Design and Evaluation of Linear Intracavitary Ultrasound Phased

Over the past decade, numerous minimally invasive thermal procedures have been investigated to treat benign prostate hyperplasia and prostate cancer. These hyperthermia and thermal surgery techniques have employed RF energy, microwave radiation, laser, and acoustic energy to heat and necrose a localized region of tissue. Of these methods, ultrasound has shown considerable promise due to its ability to produce more precise and deeper foci in vivo. In addition, recent advances in ultrasound amplifier and fabrication technologies have prompted refined clinical heating devices. In this study, a linear, transrectal, ultrasound phased array capable of ablating large tissue volumes was fabricated and evaluated in tissue. The device was designed to be MRI compatible for use with MRI thermometry and guidance. The intracavitary applicator increases treatable tissue volume by utilizing an ultrasonic motor to provide up to a 100-degree mechanical rotation angle to a 62-element 1 D ultrasound array. An aperiodic array geometry was used to reduce grating lobes. The applicator built under constraints of the small rectal dimensions also incorporated a 1 mm thin custom-fabricated Kapton flexible circuit to interconnect the elements of the phased array to the ultrasound driving system. The Kapton interconnect reduced cable crosstalk and hence also improved the acoustic efficiency of the array. The array was driven by a multi-channel ultrasound amplifier system, which incorporates power and phase feedback circuitry to improve thermal focusing. MRI guided in vivo and ex vivo experiments were performed to verify the array's large volume ablative capabilities. Ex vivo bovine experiments were performed to assess the focusing range of applicator. The array generated foci in a 3 cm (2 to 5 cm from the array surface along the axis normal to the array) by 5.5 cm (along the long axis of the array) by 6 cm (along the transverse axis of the array at a depth of 4 cm) volume. Following ex vivo studies, in vivo rabbit thigh experiments were performed to evaluate the lesion producing capabilities in perfused tissue. The array generated 3 cm x 2 cm x 2 cm lesions with 8-12 half minute sonications equally spaced in the volume. The results indicate that transrectal ultrasound coagulation of the whole prostate is feasible with the method developed.