The fractional signal intensity change (Delta S/S) observed during activation in T(2)-weighted fMRI of the spinal cord has previously been shown to depend linearly on the echo time (TE) but to have a positive value of roughly 2.5% extrapolated to zero TE. In this study we investigated the origin of this finding by measuring the Delta S/S in spinal fMRI with very short TEs. Our results demonstrate that the Delta S/S does not approach zero, but has a value as high as 3.3% at TE = 11 ms. At TEs > 33 ms we observed the linear relationship between Delta S/S and TE as in previous studies. These data demonstrate that there is a non-BOLD contribution to signal changes observed in spinal fMRI. We hypothesize that this contribution is a local proton density increase due to increased water exudation from capillaries with increased blood flow during neuronal activation, and term this effect "signal enhancement by extravascular protons" (SEEP).
Copyright 2002 Wiley-Liss, Inc.