Abstract No. 160 MR Susceptibility-Weighted Imaging During Transurethral Prostate Sono-Ablation Procedures for Guidance of Device Placement to Counter Effect of Intra-Prostatic Calcifications

Abstract

Men seeking MRI-guided transurethral ultrasound ablation (TULSA) for treatment of prostate cancer (PCa) are screened for calcifications. Large calcifications can block ultrasound (US) and cause inadequate heating. Smaller calcifications (≤3 mm) can be accommodated by aligning the ultrasound applicator (UA) with calcifications between transducer elements. We compare calcification detection and size measurement using susceptibility-weighted MRI (SWI) vs. computed tomography (CT). We also assess the impact of calcifications on ablation coverage benchmarked against a previously published pivotal trial. Pre-TULSA screening for calcifications (calcs) was performed with CT for 17 men. The mean (range) of CT slice thickness was 1.6 (0.6-5.0) mm. When relevant calcs were detected, intraprocedural SWI (1.0-mm slices) was used to guide positioning of UA elements with respect to calcs. Calcification in-plane and through-plane diameters were measured on CT and SWI, and their impact on ablation coverage was assessed using MR thermometry. Maximum temperature and thermal dose maps from MR thermometry were segmented in 90-degree quadrants across the active UA elements, with presence of calcification assessed based on SWI magnitude and phase images. 25 prostatic calcs were identified across 17 men. All calcs identified on CT were visible on SWI. In 4 men, SWI phase images were used to differentiate calcs from iron. Mean (range) diameter was 3.8 (1.4-10.4) mm on CT and 4.0 (2.2-10.8) mm using SWI. SWI diameters were larger by median 22% in at least one dimension for all men and in 42/50 measurements overall. With SWI-guided UA placement in 9 men, median (IQR) proportion of target volume reaching boiling temperature was 4.0 (2-8.0) % and 3.0 (0-7.0) % in sectors with and without calcs based on MR thermometry. Adequate thermal dose was achieved in men with and without calcs: 98 (93-100) % vs 98 (95-100) %. In the pivotal trial of 115 patients, which did not have SWI-guided UA placement, 93% (90-98%) of quadrants with calcs reached adequate thermal coverage compared with 98 (93-100)% overall and 6 (1-9)% with calcs had target volume approaching boiling vs 0 (0-5) % overall. Intraprocedural SWI detected all CT-identified calcifications, tending to overestimate the diameter but guiding effective device positioning. CT remains the gold standard for pre-TULSA calcification screening, and further studies are warranted to optimize SWI protocols, correlate the size of calcifications on CT and SWI, and predict their impact on ablation coverage.