Arterial spin labeling MR perfusion

Last revised by Rohit Sharma on 4 Apr 2024

Arterial spin labeling (ASL) MR perfusion is an MR perfusion technique which does not require intravenous administration of contrast (unlike DSC perfusion and DCE perfusion). Instead, it exploits the ability of MRI to magnetically label arterial blood below the imaging slab. The parameter most commonly derived is cerebral blood flow (CBF). It is a non-invasive and non-ionizing MRI technique that measures tissue perfusion (blood flow), by using magnetically-labeled arterial blood water protons as an endogenous tracer.

ASL is a very suitable technique to use in pediatrics, in which the use of radioactive tracers may be restricted. It is also safe to use in patients with impaired renal function and those who may need serial follow-up 1.

A number of techniques have been described to achieve ASL perfusion, classified based on the magnetic labeling process 2:

  • pulsed (PASL)

  • continuous (CASL)

  • pseudocontinuous (PCASL)

  • velocity-selective ASL (VS-ASL) 

Basic principles 

The main idea in ASL is to obtain a labeled image or tagged image and a control image, in which the static tissue signals are identical but the magnetization of the inflowing blood is different. The water molecules in the arterial blood are magnetically labeled (tagged) by using a radiofrequency pulse that saturates water protons. Subtraction between labeled (tagged) and control images eliminate the static signals and the remaining signals are linear measures to the perfusion, which is proportionate to the cerebral blood flow (CBF). ASL signal-to-noise ratio is very low, because the signals from the tagged blood is only 0.5-1.5% of the entire tissue signals. Echo planar imaging (EPI) is used for ASL acquisition because of its high signal to noise ratio. EPI can lead to distortions in regions of high magnetic field. Three-dimensional sequences have been introduced to ASL acquisition to increase the SNR and provide less image distortion. ASL data must be acquired before gadolinium administration since gadolinium will cause T1 shortening leading to a decrease in the measurable signals in both the labeled and controlled images 1,4.

Clinical applications 

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