ABC | Volume 113, Nº6, December 2019

Original Article Ket et al. Myocardial ischemia by SPECT and CCTA Arq Bras Cardiol. 2019; 113(6):1092-1101 authors used another methodology with a higher slice CT system (320 detectors), as well as a slightly different protocol, which is not a problem, according with recommendations. 37 In contrast, other studies compared CCTA with SPECT and PET perfusion imaging with invasive catheterization with FFR, as a gold standard. Interestingly, perfusion PET was the exam that better correlated with the gold reference, whereas CCTA and SPECT performed similarly, showing that anatomic measures are not substitutes for functional assessment and that, even when the best method for anatomy assessment is used, functional assessment of coronary lesions is required. 36,38-40 Another finding of the study that needs discussion is the presence of 10 patients (28%) with abnormal SPECTwho did not present significant obstructive CAD on CCTA. Considering that CCTA is the anatomical method of reference in this study, we observed a high number of “false-positive” myocardial perfusion scintigraphy findings.We believe that a large part of these findings may be related with microcirculation disease (40%), since it was not possible to identify another cause that could explain them. The other findings (60%) were explained by anatomy assessment by CCTA. The best example is the case of a patient with myocardial bridge in which CT provided the anatomical substrate for the diagnosis of underlying myocardial ischemia detected by both SPECT and CT, already previously published by our research group. 41 With regard to scintigraphy, we observed that one of the studies presented low levels of the tracer, due to tracer leakage, that was not detected during the study and, therefore, was not excluded from the analysis. We believe that further studies need to be conducted in order to better clarify these findings, because they will affect clinical decision-making. There are several factors that can be potentially responsible for disagreements between the tests. Some of them are obvious, such as differences in spatial resolution between the techniques (CT has submillimeter resolution, whereas SPECT has a resolution of 6 mm) and the distinct contrast properties used: the 99m Tc-sestamibi exhibits a roll-off phenomenon, in which there is a limitation of its regional distribution when the flow is increased above certain threshold, while the same does not occur with iodinated contrast. 9,42-52 In the Brazilian context, in spite of the absence of nuclear medicine services, combined CCTA and myocardial perfusion imaging is available, thus we consider this method as a simple and enforceable strategy. Some aspects should be considered, such as the use of beta-blockers to reduce heart rate for CCTA imaging, which can have a relative influence on the ischemic area detectable by SPECT, especially in cases of microcirculation disease. Another aspect is obesity, because in these patients the quality of the images is worsened, which can cause disagreement between the techniques. Another point is that, in order to perform CT perfusion, the patient needs to be inside the equipment in the stress phase, which makes the use of pharmacological stress mandatory. If physical stress could be used, perhaps the results would have been different from what we found. 35,42,44 For CCTA, undoubtedly, the greatest limitation is exposure to radiation and iodinated contrast media, which are agents with potential adverse events. This protocol optimization, with new equipment, may be capable of reducing the levels of exposure; however, even so, the protocol shall only be adopted in selected patients, where information can be complemented. Studies using 320 detectors have shown that the combination of CT perfusion and CCTA can promote lower radiation exposure compared to the conventional protocol for myocardial perfusion imaging (9 mSv and 13 mSv, respectively). 35,36 Standardization of CT analysis is still a limitation, and the use of automatic analysis software is one of the priorities for technology development, since there are no polar maps yet, as in nuclear medicine, to display ischemic and normal patients for quantification of the level of ischemia, with validated and widely available software. Among other limitations of our study, as we detailed throughout the discussion, is the small number of individuals recruited. We believe that this is a partial limitation and should encourage further studies in different populations. We also took into account the false-positive scintigraphy results that might have influenced its performance, because we believe that the majority of cases can be explained by anatomy. Last but not least, one could imagine that the use of CCTA as an anatomical test would be limiting. In this case, numerous studies have compared CTA and catheterization with excellent results, which validates this approach. Conclusion Myocardial perfusion assessment byCCTA, after dipyridamole stress, is feasible and simple, with satisfactory results, when compared with SPECT, for obstructive CAD detection. Combined assessment of anatomy and stress perfusion by CCTA shows good capacity for detecting significant obstructive CAD, while ruling out SPECT false-positive findings. Author contributions Conception and design of the research, Analysis and interpretation of the data, Statistical analysis and Writing of the manuscript: Ker WS, Mesquita CT, Nacif MS; Acquisition of data: Ker WS, Neves DG, Mesquita CT, Nacif MS; Critical revision of the manuscript for intellectual content: Ker WS, Magalhães TA, Santos AASMD, Mesquita CT, Nacif MS. Potential Conflict of Interest No potential conflict of interest relevant to this article was reported. Sources of Funding There were no external funding sources for this study. Study Association This article is part of the thesis of master submitted by Wilter dos Santos Ker, fromUniversidade Federal Fluminense. Ethics approval and consent to participate This study was approved by the Ethics Committee of the Programa de Pós-graduação Ciências Cardiovasculares under the protocol number 392,966. All the procedures in this study were in accordance with the 1975 Helsinki Declaration, updated in 2013. Informed consent was obtained from all participants included in the study. 1098