ABC | Volume 113, Nº4, October 2019

Review Article Magalhães et al. Principles of MPCT Arq Bras Cardiol. 2019; 113(4):758-767 additional costs and time, which often makes this diagnostic workflow impractical. In view of the above, there was a desire to performmyocardial perfusion analyses with the MDCT itself, in the same procedure as the anatomical evaluation. This approach would involve the use of just one piece of equipment, reduced support staff, and reduced time and cost for exams. The initial attempts were made in commercially available equipment and showed favorable results, which confirmed the potential of a combined analysis, providing important data for an appropriate therapeutic management of such cases. 4,19,20 Initial positive expectations were strengthened as technology progressed, including dual energy use, increased detector numbers, and improvements in spatial and temporal resolutions. 4,21-23 This favorable scenario led to the development of an international multicenter trial designed to test the validity of a combined analysis of anatomy and perfusion by CT scan with the conventional way of investigating such patients, that is, angiography associated with scintigraphy. 24 This study has shown that it is possible to perform combined CT anatomy and perfusion evaluations safely and with very favorable results. 24 Nowadays, there are tomographs in Brazil with all characteristics necessary to ensure that such images are generated in accordance with what is described in this document. In addition, similar to what happened with the standardization of coronary angiotomography procedures by the National Agency for Supplementary Health Guidelines, the creation of a document on use is also educational and may hinder the indiscriminate use of diagnostic tests, thus avoiding the waste of resources in situations with no solid scientific evidence of the benefit these could bring. Therefore, the purpose of this paper is to discuss in more depth the characteristics of ischemia CT research, the technological and software prerequisites, and to define which subgroups of patients would benefit from this exam. Physiopathological rationale of coronary tomography myocardial perfusion Myocardial CTP is based on the principles of the theory of tracer-dilution, first developed by Stewart, 25 in the 19th century. Images of the heart are taken during iodinated contrast injection to assess its transit throughmyocardial microcirculation, allowing the construction of an time-attenuation curves in the aorta and myocardium, from which myocardial blood flow (MBF) and myocardial blood volume (MBV) can be defined. 26 Based on the principles of the theory of tracer-dilution on CT, 27 the higher the concentration of iodinated contrast in the intravascular space and myocardial microcirculation, the greater its attenuation; and the opposite is also true. However, diffusion into the extracellular space increases over time, and after one minute its extracellular concentration is greater than the intravascular. 28 Therefore, to obtain an accurate perfusion assessment, images must be acquired right in the beginning of the first contrast pass. Thus, in the first-pass imaging approach, the concentration of iodinated contrast is ideally proportional to MBF in a wide range of blood flows. Areas with lower (darker) attenuation upon first contrast passage are classified as hypoperfused territories and are visually and quantitatively evaluated in comparison to adjacent myocardial territories. 26 Another study has shown that first-pass perfusion imaging by helical CT correlates well with myocardial blood flow evaluated by microspheres. 29 This study confirmed the feasibility of performing an atherosclerosis and MBF assessment in a single CT scan, with the possibility of quantification and semi-quantitative analysis of perfusion data by attenuation curves. These theoretical considerations were later indorsed in a human clinical study from 2012, which confirmed the accuracy of MDCT assessment to detect coronary obstructions that cause myocardial ischemia. 30 CT myocardial perfusion imaging – Validation Although recently inserted in the clinical evaluation, Miocardial CTP has been subject of research for several years. In 2006, George et al. 31 used a canine model to determine the correlation between induced epicardial stenosis and perfusion defects identified by CT, with myocardial perfusion having microspheres as reference. The favorable results encouraged further clinical studies comparing, the additional value of the combination between Coronary computed tomography angiography (CCTA) and Myocardial CTP with the use of CCTA alone. Rocha-Filho et al. 22 showed an increase in accuracy in the combined assessment (CCTA + Myocardial CTP) compared to CCTA alone in the diagnosis of significant coronary stenosis. Adding Myocardial CTP to the strategy improved the accuracy from 0.77 to 0.90 (area under the ROC curve) in detecting stenoses. Promising data from single-center clinical studies prompted the study CORE320. 24 This is a multicenter study in which the combined use of CCTA + Myocardial CTP to detect flow‑limiting stenoses defined by obstructions >50% associated with perfusion defects was tested, defined by the combination of myocardial perfusion scintigraphy (SPECT-MPI) and cardiac catheterization. When considering all patients, the combination protocol achieved an accuracy of 87% for the definition of disease and 93% when considering only patients with no history of previous coronary disease. Data from this same study evaluated the performance of Miocardial CTP alone in the diagnosis of significant stenosis identified by catheterization alone, compared to MPC. 32 The accuracy of MDCT, defined by the area under the ROC curve, was greater than that of MPC (0.78 vs. 0.69, p = 0.001), mainly due to the higher sensitivity of the first method. Although the isolated use of MDCT for myocardial ischemia detection is not the end goal of tomography use, a recent study published by Takx et al., 33 described the diagnostic performance of MDCT related to other methods of myocardial ischemia analysis, taking invasive coronary FFR as a reference. In a per‑patient analysis, Myocardial CTP was shown to have a 93% accuracy in detecting flow-limiting coronary stenosis, while cardiac magnetic resonance and positron emission tomography had similar accuracy (94 and 93%, respectively). These values were statistically higher when compared tomethods traditionally used in myocardial ischemia analysis, such as MPC and stress echocardiography, with accuracy of 82 and 83%, respectively. The use of dynamic myocardial perfusion in the detection of myocardial ischemia has shown encouraging results validated by different reference techniques. 26,34-39 Clinical studies evaluating dynamic Myocardial CTP using invasive FFR as reference have 759