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.
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.
The application of heat to intervertebral discs is being clinically investigated for the treatment of discogenic back pain. The purpose of this study was to develop and test the feasibility of small ultrasound applicators that can be endoscopically placed adjacent to the disc, and deliver heating energy into the disc without puncturing the annular wall. Prototype devices were fabricated using curvilinear transducers (2.5-3.5 mm wide x 10 mm long, 5.4 - 6.5 MHz) that produce a narrow penetrating beam extending along the length of the ultrasound element. The transducer was affixed to either a flexible or rigid delivery catheter, and enclosed within an asymmetric coupling balloon with water-cooling flow. Bench measurements demonstrated 35-60% acoustic efficiencies, high-power output capabilities, and lightly focused beam patterns. The heating characteristics of these devices were evaluated with ex vivo and in vivo experiments within lumbar and cervical spine segments from sheep models and human cadaveric spine. The applicators were positioned adjacent to the annular wall of the surgically exposed discs. Ultrasound energy was focused directly into the disc to avoid heating the vertebral bodies. Multi-point thermocouple probes were placed throughout the disc to characterize the resultant temperature distributions. These studies demonstrated that ultrasound energy from these applicators penetrated the annular wall of the disc, and produced thermal coagulative temperatures of >60-65°C as far as 10 mm into the tissue. This study also showed that lower power levels and temperatures delivered for 10 minutes can generate a cytotoxic thermal dose of t43°C >240 min penetrating 5-10 mm from the annular wall.
Hyperthermia and high temperature thermal therapy are currently used in the clinical treatment for a variety of cancers. Despite the increasing use of thermal therapies, heat treatments have not gained large-scale clinical acceptance, due in part to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound applicators provide superior spatial control of power deposition and heating patterns compared to other interstitial techniques. Real-time MR temperature imaging (MRTI) provides thermal therapies with an accurate, non-invasive method for measuring temperature within the body during treatment. In this study, three MR-compatible water-cooled interstitial ultrasound applicators designs were developed and evaluated for dynamic angular control of the thermal dose to a target area. Two of the applicator designs utilize an ultrasound transducer separated into three individually powered sectors allowing the user to control heating deposition for the creation of odd size and shape thermal lesions. The third design utilizes a 90o sectored transducer that can be rotated to target the thermal treatment to a specified area. Comparisons and predictions of the in vivo performance of all applicators was examined with experiments in ex vivo tissue and simulations created by a biothermal model that incorporates changes in acoustic attenuation and perfusion as a function of thermal dose. Ex vivo experiments with real-time MRTI correlated well with results from the biothermal model. The results of this study bracket the feasibility and potential in vivo performance of the applicator designs for minimally invasive cancer treatment with MRTI.