Respiration gating beam steering to break moving stones by shock wave lithotripsy: Experimental results

In this study, a mask, which is made of a pair of fan-shaped closed-cell foam barriers, is used to elongate the beam width in the focal plane of a shock wave lithotripter, generating a steerable and non-axisymmetric acoustic field, and a beam steering strategy is proposed to revolve the elongated acoustic focal zone (beam), concomitant with the mask, and allows the sound wave energy to focus on moving stone, or spreading and spalling fragments, caused by patient respiratory motion and radiation force. The elongated beam is steered to cover as much as possible the stone moving trajectory, so that more shock wave energy contributes to stone fracture, but not to the peripheral tissues, in clinics. In vitro fragmentation tests have been conducted using artificial stones sited in a tube model, and the collection of quantitative data show that beam steering is able to improve stone comminution (SC) efficacy, compared with no steering. Meanwhile, patient respiration excursion and stone spreading thresholds were found distinctive correlation to SC efficacy during SWL.