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J Am Coll Cardiol, 2003; 41:1341-1348, doi:10.1016/S0735-1097(03)00158-X © 2003 by the American College of Cardiology Foundation |






* Medical Clinic I, Aachen, Germany
Department of Medical Statistics, University Hospital, Aachen, Germany
Department of Cardiology, Amsterdam, The Netherlands
Department of Clinical Physics and Informatics, Amsterdam, The Netherlands
|| PET Center, Vrije Universiteit Medical Center, Amsterdam, The Netherlands
Manuscript received October 9, 2002; revised manuscript received November 30, 2002, accepted December 18, 2002.
* Reprint requests and correspondence: Prof. Dr. Albert C. van Rossum, Department of Cardiology, VU University Medical Center, De Boelelaan 1117, Amsterdam HV 1081, The Netherlands.
ac.vrossum{at}vumc.nl
| Abstract |
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BACKGROUND: Contrast-enhanced MRI has been shown to identify scar tissue in ischemically damaged myocardium.
METHODS: Twenty-six patients with chronic coronary artery disease and LV dysfunction (mean ejection fraction 31 ± 11%) underwent 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET), technetium-99m tetrofosmin single-photon emission computed tomography (SPECT), and ceMRI. In a 17-segment model, the segmental extent of hyperenhancement (SEH) by ceMRI, defined as the relative amount of contrast-enhanced tissue per myocardial segment, was compared with segmental FDG and tetrofosmin uptake by PET and SPECT.
RESULTS: In severely dysfunctional segments (n = 165), SEH was 9 ± 14%, 33 ± 25% (p < 0.05), and 80 ± 23% (p < 0.05) in segments with normal metabolism/perfusion, metabolism/perfusion mismatch, and matched defects, respectively. Segmental glucose uptake by PET was inversely correlated to SEH (r = 0.86, p < 0.001). By receiver operator characteristic curve analysis, the area under the curve was 0.95 for the differentiation between viable and non-viable segments. At a cutoff value of 37%, SEH optimally differentiated viable from non-viable segments defined by PET. Using this threshold, the sensitivity and specificity of ceMRI to detect non-viable myocardium as defined by PET were 96% and 84%, respectively.
CONCLUSIONS: Contrast-enhanced MRI allows assessment of myocardial viability with a high accuracy, compared with FDG-PET, in patients with chronic ischemic heart disease and LV dysfunction.
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The aim of this study was to compare ceMRI with nuclear metabolic imaging using 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) for the detection of myocardial viability in patients with chronic ischemic heart disease and LV dysfunction.
| Methods |
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FDG-PET imaging
All patients underwent hyperinsulinemic-euglycemic clamping (20). Scans were performed in a two-dimensional mode, using an ECAT EXACT HR+ (Siemens/CTI, Knoxville, Tennessee), after an intravenous injection of 370 MBq of FDG. The dynamic scan consisted of 39 frames with variable frame lengths for a total time of 60 min. All dynamic scan data were corrected for physical decay of 18F and for dead time, scatter, and random and measured photon attenuation. The images were reconstructed using filtered backprojection with a Hanning filter at the Nyquist frequency. This resulted in a transaxial spatial resolution of
7 mm full width at half maximum. Additionally, each patient underwent technetium-99m-tetrofosmin single-photon emission computed tomography (SPECT) for assessment of rest blood flow.
Data analysis.
Segmental model
For each imaging modality, an identical 17-segment model was used dividing the LV into six basal, six midventricular, and four distal segments, and the apex (21). The basal, midventricular, and distal segments were evaluated in short-axis images, whereas the apical cap was evaluated in the two-chamber long-axis view with ceMRI and in the vertical long-axis view with PET and SPECT. By convention, the most basal short-axis slice used for analysis was located just below and exclusive of the LV outflow tract.
Magnetic resonance imaging
All MRI images were first previewed on a personal computer work station, using commercial software (Radworks version 5.0, Applicare Medical Imaging, Zeist, The Netherlands). To compose the basal, midventricular, and distal segments, MRI data of a maximum of three short-axis slices were averaged. All short-axis slices were projected on the two-chamber long-axis view and were allocated to the different positions according to their relationship to the papillary muscles: the midventricular slices at the level of the papillary muscles and the basal and distal slices above or below the papillary muscles. Each basal and midventricular slice was divided into six equidistant sectors angulated 60° apart starting from the posterior insertion of the right ventricular free wall into the LV myocardium. The distal slices were segmented into four equidistant sectors angulated at 90°.
