BOLD background gradient contributions in diffusion‐weighted fMRI—comparison of spin‐echo and twice‐refocused spin‐echo sequences

Abstract

The interaction ('cross terms') between diffusion-weighting gradients and susceptibility-induced background gradient fields around vessels has an impact on apparent diffusion coefficient (ADC) measurements and diffusion-weighted functional magnetic resonance imaging (DFMRI) experiments. Monte-Carlo (MC) simulations numerically integrating the Bloch equations for a large number of random walks in a vascular model were used to investigate to what extent such interactions would influence the extravascular signal change as well as the ADC change observed in DFMRI experiments. The vascular model consists of a set of independent, randomly oriented, infinite cylinders whose internal magnetic susceptibility varies as the state changes between rest and activation. In such a network, the cross terms result in the observation of a functional increase in ADC accompanied by a descending percent signal change with increasing diffusion weighting. It is shown that the twice-refocused spin-echo sequence permits sufficient yet not total suppression of such effects compared to the standard Stejskal-Tanner spin-echo diffusion weighting under experimentally relevant conditions.

Quantification of iron in the non-human primate brain with diffusion-weighted magnetic resonance imaging

“…Interestingly, changes around the basal ganglia were readily observed on Δξ [Fe] maps, but much milder on ΔR2 maps. Although this effect is at the origin of the BOLD fMRI method, which exploits the sensitivity of gradient echo sequences (and to a lesser degree spin-echo sequences) to the resulting changes in R2* relaxivity, its contribution to the diffusion MRI signal is expected to be very small (Pampel et al, 2010), at least compared to the iron effects observed in this study. Indeed, during brain activation, water diffusion has been shown not to increase (as expected from a local field gradient model when the deoxyhemoglobin concentration decreases), but to decrease (Le Bihan et al, 2006;Tsurugizawa et al, 2013), the ADC increase observed at very low b values (Song et al, 1999) has a completely different origin, the increase in blood flow being captured through the Intra Voxel Incoherent Motion (IVIM) effect (Le Bihan et al, 1988).…”

High b -value diffusion-weighted fMRI in a rat forepaw electrostimulation model at 7 T

“…In that case, a conflict with the above single-compartment interpretation of the functional data would have to be acknowledged, although the behaviour of the signal change with b-value would still differ from that in previous high b-value DW-fMRI studies. On the other hand, the gradient-coupling simulations performed by Pampel et al (2010) suggest that the PSCR would decrease with increasing b-value and the ΔADC would be in the range 0.2-1.2%. Therefore, rather than implying the presence of a second signal source, another potential explanation for the behaviour is the coupling of the background gradients and the applied MPGs.…”

“…Recognising that gradient coupling may interfere with the desired signal behaviour, most previous high-b-value DW-fMRI studies have used a dual-SE sequence to minimise the potential influence of this mechanism (Pampel et al, 2010;Jin and Kim, 2008). In contrast, the data presented in this manuscript was intentionally acquired with a single-SE sequence so that the effects of coupling could be investigated with the more flexible rat forepaw electrostimulation model.…”

“…It is also recognised that the BOLD effect, which is related to the diffusion of water molecules through the field gradients within and surrounding blood vessels, may be perturbed by the applied MPGs so that a significant dependence on b-value may be observable. However, simulations performed by Pampel et al (2010) found that superposition of applied MPGs and blood-vessel-induced field gradients would most likely reduce, rather than increase, the fractional signal change with increasing b-value. It was therefore concluded that this mechanism is unlikely to be responsible for the DW-fMRI signal increases observed at high b-value, but may instead obscure other sources of signal change.…”

“…Another factor that has been mentioned as a possible source of DW-fMRI signal changes is the so-called "coupling" of the applied MPGs and internal field inhomogeneities (Jin and Kim, 2008;Miller et al, 2007;Yacoub et al, 2008;Pampel et al, 2010). It has long been known that the presence of subvoxel susceptibility variations can affect diffusion measurements (Stesjkal and Tanner, 1965;Zhong et al, 1991Zhong et al, , 1998.…”