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<p>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 (L<sub>1</sub>)
was modified so that the acoustic focus of the new lens (L<sub>2</sub>) 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 (L<sub>2</sub>+M<sub>8025</sub> and L<sub>2</sub>+M<sub>9030</sub>) 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 L<sub>1</sub>, 15.8 kV for L<sub>2</sub>+M<sub>8025</sub>, and 17 kV L<sub>2</sub>+M<sub>9030</sub>)
to produce equivalent acoustic pulse energy of 45 mJ in all setups, measured in the
lithotripter focal plane. Under this condition, L<sub>2</sub>+M<sub>8025</sub> and
L<sub>2</sub>+M<sub>9030</sub> generate lower peak pressure (38.2 and 36.8 MPa) with
a significantly broadened BW<sub>y</sub> (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 L<sub>1</sub>.
It is worth noting that L<sub>2</sub>+M<sub>8025</sub> and L<sub>2</sub>+M<sub>9030</sub>
produce a <italic>BW</italic><sub>x</sub> (7.6 and 7.5 mm) in the orthogonal direction
to the mask axis, which is also comparable to L<sub>1</sub>. Similarly, the beam
width of the cavitation field was broadened from 8.1 to 12.2 mm for L<sub>2</sub>+M<sub>8025</sub>,
and from 10.9 to 17.9 mm for L<sub>2</sub>+M<sub>9030</sub>, compared to the range
of 8.8 to 9.4 mm measured from L<sub>1</sub>. In comparison, L<sub>2</sub>+M<sub>8025</sub>
produces a denser and narrower bubble cloud along the y-axis than L<sub>2</sub>+M<sub>9030</sub>.
In vitro stone comminution (<italic>SC</italic>) tests in a tube holder (Diameter
= 14 mm) have demonstrated that L<sub>2</sub>+M<sub>8025</sub> and L<sub>2</sub>+M<sub>9030</sub>
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<sub>+(avg)</sub>,
and effective acoustic pulse energy, E<sub>eft</sub>, delivered to the stone, as shown
in previous studies. Furthermore, a ureter model was developed and used to assess
the performance of L<sub>2</sub>+M<sub>9030</sub>, which has the maximally elongated
<italic>BW</italic> under various static and simulated respiratory motion conditions.
The results suggest that L<sub>2</sub>+M<sub>9030</sub> can produce significantly
better <italic>SC</italic> than L<sub>1</sub> 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.</p>
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