Controversies in cardiovascular MR imaging: reasons why imaging myocardial T2 has clinical and pathophysiologic value in acute myocardial infarction

Unravelling the mechanisms of CE-SSFP in imaging myocardium at risk: The effect of relaxation times on myocardial contrast

PURPOSE

The aim of this study was to investigate the contrast mechanisms of Contrast-enhanced steady-state free-precession (CE-SSFP) through the utilization of Bloch simulations in an experimental porcine model and in patients with acute myocardial infarction.

METHODS

Six pigs and ten patients with myocardial infarction underwent CMR and tissue characterization at 1.5 T whereas a Bloch simulation framework was utilized to simulate the CE-SSFP signal formation and compare it against the actual CE-SSFP signal acquired from the experimental porcine model and the patient population. The relaxation times of remote, salvaged, and infarcted myocardium were calculated after the injection of gadolinium, at the time of CE-SSFP acquisition. Simulations were performed using the same CE-SSFP pulse sequence as used on the scanner on a set of spins with the calculated relaxation times from the CMR scans.

RESULTS

The normalized signal intensities of salvaged and infarcted myocardium obtained with simulations were lower than the corresponding normalized signal intensities obtained in vivo in pigs (p < 0.05, 134% vs 153%) and in patients (p < 0.05, 126% vs 145%). The results from simulations showed a linear relationship to the results obtained in the experimental porcine model (r2 = 0.61) and in patients (r2 = 0.69).

CONCLUSION

The T1 and T2 values of remote, salvaged, and infarcted myocardium only partly explain the signal intensities in CE-SSFP images. Bloch simulations suggest that there may be more elements that contribute to the CE-SSFP contrast. Integration of other aspects of the MR experiment into the simulation model could further help to fully unravel the mechanisms of CE-SSFP.

Single breath-hold three-dimensional whole-heart T2 mapping with low-rank plus sparse reconstruction

The purpose of the current study was to develop and evaluate a three-dimensional single Breath-hOLd cardiac T₂ mapping sequence (3D BOLT) with low-rank plus sparse (L + S) reconstruction for rapid whole-heart T₂ measurement. 3D BOLT collects three highly accelerated electrocardiogram-triggered volumes with whole-heart coverage, all within a single 12-heartbeat breath-hold. Saturation pulses are performed every heartbeat to prepare longitudinal magnetization before T₂ preparation (T₂ -prep) or readout, and the echo time of T₂ -prep is varied per volume for variable T₂ weighting. Accelerated volumes are reconstructed jointly by an L + S algorithm. 3D BOLT was optimized and validated against gradient spin echo (GraSE) and a previously published approach (three-dimensional free-breathing cardiac T₂ mapping [3DFBT2]) in both phantoms and human subjects (11 healthy subjects and 10 patients). The repeatability of 3D BOLT was validated on healthy subjects. Retrospective experiments indicated that 3D BOLT with 4.2-fold acceleration achieved T₂ measurements comparable with those obtained with fully sampled data. T₂ measured in phantoms using 3D BOLT demonstrated good accuracy and precision compared with the reference (R²  > 0.99). All in vivo imaging was successful and the average left ventricle T₂ s measured by GraSE, 3DFBT2, and 3D BOLT were comparable and consistent for all healthy subjects (47.0 ± 2.3 vs. 47.7 ± 2.7 vs. 48.4 ± 1.8 ms) and patients (50.8 ± 3.0 vs. 48.6 ± 3.9 vs. 49.1 ± 3.7 ms), respectively. Myocardial T₂ measured by 3D BOLT had excellent agreement with 3DFBT2 and there was no significant difference in mean, standard deviation, and coefficient of variation. 3D BOLT showed excellent repeatability (intraclass correlation coefficient: 0.938). The proposed 3D BOLT achieved whole-heart T₂ mapping in a single breath-hold with good accuracy, precision, and repeatability on T₂ measurements.

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.

