Is ASL Useful for Brain Mapping?
Wen-Chau Wu1,2

1Graduate Institute of Medical Device and Imaging, National Taiwan University, Taipei, Taiwan, 2Medical Imaging, National Taiwan University Hospital, Taipei, Taiwan

Synopsis

Since its introduction in 1990's, ASL has gone through abundant developments that have remarkably improved this contrast-material-free technique in data acquisition, image quality, and quantitative modeling. We now have a variety of methods to choose from and a number of review articles to refer to. However, it still seems an open question when it comes to the usefulness of ASL in brain mapping. In this presentation, the issues that have been keeping us from saying yes to the question will be reviewed. The usefulness and caveats of ASL in brain mapping will be discussed from the viewpoint of clinical application.

Introduction

Since its introduction in 1990’s (1-3), arterial spin labeling (ASL) has gone through abundant developments that have remarkably improved this contrast-material-free technique in data acquisition, image quality, and quantitative modeling. We now have a variety of methods to choose from and a number of review articles (4-6) including a recently published white paper (7) to refer to. However, it still seems an open question when it comes to the usefulness of ASL in brain mapping. In this presentation, the issues that have been keeping us from saying yes to the question will be reviewed. The usefulness and caveats of ASL in brain mapping will be discussed from the viewpoint of clinical application.

Caveats

Absolute quantification of ASL signal requires knowledge of several parameters. Among these, arterial transit time (ATT) is arguably the most explored, probably because it is usually unknown in practice and even harder to predict in disease conditions. Not knowing ATT makes it difficult to choose the time delay between label and excitation pulses which is commonly called post-labeling delay (PLD) in continuous labeling methods and inversion time (TI) in pulsed labeling methods. In theory, the optimal PLD/TI is equal to ATT. If PLD/TI is shorter than ATT, there will be intravascular signal that cannot be quantified as perfusion and delayed flow will be misinterpreted as low flow or no flow. To avoid these, one may set PLD/TI as long as possible. However, the longer PLD/TI, the more signal-to-noise (SNR) we lose and measurement reliability starts being compromised when SNR drops below some threshold. Multiple PLD/TI’s can be used to fit for ATT (8) but how to choose these PLD/TI’s still relies on knowledge of ATT because these PLD/TI’s need to cover the wash-in phase and if possible also the wash-out phase. The question is, however, if we know how to choose these PLD/TI’s, we kind of already know about the ATT. And let’s not forget that this increases the total scan time. For a fixed scan time, this means we have to divide it into multiple PLD/TI scans, which means fewer measurements and thus lower SNR for each PLD/TI scan. How would this impact the reliability of the fitted ATT? Not to mention other factors such as spin compartment (9) (does the arterial spin stay in blood stream all the time? or does it enter the interstitial space at some time? This affects the choice of relaxation time constants), labeling efficiency (10), and relaxation time constants. Is it possible to deal with all these issues on a per-subject and per-exam basis?

Applications

Enough being said about quantification, we wonder if ASL is only useful when it is absolutely quantitative. Fortunately, multiple parameters can be derived from ASL even though they may not be quantitative. For example, ATT can be estimated by using multi-PLD/TI ASL. Even though the calculated transit time may not be perfectly accurate, it would be useful enough to highlight the area with abnormal changes. This is also true with the global perfusion derived from single-PLD/TI ASL. Even though the perfusion value may not be absolutely quantitative, local increase/decrease or intravascular hyperintensities in contrast to normal/contralateral areas may provide useful information. Furthermore, ASL can be tailored to be vessel selective/encoding to depict arterial flow territories (11). Information of how arterial blood is distributed to perfusion territories may help the evaluation of collateral circulation (12,13). Another application of ASL that is not related to absolute quantification is for calibrated blood-oxygenation-level-dependent (BOLD) MRI (14,15), an application starting to gather attention in clinical neurology and neuroscience. Briefly, relative cerebral blood flow change is measured along with relative BOLD signal change. The measurement is first performed to estimate the constant of maximum signal change or ceiling effect in an iso-CMRO2 (CMRO2 = cerebral metabolic rate of oxygen) session where changes of cerebral blood flow and BOLD signal are induced (e.g., by hypercapnia) without altering CMRO2. After the calibration, task-specific CMRO2 demand can be calculated by using the ceiling effect constant and simultaneous measurement of ASL and BOLD signals.

Summary

So, is ASL useful in brain mapping? I think we should first acknowledge that ASL is not a magic technique that works in every possible way, which is probably true with all other MR techniques. If a ‘yes’ or ‘no’ is really demanded, my personal opinion is more of yes. ASL definitely has its niche in brain mapping but this does not mean it is the first choice in all applications. Sophisticated data acquisition and quantitative modeling should be considered in the context of SNR achievable with clinically reasonable scan time.

Acknowledgements

Grant sponsor: Ministry of Science and Technology, Taiwan. Grant numbers: 105-2314-B-002 -094 -MY3 and 106-2628-E-002-003-MY3.

References

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Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)