Myocardial Perfusion Imaging. Dual-Energy Approaches

The evaluation of patients presenting with symptoms sug- gestive of myocardial ischemia is one of the most common and challenging scenarios clinicians face. Despite consider- able advances in treatment, more than 50% of acute myocar- dial infarctions (AMI) resulting in death occur in patients before undergoing cardiac catheterization. Thus, risk stratifi- cation plays a central role in averting major adverse cardiac events [1]. The current WHO rating attributes more than 25% of deaths worldwide to cardiovascular disease (CVD). Despite a decreasing trend in the last decade, CVD is the leading cause of death in the United States and worldwide. On average there is approximately one CVD-related death every 40 s, resulting in the death of over 2000 Americans each day. The estimated direct and indirect cost of CVD in 2015 was $320.1 billion and is projected to be $918 billion by 2030. According to the current appropriate use criteria, coro- nary CT angiography (CCTA) is a robust imaging technique that provides a noninvasive, morphological assessment of the coronary arteries which can accurately depict coronary anatomy and atherosclerotic plaque burden. Thanks to its power to exclude significant coronary artery stenosis in patients with low and intermediate coronary artery disease (CAD) risk profiles, CCTA has become an integral part of the noninvasive diagnostic workup for the anatomic evaluation of the coronary arteries in patients with suspected CAD. A growing body of evidence has validated CCTA as the noninvasive imaging technique with the high- est sensitivity and specificity in detecting CAD, with a pooled sensitivity and specificity of 98% and 89%, respectively. These results compare favorably with alterna- tive noninvasive imaging tests, where SPECT reaches sensitivities and specificities of 88% and 61%, PET of 84% and 81%, and cardiac magnetic resonance imaging (CMR) of 89% and 76%, respectively. Although CCTA remains a morphological technique that can accurately depict coronary anatomy and atherosclerotic plaque burden, it is hampered by several limitations in the assessment of the hemodynamic significant coronary stenosis. The FAME and COURAGE trials, two major studies validating the impact of functional tests in coronary revascu- larization, have shown that the hemodynamic relevance of coronary stenosis is not adequately predicted by purely ana- tomical tests. Additionally, without functional data, ICA and CCTA can only provide limited correlation with myocardial perfusion defects. As revascularization should be guided by information on the state of myocardial perfusion, increasing efforts aim at determining the functional relevance of lesions by CCTA. Thus, noninvasive evaluation of patients with suspected CAD has started to shift focus from morphological CAD assessment to a complex, comprehensive mor- phological and functional evaluation. Furthermore, patient evaluation, management, and prognostication are more reli- able and effective when morphological and functional assess- ments are used in concert. Multiple CT techniques have the potential to provide a functional analysis. Some of these techniques are based on post-processing analysis of CCTA dataset and are focused on the direct assessment of coronary stenosis significance, such as CCTA-derived fractional flow reserve (CT-FFR) and transluminal attenuation gradient (TAG). CT-FFR relies on principles of computational fluid dynamics to calculate the ratio between the maximum coronary flow in the presence of a coronary stenosis and the hypothetical maximum coronary flow in absence of stenosis. Despite excellent results in terms of diagnostic accuracy, the only CT-FFR software that has been granted FDA approval to date requires complex offsite analysis. TAG represents the contrast attenuation gradient along the course of a coronary artery. The reliability of this technique is often hampered by extensive coronary cal- cifications or temporal inhomogeneity due to the acquisition window covering multiple heartbeats. The correlation between coronary density and the corresponding aortic attenuation at the same axial slice, formally known as CCO (corrected coronary opacification), has been proposed as a method to achieve more robust results. However, TAG and CCO have inferior diagnostic performance when compared to other functional tests. Other techniques based on CT data are focused on direct assessment of myocardial ischemia. Due to recent advance- ments in CT technology, in fact, in addition to its role in assessing coronary morphology and left ventricular function, CCTA has been utilized in the evaluation of a third aspect in the diagnostic algorithm of ischemic heart disease – myocardial perfusion. Computed tomography myocardial perfusion imaging (CTMPI) offers the possibility to directly detect the presence of perfusion defects in the myocardium following the administration of pharmacological stressing agent. Providing diagnostic information for each of these three cor- nerstones of ischemic heart disease workup, this emerging technology has the potential to become the stand-alone method for the evaluation of patients with suspected CAD using a single imaging modality and within a single imaging session.