Comparison of Cardiac MR with PET CT in Evaluation of Myocardial Viability: Single-Center Experience

Abstract

Aims and Objectives

The aim of this study was to compare the efficacy of various cardiac magnetic resonance imaging (MRI) parameters in assessing myocardial viability with positron emission tomography–computed tomography (PET-CT) as the gold standard.

Materials and Methods

This was a prospective analytical study comprising 28 adult patients with perfusion/metabolism matched or mismatched defects in PET-CT. These patients were taken up for MR evaluation, mainly assessing end-diastolic wall thickness (EDWT), regional wall motion abnormalities, and late gadolinium enhancement (LGE) for infarcted tissue. EDWT was measured manually and using semiautomatic software, and means were calculated.

Results

Of 448 (28 × 16) myocardial segments, 6% (n = 27) of the segments were considered not viable with matched defect, 56.5% (n = 253) were normal, and 37.5% (n = 168) were hibernating according to PET-CT. 51.1% (n = 86) of the hibernating segments showed no wall motion and 48% (n = 82) showed some wall motion, with a sensitivity of 66.03% (p < 0.001). There is good concordance of motion abnormalities detected using tagging with LGE. LGE and EDWT showed good sensitivity, 91.7 and 89.5%, respectively, as compared with PET CT in identifying viable but hibernating myocardium.

Conclusion

MRI parameter shows increased specificity and positive predictive value in identifying non-viable myocardium compared with PET/CT. All parameters had good sensitivity in identifying hibernating myocardium but had significantly low specificities.

Clinical relevance Cardiac MRI, when used in combination with PET CT, improves the sensitivity in identifying the viable but hibernating myocardium.

 

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Keywords

cardiac - gadolinium - myocardium - PET-CT

Introduction

Ischemic heart disease is a major problem worldwide, although the mortality attributed to this condition has gradually declined over the last decades in the west,[1] [2] [3] due to the improvement in health care and awareness among the people. However, it is still responsible for one-third of all deaths in people older than 35 years and remains the major cause of morbidity worldwide.[1]

In this era of precision medicine, imaging myocardial viability is very essential for providing adequate care to the patient. Viability of myocardium is assessed by various methods and techniques, including perfusion analysis, motion analysis, and anatomical analysis.[4] [5] [6] [7] [8] Studies have been done comparing the perfusion assessment of SPECT and cardiac magnetic resonance (CMR), proving the superiority of the latter mainly in terms of sensitivity in identifying the sub-endocardial infarct and small infarcts in the inferior wall in chronic as well as in acute infarction.[7]

Ample studies also show CMR is comparable with positron emission tomography (PET) in viability assessment.[4] [5] [6] However, widespread usage of CMR needs much more supportive evidence. The aim of this study was to evaluate the various CMR tools to assess viability in comparison to PET-CT perfusion assessment.

 

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Materials and Methods

Study Design

This was a prospective analytical study. A total of 28 consecutive adult (age >18 years) patients referred to PET-CT from the cardiology department for persistent heart failure symptoms in spite of prior surgical/percutaneous intervention, who showed matched or mismatched defects, were taken for MR evaluation typically within a week after the PET-CT completion. This study was done for a duration of one and a half years.

Patients with contraindications for MRI, with acute cardiac symptoms, with a history or suspicion of other causes of cardiomyopathy, severe arrhythmia, and structural heart diseases, were excluded from the study.

Informed consent was obtained from all the included patients. All guidelines as per the Declaration of Helsinki and good clinical practice guidelines were followed.

 

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Imaging Parameters

Patients with perfusion/metabolism matched or mismatched defects were evaluated with MR, assessing late gadolinium enhancement (LGE), functional cine images and tagging sequence to analyze the presence of scar, the presence or absence of residual regional contraction, and end-diastolic myocardial thickness in the hibernating myocardium.

 

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Image Acquisition

• PET-CT data are acquired with 13N ammonia and fluorodeoxyglucose (FDG) for calculating the perfusion and metabolic defects.

• All CMR examinations were performed on a 3T system (Ingenia; Philips Healthcare, Best, the Netherlands). MRI sequence is as follows:

• – Dark blood—axial.

