Characterization of Microvascular Dysfunction After Acute Myocardial Infarction by Cardiovascular Magnetic Resonance First-Pass Perfusion and Late Gadolinium Enhancement Imaging

Quantitative Assessment of Myocardial Ischemia With Positron Emission Tomography

Recent advances in positron emission tomography (PET) technology and reconstruction techniques have now made quantitative assessment using cardiac PET readily available in most cardiac PET imaging centers. Multiple PET myocardial perfusion imaging (MPI) radiopharmaceuticals are available for quantitative examination of myocardial ischemia, with each having distinct convenience and accuracy profile. Important properties of these radiopharmaceuticals ( 15 O-water, 13 N-ammonia, 82 Rb, 11 C-acetate, and 18 F-flurpiridaz) including radionuclide half-life, mean positron range in tissue, and the relationship between kinetic parameters and myocardial blood flow (MBF) are presented. Absolute quantification of MBF requires PET MPI to be performed with protocols that allow the generation of dynamic multiframes of reconstructed data. Using a tissue compartment model, the rate constant that governs the rate of PET MPI radiopharmaceutical extraction from the blood plasma to myocardial tissue is calculated. Then, this rate constant ( K1 ) is converted to MBF using an established extraction formula for each radiopharmaceutical. As most of the modern PET scanners acquire the data only in list mode, techniques of processing the list-mode data into dynamic multiframes are also reviewed. Finally, the impact of modern PET technologies such as PET/CT, PET/MR, total-body PET, machine learning/deep learning on comprehensive and quantitative assessment of myocardial ischemia is briefly described in this review.

Cardioprotective effects of shock wave therapy: A cardiac magnetic resonance imaging study on acute ischemia-reperfusion injury

Introduction

Cardioprotection strategies remain a new frontier in treating acute myocardial infarction (AMI), aiming at further protect the myocardium from the ischemia-reperfusion damage. Therefore, we aimed at investigating the mechano-transduction effects induced by shock waves (SW) therapy at time of the ischemia reperfusion as a non-invasive cardioprotective innovative approach to trigger healing molecular mechanisms.

Methods

We evaluated the SW therapy effects in an open-chest pig ischemia-reperfusion (IR) model, with quantitative cardiac Magnetic Resonance (MR) imaging performed along the experiments at multiple time points (baseline (B), during ischemia (I), at early reperfusion (ER) (∼15 min), and late reperfusion (LR) (3 h)). AMI was obtained by a left anterior artery temporary occlusion (50 min) in 18 pigs (32 ± 1.9 kg) randomized into SW therapy and control groups. In the SW therapy group, treatment was started at the end of the ischemia period and extended during early reperfusion (600 + 1,200 shots @0.09 J/mm2, f = 5 Hz). The MR protocol included at all time points LV global function assessment, regional strain quantification, native T1 and T2 parametric mapping. Then, after contrast injection (gadolinium), we obtained late gadolinium imaging and extra-cellular volume (ECV) mapping. Before animal sacrifice, Evans blue dye was administrated after re-occlusion for area-at-risk sizing.

Results

During ischemia, LVEF decreased in both groups (25 ± 4.8% in controls (p = 0.031), 31.6 ± 3.2% in SW (p = 0.02). After reperfusion, left ventricular ejection fraction (LVEF) remained significantly decreased in controls (39.9 ± 4% at LR vs. 60 ± 5% at baseline (p = 0.02). In the SW group, LVEF increased quickly ER (43.7 ± 11.4% vs. 52.4 ± 8.2%), and further improved at LR (49.4 ± 10.1) (ER vs. LR p = 0.05), close to baseline reference (LR vs. B p = 0.92). Furthermore, there was no significant difference in myocardial relaxation time (i.e. edema) after reperfusion in the intervention group compared to the control group: ΔT1 (MI vs. remote) was increased by 23.2±% for SW vs. +25.2% for the controls, while ΔT2 (MI vs. remote) increased by +24.9% for SW vs. +21.7% for the control group.

Discussion

In conclusion, we showed in an ischemia-reperfusion open-chest swine model that SW therapy, when applied near the relief of 50' LAD occlusion, led to a nearly immediate cardioprotective effect translating to a reduction in the acute ischemia-reperfusion lesion size and to a significant LV function improvement. These new and promising results related to the multi-targeted effects of SW therapy in IR injury need to be confirmed by further in-vivo studies in close chest models with longitudinal follow-up.

Decrease in Pancreatic Perfusion of Patients with Type 2 Diabetes Mellitus Detected by Perfusion Computed Tomography

Objectives:

The objectives of the study was to compare pancreatic perfusion by computed tomography in type 2 diabetes and non-diabetic subjects.

Material and Methods:

In this case–control study, 17 patients with type 2 diabetes and 22 non-diabetic controls were examined with a dynamic 192-slices perfusion computed tomography for estimating pancreatic perfusion parameters.

Results:

Thirty-nine patients were included (22 with Type 2 diabetes mellitus [T2DM]), with a mean age of 64 years. There were significant differences in some pancreatic perfusion parameters in patients with and without type 2 diabetes. Blood volume (BV) was lower in pancreatic head (with T2DM: 14.0 ± 3.4 vs. without T2DM: 16.1 ± 2.4 mL/100 mL; P = 0.033), pancreatic tail (with: 14.4 ± 3.6 vs. without: 16.8 ± 2.5 mL/100 mL; P = 0.023), and in whole pancreas (with: 14.2 ± 3.2 vs. without: 16.2 ± 2.5 mL/100 mL; P = 0.042). Similar behavior was observed with mean transit time (MTT) in pancreatic head (with: 7.0 ± 1.0 vs. without: 7.9 ± 1.2 s; P = 0.018), pancreatic tail (with: 6.6 ± 1.3 vs. without: 7.7 ± 0.9 s; P = 0.005), and in whole pancreas (with: 6.8 ± 1.0 vs. without: 7.7 ± 0.9 s; P = 0.016). BV in head, tail, and whole pancreas had negative correlations with age (head r: –0.352, P = 0.032; tail r: –0.421, P = 0.031; whole pancreas r: –0.439, P = 0.007), and fasting plasma glucose (head r: –0.360, P = 0.031; tail r: –0.483, P = 0.003; whole pancreas r: –0.447, P = 0.006). In a multivariate linear regression model, HbA1c was independently associated with decrease in BV in whole pancreas (β: –0.884; CI95%: –1.750 to –0.017; P = 0.046).

