4 Tesla gradient recalled echo characteristics of photic stimulation‐induced signal changes in the human primary visual cortex

Multi‐echo measurements of photic stimulation‐induced signal changes in human visual cortex were made at 4 Tesla in order to quantify the nature of the signal change and its vascular origin, and to determine the optimum echo time for detection of the changes. Utilizing high resolution images, two distinct regions (ascribed to be microvasculature and visible venous vessels) were identified as giving rise to the signal increase. The fractional signal changes in gray matter areas depended linearly on echo time (TE) in the range of 10 to 60 ms and extrapolated to virtually zero for TE = 0, indicating that in‐flow effects secondary to stimulation‐induced blood flow increases were negligible in our functional imaging studies; instead, signal change due to photic stimulation originated from the increase in the apparent transverse relaxation rate, 1/T₂*. This decrease in (1/T₂*, brought about by the alterations in hemodynamic parameters, was 1.3 ± 0.4 s⁻¹ for gray matter and 3.0 ± 0.7 s⁻¹ (averaged over 10 individuals) for venous vessels visible in the images. The optimum choice of echo time was found to be TE ≥ T₂*.