Looking at the Skull in a New Light: Rayleigh-Lamb Waves in Cranial Bone

Current knowledge on the ultrasound wave propagation in the cranial bone is restricted to far-field observations. In order to extend our understanding on how ultrasound waves propagate in the skull, we use short laser pulses to excite ultrasound waves in water-immersed ex vivo mouse and human skulls and explored their near-field. The laser pulses (10 ns duration) of 532 nm are absorbed by a small layer of black burnish deposited on the skull's inner surface and generate ultrasound waves due to the thermoelastic effect. The acoustic near-field is mapped using a needle hydrophone close to the skull surface, following a three-dimensional scanning path derived from a previous pulse-echo scan of the skull with a spherically focused ultrasound transducer. The results for mouse and human skulls show different wave propagation regimes according to their differences in size, thickness, and internal structure. Leaky and non-leaky waves have been observed for both skull samples. Zero order Lamb modes were observed in the mouse skull, whereas Rayleigh-Lamb higher order modes can be observed in the human skull sample, presumably propagating in the outer cortical bone layer. Good agreement is found between the experiments and the multilayered flat plate model.