Conclusion:

Pancreatic BV and MTT were significantly lower in patients with type 2 diabetes. BV was decreased with older age and poorer glycemic control.

Role of Cardiovascular Magnetic Resonance in Ischemic Cardiomyopathy

Ischemic heart disease is the most common cause of cardiovascular morbidity and mortality. Cardiac magnetic resonance (CMR) improves on other noninvasive modalities in detection, assessment, and prognostication of ischemic heart disease. The incorporation of CMR in clinical trials allows for smaller patient samples without the sacrifice of power needed to demonstrate clinical efficacy. CMR can accurately quantify infarct acuity, size, and complications; guide therapy; and prognosticate recovery. Timing of revascularization remains the holy grail of ischemic heart disease, and viability assessment using CMR may be the missing link needed to help reduce morbidity and mortality associated with the disease.

Cardiovascular magnetic resonance techniques for tissue characterization after acute myocardial injury

The annual incidence of hospital admission for acute myocardial infarction lies between 90 and 312 per 100 000 inhabitants in Europe. Despite advances in patient care 1 year mortality after ST-segment elevation myocardial infarction (STEMI) remains around 10%. Cardiovascular magnetic resonance imaging (CMR) has emerged as a robust imaging modality for assessing patients after acute myocardial injury. In addition to accurate assessment of left ventricular ejection fraction and volumes, CMR offers the unique ability of visualization of myocardial injury through a variety of imaging techniques such as late gadolinium enhancement and T2-weighted imaging. Furthermore, new parametric mapping techniques allow accurate quantification of myocardial injury and are currently being exploited in large trials aiming to augment risk management and treatment of STEMI patients. Of interest, CMR enables the detection of microvascular injury (MVI) which occurs in approximately 40% of STEMI patients and is a major independent predictor of mortality and heart failure. In this article, we review traditional and novel CMR techniques used for myocardial tissue characterization after acute myocardial injury, including the detection and quantification of MVI. Moreover, we discuss clinical scenarios of acute myocardial injury in which the tissue characterization techniques can be applied and we provide proposed imaging protocols tailored to each scenario.

Myocardial Blood Flow Quantified by Low-Dose Dynamic CT Myocardial Perfusion Imaging Is Associated with Peak Troponin Level and Impaired Left Ventricle Function in Patients with ST-Elevated Myocardial Infarction

Objective

To investigate the association of myocardial blood flow (MBF) quantified by dynamic computed tomography (CT) myocardial perfusion imaging (MPI) with troponin level and left ventricle (LV) function in patients with ST-segment elevated myocardial infarction (STEMI).

Materials and Methods

Thirty-five STEMI patients who successfully had undergone reperfusion treatment within 1 week of their infarction were consecutively enrolled. All patients were referred for dynamic CT-MPI. Serial high-sensitivity troponin T (hs-TnT) levels and left ventricular ejection fraction (LVEF) measured by echocardiography were recorded. Twenty-six patients with 427 segments were included for analysis. Various quantitative parameters derived from dynamic CT-MPI were analyzed to determine if there was a correlation between hs-TnT levels and LVEF on admission and again at the 6-month mark.

Results

The mean radiation dose for dynamic CT-MPI was 3.2 ± 1.1 mSv. Infarcted territories had significantly lower MBF (30.5 ± 7.4 mL/min/100 mL versus 73.4 ± 8.1 mL/min/100 mL, p < 0.001) and myocardial blood volume (MBV) (2.8 ± 0.9 mL/100 mL versus 4.2 ± 1.1 mL/100 mL, p = 0.044) compared with those of reference territories. MBF showed the best correlation with the level of peak hs-TnT (r = −0.682, p < 0.001), and MBV showed a moderate correlation with the level of peak hs-TnT (r = −0.437, p = 0.026); however, the other parameters did not show any significant correlation with hs-TnT levels. As for the association with LV function, only MBF was significantly correlated with LVEF at the time of admission (r = 0.469, p = 0.016) and at 6 months (r = 0.585, p = 0.001).

Conclusion

MBF quantified by dynamic CT-MPI is significantly inversely correlated with the level of peak hs-TnT. In addition, patients with lower MBF tended to have impaired LV function at the time of their admission and at 6 months.

Future perspectives of nanoparticle-based contrast agents for cardiac magnetic resonance in myocardial infarction

Cardiac Magnetic Resonance (CMR), thanks to high spatial resolution and absence of ionizing radiation, has been widely used in myocardial infarction (MI) assessment to evaluate cardiac structure, function, perfusion and viability. Nevertheless, it suffers from limitations in tissue and assessment of myocardial pathophysiological changes subsequent to MI. In this issue, nanoparticle-based contrast agents offer the possibility to track biological processes at cellular and molecular level underlying the various phases of MI, infarct healing and tissue repair. In this paper, first we examine the conventional CMR protocol and its findings in MI patients. Next, we looked at how nanoparticles can help in the imaging of MI and give an overview of the major approaches currently explored. Based on the presentation of successful nanoparticle applications as contrast agents (CAs) in preclinical and clinical models, we discuss promises and outstanding challenges facing the field of CMR in MI, their translational potential and clinical application.

MR findings of microvascular perfusion in infarcted and remote myocardium early after successful primary PCI

Objectives

The aim of the study was to evaluate CMR myocardial first-pass perfusion in the injured region as well as the non-infarcted area in ST-elevation myocardial infarction (STEMI) patients few days after successful primary percutaneous coronary intervention (PCI).

Materials and methods

220 patients with first time STEMI successfully treated with PCI (with or without postconditioning) were recruited from the Postconditioning in STEMI study. Contrast enhanced CMR was performed at a 1.5 T scanner 2 (1–5) days after PCI. On myocardial first-pass perfusion imaging signal intensity (SI) was measured in the injured area and in the remote myocardium and maximum contrast enhancement index (MCE) was calculated. MCE = (peak SI after contrast—SI at baseline) / SI at baseline x 100.

