Neurosciences Graduate Program, Medical Scientist Training Program, University of California, San Diego, USA; Center for Functional MRI, University of California, San Diego, USA.
Psychiatry, University of California, San Diego, USA; Center for Human Development, University of California, San Diego, USA.
Neuroimage. 2019 Jan 15;185:154-163. doi: 10.1016/j.neuroimage.2018.10.032. Epub 2018 Oct 10.
Cerebral blood flow (CBF) and blood oxygenation level dependent (BOLD) signal measurements make it possible to estimate steady-state changes in the cerebral metabolic rate of oxygen (CMRO) with a calibrated BOLD method. However, extending this approach to measure the dynamics of CMRO requires an additional assumption: that deoxygenated cerebral blood volume (CBV) follows CBF in a predictable way. A test-case for this assumption is the BOLD post-stimulus undershoot, for which one proposed explanation is a strong uncoupling of flow and blood volume with an elevated level of CBV during the post-stimulus period compared to baseline due to slow blood volume recovery (Balloon Model). A challenge in testing this model is that CBV differs from total blood volume, which can be measured with other techniques. In this study, the basic hypothesis of elevated CBV during the undershoot was tested, based on the idea that the BOLD signal change when a subject switches from breathing a normoxic gas to breathing a hyperoxic gas is proportional to the absolute CBV. In 19 subjects (8F), dual-echo BOLD responses were measured in primary visual cortex during a flickering radial checkerboard stimulus in normoxia, and the identical experiment was repeated in hyperoxia (50% O/balance N). The BOLD signal differences between normoxia and hyperoxia for the pre-stimulus baseline, stimulus, and post-stimulus periods were compared using an equivalent BOLD signal calculated from measured R* changes to eliminate signal drifts. Relative to the pre-stimulus baseline, the average BOLD signal change from normoxia to hyperoxia was negative during the undershoot period (p = 0.0251), consistent with a reduction of CBV and contrary to the prediction of the Balloon Model. Based on these results, the BOLD post-stimulus undershoot does not represent a case of strong uncoupling of CBV and CBF, supporting the extension of current calibrated BOLD methods to estimate the dynamics of CMRO.
脑血流 (CBF) 和血氧水平依赖 (BOLD) 信号测量使得使用校准的 BOLD 方法来估计脑氧代谢率 (CMRO) 的稳态变化成为可能。然而，将这种方法扩展到测量 CMRO 的动态需要一个额外的假设：去氧脑血容量 (CBV) 以可预测的方式跟随 CBF。这个假设的一个测试案例是 BOLD 刺激后下冲，一种解释是由于血液体积恢复缓慢，在刺激后期间与基线相比，CBV 水平升高，导致血流和血液体积强烈解耦（气球模型）。测试此模型的一个挑战是 CBV 与总血容量不同，总血容量可以用其他技术测量。在这项研究中，基于当受试者从呼吸正常氧气体切换到呼吸高氧气体时，BOLD 信号变化与绝对 CBV 成正比的想法，测试了在刺激后下冲期间 CBV 升高的基本假设。在 19 名受试者（8 名女性）中，在正常氧合条件下，在初级视觉皮层中测量闪烁的放射状棋盘格刺激的双回波 BOLD 反应，并在高氧合条件（50% O/平衡 N）下重复进行相同的实验。使用从测量的 R*变化计算得出的等效 BOLD 信号来消除信号漂移，比较正常氧合和高氧合时刺激前基线、刺激期和刺激后期的 BOLD 信号差异。与刺激前基线相比，从正常氧合到高氧合的平均 BOLD 信号变化在刺激后下冲期间为负（p=0.0251），与 CBV 减少一致，与气球模型的预测相反。基于这些结果，BOLD 刺激后下冲不代表 CBV 和 CBF 强烈解耦的情况，支持将当前校准的 BOLD 方法扩展到估计 CMRO 的动态。
