Iron-Sensitive Cardiac Magnetic Resonance Imaging for Prediction of Ventricular Arrhythmia Risk in Patients With Chronic Myocardial Infarction: Early Evidence

Microvascular Obstruction and Intramyocardial Hemorrhage in Reperfused Myocardial Infarctions: Pathophysiology and Clinical Insights From Imaging

Microvascular injury immediately following reperfusion therapy in acute myocardial infarction (MI) has emerged as a driving force behind major adverse cardiovascular events in the postinfarction period. Although postmortem investigations and animal models have aided in developing early understanding of microvascular injury following reperfusion, imaging, particularly serial noninvasive imaging, has played a central role in cultivating critical knowledge of progressive damage to the myocardium from the onset of microvascular injury to months and years after in acute MI patients. This review summarizes the pathophysiological features of microvascular injury and downstream consequences, and the contributions noninvasive imaging has imparted in the development of this understanding. It also highlights the interventional trials that aim to mitigate the adverse consequences of microvascular injury based on imaging, identifies potential future directions of investigations to enable improved detection of disease, and demonstrates how imaging stands to play a major role in the development of novel therapies for improved management of acute MI patients.

On the loss of image contrast in double-inversion-recovery prepared T2* MRI of Intramyocardial hemorrhage

PURPOSE

Studies have shown that double-inversion-recovery (DIR) prepared dark-blood T2*-weighted images result in lower SNR, CNR and diagnostic accuracy for intramyocardial hemorrhage (IMH) detection compared to non-DIR-prepared (bright-blood) T2*-weighted images; however, the mechanism contributing to this observation has not been investigated and explained in detail. This work tests the hypothesis that the loss of SNR on dark-blood cardiac T2*-weighted images of IMH stems from spin-relaxation during the long RF pulses in double inversion preparation, as a result, compromising image contrast for intramyocardial hemorrhage detection.

METHODS

Phantom and in-vivo animal studies were performed to test the hypothesis of the study. An agar phantom was imaged with multi-gradient-echo T2* imaging protocols with and without double-inversion-recovery (DIR) preparation. Image acquisitions were placed at different delay times (TD) after DIR preparation. SNR, T2* and Coefficient of Variation (COV) were measured and compared between DIR-prepared and non-DIR-prepared images. Canines with hemorrhagic myocardial infarctions were scanned at 3.0 T with DIR-prepared (dark-blood) and non-DIR-prepared (bright-blood) T2* imaging protocols. DIR-prepared T2* images were acquired with short, medium, and long delay times (TD). SNR, CNR, intramyocardial hemorrhage (IMH) extent, T2* and COV were measured and compared between DIR-prepared T2* images with short, medium, and long delay times (TD) to non-DIR-prepared bright-blood T2* images.

RESULTS

Phantom studies confirmed the hypothesis that the SNR loss on DIR-prepared T2* images originated from signal loss during DIR preparation. SNR followed T1 recovery curve with increased delay times (TD) indicating that SNR can be recovered with longer time delay between DIR and image acquisition. Myocardial T2* values were not affected by DIR preparation but COV of T2* was elevated. Animal studies supported the hypothesis and showed that DIR-prepared T2* images with insufficient delay time (TD) had impaired sensitivity for IMH detection due to lower SNR and CNR, and higher COV.

CONCLUSION

We conclude that lower SNR and CNR on DIR-prepared T2* images originate from signal loss during DIR preparation and insufficient recovery between DIR preparation and image acquisition. Although, the impaired sensitivity can be recovered by extending delay time (TD), it will extend the scan time. Bright-blood T2* imaging protocols should remain the optimal choice for assessment of intramyocardial hemorrhage. DIR-prepared dark-blood T2* imaging protocols should be performed with extra attention on image signal-to-noise ratio when used for intramyocardial hemorrhage detection.

