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

Abstract

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 degrees independent active sectors per tube. A flexible delivery catheter facilitated water cooling (100 ml min(-1)) within an expandable urethral balloon (35 mm long x 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 approximately 90 degrees-100 degrees acoustic output patterns from each 120 degrees 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 degrees C maximum temperature, t43 = 240 min thermal dose contours, and TTC tissue sections) that extended radially from the applicator toward the border of the prostate (approximately15 mm) during a short power application (approximately 8-16 W per active sector, 8-15 min), with approximately 200 degrees or 360 degrees 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 approximately 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 transurethral applicators include a flexible delivery catheter and that mechanical manipulation of the device using rotational motors is not required during therapy. This multi-sectored tubular array transurethral ultrasound technology has demonstrated potential for relatively fast and reasonably conformal targeting of prostate volumes suitable for the minimally invasive treatment of BPH and cancer under MR guidance, with further development warranted.

Figure 1

(a) Schematic diagram of the interior of the transurethral catheter and incorporated multisectored transducer. (b) Diagram of the tip of the multisectored transurethral ultrasound applicator with concentric water flow cooling, urethral cooling balloon, and urinary bladder placement balloon. (c) Photograph of the applicators used in this study with a translating and rotating hub for accurate positioning of the transducer assembly.

Figure 2

Screen captured sagittal MR image to demonstrate the typical applicator setup within the prostatic urethra. The applicator, endorectal coil with cooling, and prostate are labeled on the image.

Figure 3

Power delivered to two active sectors (S1, S2) on both transducers on an applicator (T1, T2) in (a) Canine 1 and (b) Canine 3, as adjusted by hand using visual MR temperature feedback.

Figure 4

Example rotational beam plot measured for a single tubular transducer segment (3 mm OD, 6 mm length, 3×120° sectors) of the transurethral ultrasound applicator, with normalized output from all three sectors. The radial distance of the scan was set at 4 mm from urethral balloon surface as the applicator was rotated 360°.

Figure 5

MR temperature and thermal dose measurements of a 2 active sector (S1, S2) heat from a multisectored transurethral ultrasound applicator at 80 s (●●●), 180 s (●–●), 355 s (– –), and 505 s (solid). The 52 °C contour throughout time (a) and the radial profile of the heating along the white arrows (S1, S2) (b,c) are shown. The thermal dose (t₄₃=240 min) at the different times (d) was calculated from the MR temperature measurements and corresponded with acute gross examination of the thermal lesion (e).

Figure 6

MR temperature and thermal dose measurements of a three active sector (S1, S2, S3) bulk ablation of a canine prostate with a multisectored transurethral ultrasound applicator at 150 s (●●●), 240 s (●–●), 360 s (– –), and 450 s (solid). The 52 °C temperature contours (a) expanded throughout the treatment and thermal conduction filled in the heating pattern at 240 s. The thermal dose contours (t₄₃=240 min) (b) fit closely to the 52 °C contour at the different time points and showed bulk ablation of the prostate in less than 7 min. Gross examination of the TTC stained prostate (c) verified thermal destruction of the tissue extending to the prostate boundary. The peak temperatures of the heating distribution from S1 (d), S2 (e), and S3 (f) were about 5 mm from the urethral cooling balloon.

Figure 7

Three coronal MR temperature images at the end of a 10 min heat treatment and gross examination of the resulting thermal lesion from a multisectored transurethral applicator with two active transducer elements. (a) The 52 °C contour was manually fitted to the border of the prostate according to visual MR temperature feedback. The thermal dose contour (t₄₃=240 min) (solid line) was calculated from the MR temperature measurements. A schematic of the approximate applicator and transducer position is overlaid on the image. The imaging slices directly above (b) and below (c) the applicator displayed the large thermal dose delivered to the ventral portion of the gland and the protection of the tissue in the dorsal gland, respectively. (d) TTC stain of the thermal lesion and the approximate location of the monitored coronal imaging slices.