Abstract
Recent study had suggested diffusion-weighted (DW) gradient-echo (GE) pulse sequence null the majority of fMRI signal arise from intravascular(IV) at 1.5 T and retains water spins with lower ADC close to tissue water. Moreover, some groups indicate that extravascular (EV) component of large vessels contribute only to GE fMRI, not to spin echo (SE) fMRI, because the 180°RF pulse in SE fMRI can refocus the dephasing effect of the static field inhomogeneity around large vessels. Since blood flow occur earlier in capillaries and small venules near the activation site and gradually extend to larger draining vein, to improve temporal specify and reduce both the IV and EV signal contribution from large vessels distant to neuronal activity, this study apply bipolar diffusion gradients into SE pulse sequence only in slice selection direction at 1.5 T in response to brief visual stimulus and compare onset time variance at the same CNR to that detected in conventional GE and non-diffusion-weighted SE. The results show onset variance is smaller in DW SE than that of GE at the same CNR but no difference between DW SE b value of 50 s mm-2 and DW SE b value of 200 s mm-2. This illustrate DW SE signal is purified and b value 50 s mm-2 is sufficient to suppress contaminated signal from large vessels. Further, onset time of DW SE with b values of 200 s mm-2 precedes that of GE (2.23±0.11 vs. 2.92±0.08, p<0.005) and time to peak (TTP) of DW SE with b values of 200 s mm-2 is also earlier than that of GE (4.62 ±0.07 vs. 4.97 ±0.19, p<0.05). Meanwhile, to take the direction of applied diffusion gradients into consideration, this study also simultaneously incorporated 3 axes bipolar diffusion gradients into SE sequence and make the same comparison as previous did. The results show earlier onset time, time to peak, and smaller onset time variance in DW SE, as previous. This is consistent with what we expected because only in voxels with strong anisotropy of flow does the gradient direction become important.