ASL: Acquisition & Analysis in the Body

Susan T. Francis¹
¹University of Nottingham, Nottingham, United Kingdom

Synopsis

This lecture will outline Arterial Spin Labelling acquisition and analysis methods used in body applications. The considerations for ASL acquisition in terms of labelling schemes and post-label delay, image readout, and methods to reduce motion effects for body ASL are discussed. Analysis methods used in the body applications to account for transit delays and correct for motion will be discussed. The application of ASL MRI across various body organs will be outlined including the kidney, placenta, pancreas and liver, and heart, with clinical applications highlighted.

Objectives

To describe the acquisition and analysis methods used for Arterial Spin Labelling (ASL) in the body and the associated challenges. To highlight the current status, recent technical advances, and clinical applications of body ASL.

Purpose

Arterial spin labelling (ASL) is an MRI technique that provides a non-contrast enhanced method to assess tissue perfusion using the intrinsic signal from water in the body. ASL is well-established technique in the brain, and applications of ASL in the body are currently growing rapidly. To date, in the body ASL has been most widely applied in the kidney (1-11), and also been demonstrated in the pancreas (12-15), liver (16-21) and heart (22-31), as well as the placenta (32-37). Since ASL is contrast free, it can be used in situations in which gadolinium-based contrast agents are contraindicated, such as in renal disease where there is the risk of nephrogenic systemic fibrosis and for dynamic studies where repeated measures are required, for example to study the temporal evolution in response to physiological manipulations using pharmaceuticals (38), oxygen (16), or exercise (39).

Methods and Results

There are a number of considerations to acquiring ASL in the body. First, is the choice of how to label the magnetisation of the inflowing blood, with options including either pulsed ASL (PASL) (40) or pseudocontinuous ASL (PCASL) (3, 41, 42), or the use of velocity-selective arterial spin labelling (VSASL) (4, 42). Secondly, for body applications the choice of image readout scheme to use must be considered (6-10), this is a trade-off in terms of the image distortions, achievable spatial resolution and the number of slices to acquire, and whether ASL is being combined with other multiparametric measures. Finally, the method with which to reduce or correct respiratory motion must be considered, with the added complication of heart motion for cardiac ASL. Due to the intrinsically low SNR of ASL, a number of signal averages must be combined and any motion between these must be minimised. Respiratory motion can be overcome using respiratory triggering, breath-holding or the use of a navigator echo (4,7,9). Alternatively, motion correction and data sorting techniques can be applied in post-processing to eliminate any motion artefacts (7-9). Further background suppression (BGS) methods can also be applied to suppress the signal from any static tissue (7,8). There have been the largest number of ASL body studies of the kidney, with this being a rapidly growing area. This is outlined by a recent systematic review [1] and consensus paper [2] by the PARENCHIMA consortium. Studies have shown a decrease in renal perfusion with diseases such as Chronic Kidney Disease (44) and Acute Kidney Injury (45). In liver disease, a decrease in liver perfusion as measured with ASL has been demonstrated in patients with compensated cirrhosis (19). Whilst recently, ASL of the placenta has been shown to be promising for the predicting the subsequence development of ischemic placenta (33). One major challenge in making body ASL a clinically useable tool is the limited number of studies to directly compare ASL variants (5,35) in terms of sensitivity, specificity, reproducibility and change with disease. Although MRI vendors are now providing ASL sequences for use in the brain, sequences available for body ASL are limited.

Conclusion

There have been a number of recent advances in body ASL in terms of image acquisition and analysis methods demonstrating the feasibility and repeatability of measures. To make ASL a more clinically useable tool for body applications there now needs to be efforts to harmonise techniques and assess variation in sensitivity, specificity and reproducibility.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)