CMR in Phenotyping the Arrhythmic Substrate

The purpose of this review is to explore the role of cardiac magnetic resonance imaging (CMR) in the evaluation of myocardial disorders that can present with significant cardiac arrhythmias. In addition, we explore recent developments in the field of CMR in the evaluation of such conditions, such as T1 and T2 mapping techniques. The importance of CMR as a diagnostic tool in the evaluation of such conditions lies in the fact that in routine clinical practice these conditions are encountered relatively frequently, and often important clinical decisions are made based on the findings of cardiac investigations such as CMR, e.g., decisions surrounding revascularization or implantable cardioverter defibrillator (ICD) therapy. In fact, CMR is now included in many practice guidelines (American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines et al. in J Thorac Cardiovasc Surg, 142(6):e153–203, 2011; Authors/Task Force members et al. in Eur Heart J, 35(39):2733–79, 2014; Epstein et al. in Heart Rhythm, 5(6):934–55, 2008; Maron et al. in J Am Coll Cardiol, 42(9):1687–713, 2003) and is routinely used in the evaluation of cardiomyopathic and ischemic myocardial conditions in many institutions. Although late gadolinium enhancement (LGE)-CMR allows a very sensitive and reproducible qualitative assessment of myocardial replacement fibrosis, it is limited in regards to absolute quantification of myocardial fibrosis, and also, the assessment of diffuse fibrosis by CMR can be technically challenging. As a result, in recent years, T1 and T2 mapping techniques have been welcomed by the CMR community, as potential methods of quantitating myocardial fibrosis accurately. Areas of diffuse myocardial fibrosis have greater T1 values (by about 10–20%) than normal tissue, before intravenous Gd is given. Post-Gd administration, T1 values are lower than normal in diffuse myocardial fibrosis. Thus, using this technique maps can be generated that explore the myocardium in detail and can detect early myocardial fibrosis, often before the presence of LGE. T2-weighted CMR identifies myocardial edema before the onset of irreversible ischemic injury and has shown to have value in risk-stratifying patients with chest pain (Eitel and Friedrich in J Cardiovasc Magn Reson, 13:13, 2011). Clinical acceptance of T2-weighted CMR has, however, been limited by well-known technical problems associated with existing techniques. T2 quantification using T2 mapping in the future should overcome these problems. Both T1 and T2 mapping techniques are discussed in further detail below. The assessment of infarct heterogeneity, as measured by peri-infarct zone (PIZ) mass and percentage PIZ (%PIZ), as compared to normal myocardium, is a valuable CMR tool for the assessment of tissue characteristics in the peri-infarct region (Morgan and Kwong in Curr Treat Options Cardiovasc Med, 17(11):53, 2015). It has recently been shown to have useful prognostic information (Watanabe et al. in Circ Cardiovasc Imaging, 7(6):887–94, 2014) in patients with coronary artery disease (CAD) and left ventricular (LV) dysfunction referred for CMR. This is discussed in further detail below. A recent paper by Neilan et al. (JACC Cardiovasc Imaging, 8(4):414–23, (2015)) studied 137 patients who underwent CMR post-resuscitated cardiac arrest and found that in cases where the cause of sudden cardiac death (SCD) was not initially clear, the use of CMR provided a diagnosis in 104 patients (76%). In a multivariable analysis, the strongest predictors of recurrent events were the presence and the extent of LGE (p < 0.001). In summary, this paper, through recent evidence and clinical examples, explores those myocardial conditions which pose a significant clinical issue in the form of potentially dangerous cardiac dysrhythmias. Early detection of predictive myocardial characteristics beyond the traditional risk factors has been welcomed by cardiology and cardiac imaging communities. Much work is in progress to make new methods such as T1 and T2 mapping more clinically available and applicable and to ensure that imaging time is not lengthened beyond that which is comfortable for the patient.