The Canadian Cardiovascular Society Classification of Acute Atherothrombotic Myocardial Infarction Based on Stages of Tissue Injury Severity: An Expert Consensus Statement

Myocardial infarction (MI) remains a leading cause of morbidity and mortality. In atherothrombotic MI (ST-elevation MI and type 1 non-ST-elevation MI), coronary artery occlusion leads to ischemia. Subsequent cardiomyocyte necrosis evolves over time as a wavefront within the territory at risk. The spectrum of ischemia and reperfusion injury is wide: it can be minimal in aborted MI or myocardial necrosis can be large and complicated by microvascular obstruction and reperfusion hemorrhage. Established risk scores and infarct classifications help with patient management but do not consider tissue injury characteristics. This document outlines the Canadian Cardiovascular Society classification of acute MI. It is an expert consensus formed on the basis of decades of data on atherothrombotic MI with reperfusion therapy. Four stages of progressively worsening myocardial tissue injury are identified: (1) aborted MI (no/minimal myocardial necrosis); (2) MI with significant cardiomyocyte necrosis, but without microvascular injury; (3) cardiomyocyte necrosis and microvascular dysfunction leading to microvascular obstruction (ie, "no-reflow"); and (4) cardiomyocyte and microvascular necrosis leading to reperfusion hemorrhage. Each stage reflects progression of tissue pathology of myocardial ischemia and reperfusion injury from the previous stage. Clinical studies have shown worse remodeling and increase in adverse clinical outcomes with progressive injury. Notably, microvascular injury is of particular importance, with the most severe form (hemorrhagic MI) leading to infarct expansion and risk of mechanical complications. This classification has the potential to stratify risk in MI patients and lay the groundwork for development of new, injury stage-specific and tissue pathology-based therapies for MI.

The role of cardiac magnetic resonance imaging in defining the prognosis of patients with acute <i>ST</i>-segment elevation myocardial infarction. Part 2. Assessment of the disease prognosis

Currently the incidence of congestive heart failure after ST-segment elevation myocardial infarction (STEMI) tends to increase. Reperfusion therapy is still the only effective method to reduce an infarct size. Therefore, there is a high unmet need of novel cardioprotective treatments that would improve outcomes in such patients. Recent advances in cardiovascular magnetic resonance (CMR) methods enabled the identification of certain new infarct characteristics associated with the development of heart failure and sudden cardiac death. These characteristics can help identify new groups of high risk patients and used as a targets for novel cardioprotective treatments. This part of the review summarizes novel CMR-based characteristics of myocardial infarction and their role in the prognostic stratification of STEMI patients.

Quantification of myocardial hemorrhage using T2* cardiovascular magnetic resonance at 1.5T with ex-vivo validation

BACKGROUND

T2* cardiovascular magnetic resonance (CMR) is commonly used in the diagnosis of intramyocardial hemorrhage (IMH). For quantifying IMH with T2* CMR, despite the lack of consensus studies, two different methods [subject-specific T2* (ssT2*) and absolute T2* thresholding (aT2* < 20 ms)] are interchangeably used. We examined whether these approaches yield equivalent information.

METHODS

ST elevation myocardial infarction (STEMI) patients (n = 70) were prospectively recruited for CMR at 4-7 days post revascularization and for 6-month follow up (n = 43). Canines studies were performed for validation purposes, where animals (n = 20) were subject to reperfused myocardial infarction (MI) and those surviving the MI (n = 16) underwent CMR at 7 days and 8 weeks and then euthanized. Both in patients and animals, T2* of IMH and volume of IMH were determined using ssT2* and aT2* < 20 ms. In animals, ex-vivo T2* CMR and mass spectrometry for iron concentration ([Fe]Hemo) were determined on excised myocardial sections. T2* values based on ssT2* and absolute T2* threshold approaches were independently regressed against [Fe]Hemo and compared. A range of T2* cut-offs were tested to determine the optimized conditions relative to ssT2*.

