Transcranial functional ultrasound imaging shows widespread hemodynamic changes induced by focused ultrasound (FUS)

Focused ultrasound (FUS) is an emerging non-invasive technique to induce a variety of biological effects in the central nervous system (CNS). Functional ultrasound imaging (fUSI) leverages ultrafast Power Doppler Imaging (PDI) to detect changes in cerebral blood volume (CBV) and thus holds promise to monitor brain responses to FUS. An integrated, non-invasive, ultrasound-based system for transcranial mapping of FUS responses could help elucidate the physiological effects of FUS in the brain and provide both feedback and understanding of the underlying mechanism. Here, we aim to demonstrate the feasibility of transcranial fUSI to characterize the immediate and short-term hemodynamic effects of transcranial FUS stimulation in the mouse brain. We used a co-aligned FUS-fUSI system to apply FUS and transcranially monitor hemodynamic changes during and immediately after stimulation in lightly anesthetized mice. We investigated the effects of varying intensities and transducer positions on the hemodynamic responses. We found widespread hemodynamic responses not confined to the focal area and show that higher FUS pressures increase the size of the brain area with a significant hemodynamic response, as well as the magnitude of change in CBV. Sham sonications, as well as FUS with lower pressures did not induce hemodynamic responses and unilateral sonications resulted in bilateral hemodynamic changes with a significantly stronger response on the ipsilateral side. FUS stimulation in mice with a cranial window showed distinct activation patterns that were frequency-dependent and different from the activation patterns observed in the transcranial model. Specifically, the widespread cortical response observed transcranially was substantially reduced in the window model, indicating that skull-mediated thermal effects are a primary driver of the cortical hemodynamic changes observed in the transcranial condition. In conclusion, fUSI was shown capable of transcranially monitoring online and short-term hemodynamic effects in the brain during and following FUS stimulation.