MR-IMAGING OF CEREBRAL ACTIVATION PERFORMED WITH A GRADIENT-ECHO TECHNIQUE AT 1.5 T - SOURCES OF ACTIVATION SIGNALS

OBJECTIVE. MR imaging of cerebral activation has been successfully per formed at 1.5 T for functional maps of the brain. However, major sourc es of activation signals in such imaging remain controversial. The pur pose of this study is to investigate anatomic and physiologic sources of activation signals in MR imaging of cerebral activation performed w ith a gradient-echo technique at 1.5 T. SUBJECTS AND METHODS. Motor co rtex activation studies (n = 8) were conducted using a gradient-echo t echnique (80/60 [TR/TE], 40 degrees flip angle). MR venograms were the n obtained at the same imaging plane to visualize the cortical veins, which were then compared with the shape and location of the activation signals. To investigate the physiologic sources of activation signal, the activation studies were repeated with different TEs (15, 30, and 60 msec), which allowed us to evaluate the blood oxygen level-dependen t effect; with different flip angles (40 degrees and 10 degrees); and without and with presaturation of adjacent sections, all of which allo wed us to evaluate inflow effect RESULTS, All activation signals were detected in the sulcus just posterior (n = 7) or lateral (n = 1) to th e motor cortex. In seven of eight studies, shape and location of these signals corresponded well with those of the cortical veins. In the ei ghth study, the correspondence was partial. Activation signals signifi cantly increased at a TE of 60 msec (p < .01), suggesting enhancement of the blood oxygen level-dependent effect at a long TE. Activation si gnals significantly decreased with a 10 degrees flip angle (p < .01) a nd with presaturation of adjacent sections (p < .01), indicating that the inflow effect was suppressed by a small flip angle and the elimina tion of unsaturated inflowing protons. CONCLUSION. Our results suggest that signals in cerebral activation obtained by MR imaging with a gra dient-echo technique at 1.5 T arise mainly from the cortical veins dra ining the activated cortex. Physiologically, both blood oxygen level-d ependent and inflow effects contribute to signal generation.