Adaptive ultrasound phased array systems for deep hyperthermia.

Abstract

Ultrasound phased arrays show increasing promise as versatile applicators for hyperthermia cancer therapy, because they can produce complicated spatially distributed energy deposition patterns without mechanical movement. However, tissue inhomogeneities can affect the focused beams produced by a phased array. Thus adaptive array systems that can correct phase errors due to inhomogeneous tissues may be needed in the clinic. In this research, the effects of phase quantization and Gaussian-distributed random phase errors on focal patterns were studied. Focal distortions caused by inhomogeneous media such as pork tissue or a polymer phantom were experimentally studied. These studies showed the sensitivity of different focal patterns to phase errors. Also, the approach of using implantable acoustic sensors for acoustic feedback was experimentally verified, where several prototype phased arrays were applied to focus through inhomogeneous media by compensating for phase errors due to tissue inhomogeneity and target movement. This study proves that ultrasound phased arrays can dramatically improve focus by adaptively overcoming phase errors. Furthermore, fundamental limits on focused ultrasound for hyperthermia were investigated by showing the unavoidable hot spots in normal tissues when the tumor volume is too large. This study determined the relationships between the upper bound of treatable volume and physical constraints such as tumor depth, attenuation rate, acoustic window size, blood perfusion rate, and tolerable temperature limit in normal tissues. Finally, a prototypic phased array was applied to produce different heating patterns in dogs' thighs in vivo.