Functional micro-ultrasound imaging of rodent cerebral hemodynamics

Healthy cerebral microcirculation is crucial to neuronal functioning. We present a new method to investigate microvascular hemodynamics in living rodent brain through a focal cranial window based on high-frequency ultrasound imaging. The method has a temporal resolution of 40 ms, and a 100 μm in-plane and 600 μm through-plane spatial resolution. We use a commercially available high-frequency ultrasound imaging system to quantify changes in the relative cerebral blood volume (CBV) by measuring the scattered signal intensity from an ultrasound contrast agent circulating in the vasculature. Generalized linear model analysis is then used to produce effect size and significance maps of changes in cerebral blood volume upon electrical stimulation of the forepaw. We observe larger CBV increases in the forelimb representation of the primary somatosensory cortex than in the deep gray matter with stimuli as short as 2 s (5.1 ± 1.3% vs. 3.3 ± 0.6%). We also investigate the temporal evolution of the blood volume changes in cortical and subcortical gray matter, pial vessels and subcortical major vessels, and show shorter response onset times in the parenchymal regions than in the neighboring large vessels (1.6 ± 1.0 s vs. 2.6 ± 1.3 s in the cortex for a 10 second stimulus protocol). This method, which we termed functional micro-ultrasound imaging or fMUS, is a novel, highly accessible, and cost-effective way of imaging rodent brain microvascular topology and hemodynamics in vivo at 100 micron resolution over a 1-by-1 cm field of view with 10s–100s frames per second that opens up a new set of questions regarding brain function in preclinical models of health and disease.

Research highlights► Functional micro-ultrasound (fMUS) is introduced as a new brain imaging modality. ► fMUS shows activation patterns in the rat cortex and deep gray upon stimulation. ► fMUS allows for quantification of the temporal evolution of the CBV response. ► Resolution: approx. 100 μm in-plane, down to 600 μm through-plane, 40 ms temporal.