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Neuroimage. 2016 Jan 15;125:848-56. doi: 10.1016/j.neuroimage.2015.10.083. Epub 2015 Oct 31.

Brain region and activity-dependent properties of M for calibrated fMRI.

Author information

1
Department of Biomedical Engineering, Yale University, New Haven, CT, USA. Electronic address: christina.shu@yale.edu.
2
Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
3
Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA; Department of Neurology, Yale University, New Haven, CT, USA.
4
Department of Biomedical Engineering, Yale University, New Haven, CT, USA; Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA.
5
Department of Biomedical Engineering, Yale University, New Haven, CT, USA; Department of Radiology and Biomedical Imaging and Magnetic Resonance Research Center, Yale University, New Haven, CT, USA. Electronic address: fahmeed.hyder@yale.edu.

Abstract

Calibrated fMRI extracts changes in oxidative energy demanded by neural activity based on hemodynamic and metabolic dependencies of the blood oxygenation level-dependent (BOLD) response. This procedure requires the parameter M, which is determined from the dynamic range of the BOLD signal between deoxyhemoglobin (paramagnetic) and oxyhemoglobin (diamagnetic). Since it is unclear if the range of M-values in human calibrated fMRI is due to regional/state differences, we conducted a 9.4T study to measure M-values across brain regions in deep (α-chloralose) and light (medetomidine) anesthetized rats, as verified by electrophysiology. Because BOLD signal is captured differentially by gradient-echo (R2*) and spin-echo (R2) relaxation rates, we measured M-values by the product of the fMRI echo time and R2' (i.e., the reversible magnetic susceptibility component), which is given by the absolute difference between R2* and R2. While R2' mapping was shown to be dependent on the k-space sampling method used, at nominal spatial resolutions achieved at high magnetic field of 9.4T the M-values were quite homogenous across cortical gray matter. However cortical M-values varied in relation to neural activity between brain states. The findings from this study could improve precision of future calibrated fMRI studies by focusing on the global uniformity of M-values in gray matter across different resting activity levels.

KEYWORDS:

Blood flow; Blood volume; Glutamate; Lactate; Neurometabolic coupling; Neurovascular coupling

PMID:
26529646
PMCID:
PMC4691415
DOI:
10.1016/j.neuroimage.2015.10.083
[Indexed for MEDLINE]
Free PMC Article
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