| Monte-Carlo simulations were used to investigate contrast in phase cycled balanced steady state free precession (b-SSFP) fMRI acquisitions. Additionally, the b-SSFP fMRI contrast mechanism and its dependence on vascular architecture was explored for randomly oriented microvasculature and large transport vessels. b-SSFP fMRI contrast is dominated by intra-vascular T 2 changes in the pass-band (pbSSFP) and susceptibility-induced field offsets in the transition-band (tbSSFP) at short TR values. Neurovascular coupling is better in pbSSFP than tbSSFP because of a significant extra-vascular diffusion contribution to fMRI contrast. Contrast is reduced from the steady state (SS) value in phase cycled acquisitions and steadily increases throughout the readout window, suggesting the use of late echoes in the readout train to encode the k-space center to maximize contrast. This contrast is further increased to 82% of the SS value by using a 500 ms inter-acquisition delay and a late TE readout, without seriously reducing neurovascular coupling. |
