Original ArticleNon-contrast cardiac computed tomography can accurately detect chronic myocardial infarction: Validation study
Introduction
The principal cause of morbidity and mortality in all industrialized nations is coronary artery disease (CAD). In approximately half of these patients, the initial presentation of CAD is either myocardial infarction (MI) or death.1 Unrecognized MI leads to less effective management and increased major adverse cardiac events (MACE). Population-based studies have revealed that one-fourth of MIs demonstrated by Q waves on the ECG are clinically unrecognized and estimates of the frequency of unrecognized MI detected in the Framingham study may well underestimate the true prevalence of the condition.2 The 10-year mortality from an unrecognized MI has been estimated to be 45% to 55%3,4 comparable to or even higher than in patients with diagnosed MI. Given the implications of unrecognized MI, studies are needed to address screening strategies, risk stratification, and the role of post-infarction therapies in these patients.5
Nuclear myocardial perfusion imaging (MPI) has been widely used as a non-invasive method for the diagnosis of CAD. In comparative studies with autopsy findings, thallium (Tl)-201 myocardial single-photon emission computed tomography (SPECT) has been shown to accurately assess the extent of MI6 and myocardial viability for the selection of therapy and for predicting prognosis.7, 8, 9 Although rest Tc-99m-methoxyisobutyl isonitrile (MIBI) underestimates the extent of viable myocardium compared with Tl-201 imaging, however several studies show that sublingual administration of nitroglycerin before a rest Tc-99m-MIBI study improves the detection of defect reversibility in exercise-induced perfusion defects and thus improves the assessment of myocardial viability in comparison with the standard Tc-99m-MIBI exercise–rest protocol.10,11
Coronary artery calcium (CAC) score measured through non-contrast cardiac computed tomography (CCT) scanning has been shown to predict future coronary events in both asymptomatic and symptomatic patients.12 Non-contrast CCT has also been used for assessing the pericardial fat,13 liver fat, and determining the field of view for CCT angiography (CCTA) to reduce radiation dose.14 The potential role of non-enhanced CCT to detect chronic MI, however, has not been well studied. Areas of chronic left ventricular (LV) infarction are readily apparent on non-contrast CCT as regions of hypo-attenuation either due to deposition of fibrous or adipose tissue15,16 or probably reduced capillary density.17
We conducted this study to evaluate the accuracy and feasibility of using non-contrast CCT to detect chronic MI in patients with irreversible perfusion defect on MPI. We also define the optimal cutoff value for LV myocardium Hounsfield unit (HU) to detect chronic MI.
Section snippets
Study Population
Our study included 122 consecutive symptomatic patients (with exertional angina or dyspnea) who were referred for coronary CAC scanning at our institution after undergoing an MPI study. Patients with reversible perfusion defects on MPI were excluded. Subjects with arrhythmia, renal failure, or hemodynamic instability were excluded. The characteristics of the study population are summarized in Tables 1 and 2.
Demographic data and clinical history were retrieved from the clinical records for all
Results
A total of 122 patients were studied, including 62 patients (51%) with an irreversible perfusion defect on MPI and 60 patients with a normal MPI. The mean age for this patient cohort was 63.5 ± 12 years and 71(58%) were males.
Out of the 62 patients with irreversible perfusion defect on MPI, non-contrast CCT accurately detected infarct in 57 subjects. Thus, on a per patient level and using MPI as the reference standard, non-contrast CCT showed a sensitivity of 92% (95% CI, 82.2-97.3),
Discussion
Contrast-enhanced CCTA has been shown to be comparable to MPI and contrast-enhanced cardiac magnetic resonance (CMR) in detecting fixed perfusion defect or scar as areas of late contrast enhancement.20, 21, 22, 23 Hypo-enhanced regions on contrast-enhanced CCTA have been shown to closely relate to delayed enhancement of iodinated contrast.24,25 The aim of this study was to evaluate the ability and feasibility of non-contrast CCT scans to detect chronic MI in patients with irreversible perfusion
Limitation
The primary limitation of this study is that we used irreversible perfusion defects on standard rest MIBI as the reference for detecting chronic MI on CAC scan. However, it should be noted that rest Tc-99m-MIBI underestimates the extent of viable myocardium compared with Tl-201 imaging, and thus some of the irreversible perfusion defects might represent hibernating viable myocardium. Thus, in this study some of the true positives on CAC scan could actually have been false positives. Sublingual
Conclusion
This study demonstrates that the non-contrast CCT has an excellent agreement with MPI in detecting chronic MI. Further prospective studies are warranted to evaluate the role of non-contrast CCT in infarct assessment for the management of at-risk patients.
Acknowledgments
None of the authors have received any funding for this study from any institution.
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See related editorial, doi:10.1007/s12350-010-9325-0.