REPRODUCIBILITY AND POSTPROCESSING OF GRADIENT-ECHO FUNCTIONAL MRI TOIMPROVE LOCALIZATION OF BRAIN ACTIVITY IN THE HUMAN VISUAL-CORTEX

High reproducibility of human FMRI studies is imperative for potential clinical applications of this new method for mapping human brain func tions, So far, published data are not comparable quantitatively (even at the same field strength) as differences in sequence design and para meters as well as statistical methods applied to enhance function rela ted image contrast, in particular, to extract the size of the ''activa ted areas,'' are manifold, We present a study on reproducibility of gr adient-echo FMRI in the human visual cortex using thee different thres hold strategies for correlation analysis that shows that, (a) applying adaptive correlation thresholds results in higher reproducibility com pared to a fixed (0.5) threshold; (b) highly reproducible data can be obtained on a clinical 1.5 T MRI system, at least for repeated single subject studies (i.e., standard deviation of 2-30% for signal enhancem ent in 72-94% of the studies and 5-50% for activated area size in 63-8 8% of the studies, respectively, depending on threshold strategies); h owever, depending also on subject cooperation; (c) reproducibility acr oss groups (alpha=const.) is worse, i.e., standard deviations are with in 33-45% for signal enhancement and 41-74% for activated area size, r espectively; (d) SNR is maximum at about 30 degrees flip angle, sugges ting significant contributions from T-1-effects for larger flip angles . Various technical, methodological, and physiological factors are inf luencing variability of signal enhancement and apparently activated ar ea size, which should be taken into account if interpreting FMRI data quantitatively.