Aberdeen Biomedical Imaging Centre, University of Aberdeen, UK.
Neuroimage. 2012 Feb 1;59(3):2401-12. doi: 10.1016/j.neuroimage.2011.08.110. Epub 2011 Sep 18.
Hyperoxia or hypercapnia provides a useful experimental tool to systematically alter the blood oxygenation level dependent (BOLD) contrast. Typical applications include calibrated functional magnetic resonance imaging (fMRI), BOLD sensitivity mapping, vessel size imaging or cerebrovascular reactivity mapping. This article describes a novel biophysical model of hyperoxic and hypercapnic BOLD contrast, which accounts for the magnetic susceptibility effects of molecular oxygen that is dissolved in blood and tissue, in addition to the well-established effects caused by the paramagnetic properties of deoxyhaemoglobin. Furthermore, the concept of vascular component analysis (VCA) is introduced and is shown to provide a computationally efficient tool for investigating the vascular specificity of hyperoxic and hypercapnic BOLD contrast. A theoretical investigation of gradient and spin echo BOLD contrast based on computer simulations was performed to compare three different conditions (hypercapnia induced by breathing 6% CO2, hyperoxia induced by breathing 100% O2, and simultaneous hypercapnia and hyperoxia induced by breathing carbogen, i.e. 5% CO2 in 95% CO2) with baseline (breathing air). Simulations were carried out for different levels of metabolic oxygen extraction fraction (OEF) ranging from 0 to 0.5. The key findings can be summarised as follows: (i) for hyperoxia the susceptibility of dissolved O2 may lead to a significant arterial BOLD contrast; (ii) under normoxic conditions the susceptibility of dissolved O2 is negligible; (iii) an almost complete loss of BOLD sensitivity may occur at lower OEF values in all parts of the vascular tree, whereas hyperoxic BOLD sensitivity is largely maintained; (iv) under hyperoxic conditions, a transition from positive to negative BOLD contrast occurs with decreasing OEF values. These findings have important implications for experimental applications of hyperoxic and hypercapnic BOLD contrast and may enable new clinical applications in ischemic stroke and other forms of acquired brain injury.
氧合或高碳酸血症为系统改变血氧水平依赖（BOLD）对比提供了一种有用的实验工具。典型的应用包括校准功能磁共振成像（fMRI）、BOLD 敏感性映射、血管大小成像或脑血管反应性映射。本文描述了一种新的氧合和高碳酸血症 BOLD 对比的生物物理模型，该模型考虑了溶解在血液和组织中的分子氧的磁化率效应，以及脱氧血红蛋白的顺磁性特性所引起的已确立的效应。此外，引入了血管成分分析（VCA）的概念，并证明它为研究氧合和高碳酸血症 BOLD 对比的血管特异性提供了一种计算效率高的工具。基于计算机模拟对梯度和自旋回波 BOLD 对比进行了理论研究，以比较三种不同的条件（呼吸 6%CO2 引起的高碳酸血症、呼吸 100%O2 引起的高氧血症以及呼吸碳化氧（即 5%CO2 在 95%CO2 中）引起的同时高碳酸血症和高氧血症）与基线（呼吸空气）。模拟了代谢性氧提取分数（OEF）从 0 到 0.5 的不同水平。主要发现可概括如下：（i）对于高氧血症，溶解 O2 的磁化率可能导致显著的动脉 BOLD 对比；（ii）在正常氧合条件下，溶解 O2 的磁化率可以忽略不计；（iii）在血管树的所有部分，随着 OEF 值的降低，BOLD 敏感性可能几乎完全丧失，而高氧 BOLD 敏感性则基本保持；（iv）在高氧条件下，随着 OEF 值的降低，BOLD 对比从正变为负。这些发现对高氧和高碳酸血症 BOLD 对比的实验应用具有重要意义，并可能为缺血性中风和其他形式的获得性脑损伤提供新的临床应用。
