Effects of beam asymmetry on ultrasonic thermal lesions

This report describes therapeutic ultrasound transducers comprising a spherically focused annular array with a central rectangular cutout, which houses a diagnostic ultrasound array used for aiming and monitoring. Simulations have been conducted to compute the axially-asymmetric, multi-lobed beams produced by these transducers and to evaluate the spatio-temporal distribution of induced temperature rises in treated tissues. It is shown that, for typical 5-s exposures, thermal conduction can spatially smooth these temperature rises to produce useful lesion features. Thermal conduction can also reduce undesired effects due to spurious axial maxima in intensity, which can occur when the focal length of the therapy array is varied electronically.