Four types of transurethral applicators were devised for thermal ablation of prostate combined with MR thermal
monitoring: sectored tubular transducer devices with directional heating patterns; planar and curvilinear devices with
narrow heating patterns; and multi-sectored tubular devices capable of dynamic angular control without applicator
movement. These devices are integrated with a 4 mm delivery catheter, incorporate an inflatable cooling balloon (10
mm OD) for positioning within the prostate and capable of rotation via an MR-compatible motor. Interstitial devices
(2.4 mm OD) have been developed for percutaneous implantation with directional or dynamic angular control. In vivo
experiments in canine prostate under MR temperature imaging were used to evaluate the heating technology and develop
treatment control strategies. MR thermal imaging in a 0.5 T interventional MRI was used to monitor temperature and
thermal dose in multiple slices through the target volume. Sectored tubular, planar, and curvilinear transurethral
devices produce directional coagulation zones, extending 15-20 mm radial distance to the outer prostate capsule.
Sequential rotation and modulated dwell time can conform thermal ablation to selected regions. Multi-sectored
transurethral applicators can dynamically control the angular heating profile and target large regions of the gland in short
treatment times without applicator manipulation. Interstitial implants with directional devices can be used to effectively
ablate the posterior peripheral zone of the gland while protecting the rectum. The MR derived 52 °C and lethal thermal
dose contours (t43=240 min) allowed for real-time control of the applicators and effectively defined the extent of thermal
damage. Catheter-based ultrasound devices, combined with MR thermal monitoring, can produce relatively fast and
precise thermal ablation of prostate, with potential for treatment of cancer or BPH.
The purpose of this study is to further investigate the approach of DWI to estimate the cell viability immediately after treatment. In this work, we reported the result from 12 canine prostate experiments underwent cryoablation or hyperthermic therapy. The lesion detected by diffusion-weighted imaging was evaluated through apparent diffusion coefficient (ADC) value, image contrast, and lesion contour compared to contrast enhanced imaging and histology.
Magnetic Resonance Imaging (MRI) is a promising tool for visualizing the delivery of minimally invasive cancer
treatments such as high intensity ultrasound (HUS) and cryoablation. We use an acute dog prostate model to correlate
lesion histopathology with contrast-enhanced (CE) T1 weighted MR images, to aid the radiologists in real time
interpretation of in vivo lesion boundaries and pre-existing lesions. Following thermal or cryo treatments, prostate glands
are removed, sliced, stained with the vital dye triphenyl tetrazolium chloride, photographed, fixed and processed in
oversized blocks for routine microscopy. Slides are scanned by Trestle Corporation at .32 microns/pixel resolution, the
various lesions traced using annotation software, and digital images compared to CE MR images. Histologically, HUS
results in discrete lesions characterized by a "heat-fixed" zone, in which glands subjected to the highest temperatures are
minimally altered, surrounded by a rim or "transition zone" composed of severely fragmented, necrotic glands,
interstitial edema and vascular congestion. The "heat-fixed" zone is non-enhancing on CE MRI while the "transition
zone" appears as a bright, enhancing rim. Likewise, the CE MR images for cryo lesions appear similar to thermally
induced lesions, yet the histopathology is significantly different. Glands subjected to prolonged freezing appear totally
disrupted, coagulated and hemorrhagic, while less intensely frozen glands along the lesion edge are partially fragmented
and contain apoptotic cells. In conclusion, thermal and cryo-induced lesions, as well as certain pre-existing lesions
(cystic hyperplasia - non-enhancing, chronic prostatitis - enhancing) have particular MRI profiles, useful for treatment
and diagnostic purposes.
Thermal ablation is a minimally-invasive treatment option for benign prostatic hyperplasia (BPH) and localized prostate cancer. Accurate spatial control of thermal dose delivery is paramount to improving thermal therapy efficacy and avoiding post-treatment complications. We have recently developed three types of transurethral ultrasound applicators, each with different degrees of heating selectivity. These applicators have been evaluated in vivo in coordination with magnetic resonance temperature imaging, and demonstrated to accurately ablate specific regions of the canine prostate. A finite difference biothermal model of the three types of transurethral ultrasound applicators (sectored tubular, planar, and curvilinear transducer sections) was developed and used to further study the performance and heating capabilities of each these devices. The biothermal model is based on the Pennes bioheat equation. The acoustic power deposition pattern corresponding to each applicator type was calculated using the rectangular radiator approximation to the Raleigh Sommerfield diffraction integral. In this study, temperature and thermal dose profiles were calculated for different treatment schemes and target volumes, including single shot and angular scanning procedures. This study also demonstrated the ability of the applicators to conform the cytotoxic thermal dose distribution to a predefined target area. Simulated thermal profiles corresponded well with MR temperature images from previous in vivo experiments. Biothermal simulations presented in this study reinforce the potential of improved efficacy of transurethral ultrasound thermal therapy of prostatic disease.
Two catheter-based transurethral ultrasound applicators designed for selective thermal coagulation of prostate tissue were evaluated. The first applicator utilized two 3.5 mm piezoelectric sectored tubes with the active transducer surface forming 90°. The second applicator's transducer assembly consisted of a linear array of 3.5 x 10 mm planar transducer elements. Both applicators operated at 8 MHz and were positioned on a 4 mm diameter catheter within an integrated expandable balloon (10 mm). Manual rotation of the transducer assembly within the balloon allowed for angular control and/or sweeping of the treatment volume. Ambient temperature degassed cooling water (~120 ml/min) was circulated inside the balloon to preserve the urethral mucosa. Acoustic efficiencies of 20-54% and acoustic beam distributions were measured. The thermal treatment characteristics of the applicator were investigated in vivo (canine prostate) under MRI guidance in an interventional open magnet (0.5 T). Magnetic resonance thermal imaging (MRTI) monitored the treatments (GRE phase mapping, multiple planes, 15 sec update intervals). Post-treatment imaging (T1 w/contrast) and TTC staining of the prostate were used to verify zones of thermal damage. Single sonications lasting 8-15 min produced coagulated zones of tissue extending to the outer boundary of the prostate while preserving 2-3 mm of urethral mucosa. Multiple sonications in sequence produced larger contiguous sectors of coagulated tissue (~ 3/4 of the gland). In summary, highly directional transurethral applicators under MRI guidance were able to produce selective and controllable thermal coagulation.
MRI compatible, multi-element ultrasound applicators were fabricated using cylindrical piezoceramic transducers sectored to 180 degrees to provide angular directional heating. The applicators were designed to be inserted into standard 13 or 14 gage brachytherapy catheters integrated with water-cooling. Two applicators were inserted transperinealy into the posterior region of a canine prostate. Power output ranged from 5-15 W per element during the 15 minute heating period. Phase-sensitive gradient-recalled MR imaging was used to monitor the treatment in real-time on a 0.5 Tesla MRT system. Gadolinium-enhanced T1 weighted images and diffusion-weighted images were obtained to view the regions which had been ablated during the heating procedure. Upon euthanasia, the prostate was removed, axially sectioned, and stained with TTC to reveal any regions of remaining viable tissue. Results from this study indicated a large volume of ablated tissue within the prostate which was highly correlated to the regions in the T1-weighted and diffusion-weighted images which had decreased intensity, and to the 52C contour displayed in the images obtained during the treatment. This study demonstrates the ability to control thermal coagulation within a targeted tissue volume while protecting surrounding tissue from thermal damage.