The stripe artifact in transcranial ultrasound imaging.

Objective: Transcranial images are affected by a "stripe artifact" (also known as a "streak artifact"): two dark stripes stem radially from the apex to the base of the scan. The stripes limit the effective field of view even on patients with good temporal windows. This study investigated the angle dependency of ultrasound transmission through the skull to elucidate this artifact.

Methods: In vivo transcranial images were obtained to illustrate the artifact. In vitro hydrophone measurements were performed in water to evaluate transcranial wavefronts at different incidence angles of the ultrasound beam. Both a thin acrylic plate, as a simple bone model, and a human temporal bone sample were used.

Results: The imaging wavefront splits into two after crossing the solid layer (acrylic model or skull sample) at an oblique angle. An early-arrival wavefront originates from the direct longitudinal wave transmission through water-bone interfaces, while a late-arrival wavefront results from longitudinal-to-transverse mode conversion at the water-bone interface, propagation of the transverse wave through the skull, and transverse-to-longitudinal conversion at the bone-water interface. At normal incidence, only the direct wavefront (without mode conversion) is observed. As the incidence angle increases, the additional "mode conversion" wavefront appears. The amplitude of the transcranial wavefront decreases and reaches a minimum at an incidence angle of about 27°. Beyond that critical angle, only the mode conversion wavefront is transmitted.

Conclusions: The stripes are a consequence of the angle-dependent ultrasound transmission and mode conversion at fluid-solid interfaces such as between the skull and the surrounding fluidlike soft tissues.