Origin of the signal undershoot in BOLD studies of the visual cortex.
The nature of the signal undershoot observed in the inter-stimulation intervals in fMRI studies using a block paradigm consisting of alternating periods of visual stimulation and rest was investigated using the following single shot EPI sequences: gradient echo (GRE), spin echo (SE), spin echo with additional flow dephasing gradients and inversion recovery (IR) prepared SE. Both the GRE and SE sequences showed a significant signal undershoot during the inter-stimulation intervals. DeltaR(2)*/DeltaR(2) ratios of 3.7 +/- 0.9 and 3.1 +/- 0.7 were measured in the stimulation and inter-stimulation periods, respectively, with the latter being lower than that which would be consistent with a pure extra-vascular effect arising from an elevated venous blood volume and a normal deoxyhaemoglobin content post-stimulation. The addition of dephasing gradients to the SE sequence in order to attenuate the signal from spins flowing within larger vessels produced a four-fold reduction in the number of activated pixels but had little effect on the time intensity profile. Our interpretation of these results is that both extra- and intra-vascular BOLD effects are present in the inter-stimulation intervals and the lack of any effect of the dephasing gradient on the time-intensity profile indicates that the intra-vascular component probably occurs mainly in smaller vessels, such as venules, which are affected relatively little by the relatively weak dephasing gradient (b = 29 s/mm(2)) used in this study. For the IR-SE sequence the DeltaR(2) measured during the inter-stimulation intervals was similar to that seen with the SE BOLD sequence and thus was consistent with a residual BOLD effect, implying that perfusion changes in the capillary vessels did not contribute significantly to the signal undershoot.
Jones
R A
RA
Max Planck Institute für Psychiatrie, Kraepelinstr. 10, D-80804 München, Germany. jones@rmsb.u-bordeaux2.fr
D000328:Adult; D002560:Cerebrovascular Circulation / Q000502:physiology; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010100:Oxygen / Q000097:blood*; D010775:Photic Stimulation; D012815:Signal Processing, Computer-Assisted; D014793:Visual Cortex / Q000033:anatomy & histology / Q000098:blood supply*
D010100:Oxygen