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A technique is described for discriminating blood-oxygen-level-dependent (BOLD) signal changes originating from large venous vessels and those that arise from the cortical parenchyma based on examining the temporal delay of each pixel's response. Photic stimulation experiments were performed with a conventional 1.5 T scanner and correlated each pixel's time-course with sine and cosine functions at the frequency of the stimulus. It was found that the signal in pixels anatomically associated 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 for the largest vessels, which are attributed to bulk susceptibility shifts.
Lee et al. (Thu,) studied this question.
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