Integration of deployable fluid lenses and reflectors with endoluminal therapeutic ultrasound applicators: Preliminary investigations of enhanced penetration depth and focal gain

Abstract

Purpose Catheter-based ultrasound applicators can generate thermal ablation of tissues adjacent to body lumens, but have limited focusing and penetration capabilities due to the small profile of integrated transducers required for the applicator to traverse anatomical passages. This study investigates a design for an endoluminal or laparoscopic ultrasound applicator with deployable acoustic reflector and fluid lens components, which can be expanded after device delivery to increase the effective acoustic aperture and allow for deeper and dynamically adjustable target depths. Acoustic and biothermal theoretical studies, along with benchtop proof-of-concept measurements, were performed to investigate the proposed design. Methods The design schema consists of an array of tubular transducer(s) situated at the end of a catheter assembly, surrounded by an expandable water-filled conical balloon with a secondary reflective compartment that redirects acoustic energy distally through a plano-convex fluid lens. By controlling the lens fluid volume, the convex surface can be altered to adjust the focal length or collapsed for device insertion or removal. Acoustic output of the expanded applicator assembly was modeled using the rectangular radiator method and secondary sources, accounting for reflection and refraction at interfaces. Parametric studies of transducer radius (1–5 mm), height (3–25 mm), frequency (1.5–3 MHz), expanded balloon diameter (10–50 mm), lens focal length (10–100 mm), lens fluid (silicone oil, perfluorocarbon), and tissue attenuation (0–10 Np/m/MHz) on beam distributions and focal gain were performed. A proof-of-concept applicator assembly was fabricated and characterized using hydrophone-based intensity profile measurements. Biothermal simulations of endoluminal ablation in liver and pancreatic tissue were performed for target depths between 2–10 cm. Results Simulations indicate that focal gain and penetration depth scale with the expanded reflector-lens balloon diameter, with greater achievable performance using perfluorocarbon lens fluid. Simulations of a 50 mm balloon OD, 10 mm transducer outer diameter (OD), 1.5 MHz assembly in water resulted in maximum intensity gain of ~170 (focal dimensions: ~12 mm length × 1.4 mm width) at ~5 cm focal depth and focal gains above 100 between 24–84 mm depths. A smaller (10 mm balloon OD, 4 mm transducer OD, 1.5 MHz) configuration produced a maximum gain of 6 at 9 mm depth. Compared to a conventional applicator with a fixed spherically-focused transducer of 12 mm diameter, focal gain was enhanced at depths beyond 20 mm for assembly configurations with balloon diameters ≥ 20 mm. Hydrophone characterizations of the experimental assembly (31 mm reflector/lens diameter, 4.75 mm transducer radius, 1.7 MHz) illustrated focusing at variable depths between 10–70 mm with a maximum gain of ~60 and demonstrated agreement with theoretical simulations. Biothermal simulations (30 s sonication, 75°C maximum) indicate that investigated applicator assembly configurations, at...

Deployable cylindrical phased-array applicator mimicking a concentric-ring configuration for minimally-invasive delivery of therapeutic ultrasound

“…Such deployable device assemblies have been devised using cylindrical ultrasound radiators and expandable reflective balloons, as used for intravascular delivery and intracardiac expansion for treatment of atrial fibrillation (Meininger et al 2003, Schmidt et al 2007, Nakagawa et al 2007, Sinelnikov et al 2009). Our group has investigated combining cylindrical transducer sources with an expandable conical balloon that contains multiple luminal compartments filled with water, air, or perfluorocarbon fluid (Adams et al 2017, Adams et al 2017). In this assembly configuration, acoustic energy emitted by the cylindrical radiator is reflected off the transverse conical balloon boundary, redirected towards the tip of the device, and focused by refraction as it travels across the perfluorocarbon-filled fluid lens compartment into tissue.…”

Therapeutic Systems and Technologies: State-of-the-Art Applications, Opportunities, and Challenges

“…A transurethral ultrasound ablation system integrated with MRI thermometry has been clinically applied for treatment of benign prostatic hyperplasia [46]. Interstitial devices are under development for treatment of targets in the brain [47] and liver [47] and deployable applicators are under investigation for targeting pancreatic tumors via an endogastric approach [48].…”

An endoluminal cylindrical sectored‐ring ultrasound phased‐array applicator for minimally‐invasive therapeutic ultrasound

“…Endoluminal and interstitial catheter‐based ultrasound applicators have been developed for hyperthermia and thermal ablation from placement within the body. 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 While more invasive than the extracorporeal counterparts, these devices are less affected by surrounding intervening tissues and can deliver energy to hard‐to‐reach sites. Minimally‐invasive phased array transducers have been used in simulation, pre‐clinical or clinical studies, for prostate ablation, 13 , 43 , 44 liver ablation, 15 cardiac ablation, 40 , 45 , 46 renal tumor ablation, 47 and endoluminal applicator for esophageal ablation.…”

“…Recent advances in these expandable ultrasound applicators have incorporated cylindrical applicators with deployable reflective balloons, perfluorocarbon liquid lenses, and segmented or phased arrays. 35 , 48 Dual‐layered parabolic balloon reflectors enclosing ultrasound transducers and producing annular ablation patterns were used for treating atrial fibrillation and intravascular delivery. 49 , 50 , 51 , 52 Our group has investigated combining cylindrical transducers with an expandable conical balloon with two compartments to hold water and air, 53 whereas the ultrasound energy is reflected from the water‐air interface and redirected toward the tissue.…”

“…Endoluminal and interstitial catheter‐based ultrasound applicators have been developed for hyperthermia and thermal ablation from placement within the body 34–42 . While more invasive than the extracorporeal counterparts, these devices are less affected by surrounding intervening tissues and can deliver energy to hard‐to‐reach sites.…”