The role of internal reflection in transskull phase distortion

Phase distortion due to reflection in transcranial ultrasound propagation is investigated. Understanding of these phase-dependent properties is motivated by efforts to construct a reliable prediction model for noninvasive ultrasound therapy in the brain. The present study measures the phase of an ultrasound wave after propagation through an ex vivo human skull and considers the dependence of this phase on reflections between the transducer and the skull surface in addition to reflections within the skull. Experiments are performed using a human calvarium fragment placed between an underwater ultrasonic transducer and a polyvinylidene difluoride hydrophone. Data are presented indicating the ultrasound phase dependence as a function of burst length and the distance of the transducer element from the skull at a driving frequency of 0.5 MHz. Experimental results are compared with predictions obtained from a propagation model which considers transmission at the skull interfaces as well as multiple reflections within the skull. It is concluded that by using short ultrasound bursts a distance may be indicated that beyond which the contributions of transducer reflections on the phase of the propagating wave may be neglected. Additionally, a comparison of the measurements with simulated data supports the contention that for reasonably small incident angles, reflection within the skull causes minimal phase shift.