Contrast-Free Transcranial Functional Ultrasound Neuroimaging

Transcranial functional ultrasound is a relatively new technology for assessing functional responses in the brain. So far, transcranial functional ultrasound without contrast agents has only been applied to human imaging during surgery with the skull removed or in imaging neonates. Here, we demonstrate the feasibility of transcranial functional ultrasound through the adult skull. To overcome the severe attenuation from the skull we use a previously developed a compound Barker coded excitation method. For clutter filtering, we use an adaptive demodulation motion correction method with singular value decomposition (SVD) filtering. As a demonstration, we assess cerebrovascular reactivity induced by a breath hold. We measured the power Doppler signal in the vasculature surrounding the midbrain in a healthy adult volunteer during a five minute task of alternating free breathing and breath holding periods. We showed that the power Doppler signal in the blood vessels was highly correlated to the breath hold task (ρ=0.53) and the oxygen saturation (ρ=0.61) as measured by finger pulse oximeter. We also observed a delayed vasodilatory response in the power Doppler signal that reflected the delayed drop in oxygen saturation from the breath hold. These results demonstrate that contrast-free transcranial functional imaging in adults is possible using coded excitation to increase SNR and blood flow sensitivity and with appropriate motion compensation and clutter filtering techniques.