Results

There were no significant differences in first-pass perfusion between patients treated with standard PCI and patients treated with additional postconditioning. The injured myocardium showed a significantly lower MCE compared to remote myocardium (94 ± 55 vs. 113 ± 49; p < 0.001). When patients were divided into four quartiles of MCE in the injured myocardium (MCE injured myocardium), patients with low MCE injured myocardium had: significantly lower ejection fraction (EF) than patients with high MCE injured myocardium, larger infarct size and area at risk, smaller myocardial salvage and more frequent occurrence of microvascular obstruction on late gadolinium enhancement. MCE in the remote myocardium revealed that patients with larger infarction also had significantly decreased MCE in the non-infarcted, remote area.

Conclusion

CMR first-pass perfusion can be impaired in both injured and remote myocardium in STEMI patients treated with primary PCI. These findings indicate that CMR first-pass perfusion may be a feasible method to evaluate myocardial injury after STEMI in addition to conventional CMR parameters.

Assessment of the longitudinal changes in infarct heterogeneity post myocardial infarction

Background

Infarct heterogeneity, as assessed by determination of the peri-infarct zone (PIZ) by cardiac magnetic resonance imaging, has been shown to be an independent predictor for the development of cardiac arrhythmias and mortality post myocardial infarction (MI). The temporal evolution of the PIZ post MI is currently unknown. Thus, the main objective of our study was to describe the temporal evolution of the PIZ over a 6 month time period in contemporarily managed ST elevation myocardial infarction (STEMI) patients. Further, given the poor prognosis associated with microvascular obstruction (MVO) post STEMI, we sought to compare the temporal evolution of the PIZ in patients with and without MVO. We hypothesized that patients with MVO would show a relative persistence of PIZ over time when compared to those without MVO.

Methods

Twenty-one patients post primary percutaneous coronary intervention were enrolled and treated with evidence based therapy. Each patient had three cardiac MRI scans at 48 h, 3 weeks and 6 months post infarction. Repeated Measures Analysis of Variance (ANOVA) was used to assess the evolution of core infarct size and peri-infarct zone size across the three time frames.

Results

The patients in this study were predominantly male, with ~40 % LAD territory infarction and a mean LVEF of 46 ± 7 %. Core infarct size and PIZ size both decreased significantly across the three time frames. The presence of microvascular obstruction (MVO), a known adverse prognostic factor, influenced PIZ size. Both patients with and without MVO had a significant reduction in core infarct size over time. Patients with MVO did not have a significant change in PIZ size over time (11.9 ± 6.8 %, 12.2 ± 7.5 %, 10.7 ± 6.6 % p = 0.77). In contrast, non-MVO patients did have a significant decrease in PIZ size over time (7.0 ± 5.5 %, 7.1 ± 6.5 %, 2.7 ± 2.6 %, p = 0.01).

Conclusions

Peri-infarct zone size, like core infarct size, varies depending upon the timing of measurement. Patients with MVO displayed a persistence of the PIZ over time.

Microvascular Obstruction Evaluation Using Cardiovascular Magnetic Resonance (CMR) in ST-Elevated Myocardial Infarction (STEMI) Patients

BACKROUND

Restoration of blood flow in epicardial coronary artery in patients with acute myocardial infarction can, but does not have to restore efficient blood flow in coronary circulation. The aim of the study was a direct comparison of microvascular obstruction (MVO) detected by rest and stress perfusion imaging and gadolinium enhancement obtained 2 min. (early MVO) and 15 min. (delayed MVO) post contrast.

MATERIAL/METHODS

106 patients with first anterior myocardial infarction were studied. Cardiovascular magnetic resonance (CMR) was performed 5±2 days after primary percutaneous coronary intervention (pPCI). Stress and rest perfusion imaging was performed as well as early and delayed gadolinium enhancement and systolic function assessment. Scoring of segmental function, perfusion defect, MVO and scar transmurality was performed in 16 segment left ventricular model.

RESULTS

The prevalence of MVO varies significantly between imaging techniques ranging from 48.8% for delayed MVO to 94% with stress perfusion. Median sum of scores was significantly different for each technique: stress perfusion 13 (7; 18), rest perfusion 3 (0.5; 6), early MVO 3 (0; 8), delayed MVO 0 (0; 4); p<0.05. Infarct size, stress and rest perfusion defects were independent predictors of LV EF at discharge from hospital.

CONCLUSIONS

Imaging protocol has a significant impact on MVO results. The study is the first to describe a stress-induced MVO in STEMI patients. Further research is needed to evaluate its impact on a long term prognosis.

Six-Year Prognostic Value of Microvascular Obstruction After Reperfused ST-Elevation Myocardial Infarction as Assessed by Contrast-Enhanced Cardiovascular Magnetic Resonance

Although recent studies showed the prognostic value of cardiac magnetic resonance (CMR) parameters especially microvascular obstruction (MO) after reperfused ST-elevation myocardial infarction (STEMI), a study assessing their prognostic significance for long-term follow-up is missing so far. The objective of this study was to determine the prognostic impact of MO on long-term prognosis after reperfused first STEMI in a setting allocating CMR-assessed parameters to hard clinical events only. In 249 patients, CMR was performed after reperfused STEMI, and hereby, left ventricular ejection fraction (LVEF), infarct size (IS), and the amount of MO were quantified. Follow-up (median 6.0 years) was obtained regarding occurrence of major adverse cardiac events (MACE). MACE occurred more often in patients showing presence of MO (MO vs no MO: n = 61 [54%] vs n = 12 [9%], p <0.0001). By multivariate analysis, the extent of MO remained the strongest predictor (p <0.001) for occurrence of MACE and provided incremental prognostic value over clinical variables and LVEF (p = 0.028, c-index increase from 0.723 to 0.817). In conclusion, CMR-assessed MO proves predictive for assessment of 6-year prognosis in patients after reperfused first STEMI and provides incremental prognostic information over clinical variables and LVEF in a setting based on hard end points.

