MODULATION OF SIGNAL CHANGES IN GRADIENT-RECALLED ECHO FUNCTIONAL MRIWITH INCREASING ECHO TIME CORRELATE WITH MODEL-CALCULATIONS

Based on systematic in vivo studies analysed by fuzzy clustering, we p rove the complex dependence of functional magnetic resonance imaging ( fMRI) signal changes on echo time (TE) (ranging from 42 ms up to 160 m s) in the human visual cortex at 1.5 Tesla, We obtain a steady increas e of signal enhancement with increasing TE, except at TE = 130 ms wher e the signal increase is significantly less than at 100 and 160 ms, re spectively, caused by signal dephasing of spins in the vascular enviro nment of the cortical region imaged, A comparison with model calculati ons reveals that vessels with a diameter of about 0.5 to 1 mm may be t he main source of gradient-recalled echo fMRI signal changes at 1.5 Te sla at the given spatial resolution, In addition to conventional corre lation analysis, fuzzy cluster analysis has been applied to evaluate f MRI data sets, Our results also indicate that, despite the similar tem poral pattern of the functional response, it is possible to differenti ate between areas which show higher signal enhancement (cluster 1, hig her blood volume fraction:, and lower signal enhancement (cluster 2, l ower blood volume fraction), reflecting the different vascular environ ment, Therefore, fuzzy cluster analysis may help to extract functional information from activated areas closer to the actual neuronal activa tion, (C) 1997 Elsevier Science Inc.