Investigating the spatial specificity of S2-SSFP fMRI: A Monte Carlo simulation approach

The desirable spatial specificity of spin echo (SE) fMRI cannot be efficiently utilized in high fields due to specific absorption rate (SAR) and B1 inhomogeneity problems. Consequently, S2-SSFP fMRI has been suggested as an alternative to mitigate these problems. Nevertheless, no accurate analysis has been performed thus far to evaluate spatial specificity of this technique. To study spatial specificity, we performed Monte Carlo simulations for evaluating the micro-vasculature contribution in functional contrast along with vessel size sensitivity estimations for a range of relevant imaging parameters. Results showed a spatial specificity at the level of SE fMRI. Simulations further revealed that similar to SE fMRI, an effective echo time (TE) close to the tissue T₂ maximizes the micro-vasculature contribution in the obtained contrast. The amount of this contribution, however, showed a slight decrease at ultra-high fields compared to SE fMRI. As for vessel size sensitivity, simulations presented a pattern for S2-SSFP similar to SE fMRI but with a minor shift toward larger vessels. These results are in general agreement with reported experimental studies. Our findings also suggest that the effect of older pathways, rather than primary SE pathway, might be responsible for the observed discrepancies between S2 and SE. Based on this study, provided that optimum experimental parameters are used, S2, with its desirable micro-vasculature contribution and high sensitivity to small vessels, is a promising low SAR approach to replace SE fMRI in high field.