| Ultrasound induced hyperthermia is a useful adjuvant to radiation therapy in the treatment of prostate cancer. A uniform thermal dose (43°C for 30 minutes) is required within the targeted cancerous volume for effective therapy. This imposes specific ultrasound transducer design requirements. Although phased arrays have been previously used to overcome the inability of nearfield focusing of large cylindrical piezoelectric transducers, the lack of an acoustically inhomogeneous three dimensional (3D) prostate model and economical computational methods have made it difficult to predict the appropriate shape of the array for better focusing and steering. This research utilizes the k-space computational method and a 3D, inhomogeneous, large scale, and coarse grid human prostate model to design an intracavitary probe for hyperthermia treatment of prostate cancer. Magnetic resonance imaging (MRI) thermometry and automatic feedback controlling were also used to accomplish the therapy. To achieve this, a 3D prostate model utilizing imaging data from the Visible Human ProjectRTM was used to determine acoustical parameters of glandular, connective, fat, and muscle tissues. The acoustical model included sound speed, density, and absorption parameters, and was determined depending on optical parameters of each pixel of image layers. The k-space computational method used this coarse grid and inhomogeneous tissue model to simulate ultrasound wave propagation to predict the steady state pressure wavefield of the designed phased array. To insure the uniformity and spread of the pressure in the length of the array, and the steering and focusing capability in the width of the array, the equal sized elements of the phased array were 1 x 14 mm. The anatomical measurements of the prostate were used to predict the final phased array specifications (4 x 20 planar array, 1.2 MHz, element size = 1 x 14 mm, array size = 56 x 20 mm). A single input single output, switching, feedback controller was developed to control hyperthermia temperatures from the probe. Good agreement between the exposimetry and the k-space computational method results was shown. As an example, the -3 dB distances of the focal volume were 22.0 and 20.0 mm in the propagation direction for k-space prostate simulation and exposimetry results, respectively. Temperature simulations indicated that the rectal wall temperature was elevated less than 2°C during hyperthermia treatment. Steering and focusing ability of the designed probe in both azimuth and propagation directions were found to span the whole prostate volume with minimal grating lobes (-10 dB reduction from the main lobe) and minimal heat damage to the rectal wall. Ex-vivo controlled hyperthermia experiments showed that the rise time was reduced by a factor of two when doubling the driving power. With a desired temperature plateau of 43.0°C, the MRI temperature results at the steady state were 42.9 +/- 0.38°C and 43.1 +/- 0.80°C for ex-vivo and in-vivo experiments, respectively. Unlike conventional computational methods, the k-space method provides a powerful tool to predict the pressure wavefield and temperature rise in sophisticated, large scale, 3D, inhomogeneous and coarse grid models. [Research supported by the Department of Defense Congressionally Directed Medical Prostate Cancer Research Program.]... |
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