Advanced Contrast Mechanisms: Diffusion & DTI of the Heart
Christopher Nguyen1

1Massachusetts General Hospital, United States

Synopsis

Diffusion MRI is powerful tool to interrogate the myocardial microstructure revealing fiber architecture. Technical advances in the past 5 years have enabled in vivo free breathing diffusion MRI of the heart and is primed to make new discoveries in patient populations as an orthogonal quantitative tool alongside conventional methods such as relaxometry mapping and function.

Target Audience

Scientists, engineers, and clinicians interested in learning more about diffusion MRI of the heart.

Trainees are most welcome!

Outcome Objectives

Audience will be able to recognize the following key aspects of diffusion MRI in the heart:

  1. Technical challenges and difficulties
  2. Myocardial microstructure representations by diffusion MRI
  3. Potential clinical applications

Purpose

Diffusion MRI of the heart has recently gained traction given the technical advances in hardware (high performance gradient systems) and pulse sequence design (second order motion compensation, navigator stimulated echo, etc). These technical advances have primarily dealt with the most challenging aspect of diffusion MRI, which is its extreme sensitivity to bulk motion. Given the beating heart is moving about 1Hz, performing in vivo diffusion MRI in the heart seems daunting.

However despite all the technical challenges, diffusion MRI in the heart is powerful in identifying microstructural deficiencies in the myocardium and has the added benefit of being contrast free. Diffusion MRI has shown promise in identifying fibrosis, classifying different forms of non-ischemic cardiomyopathy, and even predicting functional recovery after intervention.

Methods

Single shot echo planar imaging due to this rapid acquisition still remains the most ubiquitous readout for diffusion MRI in the heart. Most differences found in diffusion MRI techniques of the heart can be found in the diffusion encoding and how to mitigate the extreme signal decay due to bulk motion effects. Analysis of diffusion MRI has largely been focused on looking at fiber orientation (both sheet and fiber angles) and fiber tractography.

Results

The effect of motion will be shown in regard to different diffusion encodings and key results from recent literature will be discussed including stem cell therapy evaluation, differentiation between various cardiomyopathies, and detection of fibrosis.

Discussion

Bulk motion induced signal decay confounding diffusion weighted contrast can be largely mitigated by second order motion compensation spin echo and dual heart beat stimulated echo acquisition. Modeling of the diffusion tensor accurately reflects underlying myocardial fiber orientation without the need of higher order models due to lack of fiber crossings, which are usually found in the brain. Calculation of helix angle, helix angle transmurality, sheet angle (E2), and other fiber orientation angles are largely congruent across most groups performing diffusion MRI but differences may still exist in the representation of such parameters (such as windowing or range). Future technical advances will be made in how to analyze the diffusion MRI-based myocardial microstructure for the entire heart and register between groups of subjects.

Conclusion

Diffusion MRI is a wonderful tool to probe the myocardial microstructure and can be leveraged to identify various cardiomyopathies. It has the added benefit of using endogenous contrast mechanisms allowing for full use in renal insufficient patients. With the recent technical advances, cardiac diffusion MRI is in an exciting phase to reveal unique biology and pathophysiology without worrying about the inherent technical challenges it has faced the last 20 years.

Acknowledgements

No acknowledgement found.

References

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