The effects of curved tissue layers on the power deposition patterns of therapeutic ultrasound beams.

A numerical model for calculating the ultrasonic power deposition in layered media was developed and experimentally tested. The new model takes into account the ultrasound wave reflection and refraction occurring at tissue interfaces. In order to study the effects of soft tissue interfaces on the resulting ultrasonic power deposition patterns, the tested model was applied to simulate four different treatments situations in ultrasound therapy (such as hyperthermia). It was found that in most cases the effect of soft tissue interfaces could be ignored during ultrasound treatments. However, strongly curved tissue interfaces such as those encountered during brain treatments could have significant effects on the location of the focus and the magnitude of local power deposition if sharply focused ultrasound beams are used. The maximum lateral displacement of the focus in a brain treatment was 3 mm. This is too large to be ignored during therapy and thus the effect of tissue interfaces should be taken into account when certain ultrasound treatments are planned and executed.