Effect of microvascular obstruction and intramyocardial hemorrhage by CMR on LV remodeling and outcomes after myocardial infarction: a systematic review and meta-analysis

The goal of this systematic analysis is to provide a comprehensive review of the current cardiac magnetic resonance data on microvascular obstruction (MVO) and intramyocardial hemorrhage (IMH). Data related to the association of MVO and IMH in patients with acute myocardial infarction (MI) with left ventricular (LV) function, volumes, adverse LV remodeling, and major adverse cardiac events (MACE) were critically analyzed. MVO is associated with a lower ejection fraction, increased ventricular volumes and infarct size, and a greater risk of MACE. Late MVO is shown to be a stronger prognostic marker for MACE and cardiac death, recurrent MI, congestive heart failure/heart failure hospitalization, and follow-up LV end-systolic volumes than early MVO. IMH is associated with LV remodeling and MACE on pooled analysis, but because of limited data and heterogeneity in study methodology, the effects of IMH on remodeling require further investigation.

Determining microvascular obstruction and infarct size with steady-state free precession imaging cardiac MRI

PURPOSE

In cardiac MRI (cMRI) injection of contrast medium may be performed prior to the acquisition of cine steady-state free precession (SSFP) imaging to speed up the protocol and avoid delay before late Gadolinium enhancement (LGE) imaging. Aim of this study was to evaluate whether a condensed clinical protocol with contrast cine SSFP imaging is able to detect early microvascular obstruction (MO) and determine the infarct size compared to the findings of LGE inversion recovery sequences.

MATERIALS AND METHODS

The study complies with the Declaration of Helsinki and was performed following approval by the ethic committee of the University of Erlangen-Nuremberg. Written informed consent was obtained from every patient. 68 consecutive patients (14 females/54 males) with acute ST-elevation myocardial infarction (STEMI) treated by percutaneous coronary revascularization were included in this study. CMRI was performed 6.6±2 days after symptom onset and MO and infarct size in early contrast SSFP cine imaging were compared to LGE imaging.

RESULTS

MO was detected in 47/68 (69%) patients on cine SSFP and in 41/68 (60%) patients on LGE imaging. In 6 patients MO was found on cine SSFP imaging but was not detectable on LGE imaging. Infarct size on cine SSFP showed a strong agreement to LGE imaging (intraclass correlation coefficient [ICC] of 0.96 for enddiastolic, p<0.001 and 0.96 for endsystolic, p<0.001 respectively). Significant interobserver agreement was found measuring enddiastolic and endsystolic infarct size on cine SSFP imaging (p<0.01).

CONCLUSIONS

In patients after STEMI infarct size and presence of MO can be detected with contrast cine SSFP imaging. This could be an option in patients who are limited in their ability to comply with the demands of a cMRI protocol.

Intramyocardial hemorrhage: an enigma for cardiac MRI?

Cardiovascular magnetic resonance (CMR) is a useful noninvasive technique for determining the presence of microvascular obstruction (MVO) and intramyocardial hemorrhage (IMH), frequently occurring in patients after reperfused myocardial infarction (MI). MVO, or the so-called no-reflow phenomenon, is associated with adverse ventricular remodeling and a poor prognosis during follow-up. Similarly, IMH is considered a severe damage after revascularization by percutaneous primary coronary intervention (PPCI) or fibrinolysis, which represents a worse prognosis. However, the pathophysiology of IMH is not fully understood and imaging modalities might help to better understand that phenomenon. While, during the past decade, several studies examined the distribution patterns of late gadolinium enhancement with different CMR sequences, the standardized CMR protocol for assessment of IMH is not yet well established. The aim of this review is to evaluate the available literature on this issue, with particular regard to CMR sequences. New techniques, such as positron emission tomography/magnetic resonance imaging (PET/MRI), could be useful tools to explore molecular mechanisms of the myocardial infarction healing process.

Head-to-head comparison of 1 week versus 6 months CMR-derived infarct size for prediction of late events after STEMI

Infarct size (IS) at 1 week after ST-elevation myocardial infarction (MI) diminishes during the first months. The incremental prognostic value of IS regression and of scar size (SS) at 6 months is unknown. We compared cardiovascular magnetic resonance (CMR)-derived IS at 1 week and SS at 6 months after MI for predicting late major adverse cardiac events (MACE). 250 patients underwent CMR at 1 week and 6 months after MI. IS and SS were determined as the extent of transmural late enhancement (in >50 % of wall thickness, ETLE). During 163 weeks, 23 late MACE (cardiac death, MI or readmission for heart failure after the 6 months CMR) occurred. Patients with MACE had a larger IS at 1 week (6 [4-9] vs. 3 [1-5], p < .0001) and a larger SS at 6 months (5 [2-6] vs. 3 [1-5], p = .005) than those without MACE. Late MACE rates in IS >median were higher at 1 week (14 vs. 4 %, p = .007) and in SS >median at 6 months (12 vs. 5 %, p = .053). The C-statistic for predicting late MACE of CMR at 1 week and 6 months was comparable (.720 vs. .746, p = .1). Only ETLE at 1 week (HR 1.31 95 % CI [1.14-1.52], p < .0001, per segment) independently predicted late MACE. CMR-derived SS at 6 months does not offer prognostic value beyond IS at 1 week after MI. The strongest predictor of late MACE is ETLE at 1 week.

Imaging the myocardial microcirculation post-myocardial infarction

The myocardial microcirculation provides the vital pressure control and metabolic homeostasis for normal muscle function. Microvascular dysfunction is implicated in chronic cardiac disease and can signify higher risk, but its effect in acute myocardial infarction (AMI) can be profound. Modern management of AMI is focussed entirely on timely epicardial coronary patency, but as a result can leave microcirculatory devastation in its wake. The 'no-reflow' phenomenon occurs in up to 40 % of those successfully reperfused following an ST-elevation AMI (STEMI), and reflects significant microvessel injury that at its most severe involves both microvascular obstruction (MVO) and intramyocardial haemorrhage. Myocardial contrast echocardiography and cardiac magnetic resonance imaging have both led the field in establishing MVO as the prime determinant of adverse left ventricular (LV) remodeling, LV dysfunction, heart failure and increased mortality. These imaging techniques will be essential to support future research endeavours and shift focus to the maintenance of microvascular flow in AMI.