• – Bright blood balanced turbo field echo—axial.

• – Cine—horizontal long axis.

• – Cine—vertical long axis.

• – Cine—short axis.

• – Dynamic perfusion.

• – Early gadolinium enhancement.

• – LGE.

• – Tagging—grid.

 

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Image Analysis

PET-CT perfusion data and CMR were analyzed by two nuclear medicine specialists and two radiologists, respectively. Any discrepancy was sorted by mutual consensus. Radiologists were blinded to the PET-CT results to improve objectivity. Image reporting was done after segmentation of the myocardium as per the American Heart Association (AHA) model ([Fig. 1]). The AHA model helped to compare the MRI and PET-CT data objectively.

Fig. 1 Polar plot of myocardial segments of the left ventricle (the American Heart Association model).

Patients with defects in the 13N ammonia perfusion study and FDG study were deemed as matched defects in the particular segment ([Fig. 2]). However, no defects in the FDG study were deemed as mismatched defects in the particular segment. Segment-wise data were collected for all the segments of myocardium, similar to the MRI data ([Fig. 3]).

Fig. 2 (A–F) A 53-year-old man with chest pain and dyspnea for the past 6 months with an ejection fraction of 30%. PET NH3 vertical long axis stack images showed perfusion defect along the anterior wall of the left ventricle; corresponding FDG images show minimal uptake, suggesting a matched defect–non-viable myocardium. (G-I) Cine short-axis images of the same patient at the end-diastolic phase show diffuse thinning of myocardium more pronounced along the septum (end-diastolic wall thickness 3.7 mm)–non-viable (G). On tagging (H) and cine sequences (not shown); there was grade 4–severe hypokinesia. (I) Vertical long–axis section shows diffuse myocardial thinning along the anterior wall of the left ventricle with subtle but near–complete enhancement (grade 3) s/o non-viable myocardium in concordance with PET result.

Fig. 3 (A–F) A 62-year-old male with chronic LV dysfunction short-axis stacks. PET NH3 (A–C) shows a defect in the basal inferior segment, corresponding FDG images (D–F) show uptake, suggesting a mismatch defect—viable but hibernating myocardium. (G–I) Cine short-axis images of the same patient showing thinning with T2 hypointensity in the basal inferior segment end-diastolic wall thickness was ∼4.7 mm (G), showing subendocardial late gadolinium enhancement (<25%) (H). On tagging sequence, poor grid crunching in the basal inferior segment is seen (I).

A Likert scale was utilized in reporting regional wall motion abnormalities (RWMA) and LGE. Scores 1 to 5 represent normokinesia to akinesia or dyskinesia. Scores 1 to 4 were assumed as characteristics of viable myocardium. Score 5 is assumed to be the characteristics of non-viable myocardium. RWMA was analyzed using both cine and grid sequence (tagging). The grid sequence compartmentalized the myocardial thickness into three layers from the inner to outer surface. Score 1: if all the layers are contracting equally, Score 2: if the outer 2 layers are contracting, Score 3: if only the outer layer is contracting, Score 4: if there is sluggish contraction not enough to place in any of the grades, and Score 5: if no contraction or dyskinetic contraction. Grading was given to reduce the subjectivity between the observers, and the grid sequence was also helpful for the same purpose.

Grades 0 to 4 were given according to the extent of the enhancement from subendocardial to transmural. In our study, LGE was graded according to the percentage of transmural involvement into grade 1 (0–25%), grade 2 (26–50%), grade 3 (51–75%), and grade 4 (76–100%). Grades 1 and 2 were considered viable and grades 3 and 4 were considered non-viable.[9]

Left ventricular ejection fraction (EF), end-diastolic and systolic volumes, and stroke volumes were calculated for each patient along with the amount of enhancement for the entire myocardium. Semiautomatic analysis was used for calculating the EF from the MRI data.

 

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Statistical Analysis

Analysis was done for individual myocardial segments with comparison of various cardiac MRI parameters with PET-CT data. All the statistical tests were two-sided and were performed at a significance level of α = 0.05. Concordance and discordance were calculated by the method of the kappa test of agreement. Sensitivity and specificity of each test were assessed using the area under the curve generation for both modalities.

