Li G et al, 2014: Effect of the Body Wall on Lithotripter Shock Waves
Li G, McAteer JA, Williams JC Jr, Berwick ZC
Department of Anatomy and Cell Biology, Indiana University School of Medicine , Indianapolis, Indiana
PURPOSE: Determine the influence of passage through the body wall on the properties of lithotripter shock waves (SWs) and the characteristics of the acoustic field of an electromagnetic lithotripter.
METHODS: Full-thickness ex vivo segments of pig abdominal wall were secured against the acoustic window of a test tank coupled to the lithotripter. A fiberoptic probe hydrophone was used to measure SW pressures, determine shock rise time and map the acoustic field in the focal plane.
RESULTS: Peak positive pressure on axis was attenuated roughly proportional to tissue thickness - approximately 6% per cm. Irregularities in the tissue path affected the symmetry of focusing, shifting the maximum peak positive pressure laterally by as much as ~2mm. Within the time resolution of the hydrophone (7-15 ns), shock rise time was unchanged, measuring ~17-21 ns with and without tissue present. Mapping of the field showed no effect of the body wall on focal width, regardless of thickness of the body wall.
CONCLUSIONS: Passage through the body wall has minimal effect on the characteristics of lithotripter SWs. Other than reducing pulse amplitude and having the potential to affect the symmetry of the focused wave, the body wall has little influence on the acoustic field. These findings help to validate laboratory assessment of lithotripter acoustic field and suggest that the properties of SWs in the body are much the same as have been measured in vitro.
J Endourol. 2014 Jan 8. [Epub ahead of print]. PMID:24308532 [PubMed - as supplied by publisher]. FREE ARTICLE
The effect of the body wall on the physical properties of shockwaves is indeed highly interesting and important for the settings of treatment variables during SWL. In this experimental study with shockwaves delivered through the pig abdominal wall, some interesting observations were made. The positive peak pressure (P+) was attenuated by approximately 6% per cm tissue thickness. Also the negative pressure (P-) was reduced, but these measurements were less precise. The acoustic field was slightly affected by a small lateral displacement, but the focal width was essentially unchanged.
From a clinical point of view it appears valuable to include the muscle/fat-volume (distance) that the shockwaves have to pass in order to determine the most efficient power setting. In different treatment algorithms the distance from skin to stone are included, but mainly for selecting those patients that are considered poor candidates for SWL. But would it be appropriate to allow a higher shockwave power for patients with a significant loss of energy can be expected? This is something that needs future attention and research, because I have a feeling that many patients with a long shockwave path through muscle and fat are treated with inappropriately low shockwave energy.