CMR of microvascular obstruction and hemorrhage in myocardial infarction

Microvascular obstruction (MO) or no-reflow phenomenon is an established complication of coronary reperfusion therapy for acute myocardial infarction. It is increasingly recognized as a poor prognostic indicator and marker of subsequent adverse LV remodeling. Although MO can be assessed using various imaging modalities including electrocardiography, myocardial contrast echocardiography, nuclear scintigraphy, and coronary angiography, evaluation by cardiovascular magnetic resonance (CMR) is particularly useful in enhancing its detection, diagnosis, and quantification, as well as following its subsequent effects on infarct evolution and healing. MO assessment has become a routine component of the CMR evaluation of acute myocardial infarction and will increasingly play a role in clinical trials of adjunctive reperfusion agents and strategies. This review will summarize the pathophysiology of MO, current CMR approaches to diagnosis, clinical implications, and future directions needed for improving our understanding of this common clinical problem.

Cardiac magnetic resonance derived late microvascular obstruction assessment post ST-segment elevation myocardial infarction is the best predictor of left ventricular function: a comparison of angiographic and cardiac magnetic resonance derived measurements

Microvascular obstruction (MVO) is a strong independent predictor of left ventricular remodelling and mortality following ST-segment elevation myocardial infarction (STEMI). Microvascular obstruction can be identified at angiography or with gadolinium-enhanced cardiac MRI (CMR). First-pass perfusion CMR also allows a novel quantitative evaluation of myocardial blood flow (MBF) that might provide superior predictive data in the assessment of MVO. We sought to compare angiographic and CMR derived methodologies in the assessment of MVO to determine the optimal methodology that best predicts the surrogate outcome marker of left ventricular function post STEMI. Following primary-PCI angiographic assessment of 'no-reflow' with TIMI myocardial perfusion grade (TMPG) and myocardial blush grade (MBG) were documented. Assessment of CMR derived MVO was assessed on day 3, with MVO on first-pass perfusion imaging termed 'early MVO' and on late gadolinium enhancement, 'late MVO'. Furthermore on the same day 3 CMR scan, myocardial blood flow in the infarct region was quantified at adenosine stress and rest utilizing standard perfusion imaging sequences. Assessment of remodelling, structure and function was undertaken via standard CMR imaging assessment on day 90 post-STEMI and was used as the surrogate marker for long term clinical outcome. Forty patients (age 59 ± 12 years, 84% males) were appraised. Late MVO had the strongest correlation with LVEF at 90 days compared to the CMR parameters of early MVO, stress infarct region MBF and rest infarct region MBF (r = -0.754, r = -0.588, r = 0.595 and r = 0.345 respectively). Of the angiographic parameters used to assess MVO, TMPG had the strongest relationship with MVO when assessed via CMR. Myocardial blush grade however showed no relationship to CMR derived assessment of MVO. On multivariate analysis, of all angiographic and CMR variables, late MVO was the strongest predictor of LVEF at 90 days (p = 0.004). Cardiac magnetic resonance imaging derived assessment of microvascular obstruction on late gadolinium enhancement strongly predicts left ventricular function following STEMI at 90 days.

The role of cardiovascular magnetic resonance in patients with acute coronary syndromes

Cardiovascular magnetic resonance (CMR) imaging is a recognized technique for characterization of myocardial tissue in stable ischemic heart disease. In addition, CMR is emerging as a noninvasive imaging tool that can provide supporting information to guide treatment in acute coronary syndromes (ACSs). The advantages of using CMR acutely could potentially include triage/differential diagnosis in patients presenting with chest pain and troponin rise but without diagnostic electrocardiogram changes, assessment of severity of myocardial injury (irreversible vs reversible damage) in patients with ST-elevation myocardial infarction and non-ST-elevation myocardial infarction, and risk stratification and assessment of prognosis in patients with ACS. This review evaluates a potential clinical role of CMR in the acute setting, highlighting its advantages and limitations. This critical approach emphasizes areas of uncertainty and ongoing controversies but aims to equip the reader to evaluate the potential clinical application and the practicalities of CMR in patients presenting with ACS.

Detection and characteristics of microvascular obstruction in reperfused acute myocardial infarction using an optimized protocol for contrast-enhanced cardiovascular magnetic resonance imaging

Several cardiovascular magnetic resonance imaging (CMR) techniques are used to detect microvascular obstruction (MVO) after acute myocardial infarction (AMI). To determine the prevalence of MVO and gain more insight into the dynamic changes in appearance of MVO, we studied 84 consecutive patients with a reperfused AMI on average 5 and 104 days after admission, using an optimised single breath-hold 3D inversion recovery gradient echo pulse sequence (IR-GRE) protocol. Early MVO (2 min post-contrast) was detected in 53 patients (63%) and late MVO (10 min post-contrast) in 45 patients (54%; p = 0.008). The extent of MVO decreased from early to late imaging (4.3 ± 3.2% vs. 1.8 ± 1.8%, p < 0.001) and showed a heterogeneous pattern. At baseline, patients without MVO (early and late) had a higher left ventricular ejection fraction (LVEF) than patients with persistent late MVO (56 ± 7% vs. 48 ± 7%, p < 0.001) and LVEF was intermediate in patients with early MVO but late MVO disappearance (54 ± 6%). During follow-up, LVEF improved in all three subgroups but remained intermediate in patients with late MVO disappearance. This optimised single breath-hold 3D IR-GRE technique for imaging MVO early and late after contrast administration is fast, accurate and allows detection of patients with intermediate remodelling at follow-up.

Relation of cardiac troponin I and microvascular obstruction following ST-elevation myocardial infarction

BACKGROUND

Presence of microvascular obstruction (MVO) following primary percutaneous coronary intervention (pPCI) for ST-elevation myocardial infarction (STEMI) confers higher risk of left-ventricular remodelling and dysfunction. Measurement of cardiac troponin I (cTnI) after STEMI reflects the extent of myocardial destruction. We aimed to explore whether cTnI values were associated with presence of MVO independently of infarct size in STEMI patients receiving pPCI.

METHODS

175 patients with STEMI were included. cTnI was sampled at 24 and 48 h. MVO and infarct size was determined by delayed enhancement with cardiac magnetic resonance at five to seven days post index event.

RESULTS

The presence of MVO following STEMI was associated with larger infarct size and higher values of cTnI at 24 and 48 h. For any given infarct size or cTnI value, there was a greater risk of MVO development in non-anterior infarctions. cTnI was strongly associated with MVO in both anterior and non-anterior infarctions (P < 0.01) after adjustment for covariates (including infarct size); and was reasonably effective in predicting MVO in individual patients (area-under-the-curve ≥0.81).