 

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Results

A total of 28 patients aged between 31 and 87 years (mean = 51.5 years) were included in the study. All the patients were symptomatic at the time of study, with 71.4% having dyspnea, 21.4% having orthopnea, 21.4% having chest pain, and 17.9% having pedal edema as chief presenting complaints. Out of these, 32.14% (n = 9) had triple vessel disease. The distribution of vessel disease is as follows. Nearly all the patients had a previous history of acute coronary syndrome and were treated either with coronary artery bypass graft (CABG) (46.4%; n = 13) or percutaneous coronary intervention (50%; n = 14). All of them were on anti-failure drugs at the time of the study.

Mean EF calculated using echocardiography was 27.54% (27–32%) with SD of 8.47, and mean EF calculated using cardiac MRI semiautomatic software was 28.96% with SD of 12.05. There is good agreement between EF MRI and EF echo (p < 0.005).

According to the PET results, 6% (n = 27) of the segments are considered not viable with a matched defect. Remaining segments were considered viable, which were either normal (56.5%; n = 253) or hibernating—having a mismatched defect of 37.5% (n = 168). Out of 168 mismatch segments, 79.8% (n = 134) had normal wall thickness, whereas 55.6% (n = 15) of the matched segments also showed normal wall thickness. End-diastolic wall thickness (EDWT) results had very good sensitivity but poor specificity as far as PET results are concerned (p < 0.005).

Distribution of LGE Grading with Pet Results

Out of 168 mismatched segments, 33.3% (n = 56) showed no enhancement; 23.8% (n = 40) showed subendocardial enhancement; 28% (n = 47) showed transmural enhancement but less than half of myocardial thickness; 14.8% (n = 15) of the mismatched myocardium were considered non-viable by LGE ([Fig. 4]); 55% (n = 15) of the myocardium considered non-viable by the PET was considered viable according to LGE, a majority of them showing transmural enhancement involving less than half of the myocardium (37%, n = 10). Out of 253 normal myocardium according to PET, 83% (n = 210) showed no enhancement; 13.1% (n = 33) showed enhancement but considered viable; and 4% (n = 10) were considered non-viable.

Fig. 4 (A–F) A 57-year-old male with LV dysfunction. PET NH3 short-axis images show perfusion defect in the basal inferolateral segments (A); corresponding FDG images also show poor uptake (B) s/o matched defect–non-viable myocardium. Cine short-axis image (C) of the same patient in end-diastolic phase shows focal thinning in the basal inferolateral segment; however, average end-diastolic wall thickness of 6.8 mm (F) which is suggestive of a viable myocardial segment. LGE sequence (D) shows grade 2 (less than half of myocardial thickness) late gadolinium enhancement, suggesting viable myocardium discordant with the PET results. Tagging sequence (E) of the same patient showed grade II contraction.

Out of 168 mismatched segments, 68.5% (n = 115) showed some wall motion, whereas 31.5% (n = 53) did not show any movement or dyskinetic movement; 29.6% (n = 8) also showed some movement. Using the tagging sequence, this percentage was reduced, thereby increasing the specificity, as only 11.1% of the matched myocardium showed some movement. But tagging did not show any significant difference in mismatched myocardium, as 51.2% of mismatched myocardium did not show any movement, thereby drastically reducing its sensitivity for the identification of mismatched myocardium identified by PET.

Out of 168 mismatch segments, 79.8% (n = 134) had normal wall thickness; 55.6% (n = 15) of the matched segments also showed normal wall thickness and 33.3% (n = 56) of them showed no enhancement; 14.8% (n = 15) of the mismatched myocardium were considered non-viable by LGE ([Fig. 5]).

Fig. 5 Distribution of late gadolinium enhancement grading with PET results.

 

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LGE versus PET in Variable LV Function

Patients were divided into two groups based on left ventricular (LV) EF echo <30% and >30%. Increased scar detection by cardiac MRI is seen in reduced LV function <30%. Overall, MRI detects a larger number of scars than PET. There is also increased accuracy, detecting scars in the lower LV functions (EF < 30%). There is a mild increase in specificity in higher LV function but is not significant (p > 0.005).

