Arterial versus Total Blood Volume Changes during Neural Activity-Induced Cerebral Blood Flow Change: Implication for BOLD fMRI

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Abstract
Quantifying both arterial cerebral blood volume ( CBVa) changes and total cerebral blood volume ( CBVt) changes during neural activation can provide critical information about vascular control mechanisms, and help to identify the origins of neurovascular responses in conventional blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI). Cerebral blood flow ( CBF), CBVa, and CBVt were quantified by MRI at 9.4T in isoflurane-anesthetized rats during 15-s duration forepaw stimulation. Cerebral blood flow and CBVa were simultaneously determined by modulation of tissue and vessel signals using arterial spin labeling, while CBVt was measured with a susceptibility-based contrast agent. Baseline versus stimulation values in a region centered over the somatosensory cortex were: CBF = 150 ± 18 versus 182 ± 20 mL/100 g/min, CBVa = 0.83 ± 0.21 versus 1.17 ± 0.30 mL/100 g, CBVt = 3.10 ± 0.55 versus 3.41 ± 0.61 mL/100 g, and CBVa/ CBVt = 0.27 ± 0.05 versus 0.34 ± 0.06 ( n = 7, mean ± s.d.). Neural activity-induced absolute changes in CBVa and CBVt are statistically equivalent and independent of the spatial extent of regional analysis. Under our conditions, increased CBVt during neural activation originates mainly from arterial rather than venous blood volume changes, and therefore a critical implication is that venous blood volume changes may be negligible in BOLD fMRI.