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Arterial Spin Labeling

0. About

For a long time, I was thinking of a handbook of Arterial Spin Labeling (ASL), since I didn't know any tricks when I stepped in to the field. Therefore, here is a zero to one ASL handbook. Experienced reader can also use this as a handbook through the ASL Index and Researchers in ASL.

1. Outline

ASL is non-invasive perfusion imaging method based Magnetic Resonance Imaging (MRI). It can measure the blood supply to local tissues. More specifically, it may include the blood in the vessel, but not always. Compared to other perfusion imaging methods, ASL uses the water in the arterial blood as endogenous tracer. Therefore, it doesn't require the injection of contrast agent, which makes ASL safe for the child and elderly, repeated scan for one subject without dose limitation, and more acceptable for large cohort studies.

Historically, ASL was developed from and currently still used for MR angiography, which shows vessels. Because of the small signal in ASL, there are usually a pair of images were acquired: control ('normal blood') and label ('tagged blood'). The paired images were subtracted to remove the background and to get perfusion contrast.

2. ASL sequence

Technically, ASL sequence can be divided into arterial labeling, preparation pulses, and acquisition modules.

2.1 Arterial labeling module

The perfusion contrast is mainly generated by the arterial labeling module. In short, the pCASL is the dominate labeling method and recommended method by the ISMRM perfusion study group, especially for brain imaging. Meanwhile, velocity selective labeling is growing and it shows benefits in abdominal imaging.

  • 2.1.1. continuous ASL, which requires additional hardware and therefore, limits its applications.

  • 2.1.2. pulsed ASL, which gives high stability and labeling efficiency, but reduced SNR.

  • 2.1.3. pseudo continuous ASL which gives high SNR, but off-resonance sensitive.

  • 2.1.4. velocity selective ASL which doesn't label blood spatially as the above three, but label the blood according to velocity.

2.2 ASL preparation module

Preparation pulses help to improve the SNR and stability of ASL images, including:

2.3 ASL acquisition module

The acquisitions of ASL mainly focus single shot and high SNR, including FSE, GRASE, stack-of-spiral, GRE, SSFP, and radial.

2.4 ASL artifacts

3. ASL reconstruction

4. ASL model and quantification

5. ASL postprocessing and enhancement

6. ASL applications

ASL was validated and compared with other perfusion imaging methods, such as PET ASL is used for several diseases ( e.g. Alzheimer's Diseases, Parkinson's Disease, Small Vascular Disease, stroke) and brain developments (e.g. placenta, newborn, infant, and adult).