Dynamic relationship between changes in CBF and CBV during somatosensory stimulation: Implications for calibrated fMRI

Abstract

The relationship between dynamic changes in cerebral blood flow (CBF) and volume (CBV) are very important for calibrating fMRI so that changes in oxidative metabolism can be determined 1. Although recent animal studies 2, 3 have investigated dynamic changes in CBF and CBV (CBF/CBF and CBV/CBV), the studies were limited by use of either non-quantitative methods or the CBF and CBV data had different spatial resolutions. Here towards the goal of calculating dynamic changes in cerebral metabolic rate for oxygen consumption (CMRo2) quantitatively, we demonstrate the combined method of the modified functional MRI for CBF 4 with contrast agent for CBV 5 to clarify the relationship between dynamic CBF/CBF and CBV/CBV of rat somatosensory cortex during forepaw stimulation with different stimulus duration (4,8,16,32 seconds). The figure 1 shows the relationship between dynamic CBF/CBF and CBV/CBV for two stimulus duration (4 (blue) and 16 (red) seconds). The filled and opened symbols represent the period of transition from stimulus onset to peak and transition from stimulus offset to baseline, respectively. The dotted lines show the value of the parameter ranging from 0.05 to 0.4 in CBV=CBF∧. The relationship between dynamic CBF/CBF and CBV/CBV was different throughout different stimulus durations and became complex as the stimulus duration became longer. The values of a ranged from 0.05 to 0.15 during transition from stimulus onset to a peak, whereas the value ranged from 0.15 to 0.45 during transition to baseline after stimulus offset. Thus the Grubb's power exponent, =0.38 6, should be used with caution when being used for predicting changes in CMRo2 during transition 7 and even steady state 8 since the values of a ranged from 0.10 to 0.25 under steady state throughout stimulus durations. This is the first study to directly investigate the relationship between the dynamic changes in CBF and CBV during transition periods (i.e. onset and offset of stimulation) using only MRI methods, which allow us to measure the transient changes with same spatial and temporal resolution in same subject. The modified fMRI method with contrast agent has a great potential to predict the dynamic changes in CMRo2 without any further perturbations or assumption.