DISCRIMINATION OF LARGE VENOUS VESSELS IN TIME-COURSE SPIRAL BLOOD-OXYGEN-LEVEL-DEPENDENT MAGNETIC-RESONANCE FUNCTIONAL NEUROIMAGING

A technique is described for discriminating blood-oxygen-level-depende nt (BOLD) signal changes originating from large venous vessels and tho se that arise from the cortical parenchyma based on examining the temp oral delay of each pixel's response, Photic stimulation experiments we re performed with a conventional 1.5 T scanner and correlated each pix el's time-course with sine and cosine functions at the frequency of th e stimulus. It was found that the signal in pixels anatomically associ ated with gray matter was delayed between 4 and 8 s compared with the stimulus, whereas the signal in pixels correlated with visible vessels and sulci was generally delayed from 8 to 14 s, This larger delay is consistent with the longer time required for blood to reach the larger vessels. Finally, stimulus-induced NMR phase changes were observed fo r the largest vessels, which are attributed to bulk susceptibility shi fts.