Shear-mode Transcranial Imaging

Ultrasound brain imaging holds the potential to provide a low cost, and portable method for imaging blood flow, detecting hemorrhaging, and diagnosing certain brain disorders. However, distortion and low signal to noise ratios (SNR) caused by the skull have severely limited the use of existing clinical devices such as transcranial Doppler sonography (TCD) and transcranial color coded sonography (TCCS). Signal degradation is caused by reflection, refraction, attenuation, and scattering by the skull. Our recent work, however, indicates that under certain conditions it is possible to propagate ultrasound through the skull with reduced distortion and higher signal amplitudes by using high incident angles. Both numeric and experimental investigation suggest this is due to the behavior of shear modes induced in the skull bone. When the ultrasound angle of entry is beyond Snell's critical angle for the longitudinal pressure wave, propagation in the bone is purely due to a shear wave. This wave then converts back to a longitudinal acoustic wave in the brain. This conversion from a longitudinal wave (skin) to a shear wave (skull) and again to a longitudinal wave (brain) does not necessarily produce a highly distorted or small-amplitude wave. Preliminary data shows that a signal obtained through the skull at high angles may be less distorted than a longitudinal one. This proposal investigates the idea that substantial improvement of transcranial ultrasound imaging can be achieved by propagating through the skull as a shear wave as opposed to a longitudinal acoustic mode. Simultaneously, a multi-cycle coded excitation sequence, devised in the present work, will significantly increase overall signal strength. The resulting images are expected to experience reduced distortion and increased an SNR, allowing clearer and more accurate brain images. The investigation will test the application of the transcranial shear mode to a number of imaging problems including the vessel detection, tumor detection, tissue morphology, and hemorrhaging in the brain. This study will provide new data on physical properties of human skulls, and will include a valuable assessment of the ability of ultrasound to detect features in the brain. The high-risk proposal could provide considerable benefits to clinical diagnostics of the brain. It could potentially offer a non-ionizing imaging method that could in operate clinically, while introducing a new technique into medical and biological imaging.