The interaction between magnetization transfer and blood-oxygen-level- dependent effects

Low-power off-resonance spin-echo magnetization transfer (MT) imaging experiments with a long repetition time (TR) were performed on rat brain for a range of arterial PCO₂ levels. The measured magnetization transfer ratio decreased with increased arterial PCO₂ levels. When performing blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI)-type data analysis in which signal intensities were normalized to the normocapnic state, the CO₂-based BOLD effect was much stronger with than without saturation. This increased effect is a consequence of the fact that the MT effect reduces the signal intensity in tissue more than in blood, thereby amplifying the contribution of the intravascular BOLD signal change to the overall BOLD effect. The results offer a potential approach to measure absolute cerebral blood volume in vivo and to amplify the BOLD effects for fMRI studies.