Ultrasound-induced fragmentation of connective tissue guided by passive acoustic mapping.

Abstract

To date, studies on the mechanical bio-effects of acoustic cavitation have been confined to soft tissues such as the liver or prostate. In the present work, acoustic cavitation caused by high intensity focused ultrasound (HIFU) is used to disrupt the highly collagenous, acellular connective tissue of the intervertebral disk (IVD). The IVD is comprised of three regions: a highly hydrated, amorphous nucleus pulposus (NP) enclosed circumferentially by the lamellar structure of the annulus fibrosus, and capped by cartilaginous endplates. In experiments using ex vivo bovine coccygeal disks as a model, two confocally-aligned HIFU transducers were positioned with their focus inside the disk, and driven at 500 kHz. Cavitation activity was localized and monitored using both a 3.5 MHz PCD and a 2-D passive mapping array and was found to be confined to the NP. Treatment parameters were optimized to minimize treatment time and thermal damage to adjacent tissue. Disruption of the tissue after treatment was determined visually and suggests that it should be possible to remove disrupted tissue through a small gauge needle, thus enabling several proposed treatments for disk degeneration.