Shoar K et al, 2017: Tracking kidney stones in a homogeneous medium using a trilateration approach.
Shoar K, Turney BW, Cleveland RO.
Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Oxford OX3 7DQ, United Kingdom.
Nuffield Department of Surgical Sciences, University of Oxford, Oxford OX3 9DU, United Kingdom.
Shock wave lithotripsy is a non-invasive procedure by which kidney stones are fragmented by thousands of shock waves. Currently, many shock waves are delivered to the body that do not impact the stone, but do result in tissue trauma. This motivates developing a monitoring system to locate kidney stones, with the goal of gating shock waves not aligned with the stone, and hence, reducing renal trauma during lithotripsy. The system consists of a circular array housing twenty-two 0.5 MHz transducers that can be mounted on a clinical lithotripter. It was deployed in a water tank and tested with two stone models made from gypsum cement and a stone model fragment. An algorithm consisting of threshold detection, automatic rejection of weak signals, and triangulation was developed to determine the location of stones. The results show that within ±15 mm of the focus of the lithotripter, the accuracy was better than 4 mm in the lateral directions and 2 mm in the axial direction. Using off-the-shelf hardware, the algorithm can calculate stone positions every 1 s allowing for real-time tracking during lithotripsy.
J Acoust Soc Am. 2017 Dec;142(6):3715. doi: 10.1121/1.5017718. FREE ARTICLE
It is indeed an uncomfortable and annoying situation when the target stone moves in and out of focus in an un-controlled way. Not only will the treatment efficacy be low and the surrounding organs exposed to unnecessary trauma, but there is also a loss of harmony in the treatment.
Different solutions have been suggested to cope with this problem such as respiratory trigging and ultrasound tracking. The movement of the stone also can be reduced by a belt or an abdominal plate. Unfortunately none of these procedures results in a 100% hit rate. These techniques therefore either never have got general acceptance or are used only infrequently.
It was therefore with great interest I read this article that describes a device comprised of an array of transducers emitting and receiving signals and thereby enabling administration of the shockwave when the stone is in focus and only then. This is a new and fascinating idea.
So far this carefully conducted study has only considered the conditions in water as in the original Dornier HM3 lithotripter. It will be highly interesting to see if the authors can make an adaptation to in vivo conditions and with other lithotripters. It will thereby be necessary to take into account not only the scattering from one or several stones, but also that from tissues in the body.
If such a project is met with success it will definitely fill a technical gap, because if the hit rate can be increased or optimized this is a really desirable development.