Concurrent fNIRS and fMRI processing allows independent visualization of the propagation of pressure waves and bulk blood flow in the cerebral vasculature.

Blood Oxygen Level Dependent (BOLD) functional magnetic resonance imaging (fMRI) measures changes in blood oxygenation, which is affected by physiological processes, including cardiac pulsation, breathing, and low frequency oscillations (LFO). It is challenging to identify spatial and temporal effects of these processes on the BOLD signal because the low sampling rate of BOLD leads to aliasing of higher frequency physiological signal components. In this study, we used concurrent functional near infrared spectroscopy (fNIRS) and fMRI on 6 subjects during a resting state scan. To reduce aliasing, the BOLD fMRI acquisition was repeatedly performed on a set of sequentially acquired slice stacks to lower the TR to 0.5s while retaining high spatial resolution. Regressor interpolation at progressive time delays (RIPTiDe) method was used, in which physiological signal acquired by fNIRS (without aliasing) and its temporal shifts were used as regressors in the fMRI analysis to determine the magnitude and timing of the effects of various physiological processes on the BOLD signal. The details of the timing of the passage of the cardiac pulsation wave and of the cerebral blood itself were mapped. The result suggests that the cardiac signal affects the voxels near large blood vessels (arteries and veins) most strongly, while LFO mostly affected the drainage veins. We hypothesize that this could be the result of differences in the cerebral blood path lengths, and differences in the dynamics of the propagation of the signals. Together these results validate and extend a novel imaging technique to dynamically track the pulse-wave and bulk blood flow with concurrent fMRI and fNIRS.