Arterial Spin Labeling (ASL)
Arterial Spin Labeling or ASL is an MRI method that yields images of cerebral perfusion without the use of exogenous contrast materials. This is done non-invasively, by using an RF spin labeling method. Arterial spin tagging is achieved using a clever spin inversion method referred to as pCASL or pseudo continuous ASL. Following the tagging sequence, a single shot echo planar gradient echo image is acquired with the resolution and timing below. 30 dynamic scans are acquired with a control and labeling scan in each dynamic interval (TR=4000). The control scan is acquired with a sham labeling pulse that has the same RF and gradients applied to control for magnetization transfer effects, but with the phase of the individual RF pulses in the pulse train alternating by 180 degrees so that no spin inversion will occur. CBF is calculated from the difference between the tagged and control images. This method can be used to measure cerebral blood flow (CBF) at the tissue level either in a quantitative way yielding perfusion in ml/min/gm or in a relative way to reflect regional variation in perfusion. We use a modified theoretical model 7 derived from the general two-compartment kinetic model 8, yielding the formula for CBF as given in 9. The signal to noise ratio for the ASL experiment is low with the difference between the labeled and unlabeled images being in the range of 1-2% signal change. Consequently it is necessary to average a large number of acquired images in both the labeled and unlabeled condition to calculate a CBF map with reliable contrast to noise for quantitative measurements. Typically 40 to 50 images are acquired in each state for subtraction and processing. With a 4 second TR this means a typical ASL perfusion measurement in a human subject will take 5 to 6 minutes to complete.