Pre-procedural identification of arrhythmogenic substrates can be critical when performing ventricular tachycardia (VT) ablation procedures. This is particularly true in the setting of non-ischemic cardiomyopathy (NICM) which may have complex, patchy, and multifocal scar extending through the endocardial, intramural, and epicardial myocardium. Established techniques using late-gadolinium-enhanced cardiac magnetic resonance (LGE-CMR) or computed tomography rely on assessment of wall thinning, hypokinesis/akinesis, and cardiac-scar deposition; however, these gross structural and functional changes may be preceded by subtler perturbations in myocardial function which can be detected through strain imaging. Strain imaging also affords an opportunity to examine the mechanical properties of distinct myocardial layers by taking advantage of the longitudinal and circumferential distortion during systole. The use of CMR strain imaging in NICM has been validated across other imaging modalities and is gaining a foothold as a meaningful clinical tool [1, 2], but this modality has been underutilized as a tool for electrophysiologists.

With this in mind, Raja and colleagues set out to examine the correlation between CMR strain imaging and electroanatomic mapping (EAM) data obtained during VT ablation procedures. For further assessment of its clinical utility, findings were compared to the current gold standard for cardiac scar assessment, LGE-CMR. In this multicenter, retrospective report, nineteen consecutive patients with primarily idiopathic NICM underwent LGE-CMR imaging prior to ablation procedures and EAM, LGE-CMR, and CMR strain data were analyzed off-line to assess for agreement. LGE-CMR imaging demonstrated primarily septal scarring involving the intramural and/or epicardial layers. The authors assessed strain using a feature-tracking algorithm which allows for analysis of strain in multiple dimensions based off standard cine imaging. This semi-automated process can be obtained with less post-processing time than other techniques and does not require dedicated sequencing; as with other strain analysis techniques, discrepancies between different software packages using feature tracking have been reported [3].

Agreement between EAM and strain parameters was assessed by examining the global strain with the total tissue area displaying abnormal EAM voltages (defined by bipolar voltage < 1.5 mV and unipolar voltage < 8.3 mV) as well as by examining the segment-by-segment agreement of abnormal vs normal parameters between EAM, LGE-CMR, and CMR strains. There was only moderate agreement between the percentage of segmental abnormalities with combined circumferential and longitudinal strain with the % area of unipolar abnormalities (r =  + 0.5). LGE-CMR scar area had poor correlation with EAM defined scar (p > 0.05 for bipolar and unipolar maps). When looking at segmental agreement, abnormal peak strain showed agreement with EAM with high concordance rates of segments with abnormal peak longitudinal strain and segments with abnormal bipolar (92%) or bipolar or unipolar voltages (95%). A composite analysis of circumferential and longitudinal strain abnormalities demonstrated greater concordance than LGE-CMR in identifying segments with either abnormal bipolar or unipolar voltage (63% vs 89%).

The superior performance of strain over LGE-CMR in this study in part was driven by underdetection of scar by LGE-CMR among patients with a low burden (< 20% area) of abnormal bipolar or unipolar voltage. This is consistent with prior studies demonstrating the increased sensitivity of strain imaging for early detection of fibrosis [4]. Additionally, there were notable differences in EAM/strain correlation when comparing circumferential vs longitudinal strain which may be attributed to the complex nature of myocardial fiber orientation and scar deposition patterns in patients with NICM.

The authors should be congratulated on this manuscript which represents an important first step into understanding the potential application of CMR strain imaging to aide with VT ablations. Further work is needed to improve the interobserver agreement of strain assessment, to identify appropriate cut-offs for EAM and strain parameters, and better correlate different strain parameters with scar location/transmurally. Accurate assessment of cardiac strain through feature tracking is dependent upon cine images with high spatial and temporal resolution and is negatively impacted by atrial arrhythmias, intraventricular conduction abnormalities, and device-related imaging artifacts, all of which are often present in patients with advanced cardiomyopathy undergoing VT ablation. While LGE-CMR underperformed compared to strain imaging in this study, it offers multiple advantages over strain imaging including the ability to create complex 3D reconstructions of the myocardial architecture and scar including detailed delineation of scar transmurality and depth. Theoretical risks of additional magnet exposure required for cine imaging in patients with cardiac devices may be a barrier to more widespread use, and the risk/benefit ratio of additional strain imaging in these patients remains unclear. Given the complexities of NICM, it is possible that a combination of imaging parameters, including LGE-CMR, cine, and strain assessment used in complement, will provide the truest assessment of arrhythmogenic substrates. This novel study by Raja and colleagues is an encouraging first step to better understand the strengths and limitations of cardiac strain to help guide VT ablation procedures.