Transurethral high-intensity ultrasound for treatment of stress urinary incontinence (SUI): simulation studies with patient-specific models

Abstract

Background: Stress urinary incontinence (SUI) is prevalent in adult women, attributed to weakened endopelvic supporting tissues, and typically treated using drugs and invasive surgical procedures. The objective of this in silico study is to explore transurethral high-intensity ultrasound for delivery of precise thermal therapy to the endopelvic tissues adjacent to the mid-urethra, to induce thermal remodeling as a potential minimally invasive treatment alternative.

Methods: 3D acoustic (Rayleigh-Sommerfeld) and biothermal (Pennes bioheat) models of the ultrasound applicator and surrounding tissues were devised. Parametric studies over transducer configuration [frequency, radius-of-curvature (ROC)] and treatment settings (power, duration) were performed, and select cases on patient-specific models were used for further evaluation. Transient temperature and thermal dose distributions were calculated, and temperature and dose metrics reported.

Results: Configurations using a 5-MHz curvilinear transducer (3.5 × 10 mm, 28 mm ROC) with single 90 s sonication can create heated zones with 11 mm penetration (>50 °C) while sparing the inner 1.8 mm (<45 °C) radial depth of the urethral mucosa. Sequential and discrete applicator rotations can sweep out bilateral coagulation volumes (1.4 W power, 15° rotations, 600 s total time), produce large volumetric (1124 mm³ above 60 EM_{43 °C}) and wide angular (∼50.5° per lateral sweep) coverage, with up to 15.6 mm thermal penetration and at least 1.6 mm radial urethral protection (<5 EM_{43 °C}).

Conclusion: Transurethral applicators with curvilinear ultrasound transducers can deliver spatially selective temperature elevations to lateral mid-urethral targets as a possible means to tighten the endopelvic fascia and adjacent tissues.

Keywords: Thermal therapy; high-intensity ultrasound; stress urinary incontinence; therapeutic ultrasound; thermal ablation.

Figure 1.

Schema showing (a) coronal and (b) axial diagrams of transurethral ultrasound applicator and placement (c) for delivery of precision thermal therapy to lateral zones off the female mid-urethra for the treatment of Stress Urinary Incontinence. (c) The position of urethral transducer, sequential rotation directions, and sonications with the order from 1 to 3 on one side and 4 to 6 on the opposite side represent a proposed heating strategy, with heated zones shown in red contours.

Figure 2.

Cross-sectional slice of 3D generic model and data metrics for parametric study based upon final thermal dose accumulation after sonications: the urethral sparing was defined based on thermal dose contour of 5 EM43°C (magenta contours) and therapy penetration depth was defined based on the contours of 10EM43°C, 60EM43°C (shown in yellow contours) or 240EM43°C.

Figure 3.

Patient-specific model generation and simulation workflow, shown in the sequential order of MRI image segmentation, 3D volume generation, FEM meshing, and numerical acoustic and biothermal simulation to calculate transient temperature elevations and thermal dose accumulation (dose volumes of 60EM43°C and 240EM43°C are shown in yellow and orange, respectively).

Figure 4.

Parametric studies of transducer frequency and radius of curvature (ROC): 90s sonication with 3.5 mm x10 mm curvilinear transducers (acoustic power=1.4W): (a-c) temperature distributions at frequencies of 5MHz, 6MHz and 7MHz, respectively, with ROC=20mm, indicating larger heated zones generated as frequency increased for constant power; (d-f) temperature distributions with different curvilinear ROCs of 12 mm, 28 mm, and ∞ (planar applicator), respectively, operating at 5MHz. Temperature contours of 45°C and 50°C are shown to demonstrate the sparing distance from the balloon and the penetration depth, respectively.

Figure 5.

Summary of parametric studies on frequency and radius of curvature (ROC): (a-c) the sparing, peak temperature and penetration depth, respectively, of varying powers and frequencies (5–7MHz) when ROC is fixed as 20mm; (d-f) the sparing, peak temperature and penetration depth resulting from varying powers and ROC (12mm to ∞ (planar)) when frequency is fixed at 5MHz.

