Catheter-based ultrasound applicators for selective thermal ablation: progress towards MRI-guided applications in prostate

Abstract

High-temperature thermal therapy is emerging as a feasible treatment option for prostate cancer and benign prostatic hyperplasia. Previous investigations have demonstrated distinct advantages of catheter-based ultrasound technology over other heating modalities for thermal ablation therapies, with significant potential for better spatial control and faster heating times. The purpose of this study was to develop ultrasound devices and techniques specifically for treating prostate cancer in conjunction with magnetic resonance thermal imaging (MRTI) to monitor and control treatment progression. Directional transurethral applicators have been designed with arrays of sectored tubular (90 degrees active acoustic sector) or with narrow planar transducer segments and integrated with a flexible delivery catheter with a cooling balloon. This applicator can be rotated within the prostatic urethra to target specific regions during treatment. MRI compatible catheter-cooled interstitial ultrasound applicators with 180 degrees active acoustic sectors were developed specifically to treat the prostate. These applicators may be implanted through the perineum into the posterior portion of the prostate, with their heating energy directed away from the rectum. Both heating strategies were evaluated via biothermal simulations and in vivo experiments within canine prostate (n = 3). During the in vivo studies, MRTI was used to monitor treatment temperatures, cytotoxic thermal doses (t43 > 240 min) and corresponding maximum temperature thresholds (Tmax > 52 degrees C) within three imaging planes simultaneously. Urethral and endorectal cooling was employed with both treatment strategies to provide further protection of the urethral mucosa and rectum from thermal damage. Results using the transurethral applicators demonstrated that narrow zones of coagulation (approximately 30 degrees sector for planar, approximately 90 degrees for tubular), extending up to 20 mm from the urethra to the periphery of the prostate gland, could be produced within 10-15 min. Further, rotation of the applicator during treatment could be used to destroy larger regions in the prostate. Experiments using multiple interstitial directional applicators (approximately 180 degrees active sectors), implanted within the posterior margin of the prostate with the energy directed away from the rectum, produced contiguous zones of thermal coagulation which extended from the posterior prostate toward the anterior-lateral periphery of the gland. Both transurethral and interstitial treatment strategies demonstrated significant potential for thermal ablation of localized prostate cancer, particularly when MRTI is used to guide and assess treatment.

MR thermometry

“…The small nonlinear temperature dependence on the equilibrium magnetization is often neglected (26) or, alternatively, Eq. [7] is modified to (27):…”

“…Moving the focus of the beam through the tissue allows ablation of irregularshaped regions. Interstitial, transurethral (7), and transrectal ultrasound use MRI-compatible, multielement ultrasound transducers and provide spatial control over the heating area.…”

Interstitial devices for minimally invasive thermal ablation by high-intensity ultrasound

“…Angle resolution is then limited to each applicator which is specially designed for a given angle opening. Some authors have also proposed using two or three transducers (sectorial or not) placed along an axis to obtain dynamic axial control of heat deposit 61,56 . It would then be possible to adjust the shape and axial dimensions of coagulation necroses by modulating the power emitted by each transducer.…”

“…These applicators are usually small (1.5 to 2.5 mm in diameter) to facilitate insertion into a catheter 56 or placement in direct contact with tissue 23 . At present, this technique is proposed for treatment of benign prostate hypertrophy (BPH) 20 and prostate cancer [56][57][58] ; to correct some forms of cardiac arrhythmia 27 and for spinal treatments 59 . In general, the acoustic power of these applicators consists in several acoustic watts and treatment time lasts 15 to 20 minutes to reach temperatures of about 55°C in exposed tissue.…”

“…As with focused extra-corporeal transducers, interstitial applicator design must meet the constraints imposed by the guidance methods. Either an ultrasound imaging cell must be integrated 55 , or the materials used must be compatible with an MR imaging environment 56,46,44 .…”