CONCLUSION

Presence of MVO is reflected in levels of cTnI sampled at an early time-point following STEMI and this association persists after adjustment for infarct size.

Angiographic assessment of microvascular perfusion--myocardial blush in clinical practice

Assessment of myocardial "blush" by either Myocardial Blush Grade or TIMI Myocardial Perfusion Grade, is the angiographic method currently preferred to confirm myocardial tissue-level perfusion after primary percutaneous intervention. This review focuses on the utility of angiographic "blush" as a simple, widely available, and virtually costless technique for the immediate diagnosis of microvascular impairment at the time of acute catheterization. We comprehensively outline the available evidence behind the "blush," its use in clinical practice, and draw comparisons with other new technologies for assessment of microvascular integrity.

CMR for characterization of the myocardium in acute coronary syndromes

The utility of cardiac magnetic resonance imaging (CMR) as a diagnostic technique is well established. CMR enables tissue characterization, distinction between myocardial scar tissue and viable tissue, and evaluation of myocardial perfusion and contractile function. To date, CMR has been mostly applied in the assessment of stable disease; however, a role for CMR in the acute setting is also emerging. An accurate appraisal of the myocardium with CMR in the first hours after the onset of chest pain could provide supporting information to standard diagnostic tools, such as electrocardiography and measurement of blood biomarkers, which could help guide the selection of appropriate treatment. The aims of this integrated approach include positive identification of an ischemic syndrome, estimation of downstream areas at risk of damage, evaluation of epicardial artery patency and small vessel integrity, quantification of infarct size, and determination of myocardial function. This Review critically evaluates both established and emerging CMR techniques, and relates the imaging findings to the underlying pathophysiological processes in acute coronary syndromes. A more thorough understanding of CMR techniques will clarify their potential clinical applications and limitations, and assess the practicality of CMR in the setting of acute coronary syndromes, where early intervention is crucial to save myocardium at risk of irreversible injury.

Cardiovascular magnetic resonance imaging of myocardial infarction, viability, and cardiomyopathies

Cardiovascular magnetic resonance provides the opportunity for a truly comprehensive evaluation of patients with a history of myocardial infarction, with regard to characterizing the extent of disease, effect on left ventricular function, and degree of viable myocardium. The use of contrast-enhanced cardiac magnetic resonance (CMR) imaging for first-pass perfusion and late gadolinium enhancement is a powerful technique for delineating areas of myocardial ischemia and infarction. Using a combination of T2-weighted and contrast-enhanced CMR images, information about the acuity of an infarct can be obtained. There is extensive published data using contrast-enhanced CMR to predict myocardial functional recovery with revascularization in patients with ischemic cardiomyopathies. In addition, CMR imaging in patients with cardiomyopathies can distinguish between ischemic and nonischemic etiologies, with the ability to further characterize the underlying pathology of nonischemic cardiomyopathies.

Microvascular obstruction: underlying pathophysiology and clinical diagnosis

Successful restoration of epicardial coronary artery patency after prolonged occlusion might result in microvascular obstruction (MVO) and is observed both experimentally as well as clinically. In reperfused myocardium, myocytes appear edematous and swollen from osmotic overload. Endothelial cell changes usually accompany the alterations seen in myocytes but lag behind myocardial cell injury. Endothelial cells become voluminous, with large intraluminal endothelial protrusions into the vascular lumen, and together with swollen surrounding myocytes occlude capillaries. The infiltration and activation of neutrophils and platelets and the deposition of fibrin also play an important role in reperfusion-induced microvascular damage and obstruction. In addition to these ischemia-reperfusion-related events, coronary microembolization of atherosclerotic debris after percutaneous coronary intervention is responsible for a substantial part of clinically observed MVO. Microvascular flow after reperfusion is spatially and temporally complex. Regions of hyperemia, impaired vasodilatory flow reserve and very low flow coexist and these perfusion patterns vary over time as a result of reperfusion injury. The MVO first appears centrally in the infarct core extending toward the epicardium over time. Accurate detection of MVO is crucial, because it is independently associated with adverse ventricular remodeling and patient prognosis. Several techniques (coronary angiography, myocardial contrast echocardiography, cardiovascular magnetic resonance imaging, electrocardiography) measuring slightly different biological and functional parameters are used clinically and experimentally. Currently there is no consensus as to how and when MVO should be evaluated after acute myocardial infarction.

Microvascular obstruction remains a portent of adverse remodeling in optimally treated patients with left ventricular systolic dysfunction after acute myocardial infarction

BACKGROUND

Microvascular obstruction (MO) is associated with large acute myocardial infarction and lower left ventricular (LV) ejection fraction and predicts greater remodeling, but whether this effect is abolished by contemporary antiremodeling therapies is subject to debate. We examined the influence of several infarct characteristics, including MO, on LV remodeling in an optimally treated post-acute myocardial infarction cohort, using contrast-enhanced cardiac magnetic resonance.

METHODS AND RESULTS

One hundred patients (mean age, 58.9+/-12 years, 77%men) underwent contrast-enhanced cardiac magnetic resonance at baseline (approximately 4 days) and at 12 and 24 weeks. The effects on LV remodeling (ie, change in LV end-systolic volume index [DeltaLVESVi]) of infarct site, transmurality, endocardial extent, and the presence of early and late MO were analyzed. Mean baseline infarct volume index decreased from 34.0 (21.2) mL/m(2) to 20.9 (12.9) mL/m(2) at 24 weeks (P<0.001). Infarct site had no influence on remodeling, but greater baseline infarct transmurality (r=0.47, P<0.001) and endocardial extent (r=0.26, P<0.01) were associated with higher DeltaLVESVi. Early MO was seen in 69 patients (69%) and persisted as late MO in 56 patients (56%). Patients with late MO underwent significantly greater remodeling than those without MO (DeltaLVESVi, +4.1 [13.4] versus -7.0 [12.7] mL/m(2), respectively, P=0.001); those with early MO only displayed an intermediate DeltaLVESVi (-4.9 [13.0] mL/m(2)). Importantly, late MO was seen frequently despite optimal coronary blood flow having been restored at angiography.

