Matrix Pencil Estimation of Guided Waves Dispersion in a Human Skull

Guided Ultrasonic Waves (GUWs) have been recently proposed as an alternative to more classical ultrasonic-based techniques in brain imaging and therapy. The efficient and effective use of GUWs is strongly related to the a priori knowledge of the Lamb wave-type dispersion behavior of the cranial region adjacent to the inspected brain location. In this context, few studies have been proposed in which only A(0) and quasi Rayleigh modes are reliably detected, whereas different high-order supersonic modes cannot be clearly distinguished due to severe scattering associated to the spongeous structure of the diploe. In this work, it is demonstrated that high order modes can be excited and recorded at low and mid frequencies (<1.0 MHz) in a dry human skull. Specifically, Lamb waves are generated in the upper parietal region of the skull by using short tonebursts, while their corresponding velocity fields are recorded by means of an infrared scanning laser Doppler vibrometer. The frequency-wavenumber spectra are obtained from the recorded wave fields using the Matrix Pencil Method. Finally, the different Lamb modes in the spectra are classified using a three-layered Semi-Analytical Finite Element model in which the thickness of the cortical and trabecular layers is assumed from CT-scan images of the scanned area. The obtained results indicate that the elastic properties of the brain can be potentially assessed by totally non-invasive procedure. In addition, they open up to the possibility to penetrate inaccessible regions of the brain for imaging purposes by exploiting their intrinsic radiation properties.