Ultra High Resolution CT to Assess Role of Intramyocardial Fat and Delayed Enhancement in Ventricular Arrhythmogenesis

The investigators aim to investigate the role of intramyocardial fat on ventricular arrhythmogenesis. Intramyocardial fat deposition has been frequently observed in patients with ischemic heart disease and is readily detectable by multi-detector computed tomography (MDCT) with high sensitivity and specificity, unlike other modalities. Like intramyocardial fat, reentrant ventricular tachycardia (VT) tends to occur late after the onset of myocardial infarction and the investigators hypothesize that lipomatous metaplasia within the infarct may precipitate later onset ventricular arrhythmias (VA). Prior studies have shown that intramyocardial fat correlates with slow myocardial conduction velocity and with critical circuits for VA but the investigators do not know the causal relationship between intramyocardial fat and future risk of VA. Intramyocardial fat deposition or lipomatous metaplasia has been frequently observed in patients with ischemic heart disease and is readily detectable by multi-detector computed tomography (MDCT) with high sensitivity and specificity. Like intramyocardial fat, reentrant ventricular tachycardia (VT) tends to occur late after the onset of myocardial infarction and the investigators hypothesize that there may be a causal relationship. Prior studies have shown that intramyocardial fat correlates with slow myocardial conduction velocity and with critical circuits for VT in patients referred for VT ablation who already manifest VA. However, the correlation of intramyocardial fat on CT with late gadolinium enhancement (LGE) on cardiac magnetic resonance (CMR) in a general population of patients with ischemic cardiomyopathy with no prior history of VA has not been reported. Specifically, it is unknown whether the presence, distribution and/or volume of fat is an independent predictor of VA. Further CMR is less widely available in medical centers, and is more expensive with longer scanning times compared to CT. CT provides higher spatial resolution, is widely available and is not as susceptible to magnetic interference from internal cardiac defibrillator (ICD) generators and thus makes it an attractive imaging modality for risk stratification, particularly longitudinally over time. Hypothesis: The investigators' objective is to define the prevalence and distribution of intramyocardial fat in patients with ischemic heart disease scheduled for or with in-situ implantable defibrillators. Further, the investigators aim to assess the independent association of intramyocardial fat with VA and determine whether it adds any utility above LGE measured by CMR. Finally, the investigators will assess how well delayed enhanced CT correlates with LGE on MRI and test its association with ventricular arrhythmias. Importance: The significance of the investigators' research is that the investigators will: 1) test whether intramyocardial fat on CT can be used as a non-invasive tool for sudden cardiac death risk stratification in patients who have or are scheduled to undergo ICDs, and 2) define whether delayed enhancement CT is comparable to the current non-invasive gold standard of CMR for identifying myocardial scar.