Inflow effects on hemodynamic responses characterized by event-related fMRI using gradient-echo EPI sequences

The purpose of this study is to determine whether blood inflow impacts the temporal behavior of BOLD-contrast fMRI signal changes in a typical event-related paradigm. The inflow contributions in the hemodynamic response to repeated single trials of short visual stimulation were assessed with a gradient-echo EPI sequence by altering the flip angle (FA) from 30°to 90°at a repetition time of 1 s. For each FA condition (30°, 60°, and 90°), 30 trials were performed on 15 healthy volunteers on a 3T MRI scanner. Comparing the percent BOLD contrast, prominent inflow effects were found with statistical significance between the 90°- and 30°-FA conditions (0.73±0.15 versus 0.67±0.12%, p=0.028). BOLD responses with FA=30°exhibited latencies significantly slower than those with FA=90°(3.69±0.39 s versus 3.37±0.28 s, p=0.001). The falling time of the 30°-FA responses was earlier but not statistically different from that of the 90°-FA (8.17±1.04 s versus 8.03±1.15 s, p=0.3). Using a voxelwise analysis, the latency variations of the activated visual areas were determined at several contrast-to-noise ratio (CNR) levels (controlled by averaging different numbers of randomly selected trials). The latency variations from the 90°-FA responses were greater at lower CNR but similar at higher CNR levels when comparing to the 30°-FA ones. This study suggests that inflow effects contribute to the BOLD signal, resulting in hemodynamic response with shorter latency.