Published online by Cambridge University Press: 05 March 2013
Assessment of regional cerebral perfusion provides highly desirable information for diagnosis and management of cerebrovascular disease and acute stroke. Over the past decades, several approaches have been used to image regional perfusion in cerebrovascular disease, including positron emission tomography (PET), single-photon emission computed tomography (SPECT), xenon-enhanced X-ray computed tomography (XeCT), and MR imaging (MRI). The majority of these methods utilize an exogenous tracer administered intravenously or by inhalation. Most of the MRI studies of cerebral hemodynamics in cerebrovascular disease and stroke have also relied on dynamic tracking of susceptibility-related signal changes accompanying the passage of an exogenous bolus of intravenous contrast agent such as gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA). Dynamic susceptibility contrast (DSC) imaging primarily measures blood volume and transit times, but cerebral blood flow (CBF) can be estimated from these parameters based on the central volume principle.[2,3]
Arterial spin labeling (ASL) perfusion MRI is an emerging technology to directly measure CBF using magnetically labeled arterial blood water as endogenous tracer.[4,5] The methodological scheme of ASL is analogous to that used in steady-state PET or SPECT. Arterial blood water is magnetically labeled proximal to the tissue of interest, and perfusion is determined by pair-wise comparison with separate images acquired without labeling. Arterial blood water has a decay rate of T1, sufficiently long to detect perfusion of the microvasculature and tissue but short enough to monitor dynamic changes. As ASL does not require administration of contrast agents or radioactive tracers, it may be more convenient than other approaches, and it can be repeated as often as required in the same imaging session without accumulative effects. Consequently, ASL perfusion contrast can used to monitor CBF changes in response to pharmacological manipulation or task activation. Furthermore, ASL can provide quantitative tissue-specific perfusion values in classical units of milliliters per gram tissue per minute.