Figure 6.

Thermal simulation of maximum temperature and thermal dose of generic tissue using the curvilinear ultrasound applicator for sequential three-shot sonication patterns. (a-c) Maximum temperature with 30°, 20° and 15° sequential rotations are shown respectively, with 45°C (magenta) and 50°C (black) contours. (d-e) Corresponding thermal dose with 30°, 20° and 15° sequential rotations are shown, respectively, with contours of 10 EM43°C (magenta), 60 EM43°C (white) and 240 EM43°C (red). (The display of thermal dose was set at a maximum threshold of 240 EM43°C for visualization purposes).

Figure 7.

Cumulative maximum temperature distributions of the central axial plane at the end of each sequential sonication at (a) 120 s, (b) 210 s, and (c) 300 s for rotations of 15°, and repeated on the opposite lateral side for a total procedure time of (d) 600 s. Temperature contours of 45°C and 50°C are shown, penetrating antero-laterally into surrounding connective tissues. Critical anatomy including the pubic bones (B), vaginal wall (V), and outer boundary of urethral stromal and muscle (U) are highlighted.

Figure 8

Cumulative thermal dose distributions, corresponding to the temperature elevations of Fig. 7, of the central axial plane at the end of each sequential sonication at (a) 120 s, (b) 210 s, and (c)300 s for rotations of 15°, and repeat on the opposite side for total procedure time of (d) 600 s. Thermal dose contours of 10EM_43°C, 60EM_43°cand 240EM_43°c are shown penetrating into the surrounding connective tissue. Critical anatomy including the pubic bones (B), vaginal wall (V), and outer boundary of urethral stromal and muscle (U) are highlighted. (The display of thermal dose was set at a maximum threshold of 240 EM_43°C for visualization purposes).

Figure 9.

(a) Maximum temperature and (b) thermal dose distributions on the central coronal plane aligned with the axis of the applicator after 600s sonication to target both sides of the mid-urethra. A slightly smaller heated zone was observed in the direction of the second side treated (right side in figure), due to the presence of urethral cooling for a longer time prior to sonication. Urethral muscular stroma (U) and bladder (Bl) are highlighted. (The display of thermal dose was set at a maximum threshold of 240 EM_43°c for visualization purposes).

Figure 10.

(a) Maximum temperature and (b) thermal dose of patient specific model II on the central axial plane after 600s sonication. Temperature contours of 45°C and 50°C were shown in figure a, and dose contours of 10EM_43°C, 60EM_43°C and 240EM_43°C were shown in (b). Note due to patient positioning and anatomy, pelvic bone is delineated on one side only in this plane. (B denotes pubic bone, V denotes vagina, and U denotes urethral mucosa and surrounding layers of connective tissue. The display of thermal dose was set at a maximum threshold of 240 EM_43°C for visualization purposes).

Figure 11.

3D Thermal dose distribution of patient specific simulations for (a) model I and (b) model II. 60EM43°C and 240EM43°C isosurfaces are shown in yellow and orange respectively, indicating ultrasound energy and therapeutic temperature and dose levels were delivered selectively to the target region with deep penetration, without thermal damage on sensitive structures such as bone and vagina (note that thermal distributions are behind the bone in (a)). (B represents pubic bone, V denotes vagina, and U denotes urethral mucosa and surrounding layers of connective tissue).

Figure 12.

Maximum temperature (a) and thermal dose (b) of patient specific model I on the central axial plane after 600s sonication with lower conservative applied power of 1.05 W. Temperature contours of 45°C and 48°C are shown in (a), and dose contours of 10EM_43°c, 30EM_43°c and 60EM_43°C are shown in (b). (B denotes pubic bone, V denotes vagina, and U denotes urethral mucosa and surrounding layers of connective tissue. The display of thermal dose was set at a maximum threshold of 60 EM_43°C for visualization purposes).