## 4 Additional CT-Measurable Parameters for Cardiovascular Risk Stratification
### Myocardial Scar  
An emerging application of non-contrast cardiac CT in detecting myocardial scars due to an occult MI has recently been demonstrated. The 10-year mortality related to unrecognized MI is estimated to be 45-50% [150]. Therefore, recognizing occult MI is crucial to identify that subgroup of patients who could benefit from intensive medical treatments, coronary revascularization procedures, or implantable cardioverter defibrillator implantation. The evidence of an occult MI also has prognostic value since it is considered a predictor of MACE recurrence and cardiac death. The areas of old MI appear hypo-attenuated due to fatty replacement on non-contrast cardiac CT or pre-contrast cardiac CT [151]. Specifically, myocardial fat associated with an old MI appears on CT as an area of subendocardial hypoattenuation following the culprit coronary artery [152; 153]. In this regard, [PERSON] et al. showed that left ventricle myocardial fat can be observed at least after three years from the MI with a subendocardial location [154]. Furthermore, in patients with old MI the different CT attenuation between normal and necrotic myocardium is more significant compared to patients with acute MI, suggesting that cardiac CT may help differentiate between recent and old MI [155]. Another interesting study evaluated the ability of non-contrast cardiac CT to identify chronic MI in patients with evidence of no-reversible perfusion alterations on nuclear myocardial imaging [156]. Sixty-two patients with non-reversible perfusion defects underwent CAC scanning and MI was identified as a hypo-attenuation area on CT. The study showed that non-contrast cardiac CT was able to identify prior MI in 57 patients, with a sensitivity of 92% [156]. Furthermore, the delayed enhancement cardiac CT, which shares a similar pathophysiological basis with delayed enhancement cardiac magnetic resonance, provides more specific information in this context. A delayed phase CT scan is generally performed 5-15 min after CCTA with the administration of an additional contrast medium. Iodinated contrast is an extracellular contrast agent with late washout from areas of abnormal myocardium, such as fibrosis or scar [157]. Therefore, cardiac CT with delayed enhancement can be used to identify myocardial scar or fibrosis in both ischemic and non-ischemic cardiopathy, similar to CMR with late gadolinium enhancement, which is actually the gold standard for tissue characterization. Compared with CMR, cardiac CT has some advantages, such as higher availability and shorter examination time. It can also be helpful in patients with CMR contraindications (i.e., claustrophobia and metallic implants). Several studies demonstrated that contrast-enhanced cardiac CT allows the identification and quantification of MI, showing a good agreement with CMR [158; 159; 160]. Specifically, two different contrast-enhancement patterns can be observed in case of MI: early hypo-enhancement, which is correlated to the microvascular obstruction in acute MI, and delayed hyper-enhancement, which is typical of chronic MI. Therefore, the ability of cardiac CT to characterize ischemic myocardium can be helpful in the clinical reality to stratify cardiovascular risk and also to manage clinical decisions. In addition, delayed enhancement cardiac CT may facilitate, in patients with acute chest pain and elevated cardiac biomarkers, the differentiation of myocarditis and ACS through the detection of the characteristic mid-wall or subepicardial delayed enhancement and the absence of significant coronary stenosis in patients with myocarditis. Indeed, [PERSON] et al. demonstrated in patients with acute chest pain and elevated cardiac troponin with negative triple rule out (TRO) CT scan that late contrast enhancement CT can increase the diagnostic rate identifying myocarditis, myocardial infarction with non-obstructed coronary artery (MINOCA) and Takotsubo cardiomyopathy [161]. For instance, [PERSON] et al.