Guest Editor
Karl-Olof Lövblad
Arterial spin-labeling (ASL) is a technique that has been around for a number of years but which remained dormant until recent enthusiasm developed again about its use and, finally, acceptance came over the last few years. From the start it was, in theory, a revolution: obtaining perfusion images with MR without the use of contrast media. However, initial imaging sequences allowed only the acquisition of one section, which limited its use. Though, even then, various versions were available such as EPISTAR. This first revolution was made possible with the development of echo-planar capable 1.5T scanners. However, it would be the advent of clinical 3T scanners, as well as the development of parallel and other faster imaging techniques, that would allow ASL to be implemented in a more satisfactory way. Since then, interest in its use for research and clinical applications has grown almost exponentially. ASL provides maps of cerebral blood flow after tagging of blood in the cervical vessels. This has been validated by PET for many ASL sequences, and maps seem in most cases to correspond with this reference standard. The fact that no contrast is needed can be of interest when contrast-induced nephropathy is feared. The main application, beside research purposes, seems currently to be in cerebrovascular diseases, dementia, and epilepsy.
Epilepsy: Drug resistant epilepsy that may require aggressive treatment, and thus imaging, is a challenge. ASL can demonstrate changes associated with epilepsy in cases of hypoperfusion in drug-resistant temporal epilepsy and can even demonstrate hyperperfusion in cases of ictal episodes.
Dementia: This is one area where, recently, there has been a lot of exciting news. Indeed, the idea that ASL could replace any other kind of contrast imaging (MR or PET) in patients with dementia is very seductive. These patients represent a potentially very large group that could need screening in case a potentially effective therapy emerges and needs to be tested and/or validated. Dementia may be due to a multitude of causes, and ASL has even been used in hydrocephalus.1
Stroke: In ischemia, the use of perfusion techniques has long been established, be it with PET/SPECT or even more definitively, recently, with techniques such as contrast CT and MR perfusion techniques. ASL has the advantage of providing maps of cerebral blood flow—however, with a rather low resolution of perfusion in the core: very often on ASL perfusion maps there is almost a central black hole effect with little information visible. Nonetheless, its use in demonstrating hypoperfusion associated with ischemia has been demonstrated to be almost as good as other techniques. Additionally, the possibility to perform so-called “territorial” ASL is of great importance because one can, on the one hand, map differential territories but also demonstrate the presence of collateral flow. Collateral flow, its absence or presence, is known to play a role in the survival of ischemic brain tissue in stroke but has been difficult to demonstrate with the usual brain perfusion techniques such as CT or MR. Clinically, it has been proven to be useful in TIA.2 Also, the presence of reperfusion or hyperperfusion may be better seen in some cases with ASL, in our experience. One important application is within the context of one-stop shopping approaches for the whole neurovascular axis: Indeed, today contrast is used for imaging of the carotid arteries with CE-MRA. Thus, using CE-MRA in combination with ASL makes possible the combination of perfusion imaging and angiography. Indeed, until now one would have had to use contrast either for the CE-MRA or the perfusion study and also eventually for implicating that one of the two sequences would be spoiled by the presence of a baseline contrast presence. It has been shown to be of clear interest in patients with Moyamoya disease3 or with more severe cases of vascular disease such as asphyxia3 or even as an auxillary diagnostic modality in extreme decreases of brain perfusion, such as is seen in brain death.4 It has also been proposed as a monitoring tool for asphyxia.5
Brain tumors: For brain tumors, ASL could also play an important role despite the lower resolution currently available. It has been shown to improve grading and treatment monitoring.6