Baseline blood oxygenation modulates response amplitude

Although BOLD fMRI provides a useful tool for probing neuronal activities, large inter-subject variations in signal amplitude are commonly observed. Understanding the physiologic basis for these variations will have a significant impact on many fMRI studies. First, the physiologic modulator can be used as a regressor to reduce variations across subjects, thereby improving statistical power for detecting group differences. Second, if a pathologic condition or a drug treatment is shown to change fMRI responses, monitoring this modulatory parameter is useful in correctly interpreting the fMRI changes to neuronal deficits/recruitments. Here we present evidence that the task-evoked fMRI signals are modulated by baseline blood oxygenation. To measure global blood oxygenation, we used a recently developed technique, T2-RelaxationUnder-Spin-Tagging MRI, yielding baseline oxygenation of 63.7±7.2% in sagittal sinus with an estimation error of 1.3%. It was found that individuals with higher baseline oxygenation tend to have a smaller fMRI signal and vice versa. For every 10% difference in baseline oxygenation across subjects, the BOLD and cerebral blood flow signal differ by -0.4% and -30.0%, respectively, when using visual stimulation. TRUST MRI is a useful measurement for fMRI studies to control for the modulatory effects of baseline oxygenation that are unique to each subject.