RESULTS

While both approaches showed many similarities, there were also differences. Compared to ssT2*, aT2* < 20 ms showed lower T2* and volume of IMH in patients and animals independent of MI age (all p < 0.005). While T2* determined from both methods were highly correlated against [Fe]Hemo (R2 = 0.9 for both), the slope of the regression curve for ssT2* was significantly larger as compared to aT2* < 20 ms (0.46 vs. 0.32, p < 0.01). Further, slightly larger absolute T2* cut-offs (patients: 23 ms; animals: 25 ms) showed similar IMH characteristics compared to ssT2*.

CONCLUSION

Current quantification methods have excellent capacity to identify IMH, albeit the T2*of IMH and volume of IMH based on aT2* < 20 ms are smaller compared to ssT2*. Thus the method used to quantify IMH from T2* CMR may influence the diagnosis for IMH.

Assessment of intramyocardial hemorrhage with dark-blood T2*-weighted cardiovascular magnetic resonance

BACKGROUND

Intramyocardial hemorrhage (IMH) within myocardial infarction (MI) is associated with major adverse cardiovascular events. Bright-blood T2*-based cardiovascular magnetic resonance (CMR) has emerged as the reference standard for non-invasive IMH detection. Despite this, the dark-blood T2*-based CMR is becoming interchangeably used with bright-blood T2*-weighted CMR in both clinical and preclinical settings for IMH detection. To date however, the relative merits of dark-blood T2*-weighted with respect to bright-blood T2*-weighted CMR for IMH characterization has not been studied. We investigated the diagnostic capacity of dark-blood T2*-weighted CMR against bright-blood T2*-weighted CMR for IMH characterization in clinical and preclinical settings.

MATERIALS AND METHODS

Hemorrhagic MI patients (n = 20) and canines (n = 11) were imaged in the acute and chronic phases at 1.5 and 3 T with dark- and bright-blood T2*-weighted CMR. Imaging characteristics (Relative signal-to-noise (SNR), Relative contrast-to-noise (CNR), IMH Extent) and diagnostic performance (sensitivity, specificity, accuracy, area-under-the-curve, and inter-observer variability) of dark-blood T2*-weighted CMR for IMH characterization were assessed relative to bright-blood T2*-weighted CMR.

RESULTS

At both clinical and preclinical settings, compared to bright-blood T2*-weighted CMR, dark-blood T2*-weighted images had significantly lower SNR, CNR and reduced IMH extent (all p < 0.05). Dark-blood T2*-weighted CMR also demonstrated weaker sensitivity, specificity, accuracy, and inter-observer variability compared to bright-blood T2*-weighted CMR (all p < 0.05). These observations were consistent across infarct age and imaging field strengths.

CONCLUSION

While IMH can be visible on dark-blood T2*-weighted CMR, the overall conspicuity of IMH is significantly reduced compared to that observed in bright-blood T2*-weighted images, across infarct age in clinical and preclinical settings at 1.5 and 3 T. Hence, bright-blood T2*-weighted CMR would be preferable for clinical use since dark-blood T2*-weighted CMR carries the potential to misclassify hemorrhagic MIs as non-hemorrhagic MIs.

Cardiovascular Magnetic Resonance in Acute ST-Segment-Elevation Myocardial Infarction: Recent Advances, Controversies, and Future Directions

Although mortality after ST-segment elevation myocardial infarction (MI) is on the decline, the number of patients developing heart failure as a result of MI is on the rise. Apart from timely reperfusion by primary percutaneous coronary intervention, there is currently no established therapy for reducing MI size. Thus, new cardioprotective therapies are required to improve clinical outcomes after ST-segment-elevation MI. Cardiovascular magnetic resonance has emerged as an important imaging modality for assessing the efficacy of novel therapies for reducing MI size and preventing subsequent adverse left ventricular remodeling. The recent availability of multiparametric mapping cardiovascular magnetic resonance imaging has provided new insights into the pathophysiology underlying myocardial edema, microvascular obstruction, intramyocardial hemorrhage, and changes in the remote myocardial interstitial space after ST-segment-elevation MI. In this article, we provide an overview of the recent advances in cardiovascular magnetic resonance imaging in reperfused patients with ST-segment-elevation MI, discuss the controversies surrounding its use, and explore future applications of cardiovascular magnetic resonance in this setting.

