Non-axisymmetric and Steerable Acoustic Field for Enhanced Stone Comminution in Shock Wave Lithotripsy
The primary goal of this dissertation was to assess the feasibility of transforming an electromagnetic (EM) shock wave lithotripter with an acoustic lens as its focusing device from the original axisymmetric pressure distribution to a non-axisymmetric steerable acoustic field. This work was motivated by the desire to better match the distribution of effective acoustic pressure and pulse energy with the trajectory and anatomical features around renal and ureteral calculi during clinical shock wave lithotripsy (SWL). The acoustic field transformation was accomplished by the design of a fan-shaped acoustic barrier (mask) placed on top of the lithotripter acoustic lens to selectively reduce the source aperture along the direction of the barrier axis, therefore effectively broadening the beam width (<italic>BW</italic>) of the lithotripter field in this preferred direction. Moreover, the geometry of the original lens (L1) was modified so that the acoustic focus of the new lens (L2) at high output voltages (necessitated by the incorporation of the mask) is closely aligned with the lithotripter focus. The mask was further driven by a motor-controlled gear system to rotate around the lithotripter axis, generating a steerable and non-axisymmetric acoustic field. In this dissertation project, a linear acoustic model was first used for parametric studies to assess the effects of mask geometry (opening angle and thickness) on beam elongation and peak pressure reduction. Based on this analysis, two mask geometries (L2+M8025 and L2+M9030) were selected for modest and maximum beam elongation within the acceptable output range of the shock wave source. The acoustic and cavitation fields of the new lens with masks, as well as the corresponding field produced by the original lens, were characterized using fiber optical probe hydrophone measurements and stereoscopic high-speed imaging. Different output voltage settings were used for each lens configuration (i.e., 14 kV for L1, 15.8 kV for L2+M8025, and 17 kV L2+M9030) to produce equivalent acoustic pulse energy of 45 mJ in all setups, measured in the lithotripter focal plane. Under this condition, L2+M8025 and L2+M9030 generate lower peak pressure (38.2 and 36.8 MPa) with a significantly broadened BWy (11.4 and 14.3 mm) along the y-axis (head-to-toe direction of the patient), which is aligned with the mask axis, compared to the high peak pressure (44.1 MPa) and moderate <italic>BW</italic> (7.5 mm) of L1. It is worth noting that L2+M8025 and L2+M9030 produce a <italic>BW</italic>x (7.6 and 7.5 mm) in the orthogonal direction to the mask axis, which is also comparable to L1. Similarly, the beam width of the cavitation field was broadened from 8.1 to 12.2 mm for L2+M8025, and from 10.9 to 17.9 mm for L2+M9030, compared to the range of 8.8 to 9.4 mm measured from L1. In comparison, L2+M8025 produces a denser and narrower bubble cloud along the y-axis than L2+M9030. In vitro stone comminution (<italic>SC</italic>) tests in a tube holder (Diameter = 14 mm) have demonstrated that L2+M8025 and L2+M9030 are more effective at off-axis positions and during simulated respiratory motion along the elongated beam direction. The results of <italic>SC</italic> also confirmed the correlation between <italic>SC</italic> and the average peak pressure, p+(avg), and effective acoustic pulse energy, Eeft, delivered to the stone, as shown in previous studies. Furthermore, a ureter model was developed and used to assess the performance of L2+M9030, which has the maximally elongated <italic>BW</italic> under various static and simulated respiratory motion conditions. The results suggest that L2+M9030 can produce significantly better <italic>SC</italic> than L1 when the elongated beam is effectively aligned with the stone/fragments in the ureter or with their motion trajectory during the course of SWL treatment. Altogether, the results of this dissertation work have demonstrated <italic>in vitro</italic> that a non-axisymmetric and steerable acoustic field can significantly enhance stone comminution under clinically relevant SWL conditions. Future work is warranted to optimize the mask design and steering protocol to maximize the benefit of such an adaptable and versatile design to improve the performance and safety of clinical EM lithotripters.
Shock wave lithotripsy
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