution. The selectable electrical matching circuitry 55 is used to select the lower frequency match for transmitting, and the higher frequency match (or filter circuit) for receiving. This is advantageous for using the transducer for harmonic imaging, where it is matched and excited at, for example, 4 MHz during transmit, and matched and filtered at 8 MHz for receive, as the crystal thickness is optimized for 8 MHz operation.
In an illustrated embodiment, the system allows frequency switching by the user to render images of higher performance for all tissues with variable density and scattering characteristics due to the electronic drivers and the wider-bandwidth and higher frequency transducer. This system allows frequency switching during imaging (both during transmit and receive) for improved imaging performance, in combination with the therapy function.
Because of the frequency switching capability, transducer, and bandwidth, the illustrated embodiment also provides a system that allows for tissue imaging and tissue characterization with different frequency bands, in combination with the therapy function. The transducer is capable of an imaging and therapy function that allows imaging at a low frequency or a higher frequency as required for the depth of penetration. For example, for longer tissue depth, the system uses a lower frequency band for imaging. For a shallow tissue depth, the system uses a higher frequency band for imaging. In an illustrated embodiment, the user uses an input device to select and change the frequencies of the therapy and imaging functions (both transmit and receive). In another embodiment the selection is automated.
Because imaging and therapy functions are available with the same device, the higher-frequency and wider bandwidth imaging capability allows the transducer to produce larger contrast ultrasound images that can be used for treatment monitoring, lesion creation visualization, and lesion imaging.
Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the spirit and scope of the invention as described and defined in the following claims.