CONCLUSIONS

Late MO on predischarge contrast-enhanced cardiac magnetic resonance remains an ominous predictor of adverse LV remodeling despite powerful antiremodeling therapy and may be useful in the risk stratification of survivors of acute myocardial infarction.

Appearance of microvascular obstruction on high resolution first-pass perfusion, early and late gadolinium enhancement CMR in patients with acute myocardial infarction

BACKGROUND

The presence and extent of microvascular obstruction (MO) after acute myocardial infarction can be measured by first-pass gadolinium-enhanced perfusion cardiovascular magnetic resonance (CMR) or after gadolinium injection with early or late enhancement (EGE/LGE) imaging. The volume of MO measured by these three methods may differ because contrast agent diffusion into the MO reduces its apparent extent over time. Theoretically, first-pass perfusion CMR should be the most accurate method to measure MO, but this technique has been limited by lower spatial resolution than EGE and LGE as well as incomplete cardiac coverage. These limitations of perfusion CMR can be overcome using spatio-temporal undersampling methods. The purpose of this study was to compare the extent of MO by high resolution first-pass k-t SENSE accelerated perfusion, EGE and LGE.

METHODS

34 patients with acute ST elevation myocardial infarction, treated successfully with primary percutaneous coronary intervention (PPCI), underwent CMR within 72 hours of admission. k-t SENSE accelerated first-pass perfusion MR (7 fold acceleration, spatial resolution 1.5 mm x 1.5 mm x 10 mm, 8 slices acquired over 2 RR intervals, 0.1 mmol/kg Gd-DTPA), EGE (14 minutes after injection with a fixed TI of 440 ms) and LGE images (1012 minutes after injection, TI determined by a Look-Locker scout) were acquired. MO volume was determined for each technique by manual planimetry and summation of discs methodology.

RESULTS

k-t SENSE first-pass perfusion detected more cases of MO than EGE and LGE (22 vs. 20 vs. 14, respectively). The extent of MO imaged by first-pass perfusion (median mass 4.7 g, IQR 6.7) was greater than by EGE (median mass 2.3 g, IQR 7.1, p = 0.002) and LGE (median mass 0.2 g, IQR 2.4, p = 0.0003). The correlation coefficient between MO mass measured by first-pass perfusion and EGE was 0.91 (p < 0.001).

CONCLUSION

The extent of MO following acute myocardial infarction appears larger on high-resolution first-pass perfusion CMR than on EGE and LGE. Given the inevitable time delay between gadolinium administration and acquisition of either EGE or LGE images, high resolution first-pass perfusion imaging may be the most accurate method to quantify MO.

The DD genotype of the angiotensin converting enzyme gene independently associates with CMR-derived abnormal microvascular perfusion in patients with a first anterior ST-segment elevation myocardial infarction treated with thrombolytic agents

INTRODUCTION

The role of the angiotensin converting enzyme (ACE) gene on the result of thrombolysis at the microvascular level has not been addressed so far. We analyzed the implications of the insertion/deletion (I/D) polymorphism of the ACE gene on the presence of abnormal cardiovascular magnetic resonance (CMR)-derived microvascular perfusion after ST-segment elevation myocardial infarction (STEMI).

MATERIALS AND METHODS

We studied 105 patients with a first anterior STEMI treated with thrombolytic agents and an open left anterior descending artery. Microvascular perfusion was assessed using first-pass perfusion CMR at 7+/-1 days. CMR studies were repeated 184+/-11 days after STEMI. The ACE gene insertion/deletion (I/D) polymorphism was determined using polymerase chain reaction amplification.

RESULTS

Overall genotype frequencies were II-ID 58% and DD 42%. Abnormal perfusion (> or = 1 segment) was detected in 56% of patients. The DD genotype associated to a higher risk of abnormal microvascular perfusion (68% vs. 47%, p=0.03) and to a larger extent of perfusion deficit (median [percentile 25 - percentile 75]: 4 [0-6] vs. 0 [0-4] segments, p=0.003). Once adjusted for baseline characteristics, the DD genotype independently increased the risk of abnormal microvascular perfusion (odds ratio [95% confidence intervals]: 2.5 [1.02-5.9], p=0.04). Moreover, DD patients displayed a larger infarct size (35+/-17 vs. 27+/-15 g, p=0.01) and a lower ejection fraction at 6 months (48+/-14 vs. 54+/-14%, p=0.03).

CONCLUSIONS

The DD genotype associates to a higher risk of abnormal microvascular perfusion after STEMI.

Myocardial fat deposition after left ventricular myocardial infarction: assessment by using MR water-fat separation imaging

PURPOSE

To prospectively investigate the prevalence of fat deposition in chronic myocardial infarction (MI) by using magnetic resonance (MR) fat-water separation imaging with sampling of the entire left ventricular (LV) myocardium. A subsidiary aim was to determine the relationship between LV fat deposition and scar characteristics, as well as regional and global cardiac functional parameters.

MATERIALS AND METHODS

Twenty-five patients with LV MI were evaluated in this prospective institutional review board-approved, Health Insurance Portability and Accountability Act-compliant study after they provided written informed consent. A 1.5-T MR system was used to perform volumetric cine, fat-sensitive, and late gadolinium-enhanced (LGE) infarct imaging. Water-fat separation was performed by using a three-point Dixon reconstruction from in- and opposed-phase black-blood gradient-echo images. Fat deposition location was compared with LGE infarct imaging by using a 17-segment model. Global and regional functional variables, LGE volumes, and fat deposition were compared by using the Pearson correlation, Student t test, and multiple regression.

RESULTS

A fat deposition prevalence of 68% was found in areas of chronic MI. The patients with fat deposition had larger infarctions (30.0 mL +/- 15.1 [standard deviation] vs 14.8 mL +/- 6.1; P = .002), decreased wall thickening (2.3% +/- 20.0 vs 37.8% +/- 34.4; P = .003), and impaired endocardial wall motion (2.9 mm +/- 2.0 vs 5.8 mm +/- 2.6; P = .007). The volume of fat deposition was correlated with infarct volume, LV ejection fraction, LV end-diastolic volume index, and LV end-systolic volume index.

