Perfusion Functional Magnetic Resonance Imaging

Introduction Imaging sequences based upon blood oxygenation level dependent (BOLD) contrast are currently the predominant method for functional magnetic resonance imaging (fMRI) of the brain. BOLD weighted sequences offer both high contrast-to-noise ratio and good temporal resolution. The BOLD signal reflects the total amount of deoxyhemoglobin (dHBO2), and is thus a complex function of cerebral blood flow (CBF), the cerebral rate of oxygen metabolism (CMRO2), cerebral blood volume (CBV), and magnetic field strength. As a result, the interpretation of changes in the BOLD signal can be complicated by variations in these physiological quantities due to factors such as age, disease, or the presence of vasoactive agents [1]. Perfusion fMRI based upon arterial spin labeling (ASL) methods offers a useful complement to BOLD fMRI. It can provide quantitative measures of both baseline and functional changes in CBF that can aid in the interpretation of the BOLD signal change. Changes in CBF are thought to be more directly linked to neuronal activity than BOLD, so that perfusion fMRI also has the potential to offer more accurate measures of the spatial location and magnitude of neural function. Perfusion fMRI also has some practical advantages, including an inherent insensitivity to low-frequency fluctuations commonly observed in fMRI experiments and the ability to take advantage of imaging sequences (e.g. spin-echo) that are insensitive to susceptibility induced off-resonance effects.