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Mancini JG et al, 2013: Assessment of a Modified Acoustic Lens for Electromagnetic Shock Wave Lithotripters in a Swine Model

Mancini JG, Neisius A, Smith N, Sankin G, Astroza GM, Lipkin ME, Simmons WN, Preminger GM, Zhong P
Duke University Medical Center, Division of Urologic Surgery, 200 Trent Drive, DUMC 3707, Durham, NC 27710, USA


Abstract

PURPOSE: The acoustic lens of the Siemens Modularis electromagnetic (EM)shock wave lithotripter has been modified to produce a pressure waveform and focal zone more closely resembling that of the original Dornier HM3 device. Herein, we assess the newly designed acoustic lens in vivo in an animal model.

MATERIALS AND METHODS: Stone fragmentation and tissue injury produced by the original and modified lenses of a Siemens lithotripter were evaluated in a swine model under equivalent acoustic pulse energy (~45 mJ) at 1 Hz pulse repetition frequency. Stone fragmentation was determined by the weight percent of stone fragments less than 2mm. For tissue injury assessment, shock wave-treated kidneys were perfused, dehydrated, cast in paraffin wax and sectioned. Digital images were captured every 120 μm and processed to determine the functional renal volume damage.

RESULTS: After 500 shocks, stone fragmentation efficiency produced by the original and modified lenses was 48 ± 12%and 52 ± 17%(p=0.60), respectively. However, after 2000 shocks, the modified lens showed significantly improved stone fragmentation of86 ± 10%, compared to 72 ± 12% for the original lens (p=0.02). Tissue injury caused by the original and modified lenses was minimal at 0.57± 0.44% and 0.25 ± 0.25% (p=0.27), respectively.

CONCLUSIONS: With lens modification, the Siemens Modularis lithotripter demonstrates significantly improved stone fragmentation with minimal tissue injury at clinically relevant acoustic pulse energy. This new lens design could potentially be retrofitted to existing lithotripters, thereby improving the effectiveness of EM lithotripters.

J Urol. 2013 Feb 25. pii: S0022-5347(13)00350-9. doi: 10.1016/j.juro.2013.02.074. [Epub ahead of print]
PMID:23485509 [PubMed - as supplied by publisher]

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Comments 1

Hans-Göran Tiselius on Monday, 26 November 2012 06:33

The optimal geometry and power characteristics of the shock wave have remained a matter of debate ever since the original HM3 lithotripter was taken out of production. Several new technical solutions were considered necessary to reduce the need of anaesthesia and to allow for efficient transmission of the shock wave power. It has so far been insufficiently understood why the HM3-lithotripter with its large focal volume and relatively low energy density had treatment results that have been difficult to meet in later generations of lithotripters. Although several explanations have been put forward, interest has recently been directed towards the importance of the focal size. It thus has been suggested that a focal width exceeding the stone diameter and a low energy density causes a better disintegration by more efficient tensile forces than a small width focus with high energy.

The modified acoustic lens described in this article is an interesting contribution to this problem. The main principle is to deliver a delayed shock wave within a single discharge. Thereby the second compressive component of the shock wave is counteracted. In order to achieve this goal the focal width was increased from approximately 7 to 10 mm. In the animal experiments better disintegration and less extensive tissue damage were interesting findings. The artificial stones that were used had a size of 5-10 mm when hit by the shock wave and had thus about the same (or smaller) diameter as the focal width.

Although the described modification has been designed for the acoustic lens system only, the technical development emphasizes the importance of the shock wave focus geometry.

Hans-Göran Tiselius

The optimal geometry and power characteristics of the shock wave have remained a matter of debate ever since the original HM3 lithotripter was taken out of production. Several new technical solutions were considered necessary to reduce the need of anaesthesia and to allow for efficient transmission of the shock wave power. It has so far been insufficiently understood why the HM3-lithotripter with its large focal volume and relatively low energy density had treatment results that have been difficult to meet in later generations of lithotripters. Although several explanations have been put forward, interest has recently been directed towards the importance of the focal size. It thus has been suggested that a focal width exceeding the stone diameter and a low energy density causes a better disintegration by more efficient tensile forces than a small width focus with high energy. The modified acoustic lens described in this article is an interesting contribution to this problem. The main principle is to deliver a delayed shock wave within a single discharge. Thereby the second compressive component of the shock wave is counteracted. In order to achieve this goal the focal width was increased from approximately 7 to 10 mm. In the animal experiments better disintegration and less extensive tissue damage were interesting findings. The artificial stones that were used had a size of 5-10 mm when hit by the shock wave and had thus about the same (or smaller) diameter as the focal width. Although the described modification has been designed for the acoustic lens system only, the technical development emphasizes the importance of the shock wave focus geometry. Hans-Göran Tiselius
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