Temporal characteristics of oxygenation‐sensitive MRI responses to visual activation in humans

Abstract

Series of single-shot blipped echo-planar images with spin-density weighting and T2* sensitivity (2.0 T, TR = 400 ms, TE = 54 ms, flip angle = 30 degrees) were used to study the temporal response profiles to repetitive visual activation (5 Hz, reversing black and white checkerboard versus darkness) for protocols comprising multiple cycles of a 1.6-s stimulus in conjunction with a 8.4-s or 90-s recovery phase and a 10-s stimulus with a 20-s or 90-s recovery phase. Analysis of the real-time data from all activated pixels resulted in a strong positive MRI response (mean values 3-6%) as well as a marked poststimulus undershoot (mean values 1-2%, duration 60-90 s) for all paradigms. Repetitive protocols with insufficient recovery periods of 8.4 s or 20 s gave rise to a wraparound effect when analyzing time-locked averages from multiple activation cycles. This problem may lead to an early signal decrease that originates from the ongoing undershoot of preceding activations folded back into the initial latency phase of a subsequent activation. When ensuring complete decoupling of responses to successive stimuli by using a 90-s recovery period, the wraparound effect vanished and an initial dip was observed in one of seven subjects for a 10-s/90-s protocol.

Coupling between neuronal activity and microcirculation: Implications for functional brain imaging

“…Some reports, mainly from studies performed at high magnetic fields, confirmed the initial dip (Ernst and Hennig, 1994;Menon et al, 1995;Logothetis et al, 1999;Yacoub and Hu, 1999;. Those confirmations, however, remained somewhat isolated because the initial dip, whose amplitude grows more rapidly with the field strength than the positive BOLD response (Yacoub and Hu, 1999), was missed in most of the low-field studies (Fransson et al, 1998;Mandeville et al, 1999a;Marota 1999, but see Yacoub and Hu, 1999) and even in some high-field experiments (Silva et al, 2000); in any case, when investigated with BOLD fMRI, the initial dip appeared to be somewhat elusive (Logothetis, 2000versus Kim, 2000. In addition, doubts were raised with respect to the accuracy of the [Hbr] and the oxyhaemoglobin concentration ͓͑HbO 2 ͔͒ time courses calculated by Malonek and Grinvald in their 1996 paper, since those had been obtained with a classical Beer-Lambert spectroscopic model, which approximates the optical path as constant rather than taking its wavelength dependency into account (Cope et al, 1991;Arridge et al, 1992).…”

Nonlinear temporal dynamics of the cerebral blood flow response

“…In this transient effect, after the end of stimulation, the BOLD signal drops from the activated state to below the steadystate resting baseline, and then slowly rises back to baseline. If sufficiently long rest periods are not allowed for the response to return to baseline before subsequent stimulation, the poststimulus undershoot is known to cause an apparent shift in baseline [Fransson et al, 1998]. Because the primary focus of this Figure 10.…”

Blood Oxygenation Level-Dependent Visualization of Synaptic Relay Stations of Sensory Pathways along the Neuroaxis in Response to Graded Sensory Stimulation of a Limb

“…Functional magnetic resonance imaging (fMRI) is widely used to investigate brain function during different motor tasks and various sensory stimulations in humans (Fransson et al, 1998(Fransson et al, , 1999Hofbauer et al, 2004Hofbauer et al, , 2006Hwang et al, 2005;Olausson et al, 2005;Seung et al, 2005;Elfgren et al, 2006;Pastor et al, 2006). The most commonly used technique relies on blood oxygenation level-dependent (BOLD) contrast, which is indirectly related to neuronal activity (Ogawa et al, 1993;Heeger and Ress, 2002;Thompson et al, 2003;Mukamel et al, 2005;Niessing et al, 2005).…”