Myocardial Perfusion
Frank Kober1

1CRMBM, CNRS, Aix-Marseille Univ, Marseille, France

Synopsis

This overview will discuss expectations, solutions, advantages and limitations related to measuring perfusion in the human heart without contrast agents. The major existing approaches to Arterial Spin Labeling (ASL) in the heart will be outlined. The reasons why myocardial ASL is challenging compared with other organs will be outlined. It should also become clear why cardiac ASL is a well-working and validated method in the rodent heart whereas human applications are still scarce.

Target Audience

Physicists, clinical researchers, physicians and technologists interested in an update on contrast-free myocardial perfusion MRI technique

Objectives

  • To understand why we would want to measure perfusion without contrast agents in the heart
  • To learn why ASL in the heart is different
  • To get an overview over past and current method developments
  • To appreciate what these techniques can do now, what they may do better than contrast-based techniques in the future, and what they probably won’t do

Purpose

This talk will give an overview over expectations, solutions, advantages and limitations related to measuring perfusion in the heart without contrast agents. I will discuss the major existing approaches to Arterial Spin Labeling (ASL) in the heart, but also recall coronary sinus flow measurements as an alternative. The reasons why myocardial ASL is challenging compared with other organs will be outlined. It should also become clear why cardiac ASL is a well-working and validated method in the rodent heart whereas human applications are still scarce.

Patient populations with cardiac disease (and heart failure in particular) also over-proportionally expose kidney failure, which is why many centers are reluctant to using contrast agents especially in these populations. Left aside the known contrast-agent-related risks, non-contrast myocardial perfusion techniques might also open new ways of improving quantification accuracy due to their ease of use and due to their free repeatability within an MRI exam. These arguments have been motivating developments in cardiac ASL despite the technical challenges.

Methods and Results

Flow-sensitive alternating recovery (FAIR) ASL techniques with various modifications have been proposed and shown feasible in the human heart since the late 90’s (1,2) when the signal-efficient balanced SSFP methods had not yet found their entry in cardiovascular MRI. The first results were considered reliable only under pharmacologic stress conditions, when myocardial blood flow is high. Since then, developments have taken advantage of more efficient readout schemes (3), and the origins of variability in the measured blood flow were analyzed more thoroughly (4). More recently, different labeling approaches are being explored such as velocity-selective labeling (VS-ASL) (5) or steady-pulsed labeling (spASL) (6,7) under free breathing to make the acquisition as efficient as possible and to benefit from a steady labeling despite the strongly pulsatile blood flow in cardiac vessels. Breathing-motion correction methods are being used for both free breathing and breath-hold techniques (8). Finally, it is noteworthy that global myocardial perfusion can also be estimated using coronary sinus flow quantification via phase contrast cine-MRI (9,10). In parallel to these developments for the human heart, a variety of cardiac ASL methods have been developed specifically for rodent myocardial perfusion mapping (11–15). Due to higher heart rate and blood flow in rodents than in humans, these methods have quickly become stable, and they are now routinely used for exploring pathologies in animal models.

Discussion

The last few years have brought a number of interesting technical improvements and better knowledge about error sources in cardiac ASL quantification methods. The spASL method promises better data collection efficiency and a free-breathing approach, but confounding signal contributions from motion, pulse profile imperfections, arterial transit times and magnetization transfer inherent to bSSFP have to be more thoroughly analyzed. The VS-ASL method is a promising approach to reduce the influence of transit time, but optimal velocity cut-off values need to be determined. Compared with quantitative bolus tracking techniques, ASL has low sensitivity in low blood flow conditions, since the relevant signal is derived from magnetization difference measurements, whose values are of the order of a few percent at rest. In the heart, cardiac and breathing motion, pulsatile blood flow, presence of large volumes of blood in the ventricular chambers, but also the generally lower detection sensitivity compared with brain MRI make the use of ASL challenging. Nevertheless, clinically relevant data on patients with coronary artery disease have been derived from ASL assessment of perfusion reserve in the past (16) showing that ASL in the myocardium is feasible in principle.

Conclusion

Because the potential of ASL for quantifying myocardial perfusion is obvious, ongoing efforts by several groups aim at improving its efficiency and robustness. While the feasibility of cardiac ASL has been proven, it is not yet ready for replacing 1st pass techniques. Cardiac ASL remains an active area of research with many interesting solutions on the horizon.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)