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Wang JC, Zhou Y. 2016: Shifting the Split Reflectors to Enhance Stone Fragmentation of Shock Wave Lithotripsy.

Wang JC, Zhou Y
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore.

Abstract

Shock wave lithotripsy (SWL) has been used widely in urology for about three decades to treat kidney calculi. Technical development to improve performance (i.e., stone fragmentation efficiency) is continuous. Low-pressure wide-focus lithotripters have already achieved promising results. In this study, the lithotripter field and profile of lithotripter shock waves were changed simultaneously using a cost-effective and convenient design. An intact parabolic reflector was split into four pieces, and each part was moved individually. By shifting the split reflectors, the focused acoustic beams were separated. As a result, the beam width in the focal region could be increased. Both numerical models of wave propagation using a k-wave approach and hydrophone measurements showed similar pressure waveforms at the focus and the distributions along and transverse to the lithotripter axis. The increase of the shifting distance from 0 mm to 7 mm resulted in the increase of -6 dB beam width from 7.1 mm to 13.9 mm and location of tensile peak on axis moving from z = -14 mm to 1 mm. The Lithotripters at 10 kV (intact reflector) and at 12 kV with the split reflectors shifted by 5 mm were compared with each other because of their similar peak positive pressures at the focus (8.07 MPa ± 0.05 MPa vs. 7.90 MPa ± 0.11 MPa, respectively). However, there were significant differences in their positive beam width (8.7 mm vs. 10.2 mm), peak negative pressure (-6.34 MPa ± 0.04 MPa vs. -7.13 MPa ± 0.13 MPa), the maximum tensile stress (7.55 MPa vs. 8.95 MPa) and shear stress (6.1 MPa vs. 7.76 MPa) in a 10-mm diameter spherical stone and bubble collapse time (127.6 μs ± 5.4 μs vs. 212.7 μs ± 8.2 μs). As a result, stone fragmentation efficiency was enhanced about 1.8-fold (57.9% ± 4.6% vs. 32.2% ± 5.6%, p < 0.05) when shifting the split reflectors. These results suggest that this new reflector design could change the characteristics of the lithotripter field and increase stone fragmentation efficiency. 

Ultrasound Med Biol. 2016 Aug;42(8):1876-89. doi: 10.1016/j.ultrasmedbio.2016.03.029. Epub 2016 May 7.

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Comments 1

Othmar Wess on Tuesday, 01 November 2016 12:59

The authors start their study by the hypothesis that focal size has a decisive impact on lithotripsy performance. They follow the idea of widening the lateral extension the focal size (-6dB value) in order to increase fragmentation efficiency while, simultaneously, side effects such as hematoma or bleedings are expected to be diminished. This hypothesis is widely spread in the lithotripsy community and associated to early results published for the original HM3 bathtub lithotripter which is often still considered the gold standard. Up to my knowledge there is no scientific proof for superiority of larger focus vs. smaller focus. There are many reasons for different performance levels of lithotripter devices such as quality of coupling, precision of targeting, energy setting etc. Peak pressure and according -6dB-focus extension is less important than total energy dose delivered to the target stones.
This paper focuses on a split reflector, which is shifted in parts in order to degrade peak pressure and spoil shock wave concentration. The calculated and measured -6dB focus can be reduced due to the diminished peak pressure and blurred pressure distribution. This, apparently, results in a diminished fragmentation efficiency which can only be compensated by an increased power level of the generator. Best fragmentation performance is reported for an intact (unmodified) reflector. The claimed 1.8–fold increased efficiency of the modified (shifted) reflector is achieved only in comparison to the original reflector at a significantly reduced (10kV vs. 12 kV) energy setting.
The claimed benefit of the reported technique could not be convincingly demonstrated. There might be other benefits of the split reflector which might be proved in future.

The authors start their study by the hypothesis that focal size has a decisive impact on lithotripsy performance. They follow the idea of widening the lateral extension the focal size (-6dB value) in order to increase fragmentation efficiency while, simultaneously, side effects such as hematoma or bleedings are expected to be diminished. This hypothesis is widely spread in the lithotripsy community and associated to early results published for the original HM3 bathtub lithotripter which is often still considered the gold standard. Up to my knowledge there is no scientific proof for superiority of larger focus vs. smaller focus. There are many reasons for different performance levels of lithotripter devices such as quality of coupling, precision of targeting, energy setting etc. Peak pressure and according -6dB-focus extension is less important than total energy dose delivered to the target stones. This paper focuses on a split reflector, which is shifted in parts in order to degrade peak pressure and spoil shock wave concentration. The calculated and measured -6dB focus can be reduced due to the diminished peak pressure and blurred pressure distribution. This, apparently, results in a diminished fragmentation efficiency which can only be compensated by an increased power level of the generator. Best fragmentation performance is reported for an intact (unmodified) reflector. The claimed 1.8–fold increased efficiency of the modified (shifted) reflector is achieved only in comparison to the original reflector at a significantly reduced (10kV vs. 12 kV) energy setting. The claimed benefit of the reported technique could not be convincingly demonstrated. There might be other benefits of the split reflector which might be proved in future.
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