Myocardial viability is a vital clinical concern in a patient of ischemic heart disease prior to any revascularization procedure. Among various modalities used for viability assessment, DCE-CMRI has been proven to be extremely accurate. The conventional cardiac viability protocol can last for 60–90 min and consists of localizers followed by dark blood and bright blood imaging for the 3 orthogonal views and the cine breath hold steady state free precision sequence for assessing wall motion, end-diastolic wall thickness, and ejection fraction. This is followed by gadolinium administration, TI scout, and phase-sensitive inversion recovery images in SA, VLA, and HLA views [11].
Although cardiac MRI is a versatile and highly accurate imaging modality, there are several practical limitations to the use of the technique. Dickstein (2008) mentioned that compared with CT scans and cardiac ultrasound, cardiac MRI examinations are time-consuming and prolonged examination durations can be expected as most routine cardiac MRI techniques are dependent upon image acquisition during breath hold to eliminate movement artifacts [12]. Both image quality and the duration of the examination are highly dependent upon the patient’s ability to do repeated performance of breath hold of 5–10 s. In patients who are unable to perform breath hold correctly, a poor image quality must be anticipated that can be proven difficult for satisfactory interpretation of images. Hence, in this study, the proposed non-breath-hold rapid cardiac MRI protocol was used. A free-breathing protocol precludes the use of cine imaging. These cine sequences provide vital information about regional wall motion abnormalities apart from assessment of cardiac function and end-diastolic wall thickness. Presence of LGE in an area of regional wall motion abnormality denotes non-viable myocardium. Recent evidence suggests that late gadolinium enhancement may be independent of regional wall motion abnormality in predicting myocardial viability [9].
The two most important predictors for predicting recovery of the ventricular function after revascularization procedures are the extent of infarct as assessed by LGE and end-diastolic wall thickness (EDWT) which can be assessed by the images in SA view in bright blood imaging sequence. However, Krittayaphong et al. in 2008 compared the value of LGE and EDWT assessed by cardiovascular magnetic resonance in predicting recovery of left ventricular function after coronary artery bypass graft surgery (CABG). The study concluded that LGE and EDWT are independent predictors for functional recovery after revascularization. However, LGE appears to be a more important factor and independent of EDWT [13]. Various studies have also postulated that delayed gadolinium enhancement can effectively demonstrate the extent of infarct and the long-term prognosis as well as the incidence of adverse events after revascularization. A study by Lee et al. in 2016 evaluated the impact of delayed gadolinium enhancement on the long-term progress in patients undergoing CABG and concluded that the extent of delayed contrast enhancement is a strong predictor of adverse cardiac events independent of left ventricular function [14]. In 2018, Lim et al. also concluded that the extent of myocardial viability as assessed by cardiac magnetic resonance late gadolinium enhancement appears to identify patients with a differential survival benefit from CABG [15]. In a study by Gerber (2012), it was concluded that the presence of dysfunctional viable myocardium by delayed contrast enhancement is an independent predictor of mortality in patients with ischemic LV dysfunction [16]. We studied feasibility of non-breath-hold abridged viability protocol to predict myocardial viability using late gadolinium enhancement as a sole marker for viability. Estimation of glucose metabolism by 18F-FDG PET has been reported to accurately differentiate between viable and non-viable myocardial tissue in many studies [17, 18] and, thus, was used as a reference method.
Out of the total evaluated 120 coronary arterial territories on DCE-CMR (3 vascular territories per patient): 15 patients had non-viable LAD territory, 13 patients had non-viable RCA territory, and 7 patients had non-viable LCX territory. When the results of the vascular territories were compared between DCE-CMR and 18F-FDG PET only 5 out of 120 vascular territories show discrepant results. Fifteen patients showed non-viable myocardium on DCE-CMR. Out of these, 14 patients were showing similar findings in 18F-FDG PET (Figs. 1 and 2). The results of this study demonstrate a moderate to strong agreement between DCE-MRI and 18F-FDG PET (Figs. 3 and 4) Agreement between two modalities was obtained in 595 myocardial segments (88.5%), resulting in a rho value of 0.62.
In our study, free-breathing DCE-CMR protocol is seen to be highly sensitive and accurate on a per coronary arterial territory basis. These results are comparable with the results of Li et al. who have compared the use of late gadolinium enhancement on breath hold cardiac MRI with a 18F-FDG PET scan [18].
Segment to segment analysis for discordant results was done by a nuclear medicine specialist (12 years’ experience) and radiologist trained in cross-sectional imaging (11 years’ experience), and the following reasons were attributed to for the discrepancy on segment to segment analysis.
MRI can delineate segments more accurately, because the border between enhanced and normal areas is distinct, whereas in nongated18F-FDG PET images, the border between normal and defect areas is less well defined. Moreover, for the semiquantitative analysis of the segments, polar plots were used which tend to average out the segments and hence give erroneous readings at the border segments. All comparative studies have the potential for anatomical misalignment. Although short-axis views were used in both modalities, the visualization of the inferior interception of the ventricles is especially difficult in patients with extensive scar tissue, because regions are not always clearly definable in 18F-FDG PET. Hence, there is a difference in specificity when comparing individual segments and coronary territories.
Assessment of wall thickness is limited by the spatial resolution of nongated18F-FDG PET, and epicardial tracer activity may mask small subendocardial defects. Therefore, MRI, with its better spatial resolution provides excellent delineation of scar tissue as compared to 18F-FDG PET. FDG is a marker for viability, whereas delayed contrast enhancement is considered a marker for scar tissue. Thus, a relatively small number of viable cells may show increased FDG uptake, indicating viability, whereas structural changes may already coexist and alter gadolinium kinetics. Therefore, 18F-FDG PET may show viability in segments with LGE, depending on the relative contribution of viable and fibrotic tissue. Third, an increase in regional signal intensity (due to gadolinium) may be easier to interpret than a regional comparison of flow and metabolism by PET. Authors postulate these reasons to explain the occurrence of three false negative cases (in coronary territory wise analysis) in our study. A high intracellular glucose level is unlikely to make the cells hungry for glucose when tracer is injected; hence, these cases are not likely to show any uptake of tracer despite the territory being viable. This mechanism could possibly explain the occurrence of two false positive cases (in coronary territory wise analysis) in our study. The traditional method used in viability imaging includes presence of regional wall motion abnormality in the segment showing delayed contrast enhancement. We have not done routine breath hold cine sequences, and thus, the study does not have data about regional wall motion abnormalities. Despite this, our results have shown that LGE alone (even when done in free breathing) can answer the clinically pertinent question about presence or absence of viable myocardium so as to decide the further line of treatment.
As per the authors’ knowledge, no other study, using a completely free-breathing protocol exists. In the present study, the short acquisition times and high spatial resolution of the DCE-CMR allowed to perform the cardiac examination in a short time with high definition of myocardial scarring. DCE-CMR as a technique stands on its own with comparable sensitivity and specificity with 18F-FDG PET and better morphological and functional delineation of the myocardium. Cardiac MRI has following additional advantages in assessment of myocardial viability in patients of IHD:
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18F-FDG PET requires elaborate patient preparation unlike DCE-CMR
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There is no radiation exposure.
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Better availability of MRI scanners as compared to PET scanners.
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Differentiation of ischemic and non-ischemic cardiomyopathies.
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Cheaper alternative to 18F-FDG PET.
The proposed modified DCE-CMR free-breathing protocol offers the advantages of markedly reduced scan time (20–25 min), better acceptability by patients, and efficient management of gantry time.