Curvilinear transurethral ultrasound applicator for selective prostate thermal therapy

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Abstract
Thermal therapy offers a minimally invasive option for treating benign prostatic hyperplasia (BPH) and localized prostate cancer. In this study we investigated a transurethral ultrasound applicator design utilizing curvilinear, or slightly focused, transducers to heat prostatic tissue rapidly and controllably. The applicator was constructed with two independently powered transducer segments operating at 6.5 MHz and measuring 3.5 mm × 10 mm with a 15 mm radius of curvature across the short axis. The curvilinear applicator was characterized by acoustic efficiency measurements, acoustic beam plots, biothermal simulations of human prostate, ex vivo heating trials in bovine liver, and in vivo heating trials in canine prostate ( n = 3 ) . Each transducer segment was found to emit a narrow acoustic beam ( max width < 3 mm ) , which extended the length of the transducer, with deeper penetration than previously developed planar or sectored tubular transurethral ultrasound applicators. Acoustic and biothermal simulations of human prostate demonstrated three treatment schemes for the curvilinear applicator: single shot (10 W, 60 s) schemes to generate narrow ablation zones ( 13 × 4 mm , 52 °C at the lesion boundary), incremental rotation (10 W, 10 ° ∕ 45 s ) to generate larger sector-shaped ablation zones ( 16 mm × 180 ° sector), and rotation with variable sonication times (10 W, 10 ° ∕ 15 – 90 s ) to conform the ablation zone to a predefined boundary ( 9 – 17 mm × 180 ° sector, 13 min total treatment time). During in vivo canine prostate experiments, guided by MR temperature imaging, single shot sonications ( 6 W ∕ transducer , 2–3 min) with the curvilinear applicator ablated 20° sections of tissue to the prostate boundary (9–15 mm). Multiple adjacent sonications (“sweeping”) ablated large sections of the prostate (180°) by using the MR temperature imaging to adjust the power ( 4 – 6.4 W ∕ transducer ) and sonication time (30–180 s) at each 10° rotation such that the periphery of the prostate reached 52 °C before the next rotation. The conclusion of this study was that the curvilinear applicator produces a narrow and penetrating ultrasound beam that, when combined with image guidance, can provide a precise technique for ablating target regions with a contoured outer boundary, such as the prostate capsule, by rotating in small steps while dynamically adjusting the net applied electrical power and sonication time at each position.