Evaluation of contrast-enhanced images
The MASS software (Medis, Leiden, The Netherlands) was used for quantitative analysis. Each myocardial sector was evaluated for the presence of hyperenhancement, defined as an area of signal enhancement
3 SD of the signal of non-enhanced myocardium. The total myocardial area and contrast-enhanced area per sector were traced manually. The extent of contrast enhancement was expressed as a percentage of the total myocardial area (
, where A denotes area) (Fig. 1). The MRI data of corresponding sectors on different short-axis slices used to compose one myocardial segment were averaged to give one final value for the segmental extent of hyperenhancement (SEH).
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FDG-PET
Data were analyzed blinded to the MRI results and patient data, using a SUN work station (SUN Microsystem, Inc.) with Siemens/CTI software. Transaxial images were reoriented according to the anatomic axis of the heart. Reconstructed slices were displayed as short-axis slices and horizontal as well as vertical long-axis slices. Short-axis slices were oriented in the same way as described for ceMRI, using the posterior insertion of the right ventricular wall with the LV as a landmark. Regions of interest (ROIs) were defined manually on each of the short-axis slices using the same segmentation model as for ceMRI. Corresponding ROIs from a variable number of slices were grouped in each patient to compose the 17 segments. For each segment, mean tracer uptake was calculated. Uptake of FDG in each segment was normalized to the myocardial segment with maximal tetrofosmin uptake.
Definition of viability by PET
Segments with normal perfusion (tetrofosmin uptake
50%) and metabolism (FDG uptake
50%) and segments with reduced perfusion (tetrofosmin uptake <50%) and normal or increased metabolism (mismatch) were considered viable. Segments with reduced perfusion and reduced metabolism (matched defect) were considered non-viable (2225). In two patients with a left bundle branch block, dysfunctional segments in the septum demonstrating FDG uptake <50% were considered viable if tetrofosmin uptake exceeded 50%, as FDG-PET may underestimate viability in the septal region in the presence of left bundle branch block (26).
Statistics. Data are expressed as the mean value ± SD. To compare the segmental results for EDWT, wall thickening, SEH, and FDG uptake by PET, depending on cardiac function, and to compare the segmental results for EDWT, wall thickening, and SEH, depending on the viability status as defined by PET, the unpaired Student t test at the Bonferroni-adjusted individual significance level (0.05/24; number of comparisons = 24) was performed. Recently described non-parametric analysis of overall sensitivities and specificities, as well as areas under the receiver operator characteristic (ROC) curves, were applied (27,28). The area under the ROC curve (AUC) was considered as a measure of accuracy of ceMRI to discriminate between viable and non-viable myocardium, as defined by PET. The ROC curve analysis was also used to assess the optimal cutoff point of the increase of SEH, as determined by ceMRI for the detection of segments with myocardial non-viability. Sensitivity and specificity were determined for viability or non-viability, as defined by PET. Computations were performed using SAS version 8.02 for Windows.
| Results |
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37% SEH was identified to yield optimal sensitivity and specificity for the differentiation of viable and non-viable segments defined by FDG-PET. Using this cutoff value, 100 segments were assessed as viable and 65 segments as non-viable by ceMRI. Compared with FDG-PET, the sensitivity and specificity of ceMRI for the identification of non-viable myocardium were 96% and 84%, respectively. Table 4 displays different sensitivity and specificity levels derived from the ROC curve for the detection of non-viable myocardium, as defined by PET, according to different thresholds of SEH, as assessed by ceMRI.
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37% in 24 segments (63%). Uptake of FDG was lower in the mismatched segments with SEH >37% compared with the mismatched segments with SEH
37% (54 ± 3% vs. 71 ± 9%; p < 0.001). Figure 4 shows representative images of a patient with akinesia of the inferior wall, subendocardial hyperenhancement on ceMRI, and a metabolism/perfusion mismatch by PET and SPECT.
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| Discussion |
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Comparison with previous studies.
