Analysis of the spatial and temporal accuracy of heating in the prostate gland using transurethral ultrasound therapy and active MR temperature feedback.

A new MRI-guided therapy is being developed as a minimally invasive treatment for localized prostate cancer utilizing high-intensity ultrasound energy to generate a precise region of thermal coagulation within the prostate gland. The purpose of this study was to evaluate in vivo the capability to produce a spatial heating pattern in the prostate that accurately matched the shape of a target region using transurethral ultrasound heating and active MR temperature feedback. Experiments were performed in a canine model (n = 9) in a 1.5 T MR imager using a prototype device comprising a single planar transducer operated under rotational control. The spatial temperature distribution, measured every 5 s with MR thermometry, was used to adjust the acoustic power and rotation rate in order to achieve a temperature of 55 degrees C along the outer boundary of the target region. The results demonstrated the capability to produce accurate spatial heating patterns within the prostate gland. An average temperature of 56.2 +/- 0.6 degrees C was measured along the outer boundary of the target region across all experiments in this study. The average spatial error between the target boundary and the 55 degrees C isotherm was 0.8 +/- 0.7 mm (-0.2 to 3.2 mm), and the overall treatment time was < or =20 min for all experiments. Excellent spatial agreement was observed between the temperature information acquired with MRI and the pattern of thermal damage measured on H&E-stained tissue sections. This study demonstrates the benefit of adaptive energy delivery using active MR temperature feedback, and an excellent capability to treat precise regions within the prostate gland with this technology.

MRI-compatible transurethral ultrasound system for the treatment of localized prostate cancer using rotational control.

Magnetic resonance imaging (MRI)-guided transurethral ultrasound therapy is a potential minimally invasive treatment for localized prostate cancer offering precise targeting of tissue within the gland, short treatment times, and the capability to quantify the spatial heating pattern delivered during therapy. A significant challenge in MRI-guided ultrasound therapy is the design and construction of MRI-compatible equipment capable of operation in a closed-bore MR imager. We describe a prototype system developed for MRI-guided transurethral ultrasound therapy and characterize the performance of the different components including the heating applicator design, rotational motor, and radio frequency electronics. The ultrasound heating applicator described in this study incorporates a planar transducer and is capable of producing high intensity ultrasound energy in a localized region of tissue. Results demonstrated that the heating applicator exhibits excellent MRI-compatibility, enabling precise MR temperature measurements to be acquired as close as 6 mm from the device. Simultaneous imaging and rotational motion was also possible during treatment using a motor based on piezoelectric actuators. Heating experiments performed in both phantoms and in a canine model with the prototype system verified the capability to perform simultaneous MR imaging and therapy delivery with this system. Real-time control over therapy using MR temperature measurements acquired during heating can be implemented to achieve precise patterns of thermal damage within the prostate gland. The technical feasibility of using the system developed in this study for MRI-guided transurethral ultrasound therapy in a closed-bore MR imager has been demonstrated.

Prostate tissue analysis immediately following magnetic resonance imaging guided transurethral ultrasound thermal therapy.

PURPOSE: Preclinical experiments were performed in an acute canine model to analyze the spatial pattern of thermal damage generated in the prostate gland following treatment with a prototype magnetic resonance imaging guided transurethral ultrasound heating system. In particular the boundary of tissue coagulation was analyzed to quantify the treatment margin resulting from this technology. MATERIALS AND METHODS: A heating device incorporating a planar 20 x 3.5 mm transducer operated at 9.1 MHz was used to deliver ultrasound energy to targeted regions in the prostate gland in 7 animals monitored with magnetic resonance imaging thermometry during heating. The animals were sacrificed approximately 45 minutes after treatment. The thermal damage pattern was evaluated using contrast enhanced magnetic resonance imaging, vital tissue staining, and whole mount hematoxylin and eosin stained histological sections. An image warping technique enabled quantitative comparison of these data. RESULTS: Regions of thermal fixation, coagulative necrosis and hemorrhage were observed in the treated prostate glands. The extent of the necrotic region was relatively insensitive to vessel cooling effects. Metabolic enzyme functionality coincided with tissue outside of the treatment area. At the edge of the thermal damage pattern the transition from coagulative necrosis to no visible damage occurred within 3 mm or less. CONCLUSIONS: The narrow extent of the thermal margin suggests that tissue sparing outside of the prostate could be an advantage of this treatment. Histological measurements showed a high level of spatial accuracy, useful for developing accurate control techniques for directional transurethral ultrasound thermal therapy in the treatment of prostate diseases.