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.