Arterial Spin-Labeled MR Perfusion Imaging Techniques

Arterial spin labeling (ASL) is a relatively new technique for MR perfusion assessment that is particularly useful in brain tissues. From a technical point of view, ASL-MRI is essentially a classic tracer experiment where the injected blood-borne tracer has been replaced by a procedure whereby water protons in the blood have been “labeled” on their way to the organ of interest. This labeling is achieved remotely by the application of radio-frequency (RF) waves that effect an inversion of the magnetization of the blood–water in the region where the RF is applied. This is achieved either for all blood within a region (pulsed ASL) or for blood flowing through a plane (continuous ASL), and for brain perfusion, it is usually performed in the neck. Conventional MR imaging can be used to capture this inverted blood signal once it has arrived in the organ, allowing time for it to arrive. However, the image will also contain contribution from the normal soft tissue in the image, which is much larger in magnitude than is the perfusion signal. Hence, the basic ASL experiment contains two images, one with labeling of blood–water and a second without: the subtraction of the two revealing the perfusion. The reliance on an endogenous tracer means that the ASL technique is noninvasive and, for most variants, readily available on existing hardware. However, the small signal magnitude relative to noise in the images and the relatively short “half-life” of the labeled water mean that careful and optimal acquisition and quantification are required. The primary goal of this chapter is to provide a complete overview of ASL from the technical issues, such as acquisition and quantification, to the clinical application in the brain including neuro-oncology, neurodegeneration, psychiatry, vascular and inflammatory disease, and epilepsy. The final section is focused on common pitfalls in ASL that should be avoided in diagnosis and a procedure to assess quality. A white paper1 regarding technical aspects of ASL has recently been published, from which technical recommendations in this chapter are drawn.