Feasibility of Functional MRI at Ultralow Magnetic Field via Changes in Cerebral Blood Volume

Up to now, no successful functional MRI was performed at ultralow magnetic fields (ULF). Therefore we tried to estimate the feasibility of such experiments with a simplified brain model. For a highly optimized ULF system we predict that functional MRI with a contrast to noise ratio of up to ≈ 7 is feasible with a measurement time of 30 minutes.

(a) Schematic cross-section of a brain with the second-order gradiometer coupled to a SQUID in a fiberglass dewar (green cylinder) situated above. The orange disk indicates the region of interest, comprising the hand region of primary motor cortex. (b) Functional contrast-to-noise ratios. CNRf(ΔGM, 30 min) as a function of polarizing time Tp and waiting time after polarization Tw for a single prepolarizing pulse followed by a free induction decay (FID) sequence, for a 10% increase in CBV accommodated by a decrease in grey matter (GM) tissue volume. (c) CNRf(ΔCSF, 30 min) for a 10% increase in resting CBV accommodated by a decrease in cerebrospinal fluid (CSF) volume.

We investigate the feasibility of performing functional MRI (fMRI) at ultralow field (ULF) with a Superconducting QUantum Interference Device (SQUID), as used for detecting magnetoencephalography (MEG) signals from the human head. While there is negligible magnetic susceptibility variation to produce blood oxygenation level dependent (BOLD) contrast at ULF, changes in cerebral blood volume (CBV) may be a sensitive mechanism for fMRI given the five-fold spread in spin-lattice relaxation time (T1) values across the constituents of the human brain. We undertook simulations of functional signal strength for a simplified brain model involving activation of a primary cortical region in a manner consistent with a blocked task experiment. Our simulations involve measured values of T1 at ULF and experimental parameters for the performance of an upgraded ULFMRI scanner. Under ideal experimental conditions we predict a functional signal-to-noise ratio of between 3.1 and 7.1 for an imaging time of 30 min, or between 1.5 and 3.5 for a blocked task experiment lasting 7.5 min. Our simulations suggest it may be feasible to perform fMRI using a ULFMRI system designed to perform MRI and MEG in situ.

1.
Buckenmaier K, Pedersen A, SanGiorgio, Scheffler K, Clarke J, Inglis B.:
Feasibility of functional MRI at ultralow magnetic field via changes in cerebral blood volume.
Neuroimage 2019;186:185.
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