CONCLUSION

There is a high prevalence of fat deposition in healed MI. It is associated with post-infarction characteristics including infarct volume, LV mass, wall thickness, wall thickening, and wall motion.

Microvascular obstruction is a major determinant of infarct healing and subsequent left ventricular remodelling following primary percutaneous coronary intervention

AIMS

We studied the time-dependent relationships between microvascular obstruction (MO), infarct size, and left ventricular (LV) remodelling after acute myocardial infarction (MI).

METHODS AND RESULTS

Forty-two consecutive patients with first-time ST-elevation MI, single-vessel disease, successfully treated with primary percutaneous coronary intervention (PCI) were included. Microvascular obstruction, infarct size, and LV remodelling were assessed by cardiac magnetic resonance. Cardiac magnetic resonance was performed at: 2 days, 1 week, 2 months, and 1 year following PCI. Microvascular obstruction was assessed by first-pass perfusion. Patients were divided into three groups according to the presence or absence of MO at 2 days and 1 week: no detectable MO at any time point (11 patients), MO detectable only at 2 days (16 patients), and MO detectable both at 2 days and 1 week (15 patients). In multivariable analysis adjusting for infarct size at 2 days, detectable MO at 1 week was an independent predictor (P = 0.003) of infarct size at 1 year follow-up, associated with adverse infarct healing, adverse LV remodelling, increased LV volumes, and lower ejection fractions when compared with the rest of the cohort.

CONCLUSION

Microvascular obstruction is an important determinant of infarct healing. The effect of MO on infarct size translated into distinct patterns of LV remodelling during long-term follow-up.

Assessment of microvascular obstruction and prediction of short-term remodeling after acute myocardial infarction: cardiac MR imaging study

PURPOSE

To evaluate which cardiac magnetic resonance (MR) imaging technique for detection of microvascular obstruction (MVO) best predicts left ventricular (LV) remodeling after acute myocardial infarction (MI).

MATERIALS AND METHODS

This study had local ethics committee approval; all patients gave written informed consent. Sixty-three patients with first acute MI, treated with primary stent placement and optimal medical therapy, underwent cine MR imaging at 4-7 days and at 4 months after MI. Presence of MVO was qualitatively evaluated at baseline by using three techniques: (a) a single-shot saturation-recovery gradient-echo first-pass perfusion sequence (early hypoenhancement), mean time, 1.09 minutes +/- 0.07 (standard deviation) after contrast material administration; (b) a three-dimensional segmented saturation-recovery gradient-echo sequence (intermediate hypoenhancement), mean time, 2.17 minutes +/- 0.26; and (c) a two-dimensional segmented inversion-recovery gradient-echo late gadolinium enhancement sequence (late hypoenhancement), mean time, 13.32 minutes +/- 1.26. Contrast-to-noise ratios (CNRs) were calculated from the signal-to-noise ratios of the infarcted myocardium and MVO areas. Univariable linear regression analysis was used to identify the predictive value of each MR imaging technique.

RESULTS

Early hypoenhancement was detected in 44 (70%) of 63 patients; intermediate hypoenhancement, in 39 (62%); and late hypoenhancement, in 37 (59%). Late hypoenhancement was the strongest predictor of change in LV end-diastolic and end-systolic volumes over time (beta = 14.3, r = 0.40, P = .001 and beta = 11.3, r = 0.44, P < .001, respectively), whereas intermediate and late hypoenhancement had comparable predictive values of change in LV ejection fraction (beta = -3.1, r = -0.29, P = .02 and beta = -2.8, r = -0.27, P = .04, respectively). CNR corrected for spatial resolution was significantly superior for late enhancement compared with the other sequences (P < .001).

CONCLUSION

By using cardiac MR imaging, late hypoenhancement is the best prognostic marker of LV remodeling, with highest CNR between the infarcted myocardium and MVO regions.

Functional recovery after acute myocardial infarction: comparison between angiography, electrocardiography, and cardiovascular magnetic resonance measures of microvascular injury

OBJECTIVES

We examined the relation between angiographic, electrocardiographic, and gadolinium-enhanced cardiovascular magnetic resonance (CMR) characteristics of microvascular obstruction (MVO), and their predictive value on functional recovery after acute myocardial infarction (AMI).

BACKGROUND

Microvascular obstruction on CMR has been shown to predict left ventricular (LV) remodeling, but it is not well known how it compares with commonly used criteria of microvascular injury, and earlier reports have produced conflicting results on the significance and extent of MVO.

METHODS

Thrombolysis In Myocardial Infarction (TIMI) flow grade, myocardial blush grade (MBG), and ST-segment resolution were assessed in 60 patients with AMI treated with primary stenting. Cardiovascular magnetic resonance was performed between 2 and 9 days after revascularization to determine early MVO on first-pass perfusion imaging, late MVO on late gadolinium-enhanced imaging, and infarct size and transmural extent. Cine imaging was used to determine LV volumes and global and regional function at baseline and 4-month follow-up.

RESULTS

Early and late MVO were both related to incomplete ST-segment resolution (p = 0.002 and p = 0.01, respectively), but not to TIMI flow grade and MBG. Of all angiographic, electrocardiographic, and CMR variables, late MVO was the strongest parameter to predict changes in end-diastolic volume (beta = 0.53; p = 0.001), end-systolic volume (beta = 8.67; p = 0.001), and ejection fraction (beta = 3.94; p = 0.006) at follow-up. Regional analysis showed that late MVO had incremental diagnostic value to transmural extent of infarction (odds ratio: 0.18; p < 0.0001).

CONCLUSIONS

In patients after revascularized AMI, late MVO proved a more powerful predictor of global and regional functional recovery than all of the other characteristics, including transmural extent of infarction.

Cardiovascular magnetic resonance for the clinical cardiologist

Cardiovascular magnetic resonance is a noninvasive imaging modality that provides superior anatomical and functional information in the absence of ionizing radiation. The cardiovascular magnetic resonance imaging program has been active at the Quebec Heart Institute at Laval Hospital for two years, now providing advanced imaging studies to over 42 referral centres from eastern and central Quebec as well as providing training for national and international fellows. The program benefits from the collborative work of cardiologists and radiologists, who both bring to the table their unique expertise. The following text reviews current clinical applications useful in the daily practice of the cardiovascular specialist.