Influence of Myocardial Hemorrhage on Staging of Reperfused Myocardial Infarctions With T2 Cardiac Magnetic Resonance Imaging: Insights Into the Dependence on Infarction Type With Ex Vivo Validation

OBJECTIVES

This study sought to determine whether T2 cardiac magnetic resonance (CMR) can stage both hemorrhagic and nonhemorrhagic myocardial infarctions (MIs).

BACKGROUND

CMR-based staging of MI with or without contrast agents relies on the resolution of T2 elevations in the chronic phase, but whether this approach can be used to stage both hemorrhagic and nonhemorrhagic MIs is unclear.

METHODS

Hemorrhagic (n = 15) and nonhemorrhagic (n = 9) MIs were created in dogs. Multiparametric noncontrast mapping (T1, T2, and T2*) and late gadolinium enhancement (LGE) were performed at 1.5- and 3.0-T at 5 days (acute) and 8 weeks (chronic) post-MI. CMR relaxation values and LGE intensities of hemorrhagic, peri-hemorrhagic, nonhemorrhagic, and remote territories were measured. Histopathology was performed to elucidate CMR findings.

RESULTS

T2 of nonhemorrhagic MIs was significantly elevated in the acute phase relative to remote territories (1.5-T: 39.8 ± 12.8%; 3.0-T: 27.9 ± 16.5%; p < 0.0001 for both) but resolved to remote values by week 8 (1.5-T: -0.0 ± 3.2%; p = 0.678; 3.0-T: -0.5 ± 5.9%; p = 0.601). In hemorrhagic MI, T2 of hemorrhage core was significantly elevated in the acute phase (1.5-T: 17.7 ± 10.0%; 3.0-T: 8.6 ± 8.2%; p < 0.0001 for both) but decreased below remote values by week 8 (1.5-T: -8.2 ± 3.9%; 3.0-T: -5.6 ± 6.0%; p < 0.0001 for both). In contrast, T2 of the periphery of hemorrhage within the MI zone was significantly elevated in the acute phase relative to remote territories (1.5-T: 35.0 ± 16.1%; 3.0-T: 24.2 ± 10.4%; p < 0.0001 for both) and remained elevated at 8 weeks post-MI (1.5-T: 8.6 ± 5.1%; 3.0-T: 6.0 ± 3.3%; p < 0.0001 for both). The observed elevation of T2 in the peri-hemorrhagic zone of MIs and the absence of T2 elevation in nonhemorrhagic MIs were consistent with ongoing or absence of histological evidence of inflammation, respectively.

CONCLUSIONS

Hemorrhagic MIs are associated with persisting myocardial inflammation and edema, which can confound staging of hemorrhagic MIs when T2 elevations alone are used to discriminate between acute and chronic MI. Moreover, given the poor prognosis in patients with hemorrhagic MI, CMR evidence for myocardial hemorrhage with persistent edema may evolve as a risk marker in patients after acute MI.

Oxygenation-sensitive cardiovascular magnetic resonance in hypertensive heart disease with left ventricular myocardial hypertrophy and non-left ventricular myocardial hypertrophy: Insight from altered mechanics and cardiac BOLD imaging

BACKGROUND

BOLD (blood oxygen level dependent) MRI can detect regional condition of myocardial oxygen supply and demand by means of paramagnetic properties.

PURPOSE

Noninvasive assessment of myocardial oxygenation by BOLD MRI in hypertensive patients with hypertension (HTN) left ventricular myocardial hypertrophy (LVMH) and HTN non-LVMH and its correlation with myocardial mechanics were performed.

STUDY TYPE

Prospective.

POPULATION

Twenty patients with HTN LVMH, 21 patients with HTN non-LVMH, and 23 normotensive controls were enrolled.

