Quantitative assessment of blood inflow effects in functional MRI signals.

Functional MRI (fMRI) signal dependence on changes in blood flow velocities were analyzed for both conventional and echo-planar (EPI) gradient-echo pulse sequences. As the flow velocity increases, the fMRI signal increases monotonically in spoiled gradient-echo sequences, while the fMRI signal may increase or decrease in conventional refocused gradient-echo sequences. A larger flip angle generates a larger inflow contribution to the fMRI signal. For conventional gradient-echo sequences, the inflow contribution to the fMRI images is dominated by the cortical draining veins, while its effect on capillaries is generally small and may be negligible in the spoiled sequences. For EPI gradient-echo sequences, the contribution from inflow effects is relatively small, as compared with the blood oxygen level-dependent (BOLD) contribution, to the fMRI signal, not only for capillaries but also for the cortical draining veins.