The results of the present study compare favorably with previous reports relating the extent of fibrosis to FDG-PET findings. Maes et al. (29) observed a similar extent of percentage volume fibrosis assessed on the basis of myocardial biopsy data (35 ± 25%) in segments showing non-viability by PET. Dakik et al. (30) demonstrated that
30% of transmural scarring correlated with a lack of improvement in function after revascularization. In a recent animal experiment studying the relationship between ceMRI and regional inotropic response, Gerber et al. (31) demonstrated that only segments with <33% transmural extent of CE had inotropic reserve during dobutamine infusion, whereas segments with a higher (>33%) transmural extent of CE did not. The results of the present study are also in close agreement with a recent report by Klein et al. (32), who evaluated ceMRI and FDG-PET for viability assessment in a similar patient population. In their study, the AUC was 0.93, with a sensitivity of 86% and a specificity of 94% for the detection of non-viability by FDG-PET. Additionally, a high correlation was reported between semiquantitative estimates of scar severity by ceMRI and PET (r = 0.91). Similarly, we observed a strong correlation between the segmental extent of scar by ceMRI and FDG uptake by PET (r = 0.86). This finding closely agrees with the results of a previous study relating the amount of fibrosis at histologic examination with regional thallium-201 activity (r = 0.85) in patients with chronic ischemic heart disease (33). Other functional parameters that have been useful for the characterization of myocardial viability, such as regional wall thickness and wall thickening (5), correlated less well with FDG-PET, which is in line with the results reported by Klein et al. (32).
A high concordance was found between ceMRI and PET for assessment of the viability status of dysfunctional segments with normal perfusion/metabolism or a matched defect (Table 3). In segments with preserved metabolism but reduced perfusion (mismatch), reflecting hibernating myocardium, the results were less explicit. Sixty-three percent of segments scored viable and 37% of segments scored non-viable by ceMRI, using the threshold value of 37% SEH. The amount of enhancement averaged 33 ± 25% in this group of segments (Table 3). A similar extent of volume fibrosis at histologic examination in segments with hibernating myocardium of >6 months duration (41.9 ± 22.1%) has been reported in a previous study (34). It should be considered that myocardial viability is a gradual phenomenon and that the dichotomous definition of viability used in the present study for ceMRI is related to the dichotomous definition of viability for FDG-PET. Thus, a segment with 50% SEH, although scored non-viable by ceMRI in the present study, demonstrates a large rim of viable myocardium. Even if recovery of function after revascularization is unlikely, restoration of blood flow may contribute to prevent further ischemic injury and improve the clinical status and prognosis of the patient (34,35). Moreover, while the cutoff value of 37% SEH is mathematically the best threshold, one might wish to choose a different cutoff value to minimize the chances of missing viable myocardium. Thus, as demonstrated in Table 4, a threshold of 50% would increase the specificity of detecting non-viable myocardium, thereby increasing the amount of segments scored viable by ceMRI.
Study limitations. As in all studies comparing different imaging modalities, there is the possibility of image misalignment, which may account for some of the discrepancies between the FDG-PET and ceMRI results. Moreover, variability may be associated with averaging of MRI as well as PET and SPECT data, which was performed in order to compose the 17 myocardial segments. The MRI sequence used is susceptible to artifacts associated with patient movement or imperfect breath-holding, which can be erroneously interpreted as areas of hyperenhancement. Recovery of myocardial function after revascularization was not assessed in the present study. Thus, conclusions on the functional recovery of dysfunctional segments deemed viable by ceMRI cannot be drawn from the present study. Others have shown that recovery of function after revascularization is related to the transmural extent of hyperenhancement in patients with ischemic cardiomyopathy (17), with a gradual decrease in functional recovery paralleled by an increasing transmurality of hyperenhancement. In the study of Kim et al. (17), 73% (44/60) of segments with no hyperenhancement or with <50% transmural hyperenhancement improved function after revascularization. Based on these results, one might speculate that most of the dysfunctional segments assessed as viable by ceMRI in our study might have recovered function after restoration of adequate blood flow. Nevertheless, determination of contractile function after revascularization would be helpful to establish the relative importance of ceMRI and FDG-PET to predict functional recovery, which is considered an important outcome variable. Another limitation is that although the prognostic relevance of FDG-PET to predict morbidity and mortality is well established (2,36), data on ceMRI are scarce (37). Thus, the prognostic relevance of ceMRI needs to be established in future studies.
Conclusions. In patients with chronic ischemic heart disease and LV dysfunction, ceMRI allows detection of myocardial viability with a high accuracy, as compared with FDG-PET. Therefore, ceMRI should be considered as an alternative technique for assessment of myocardial viability in patients with chronic coronary artery disease and may be an alternative imaging modality in centers where FDG-PET is unavailable or less economical. Future studies should be directed at assessing the prognostic value of ceMRI to predict morbidity and mortality in patients with chronic ischemic heart disease and LV dysfunction.
| Footnotes |
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| References |
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