 

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RWMA versus PET

Out of 168 mismatched segments, 68.5% (n = 115) showed some wall motion, whereas 31.5% (n = 53) did not show any movement or dyskinetic movement; 29.6% (n = 8) also showed some movement. Using tagging sequence, this percentage was reduced, thereby increasing the specificity, as only 11.1% of the matched myocardium showed some movement.

 

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RWMA versus LGE

There is good agreement of motion abnormalities with LGE. The more the extent of transmural scar, the more severe is the hypokinesia of the involved segment ([Fig. 6]).

Fig. 6 Distribution of cine regional wall motion abnormalities with late gadolinium enhancement grading with PET results.

 

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Discussion

Among patients with CAD, fraction of the population showed chronic LV dysfunction, which is potentially reversible if successfully managed by either by CABG or percutaneous coronary intervention. These patients are shown to have ischemic but viable myocardium, otherwise known as hibernating myocardium.[10] [11] Hibernating myocardium is one whose contraction is chronically depressed because of chronic ischemia as an adaptive response due to chronic ischemia and the scope of regaining function on revascularization.[12] Stunned myocardium, which have different pathophysiology, is the prolonged dysfunction after revascularization and caused by multiple mechanisms as discussed in the review article by Conti[13] who has also described various ways of differentiating it.

PET with FDG uses glucose utilization as a marker of viability by reducing the plasma FFA with glucose loading or by fasting, causing the ischemic myocardium to preferentially use the FDG, the former being performed more commonly.[14]

As discussed previously, nuclear imaging techniques were considered the gold standard for viability assessment due to their high sensitivity and negative predictive value (e.g., FDG-PET has 92 and 87% sensitivity and specificity, respectively).[15] Large multicenter trials conducted by Beanland et al[15]—PARR1 and PARR2 that compared (FDG -PET) directed revascularization with standard care—showed that patients considered viable by FDG-PET on revascularization had improvement in myocardial function.[16] On comparison with SPECT, FDG-PET had better sensitivity and a good agreement of 82% with SPECT.[17] Deciding upon the optimal management protocol for these patients is a complex, multifactorial process that has to consider not only viability but also processes such as ischemia and remodeling.[18]

In a study by Selvanayagam et al, there was a strong correlation between the extent of LGE and the recovery of RWMA at 6 months (p < 0.001). The study stated that LGE MRI is a powerful predictor of viable myocardium and improvement of function after surgery, suggesting an important role for this technique as a viability assessment tool.[19]

Krittayaphong et al in their study showed EDWT as an independent predictor of functional recovery of hibernating myocardium but to a lesser extent compared with LGE. Their study demonstrated a cut-off of 5.5 mm.[20]

 

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Limitation

• • Sample size was low, leading to a mildly low yield of mismatched segments.

• • LGE assessment is still a subjective analysis, although two radiologists were involved in reading the MRI images.

• • Follow-up of the patient for assessment of improvement of the function post-revascularization was not included in the study.

• • Only qualitative tagging was included in the study, leading to semi-objective analysis of RWMA; however, it was overcome by two observers with good interobserver agreement.

• • Viability assessment in the study was purely assessed and compared between LGE and PET findings. Myocardial wall thickness and myocardial edema were not included in the analysis for this research, which also constitute important predictors of myocardial viability and may be considered for future research.

• • PET findings were considered as the ground truth. Cardiac MRI efficacy is proven against this point. Actual pathological analysis is not technically possible, which is supposed to be the ground truth for comparing both modalities.

 

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Conclusion

Although LGE did not give a significant advantage over the PET-CT in identifying hibernating myocardium. Cardiac MRI, as a versatile tool, can provide us with various information about the patient's physiology and guide us more precisely in the patient management. Improved objectivity and precision were identified in EF analysis and RWMA, especially with the tagging sequence. This study, although with some limitations, proves that there is an important and definite place for cardiac MRI in the evaluation of patients with persistent symptoms of heart failure. Further research into other imaging features depicting cardiac viability is recommended for improving the precision of cardiac MRI in these clinical settings.

 

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Conflict of Interest

None declared.

• References

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Publication History

Article published online:
19 September 2025