Transurethral ultrasound applicators with dynamic multi-sector control for prostate thermal therapy: In vivo evaluation under MR guidance

The purpose of this study was to explore the feasibility and performance of a multi-sectored tubular array transurethral ultrasound applicator for prostate thermal therapy, with potential to provide dynamic angular and length control of heating under MR guidance without mechanical movement of the applicator. Test configurations were fabricated, incorporating a linear array of two multi-sectored tubular transducers (7.8–8.4 MHz, 3 mm OD, 6 mm length), with three 120 ° independent active sectors per tube. A flexible delivery catheter facilitated water cooling ( 100 ml min − 1 ) within an expandable urethral balloon ( 35 mm long × 10 mm diameter ) . An integrated positioning hub allows for rotating and translating the transducer assembly within the urethral balloon for final targeting prior to therapy delivery. Rotational beam plots indicate ∼ 90 ° − 100 ° acoustic output patterns from each 120 ° transducer sector, negligible coupling between sectors, and acoustic efficiencies between 41 % and 53 % . Experiments were performed within in vivo canine prostate ( n = 3 ) , with real-time MR temperature monitoring in either the axial or coronal planes to facilitate control of the heating profiles and provide thermal dosimetry for performance assessment. Gross inspection of serial sections of treated prostate, exposed to TTC (triphenyl tetrazolium chloride) tissue viability stain, allowed for direct assessment of the extent of thermal coagulation. These devices created large contiguous thermal lesions (defined by 52 ° C maximum temperature, t 43 = 240 min thermal dose contours, and TTC tissue sections) that extended radially from the applicator toward the border of the prostate ( ∼ 15 mm ) during a short power application ( ∼ 8 − 16 W per active sector, 8–15 min), with ∼ 200 ° or 360 ° sector coagulation demonstrated depending upon the activation scheme. Analysis of transient temperature profiles indicated progression of lethal temperature and thermal dose contours initially centered on each sector that coalesced within ∼ 5 min to produce uniform and contiguous zones of thermal destruction between sectors, with smooth outer boundaries and continued radial propagation in time. The dimension of the coagulation zone along the applicator was well-defined by positioning and active array length. Although not as precise as rotating planar and curvilinear devices currently under development for MR-guided procedures, advantages of these multi-sectored