Department of Neuroscience, Baylor College of Medicine, Houston, Texas.
GE Healthcare, Waukesha, Wisconsin.
Magn Reson Med. 2020 Feb;83(2):681-694. doi: 10.1002/mrm.27941. Epub 2019 Aug 18.
Functional MRI contrast has generally been associated with changes in transverse relaxivity caused by blood oxygen concentration, the so-called blood oxygen level dependent contrast. However, this interpretation of fMRI contrast has been called into question by several recent experiments at high spatial resolution. Experiments were conducted to examine contrast dependencies that cannot be explained only by differences in relaxivity in a single-spin population.
Measurements of functional signal and contrast were obtained in human early visual cortex during a high-contrast visual stimulation over a large range of TEs and for several flip angles. Small voxels (1.5 mm) were used to restrict the measurements to cortical gray matter in early visual areas identified using retinotopic mapping procedures.
Measurements were consistent with models that include 2 spin populations. The dominant population has a relatively short transverse lifetime that is strongly modulated by activation. However, functional contrast is also affected by volume changes between this short-lived population and the longer-lived population.
Some of the previously observed "nonclassical" behaviors of functional contrast can be explained by these interacting dual-spin populations.
功能磁共振成像对比通常与血氧浓度引起的横向弛豫率变化相关，即所谓的血氧水平依赖对比。然而，这种对 fMRI 对比的解释受到了最近在高空间分辨率下进行的几项实验的质疑。本实验旨在研究仅用单自旋群体弛豫率差异无法解释的对比依赖性。
在高对比度视觉刺激过程中，在人类早期视觉皮层中，在较大的 TE 范围内和多个翻转角下，获得了功能信号和对比的测量值。小体素（1.5mm）用于将测量限制在使用视场映射程序识别的早期视觉区域的皮质灰质中。
测量结果与包含 2 个自旋群体的模型一致。主要群体的横向寿命相对较短，受激活的强烈调制。然而，功能对比也受到这个短寿命群体和长寿命群体之间的体积变化的影响。
一些先前观察到的功能对比的“非经典”行为可以用这两个相互作用的双自旋群体来解释。
