P3B-6 Implications of Mode Conversion on Transcranial Ultrasound Imaging

Abstract

Transcranial ultrasound sector images are affected by a "streak artifact": two dark streaks stem radially from the apex to the maximal depth of the scan at particular angles, limiting the effective field of view and diagnostic confidence. The aim of this paper is to investigate experimentally the angle dependency of ultrasound transmission through the skull in order to understand and try to reduce this artifact. It is shown that the streaks are a consequence of the angle-dependent amplitude transmission coefficient at fluid-solid interfaces such as the interfaces between the skull bone and the surrounding fuild-like soft tissues. At a critical incidence angle of about 27°, no ultrasound wave can cross the soft tissue-skull interfaces, which explains the presence of the streaks. At higher incidence angles, mode conversion from longitudinal to transverse waves occurs in the skull, and then back to longitudinal into the brain. The influence of the incidence angle on the distortion of the transmitted beam is also qualitatively assessed. Implications for transcranial imaging are discussed.

Determination of temporal bone Isoplanatic Patch sizes for transcranial phase aberration correction

“…When an ultrasound beam is steered at relatively high angles (>30°) through a soft tissue-bone interface, it is known that mode conversion occurs from longitudinal waves in the soft tissue to transverse waves in the bone [16]. The speed of sound of transverse waves in cortical bone is about half the speed of sound of longitudinal waves [17].…”