MRI for measuring therapy efficiency after revascularisation in ST-segment elevation myocardial infarction: a systematic review and meta-regression analysis

OBJECTIVE

To summarise existing data on the relation between the time from symptom onset until revascularisation (time to reperfusion) and the myocardial salvage index (MSI) calculated as proportion of non-necrotic myocardium inside oedematous myocardium on T2-weighted and T1-weighted late gadolinium enhancement MRI after ST-segment elevation myocardial infarction (STEMI).

METHODS

Studies including patients with revascularised STEMI and stating both the time to reperfusion and the MSI measured by T2-weighted and T1-weighted late gadolinium enhancement MRI were searched in MEDLINE, EMBASE and ISI Web of Science until 16 May 2020. A mixed effects model was used to evaluate the relation between the time to reperfusion and the MSI. The gender distribution and mean age in included patient groups, the timing of MRI, used MRI sequences and image interpretation methodology were included in the mixed effects model to explore between-study heterogeneity.

RESULTS

We included 38 studies with 5106 patients. The pooled MSI was 42.6% (95% CI: 38.1 to 47.1). The pooled time to reperfusion was 3.8 hours (95% CI: 3.5 to 4.0). Every hour of delay in reperfusion was associated with an absolute decrease of 13.1% (95% CI: 11.5 to 14.6; p<0.001) in the MSI. Between-study heterogeneity was considerable (σ²=167.8). Differences in the gender distribution, timing of MRI and image interpretation among studies explained 45.2% of the between-study heterogeneity.

CONCLUSIONS

The MSI on T2-weighted and T1-weighted late gadolinium enhancement MRI correlates inversely with the time to reperfusion, which indicates that cardioprotection achieved by minimising the time to reperfusion leads to a higher MSI. The analysis revealed considerable heterogeneity between studies. The heterogeneity could partly be explained by differences in the gender distribution, timing and interpretation of MRI suggesting that the MRI-assessed MSI is not only influenced by cardioprotective therapy but also by patient characteristics and MRI parameters.

Prognostic value of the myocardial salvage index measured by T2-weighted and T1-weighted late gadolinium enhancement magnetic resonance imaging after ST-segment elevation myocardial infarction: A systematic review and meta-regression analysis

In all patients with ST-segment elevation myocardial infarction, risk stratification should be performed before discharge. The measurement of therapy efficiency with magnetic resonance imaging has been proposed as part of the risk assessment, but it has not been adopted widely. This meta-analysis was conducted to summarize published data on the prognostic value of the proportion of salvaged myocardium inside previously ischemic myocardium (myocardial salvage index) measured by T2-weighted and T1-weighted late gadolinium enhancement magnetic resonance imaging after ST-segment elevation myocardial infarction. Random and mixed effects models were used for analyzing the data of 10 studies with 2,697 patients. The pooled myocardial salvage index, calculated as the proportion of non-necrotic myocardium inside edematous myocardium measured by T2-weighted and T1-weighted late gadolinium enhancement MRI, was 43.0% (95% confidence interval: 37.4, 48.6). The pooled length of follow-up was 12.3 months (95% confidence interval: 7.0, 17.6). The pooled incidence of major cardiac events during follow-up, defined as cardiac death, nonfatal myocardial infarction, or admission for heart failure, was 10.6% (95% confidence interval: 5.7, 15.5). The applied mixed effects model showed an absolute decrease of 1.7% in the incidence of major cardiac events during follow-up (95% confidence interval: 1.6, 1.9) with every 1% of increase in the myocardial salvage index. The heterogeneity between studies was considerable (τ = 21.3). Analysis of aggregated follow-up data after ST-segment elevation myocardial infarction suggests that the myocardial salvage index measured by T2-weighted and T1-weighted late gadolinium enhancement magnetic resonance imaging provides prognostic information on the risk of major cardiac events, but considerable heterogeneity exists between studies.

