Arterial spin labelling

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Arterial spin labeling (ASL), also known as arterial spin tagging, is a

gadolinium contrast.[1] A number of ASL schemes are possible, the simplest being flow alternating inversion recovery (FAIR) which requires two acquisitions of identical parameters with the exception of the out-of-slice saturation; the difference in the two images is theoretically only from inflowing spins, and may be considered a 'perfusion map'.[2]
The ASL technique was developed by Alan P. Koretsky, Donald S. Williams, John A. Detre and John S. Leigh Jr in 1992.[3][4][5][6]

Physics

Arterial spin labeling utilizes the water molecules circulating with the brain, and using a radiofrequency pulse, tracks the blood water as it circulates throughout the brain. After a period of time in microseconds (enough to allow the blood to circulate through the brain), a 'label' image is captured. A 'control' image is also acquired before the labeling of the blood water. A subtraction technique gives a measurement of perfusion. In order to increase

proton density map with the same parameters (but longer TR to fully relax the blood spins) is recommended to be acquired as well. Alternatively, the average control image can be used to generate CBF, which is the case for Phillips
pCASL readouts. Usually background suppression is also applied to increase the SNR. Due to the different variations of each implementations, it is recommended that a large multi-scanner study should design a protocol minimizing the variety of readout methods used by each scanner.

One study has shown that although there are voxel differences when different readout methods are used, average gray matter CBF are still comparable. Differences in SNR are apparent when each voxel compared, but collectively are negligible.

importance of example(s)?
]

Continuous arterial spin labelling

In continuous arterial spin labeling (CASL), the blood water is inverted as it flows through the brain in one plane. CASL is characterized by one single long pulse (around 1–3) seconds. This may be disadvantageous for certain scanners that are not designed to maintain a radiofrequency pulse that long, and therefore would require adjustments to a

GE
scanners.

Pulsed arterial spin labelling

In pulse arterial spin labeling (PASL), blood water is inverted as it passes through a labeling slab (of 15 to 20 cm) instead of a plane. There are different variations of this implementations, including EPISTAR and PICORE and PULSAR. Most scanners have been designed to have PASL work out-of-the-box for research use.

Velocity selective arterial spin labelling

Velocity selective arterial spin labeling is a strategy that still requires validation. Velocity selective arterial spin labeling is advantageous in a population where blood flow may be impeded (e.g. stroke), because the labeling occurs closer to the capillaries. This allows the post labeling decay to be shorter.[9]

Diffusion prepared pseudocontinuous arterial spin labelling (DP-pCASL)

Diffusion-prepared pseudocontinuous ASL (DP-pCASL) is a more recent ASL variant sequence that magnetically labels water molecules and measures their movement across the blood-brain barrier complex, which allows for the calculation of the water exchange rate (kw).[10][11] kw is used as a surrogate for BBB function and permeability. Water exchange across the BBB is mediated by a number of processes, including passive diffusion, active co-transport through the endothelial membrane, and predominantly by facilitated diffusion through the dedicated water channel aquaporin-4 (AQP4). Several studies have investigated the use of DP-pCASL in cerebrovascular diseases, including acute ischemic stroke,[12] CADASIL,[13] hereditary cerebral small vessel disease [14] as well as in animal models.[15]

Analysis of ASL images

ASL maps can mainly be analyzed using the same tools to analyze

fMRI and VBM. Many ASL-specific toolboxes have been developed to assist in ASL analysis, such as BASIL (Bayesian inference for arterial spin labelling MRI), part of the FSL neuroimaging package and also Ze Wang's ASL toolbox (using MATLAB) to assist in the subtraction and averaging of the tagged/control pairs.[16] A visual quality check is often needed to make sure that the perfusion map is valid (such as correct registration, or correct segmentation of non-cerebral materials such as the dura mater). A whole brain/voxel-wise approach can be analyzed by registering the ASL map into MNI space for group comparisons. A region of interest approach can be analyzed by registering the ASL map into a selected cluster, or an atlas, like a standard (such as the Harvard-Oxford Cortical atlas) or an individual atlas developed by software like FreeSurfer. The recommended procedure of ASL registration for voxel-wise analysis is to register the perfusion map to a gray matter segmentation of each individual in a non-rigid procedure.[citation needed
]

Freesurfer
.

ASL

resting brain networks (such as the default mode network) less.[17]

Comparison with fMRI

comorbid effects with CVD.[18]
ASL imaging can be a useful tool to complement fMRI and vice versa.

Clinical use

In

Parkinson disease.[20] Additionally, DP-pCASL has promising potential for assessing blood-brain barrier integrity in patients with ischemic stroke.[21]

Although the primary form of

blood-oxygen-level dependent (BOLD) contrast,[22] ASL is another method of obtaining contrast.[23]

There have been research to apply ASL to renal imaging,[24] pancreas imaging,[25] and placenta imaging. A challenge to these sort of non-cerebral perfusion is motion due to breathing. Additionally, there is a lot less development on the segmentation of theses specific organs, so the studies are relatively small scale.

Safety

ASL is in general a safe technique, although injuries may occur as a result of failed safety procedures or human error like other MRI techniques.[26]

ASL, like other MRI modalities generate a fair amount of acoustic noise during the scan, so earplugs are advised.

References

  1. ^ Fortin F, Gaillard F. "Arterial spin labelling (ASL) MR perfusion". Radiopaedia. Retrieved 2017-10-15.
  2. ^ "Arterial spin labeling". University of Michigan. Retrieved 2017-10-27.
  3. PMID 1729691
    .
  4. S2CID 260421572
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  5. ^ Leigh, J.S., Detre, J.A., Williams, D.S., Koretsky, A.P. "Methods for measuring perfusion using magnetic resonance imaging" US Patent No. 5,402,785 (1995).
  6. PMID 22245338
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  7. PMID 25090654
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  8. PMID 24715426
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  9. PMID 25785831
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  10. PMID 33328848
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  11. PMID 30561821
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  12. PMID 38513785
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  13. PMID 37099178
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  14. PMID 36625892
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  15. PMID 27742887
    .
  16. PMID 17826940
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  17. PMID 26106930
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  18. S2CID 1359908
    .
  19. PMID 22468186
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  20. PMID 26351201
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  21. PMID 38513785
    .
  22. ISBN 978-0-87893-286-3
    .
  23. PMID 22246782
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  24. PMID 7784582
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  25. PMID 30229559
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  26. S2CID 57880401
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External links
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