Arterial Spin Labeling (ASL)What is arterial spin labeling (ASL) and how does it work?
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Arterial spin labeling (ASL) , also known as arterial spin tagging, is an MRI method for measuring perfusion using the patient's own water molecules as tracers. Unlike Dynamic Susceptibility Contrast (DSC) and Dynamic Contrast Enhanced (DCE) imaging, ASL does not require the injection of gadolinium or any other exogenous contrast material.
The basic principle of ASL is illustrated below. First, "control" images are acquired through the area of interest (here the cerebral hemispheres). Next, "tagging" pulse(s) are applied to a slab of tissue proximally (upstream) from the imaging volume that inverts the magnetization of water molecules in this slab. Over the next couple of seconds, most of these "magnetically labeled" molecules lying within vessels will flow into the imaging volume. These tagged water molecules exchange their magnetization with those in the static tissue, reducing the latter's equilibrium magnetization slightly (by 1-2%). The area of interest is re-imaged, and data from the newly "tagged" images are subtracted from the "control" images on a pixel-by-pixel basis. The final subtracted image is thus perfusion-weighted.
Generic ASL method showing basic principles of image contrast
Unlike gadolinium contrast agents, water is a diffusible tracer. This means that tagged water molecules do not remain confined to the extravascular space but freely move from capillaries into tissue parenchyma including cells during the course of an ASL experiment. This has implications for mathematical modeling and estimation of perfusion quantitatively.
In addition to measuring perfusion, ASL methods can be applied to generate images of blood vessels directly for MR angiography. ASL is also being used increasingly for to record areas of brain activation in fMRI as an alternative to BOLD (Blood Oxygen Level Dependent) techniques.
Several options are available for ASL labeling and imaging, and no single method has yet emerged as the undisputed best. The advantages and disadvantages of several techniques will be explored in later Q&A's.
References
Borogovac A, Asllani I. Arterial spin labeling (ASL) fMRI: advantages, theoretical constrains and experimental challenges in neurosciences. Int J Biomed Imaging 2012; Article ID 818456:1-13.
Diebler AR, Pollock JM, Kraft RA, et al. Arterial spin-labeling in routine clinical practice, Part 1: techniques and artifacts. AJNR Am J Neuroradiol 2008; 29:1228-1234.
Essig M, Shiroishi MS, Nguyen TB, et al. Perfusion MRI: the five most frequently asked technical questions. AJR Am J Roentgenol 2013; 200:24-34.
Ferré J-C, Bannier E, Raoult H, et al. Arterial spin labeling (ASL) perfusion: techniques and clinical use. Diagn Interv Radiol 2013; 94:1211-1223
Jahng G-H, Li K-L, Ostergaard l, Calamante F. Perfusion magnetic resonance imaging: a comprehensive update on principles and techniques. Korean J Radiol 2014; 15:554-577. (good recent review).
McGehee BE, Pollock JM, Maldjian JA. Brain perfusion imaging: how does it work and what should I use? J Magn Reson Imaging 2012; 36:1257-1272.
Borogovac A, Asllani I. Arterial spin labeling (ASL) fMRI: advantages, theoretical constrains and experimental challenges in neurosciences. Int J Biomed Imaging 2012; Article ID 818456:1-13.
Diebler AR, Pollock JM, Kraft RA, et al. Arterial spin-labeling in routine clinical practice, Part 1: techniques and artifacts. AJNR Am J Neuroradiol 2008; 29:1228-1234.
Essig M, Shiroishi MS, Nguyen TB, et al. Perfusion MRI: the five most frequently asked technical questions. AJR Am J Roentgenol 2013; 200:24-34.
Ferré J-C, Bannier E, Raoult H, et al. Arterial spin labeling (ASL) perfusion: techniques and clinical use. Diagn Interv Radiol 2013; 94:1211-1223
Jahng G-H, Li K-L, Ostergaard l, Calamante F. Perfusion magnetic resonance imaging: a comprehensive update on principles and techniques. Korean J Radiol 2014; 15:554-577. (good recent review).
McGehee BE, Pollock JM, Maldjian JA. Brain perfusion imaging: how does it work and what should I use? J Magn Reson Imaging 2012; 36:1257-1272.
Related Questions
What is arterial spin labeling (ASL) and how does it improve MRA?
Can you briefly explain the difference between the various ASL methods? Which is the best?
What is arterial spin labeling (ASL) and how does it improve MRA?
Can you briefly explain the difference between the various ASL methods? Which is the best?