FIELD STRENGTH/SEQUENCE

Cine imaging, T2* and T1 mapping sequences were achieved at 3.0T.

ASSESSMENT

Dedicated T1 mapping, T2*, and cine imaging analysis were performed by two radiologists using cvi42.

STATISTICAL TESTS

One-way analysis of variance, Kruskal-Wallis test, Bland-Altman analysis, Pearson's correlation coefficient, Spearman's rank correlation.

RESULTS

T2* values of HTN LVMH group were significantly lower versus the controls (23.78 ± 3.09 versus 30.77 ± 2.71; P < 0.001) and HTN non-LVMH group (23.78 ± 3.09 versus 28.64 ± 4.23; P < 0.001). Left ventricular peak circumferential strain were reduced in HTN LVMH patients compared with other two groups (-11.32 [-15.64, -10.3], -16.78 [-19.35, -15.34], and -19.73 [-20.57, -18.73]; P < 0.05); and longitudinal strain of HTN LVMH patients were lower than other two groups (-11.31 ± 2.91, -15.1 ± 3.06, and -18.85 ± 1.85; P < 0.05); radial strain of HTN LVMH patients were also lower than other two groups (25.03 ± 16, 40.95 ± 17.5 and 47.9 ± 10.23; P < 0.05). Extracellular volume correlated with peak circumferential, longitudinal, and radial strain (spearman rho = 0.6, 0.64, and -0.69; P < 0.05), respectively; T2* negatively correlated with peak circumferential and longitudinal strain (spearman rho = -0.43 and -0.49; P < 0.05), respectively. Patients with lower T2* values had significant decreases in myocardial mechanics (P < 0.05).

DATA CONCLUSION

HTN LVMH patients have both impaired myocardial mechanics and decreased T2* values compared with HTN non-LVMH and normotensive groups. BOLD MRI could provide a feasible assessment modality for detecting altered T2* due to the change of de-oxygenated hemoglobin and hence to the change of signal intensity in oxygenation-sensitive images.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2018;47:1297-1306.

Persistent Iron Within the Infarct Core After ST-Segment Elevation Myocardial Infarction: Implications for Left Ventricular Remodeling and Health Outcomes

Objectives

This study sought to determine the incidence and prognostic significance of persistent iron in patients post–ST-segment elevation myocardial infarction (STEMI).

Background

The clinical significance of persistent iron within the infarct core after STEMI complicated by acute myocardial hemorrhage is poorly understood.

Methods

Patients who sustained an acute STEMI were enrolled in a cohort study (BHF MR-MI [Detection and Significance of Heart Injury in ST Elevation Myocardial Infarction]). Cardiac magnetic resonance imaging including T₂* (observed time constant for the decay of transverse magnetization seen with gradient-echo sequences) mapping was performed at 2 days and 6 months post-STEMI. Myocardial hemorrhage or iron was defined as a hypointense infarct core with T₂* signal <20 ms.

Results

A total of 203 patients (age 57 ± 11 years, n = 158 [78%] male) had evaluable T₂* maps at 2 days and 6 months post-STEMI; 74 (36%) patients had myocardial hemorrhage at baseline, and 44 (59%) of these patients had persistent iron at 6 months. Clinical associates of persistent iron included heart rate (p = 0.009), the absence of a history of hypertension (p = 0.017), and infarct size (p = 0.028). The presence of persistent iron was associated with worsening left ventricular (LV) end-diastolic volume (regression coefficient: 21.10; 95% confidence interval [CI]: 10.92 to 31.27; p < 0.001) and worsening LV ejection fraction (regression coefficient: −6.47; 95% CI: −9.22 to −3.72; p < 0.001). Persistent iron was associated with the subsequent occurrence of all-cause death or heart failure (hazard ratio: 3.91; 95% CI: 1.37 to 11.14; p = 0.011) and major adverse cardiac events (hazard ratio: 3.24; 95% CI: 1.09 to 9.64; p = 0.035) (median follow-up duration 1,457 days [range 233 to 1,734 days]).

