Arterial blood contrast (ABC) enabled by magnetization transfer (MT): a novel MRI technique for enhancing the measurement of brain activation changes

Abstract

<jats:title>Abstract</jats:title><jats:p>Functional brain imaging in humans is almost exclusively performed using blood oxygenation level dependent (BOLD) contrast. This typically requires a period of tens of milliseconds after excitation of the spin system to achieve maximum contrast, leading to inefficient use of acquisition time, reduced image quality, and inhomogeneous sensitivity throughout the cortex. We utilise magnetisation transfer to suppress the signal differentially from grey matter relative to blood so that the local increase in blood volume associated with brain activation (mainly occurring in the arterioles and capillaries) will increase the measured signal. Arterial blood contrast (ABC) is additive to the residual BOLD effect, but will have its maximum value at the time of excitation. We measured brain activation using combined ABC and residual BOLD contrast at different times post-excitation and compared this to BOLD data acquired under otherwise identical conditions. We conclude that using ABC and measuring shortly after excitation gives comparable sensitivity to standard BOLD but will provide greater efficiency, spatial specificity, improved image quality, and lower inter-subject variability. ABC offers new perspectives for performing functional MRI.</jats:p>

Quantitative MRI maps of human neocortex explored using cell type-specific gene expression analysis

“…One possible explanation is that the macromolecules in the endothelial cells could give rise to an observable MT effect. Another explanation could be a mechanism of magnetization transfer studied in the context of functional MRI (Kim et al 2008, Pike et al 1992, Schulz et al 2020. In short, off-resonance MT-saturation pulses can efficiently saturate the water spins in cortical tissue, but not those in blood.…”

Combining arterial blood contrast with BOLD increases fMRI intracortical contrast

“…Intriguingly, we also show that the simple combination of saturation with a T 2 *‐weighted readout modulates the BOLD signal with minimal acquisition cost beyond a SAR increase and a slight SNR reduction. This adds an extra degree of freedom in fMRI signal optimization: the TE and tissue/venous suppression can be jointly optimized to, for example, increase contrast‐to‐noise and specificity in shorter TEs (Pfaffenrot & Koopmans, 2022 ; Schulz et al, 2020 ), thus increasing SNR and acquisition efficiency (a natural fit with spiral imaging; Kurban et al, 2022 ) or reduce the venous bias. In our current implementation and without the additional readout to negate BOLD contrast and thus isolate ABC, there is a sizable pial‐surface bias that renders the examination of cortical depth profiles challenging, similar to typical BOLD.…”

“…On‐resonance rectangular pulse‐trains with 0‐net flip‐angle can efficiently saturate a selected bandwidth of T 2 values in relation to the B 1 amplitude (Forster et al, 1995 ; Hu et al, 1992 ; van Gelderen et al, 2017 ). It was empirically found that at 95% SAR at 7 T, a pulse‐train consisting of seven phase‐modulated, nonslice‐selective, rectangular subpulses with B 1 = 10 μT (total duration = 6 ms; Priovoulos, Oliveira, et al, 2021 ; Schulz et al, 2020 ) could be repeated every 387 ms, thus allowing time for the readout of a 3D‐EPI segment.…”

“…Such approaches can provide an arterial‐based signal (Kim et al, 2008 ) with increased intracortical specificity due to the arteriole and capillary dilation, as shown in cats (Kim & Kim, 2010 ). Recently, an efficient saturation‐based arterial CBV‐weighted method was introduced at 3 T, relying on a short echo time to reduce BOLD sensitivity (Pfaffenrot & Koopmans, 2022 ; Schulz et al, 2020 ). The venous signal was further reduced through the downstream exchange of saturated water from tissue to the veins, therefore resulting in increased sensitivity to the more spatially‐specific arterial and capillary CBV change (arterial blood contrast; ABC).…”

“…The venous signal was further reduced through the downstream exchange of saturated water from tissue to the veins, therefore resulting in increased sensitivity to the more spatially‐specific arterial and capillary CBV change (arterial blood contrast; ABC). The reduced sensitivity to the (negative) tissue CBV change and the additive effect of residual BOLD resulted in comparable sensitivity to standard BOLD methods (Schulz et al, 2020 ). This saturation‐based approach requires a short TE and results in increased SAR deposition, which make application at UHF and at high‐resolution challenging.…”