Dynamic functional imaging of relative cerebral blood volume during rat forepaw stimulation

Dynamic measurements of regional changes in cerebral blood volume (CBV) were performed in rat models of hypercarbia and focal neuronal activation using T2-weighted imaging after injection of an intravascular contrast agent with a very long blood half-life. Calculated percent CBV change during hypercarbia was consistent with literature results from other non-invasive modalities. Equivalent percent CBV increases were found using spin- and gradient-echo images, suggesting proportional changes in blood volume for capillaries and small veins. During electrical stimulation of rat forepaw, focal CBV response to stimulation (24+/-4%) was significantly delayed relative to blood oxygen level dependent (BOLD) signal after both onset and cessation of stimulation. Poststimulus CBV decay was temporally consistent with the BOLD poststimulus undershoot. The use of exogenous agent increased the functional contrast-to-noise ratio relative to BOLD imaging by 5.7+/-1.3 at a magnetic field strength of 2 Tesla and 1.5+/-0.2 at 4.7 Tesla.