Measuring extracellular volume fraction by MRI: First verification of values given by clinical sequences

Purpose

To verify MR measurements of myocardial extracellular volume fraction (ECV) based on clinically applicable T1‐mapping sequences against ECV measurements by radioisotope tracer in pigs and to relate the results to those obtained in volunteers.

Methods

Between May 2016 and March 2017, 8 volunteers (25 ± 4 years, 3 female) and 8 pigs (4 female) underwent ECV assessment with SASHA, MOLLI5(3b)3, MOLLI5(3s)3, and MOLLI5s(3s)3s. Myocardial ECV was measured independently in pigs using a radioisotope tracer method.

Results

In pigs, ECV in normal myocardium was not different between radioisotope (average ± standard deviation; 19 ± 2%) and SASHA (21 ± 2%; P = 0.086). ECV was higher by MOLLI5(3b)3 (26 ± 2%), MOLLI5(3s)3 (25 ± 2%), and MOLLI5s(3s)3s (25 ± 2%) compared with SASHA or radioisotope (P ≤ 0.001 for all). ECV in volunteers was higher by MOLLI5(3b)3 (26 ± 3%) and MOLLI5(3s)3 (26 ± 3%) than by SASHA (22 ± 3%; P = 0.022 and P = 0.033). No difference was found between MOLLI5s(3s)3s (25 ± 3%) and SASHA (P = 0.225). Native T1 of blood and myocardium as well as postcontrast T1 of myocardium was consistently lower using MOLLI compared with SASHA. ECV increased over time as measured by MOLLI5(3b)3 and MOLLI5(3s)3 for pigs (0.08% and 0.07%/min; P = 0.004 and P = 0.013) and by MOLLI5s(3s)3s for volunteers (0.07%/min; P = 0.032) but did not increase as measured by SASHA.

Conclusion

Clinically available MOLLI and SASHA techniques can be used to accurately estimate ECV in normal myocardium where MOLLI‐sequences show minor overestimation driven by underestimation of postcontrast T1 when compared with SASHA. The timing of imaging after contrast administration affected the measurement of ECV using some variants of the MOLLI sequence.

Segment-by-segment assessment of left ventricular myocardial affection in Anderson-Fabry disease by non-enhanced T1-mapping

Background Anderson-Fabry disease (AFD) is an X-linked lysosomal enzyme disorder associated with an intracellular accumulation of sphingolipids, which shorten myocardial T1 relaxation times. Myocardial affection, however, varies between different segments. Purpose To evaluate the specific segmental distribution and degree of segmental affection in AFD patients. Material and Methods Twenty-five patients with AFD, 14 patients with hypertrophic cardiomyopathy (HCM), and 21 controls were included. A Modified Look-Locker Inversion Recovery sequence (MOLLI) was used for non-enhanced T1 mapping at 1.5 T in addition to standard cardiac imaging in 10-12 short axis views. T1 values were evaluated with a mixed model ANOVA and regression analysis to determine the best diagnostic cutoff values for T1 for each myocardial segment. Results Regression analysis showed the best diagnostic cutoff compared to controls in cardiac segments 1-4, 8-9, and 14. Mean differences between T1 for AFD versus HCM were greatest in segment 3, 4, and 9 (99 ms, 103 ms, 86 ms, respectively). Overall T1 times were 888 ± 70 ms and 903 ± 14 ms (AFD with and without LVH); 1014 ± 17 ms and 1001 ± 22 ms (HCM and controls, P < 0.05). Conclusion Myocardial segments are affected by a varying degree of T1 shortening in AFD patients. Segment-specific cutoff values allow the most specific detection and quantification of the extent of myocardial affection.

Dynamic Edematous Response of the Human Heart to Myocardial Infarction: Implications for Assessing Myocardial Area at Risk and Salvage

Supplemental Digital Content is available in the text.

Background:

Clinical protocols aimed to characterize the post–myocardial infarction (MI) heart by cardiac magnetic resonance (CMR) need to be standardized to take account of dynamic biological phenomena evolving early after the index ischemic event. Here, we evaluated the time course of edema reaction in patients with ST-segment–elevation MI by CMR and assessed its implications for myocardium-at-risk (MaR) quantification both in patients and in a large-animal model.