Conclusions

Persistent iron at 6 months post-STEMI is associated with worse LV and longer-term health outcomes. (Detection and Significance of Heart Injury in ST Elevation Myocardial Infarction [BHF MR-MI]; NCT02072850)

Recent Advances in Cardiovascular Magnetic Resonance: Techniques and Applications

Cardiovascular magnetic resonance imaging has become the gold standard for evaluating myocardial function, volumes, and scarring. Additionally, cardiovascular magnetic resonance imaging is unique in its comprehensive tissue characterization, including assessment of myocardial edema, myocardial siderosis, myocardial perfusion, and diffuse myocardial fibrosis. Cardiovascular magnetic resonance imaging has become an indispensable tool in the evaluation of congenital heart disease, heart failure, cardiac masses, pericardial disease, and coronary artery disease. This review will highlight some recent novel cardiovascular magnetic resonance imaging techniques, concepts, and applications.

Residual Myocardial Iron Following Intramyocardial Hemorrhage During the Convalescent Phase of Reperfused ST-Segment-Elevation Myocardial Infarction and Adverse Left Ventricular Remodeling

Supplemental Digital Content is available in the text.

Background—

The presence of intramyocardial hemorrhage (IMH) in ST-segment–elevation myocardial infarction patients reperfused by primary percutaneous coronary intervention has been associated with residual myocardial iron at follow-up, and its impact on adverse left ventricular (LV) remodeling is incompletely understood and is investigated here.

Methods and Results—

Forty-eight ST-segment–elevation myocardial infarction patients underwent cardiovascular magnetic resonance at 4±2 days post primary percutaneous coronary intervention, of whom 40 had a follow-up scan at 5±2 months. Native T1, T2, and T2* maps were acquired. Eight out of 40 (20%) patients developed adverse LV remodeling. A subset of 28 patients had matching T2* maps, of which 15/28 patients (54%) had IMH. Eighteen of 28 (64%) patients had microvascular obstruction on the acute scan, of whom 15/18 (83%) patients had microvascular obstruction with IMH. On the follow-up scan, 13/15 patients (87%) had evidence of residual iron within the infarct zone. Patients with residual iron had higher T2 in the infarct zone surrounding the residual iron when compared with those without. In patients with adverse LV remodeling, T2 in the infarct zone surrounding the residual iron was also higher than in those without (60 [54–64] ms versus 53 [51–56] ms; P=0.025). Acute myocardial infarct size, extent of microvascular obstruction, and IMH correlated with the change in LV end-diastolic volume (Pearson’s rho of 0.64, 0.59, and 0.66, respectively; P=0.18 and 0.62, respectively, for correlation coefficient comparison) and performed equally well on receiver operating characteristic curve for predicting adverse LV remodeling (area under the curve: 0.99, 0.94, and 0.95, respectively; P=0.19 for receiver operating characteristic curve comparison).

Conclusions—

The majority of ST-segment–elevation myocardial infarction patients with IMH had residual myocardial iron at follow-up. This was associated with persistently elevated T2 values in the surrounding infarct tissue and adverse LV remodeling. IMH and residual myocardial iron may be potential therapeutic targets for preventing adverse LV remodeling in reperfused ST-segment–elevation myocardial infarction patients.

Diagnostic and prognostic roles of echocardiography and cardiac magnetic resonance

Accurate prediction of sudden cardiac death due to ventricular arrhythmia remains challenging. Left ventricular ejection fraction has shown an association with increased risk of ventricular arrhythmias and is included in the recommendations for implantable cardioverter defibrillator as primary prevention. However, left ventricular ejection fraction may be normal in a large number of patients who are at risk of ventricular arrhythmias. Echocardiography remains the imaging technique of first choice to rule out the presence of structural heart disease and assess left and right ventricular function. Advances in strain echocardiography and cardiac magnetic resonance have provided important insights into the mechanisms of ventricular arrhythmias, and will be summarized in this review.