Dynamic imaging of perfusion and oxygenation by functional magnetic resonance imaging.

Cerebral blood flow can be measured with magnetic resonance imaging (MRI) by arterial spin labeling techniques, where magnetic labeling of flowing spins in arterial blood water functions as the endogenous tracer upon mixing with the unlabeled stationary spins of tissue water. The consequence is that the apparent longitudinal relaxation time (T1) of tissue water is attenuated. A modified functional MRI scheme for dynamic CBF measurement is proposed that depends on extraction of T1 weighting from the blood oxygenation level-dependent (BOLD) image contrast, because the functional MRI signal also has an intrinsic T1 weighting that can be altered by variations of the excitation flip angle. In the alpha-chloralose-anesthetized rat model at 7T, the authors show that the stimulation-induced BOLD signal change measured with two different flip angles can be combined to obtain a T1-weighted MRI signal, reflecting the magnitude of the CBF change, which can be deconvolved to obtain dynamic changes in CBF. The deconvolution of the T1-weighted MRI signal, which is a necessary step for accurate reflection of the dynamic changes in CBF, was made possible by a transfer function obtained from parallel laser-Doppler flowmetry experiments. For all stimulus durations (ranging from 4 to 32 seconds), the peak CBF response measured by MRI after the deconvolution was reached at 4.5 +/- 1.0 seconds, which is in good agreement with (present and prior) laser-Doppler measurements. Because the low flip angle data can also provide dynamic changes of the conventional BOLD image contrast, this method can be used for simultaneous imaging of CBF and BOLD dynamics.