Methods:

A total of 16 patients with anterior ST-segment–elevation MI successfully treated by primary angioplasty and 16 matched controls were prospectively recruited. In total, 94 clinical CMR examinations were performed: patients with ST-segment–elevation MI were serially scanned (within the first 3 hours after reperfusion and at 1, 4, 7, and 40 days), and controls were scanned only once. T2 relaxation time in the myocardium (T2 mapping) and the extent of edema on T2-weighted short-tau triple inversion-recovery (ie, CMR-MaR) were evaluated at all time points. In the experimental study, 20 pigs underwent 40-minute ischemia/reperfusion followed by serial CMR examinations at 120 minutes and 1, 4, and 7 days after reperfusion. Reference MaR was assessed by contrast-multidetector computed tomography during the index coronary occlusion. Generalized linear mixed models were used to take account of repeated measurements.

Results:

In humans, T2 relaxation time in the ischemic myocardium declines significantly from early after reperfusion to 24 hours, and then increases up to day 4, reaching a plateau from which it decreases from day 7. Consequently, edema extent measured by T2-weighted short-tau triple inversion-recovery (CMR-MaR) varied with the timing of the CMR examination. These findings were confirmed in the experimental model by showing that only CMR-MaR values for day 4 and day 7 postreperfusion, coinciding with the deferred edema wave, were similar to values measured by reference contrast-multidetector computed tomography.

Conclusions:

Post-MI edema in patients follows a bimodal pattern that affects CMR estimates of MaR. Dynamic changes in post–ST-segment–elevation MI edema highlight the need for standardization of CMR timing to retrospectively delineate MaR and quantify myocardial salvage. According to the present clinical and experimental data, a time window between days 4 and 7 post-MI seems a good compromise solution for standardization. Further studies are needed to study the effect of other factors on these variables.

Cardiac MR Imaging in Acute Coronary Syndrome: Application and Image Interpretation

Acute coronary syndrome (ACS) is a frequent cause of hospitalization and coronary interventions. Cardiac magnetic resonance (MR) imaging is an increasingly used technique for initial work-up of chest pain and early post-reperfusion and follow-up evaluation of ACS to identify patients at high risk of further cardiac events. Cardiac MR imaging can evaluate with accuracy a variety of prognostic indicators of myocardial damage, including regional myocardial dysfunction, infarct distribution, infarct size, myocardium at risk, microvascular obstruction, and intramyocardial hemorrhage in both acute setting and later follow-up examinations. In addition, MR imaging is useful to rule out other causes of acute chest pain in patients admitted to the emergency department. In this article, a brief explanation of the pathophysiology, classification, and treatment options for patients with ACS will be introduced. Indications of cardiac MR imaging in ACS patients will be reviewed and specific cardiac MR protocol, image interpretation, and potential diagnostic pitfalls will be discussed. ^© RSNA, 2017 Online supplemental material is available for this article.

Extent of Myocardium at Risk for Left Anterior Descending Artery, Right Coronary Artery, and Left Circumflex Artery Occlusion Depicted by Contrast-Enhanced Steady State Free Precession and T2-Weighted Short Tau Inversion Recovery Magnetic Resonance Imaging

Background—

Contrast-enhanced steady state free precession (CE-SSFP) and T2-weighted short tau inversion recovery (T2-STIR) have been clinically validated to estimate myocardium at risk (MaR) by cardiovascular magnetic resonance while using myocardial perfusion single-photon emission computed tomography as reference standard. Myocardial perfusion single-photon emission computed tomography has been used to describe the coronary perfusion territories during myocardial ischemia. Compared with myocardial perfusion single-photon emission computed tomography, cardiovascular magnetic resonance offers superior image quality and practical advantages. Therefore, the aim was to describe the main coronary perfusion territories using CE-SSFP and T2-STIR cardiovascular magnetic resonance data in patients after acute ST-segment–elevation myocardial infarction.

