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 f MRI 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.