The lag of cerebral hemodynamics with rapidly alternating periodic stimulation: Modeling for functional MRI

A mathematical model that characterizes the response of venous oxygenation to changes in cerebral blood flow (rCBF) and oxygen consumption has been pr eviously presented. We use this model to examine the dampening phenomenon i n functional MRI (fMRI) signals with rapidly alternating periodic stimulati on bursts. Using a mass balance approach, the equations for an input-output model are derived and solved using Matlab (the Math Works Inc.). Changes i n venous oxygenation are related to the results of fMRI experiments using p rogressively shorter periods of stimulation. An impulse-response function f or the model is derived in an attempt to explore the source of the lag in c erebral hemodynamics, Increasing the frequency of stimulation bursts eventu ally produces a dampening in the fMRI signal. The dampening phenomenon in f MRI signals occurs with stimulation of high frequency on-off alternation. T he dynamics of signal dampening, as well as the impulse-response function o f a blood oxygen level-dependent model, lend strong indirect support to the hypothesis that blood oxygen level-dependent contrast at the level of the venous blood pool, rather than RI inflow effects or changes in oxygenation at the level of the capillary bed, underlies the observed signal changes in fMRI. (C) 1998 Elsevier Science Inc.