Quantitative NumART2* mapping in functional MRI studies at 1.5 T.

Quantitative mapping of the effective transverse relaxation time, T2* and proton density was performed in a motor activation functional MRI (fMRI) study using multi-echo, echo planar imaging (EPI) and NumART2* (Numerical Algorithm for Real time T2*). Comparisons between NumART2* and conventional single echo EPI with an echo time of 64 ms were performed for five healthy participants examined twice. Simulations were also performed to address specific issues associated with the two techniques, such as echo time-dependent signal variation. While the single echo contrast varied with the baseline T2* value, relative changes in T2* remained unaffected. Statistical analysis of the T2* maps yielded fMRI activation patterns with an improved statistical detection relative to conventional EPI but with less activated voxels, suggesting that NumART2* has superior spatial specificity. Two effects, inflow and dephasing, that may explain this finding were investigated. Particularly, a statistically significant increase in proton density was found in a brain area that was detected as activated by conventional EPI but not by NumART2* while no such changes were observed in brain areas that showed stimulus correlated signal changes on T2* maps.