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The Prognostic Significance of Late Gadolinium Enhancement Cardiovascular Magnetic Resonance in Patients with Non-ischemic Cardiomyopathy

Late Gadolinium Enhanced (LGE) Cardiovascular Magnetic Resonance (CMR) imaging has a unique ability to characterize diverse mechanisms of non-ischemic myocardial injury. The pattern and extent of LGE findings have been studied across a wide range of cardiomyopathy states. To date, these collective findings provide strong justification for the use of LGE-CMR in the evaluation and prognostication of patients with Dilated, Hypertrophic, Restrictive and Inflammatory cardiomyopathies. This review article summarizes relevant studies in the field and highlights the clinical role of LGE-CMR in contemporary practice.

LGE imaging is typically performed in both short axis and long-axis imaging planes and is combined with spatially matched cine imaging, the latter providing high temporal resolution images of myocardial architecture and function.
Occasionally, LGE is also combined with T2-weighted imaging to identify the presence and extent of myocardial edema, a complimentary marker of acute injury (9,10).
Currently, such a comprehensive LGE-CMR study can be completed in 30 minutes by an experienced technologist.
While not the focus of this review, it is important to highlight rapidly expanding interest and clinical use of T1 mapping, a more quantitative application of LGE imaging.
Through dedicated pulse sequences a voxel-based "map" is generated whereby each is assigned a T1 value (in milliseconds). When acquired both before and following gadolinium, the registration of these images allows for voxelbased estimation of the extra-cellular volume (ECV) fraction (11). This may be particularly valuable for patients with nonischemic cardiomyopathy where fibrosis may be both regional and diffuse, the latter being challenging to characterize with conventional LGE techniques. This promises to expand the capacity of gadolinium-based CMR imaging to identify and prognosticate patients with cardiomyopathy.
The following review highlights studies identifying the prognostic value of LGE in a variety of non-ischemic cardiomyopathies. For ischemic cardiomyopathy, similar reviews can be found elsewhere (12,13).

Interpreting LGE-CMR: Visual versus quantitative techniques
It is prudent to first recognize variability by which LGE imaging has been studied by various investigators over the years, and how addressing this through standardized reporting is of priority for wide-spread clinical application as a prognostic tool. Two ways of evaluating LGE images exist; i) visual scoring with description of distribution pattern, and ii) signal-threshold based quantification using computer-assisted algorithms. While a combination of these approaches appears optimal, the latter has been challenging to introduce into routine clinical workflow. Visual scoring can be performed using a sub-segmental model (14) with good agreement with signal-threshold based techniques for total fibrosis burden.
The pattern of fibrosis is an important and signature feature associated with specific non-ischemic cardiomyopathy states. While 2-SD was originally applied for many studies, more recent studies have suggested this threshold to over-estimate fibrosis burden when compared to expert-adjusted manual threshold (16). An example of this technique is shown in Fig. 2.
Overall, the use of > 5SD has been shown to most closely approximate visually identified LGE in ischemic car-   (20). Numerous studies have now identified a 30-40% prevalence of non-ischemic fibrosis, typically described as a mid-wall septal "striae" pattern, on LGE-CMR in this referral population (5,21). While the pathophysiology of this phenomenon remains uncertain, the presence of any LGE (22) or specifically a septal striae pattern of LGE (5, 21, 23) has been associated with elevated rates of adverse cardiac outcomes in this population.   (25).

Hypertrophic cardiomyopathy (HCM)
Given high prevalence of HCM in the general population, The presence or extent of LGE seen in HCM does not appear to show strong association with underlying genotype (29).
When present, it is most typically of a mid-wall patchy distribution in hypertrophied segments (Fig. 1 Given an expanded focus on LGE extent (rather than binary presence) for risk stratification in this population, standardization for the reporting of LGE extent is of immediate priority.
Sub-segmental visual scoring is a validated option (14).
However, should signal threshold-based approaches be sought, a recent study critically examining all techniques identified the STRM > 3SD approach to most accurately reflect expert, manually adjusted thresholds for total LGE burden (16).

Cardiac sarcoidosis
The presence of granulomatous myocardial fibrosis in sarcoidosis is readily identified by LGE imaging, and may be found in 19-26% patients with systemic disease (7,39). The typical pattern of LGE identified in this population is that of dense, sub-epicardial fibrosis in the basal to mid anteroseptal and/or inferoseptal segments, however, a wide variety of LGE distribution has also been reported. The presence of LGE in this population has been associated with Sudden Cardiac Death (SCD) or ventricular arrhythmia (40), death or VT (41,42), and death, aborted SCD or appropriate ICD therapy (43).
Event hazards associated with LGE in this population have been reported to be as high as 31.6 (43 ms being predictive of all-cause mortality with a HR of 5.39 (59). This may be particularly valuable for patients with associated renal dysfunction.

Conclusions
LGE imaging has achieved a level clinical maturity that supports its routine use among patients with all forms of nonischemic cardiomyopathy. Meta-analyses have now been published summarizing strong prognostic utility in patients with NIDCM, HCM, sarcoidosis, and amyloidosis, supporting the consistent findings of numerous contributory studies. This work provided a foundation for larger, prospective cohort studies currently underway that will further clarify the role of LGE in these populations, such as the NIH-funded Hypertrophic Cardiomyopathy Registry (NCT-01915615) soon to LGE imaging in the management of patients with nonischemic cardiomyopathies is expected.