  Ultrasound waves propagating in water or soft biological tissue are strongly
reflected when encountering the skull, which limits the use of ultrasound-based
techniques in transcranial imaging and therapeutic applications. Current
knowledge on the acoustic properties of the cranial bone is restricted to
far-field observations, leaving its near-field properties unexplored. We report
on the existence of skull-guided acoustic waves, which was herein confirmed by
near-field measurements of optoacoustically-induced responses in ex-vivo murine
skulls immersed in water. Dispersion of the guided waves was found to
reasonably agree with the prediction of a multilayered flat plate model. It is
generally anticipated that our findings may facilitate and broaden the
application of ultrasound-mediated techniques in brain diagnostics and therapy.
