Characterization of contrast changes in functional MRI of the human

Contrast changes observed in functional magnetic resonance imaging in the h uman spinal cord were investigated with both motor and sensory tasks over a range of echo times. Data were acquired using a single-shot fast spin-echo sequence at 1.5 Tesla. Data were analyzed with two different correlation t hresholds and the effects of altering the order of repeated experiments was also investigated. Plots of the fractional signal change as a function of echo time yielded linear functions with slopes corresponding to relaxation rate changes of -0.30 sec(-1) with sensory stimulation and approximately -0 .50 sec(-1) with a motor task. However, the fractional signal change extrap olated to an echo time of zero was significantly greater than zero in each case and was roughly 2.5%. This suggests that in addition to the BOLD effec t there is a baseline signal change which occurs concomitant to neuronal ac tivation in the spinal cord. (C) 2001 Elsevier Science Inc. All rights rese rved.