Shu Christina Y, Herman Peter, Coman Daniel, Sanganahalli Basavaraju G, Wang Helen, Juchem Christoph, Rothman Douglas L, de Graaf Robin A, Hyder Fahmeed
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.
Neuroimage. 2016 Jan 15;125:848-856. doi: 10.1016/j.neuroimage.2015.10.083. Epub 2015 Oct 31.
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.
校准功能磁共振成像（fMRI）基于血氧水平依赖（BOLD）反应的血液动力学和代谢依赖性，提取神经活动所需的氧化能量变化。此过程需要参数M，它由脱氧血红蛋白（顺磁性）和氧合血红蛋白（抗磁性）之间BOLD信号的动态范围确定。由于尚不清楚人类校准fMRI中M值的范围是否归因于区域/状态差异，我们进行了一项9.4T的研究，以测量深度（α-氯醛糖）和轻度（美托咪定）麻醉大鼠脑区的M值，并通过电生理学进行了验证。因为BOLD信号通过梯度回波（R2*）和自旋回波（R2）弛豫率被差异性捕获，我们通过fMRI回波时间与R2'（即可逆磁化率分量，由R2*与R2之间的绝对差值给出）的乘积来测量M值。虽然R2'映射显示依赖于所使用的k空间采样方法，但在9.4T高磁场下达到的标称空间分辨率下，皮质灰质中的M值相当均匀。然而，皮质M值在不同脑状态之间随神经活动而变化。这项研究的结果可以通过关注不同静息活动水平下灰质中M值的全局均匀性，提高未来校准fMRI研究的精度。