Methods and Results—

CE-SSFP and T2-STIR data from 2 recent multicenter trials, CHILL-MI and MITOCARE (n=215), were used to assess MaR. Angiography was used to determine culprit vessel. Of 215 patients, 39% had left anterior descending artery occlusion, 49% had right coronary artery occlusion, and 12% had left circumflex artery occlusion. Mean extent of MaR using CE-SSFP was 44±10% for left anterior descending artery, 31±7% for right coronary artery, and 30±9% for left circumflex artery. Using T2-STIR, MaR was 44±9% for left anterior descending artery, 30±8% for right coronary artery, and 30±12% for left circumflex artery. MaR was visualized in polar plots, and expected overlap was found between right coronary artery and left circumflex artery. Detailed regional data are presented for use in software algorithms as a priori information on the extent of MaR.

Conclusions—

For the first time, cardiovascular magnetic resonance has been used to show the main coronary perfusion territories using CE-SSFP and T2-STIR. The good agreement between CE-SSFP and T2-STIR from this study and myocardial perfusion single-photon emission computed tomography from previous studies indicates that these 3 methods depict MaR accurately in individual patients and at a group level.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov . Unique identifiers: NCT01379261 and NCT01374321.

Effects of glycaemic variability on cardiac remodelling after reperfused myocardial infarction: Evaluation of streptozotocin-induced diabetic Wistar rats using cardiac magnetic resonance imaging

AIMS

In addition to hyperglycaemia, glycaemic variability seems to be associated with poor outcomes after acute myocardial infarction. This study explored the impact of glycaemic variability in diabetic Wistar rats subjected to myocardial ischaemia/reperfusion.

METHODS

Animals with streptozotocin-induced diabetes received insulin either to maintain stable hyperglycaemia (Dh group) or to generate glycaemic variability (Dv). After experimental myocardial ischaemia/reperfusion was surgically induced, 7T cardiac magnetic resonance imaging (CMR) was performed at weeks 1 (w1) and 3 (w3).

RESULTS

Twenty-six rats were randomized [sham group (S): n=5; control group (C): n=7; Dh group: n=6; and Dv group: n=8]. The mean amplitude of glucose reflecting glycaemic variability was higher in the Dv than in the Dh group (9.1±2.7mmol/L vs 5.9±1.9mmol/L; P<0.05). CMR assessment at w3 revealed ventricular enlargement in both Dh and Dv groups compared with the C and S groups (end-diastolic volume: 1.60±0.22 and 1.36±0.30mL/kg compared with 1.11±0.13 and 0.87±0.11mL/kg, respectively; P<0.05). Circumferential strain was altered between w1 and w3 in the remote area only in the Dv group, resulting in a lower value in this group than in the S, C and Dh groups (-0.11±0.01 vs -0.17±0.05, -0.15±0.03 and -0.16±0.03, respectively; P<0.05). In addition, at w3, oedema was also higher in the remote area in the Dv than in the C group (18.3±4.9ms vs 14.5±1.7ms, respectively; P<0.05).

CONCLUSION

In the context of experimental myocardial ischaemia/reperfusion, our results suggest that glycaemic variability might have a potentially deleterious impact on myocardial outcomes beyond the classical glucose metrics.

Fast T2 gradient-spin-echo (T2-GraSE) mapping for myocardial edema quantification: first in vivo validation in a porcine model of ischemia/reperfusion

BACKGROUND

Several T2-mapping sequences have been recently proposed to quantify myocardial edema by providing T2 relaxation time values. However, no T2-mapping sequence has ever been validated against actual myocardial water content for edema detection. In addition, these T2-mapping sequences are either time-consuming or require specialized software for data acquisition and/or post-processing, factors impeding their routine clinical use. Our objective was to obtain in vivo validation of a sequence for fast and accurate myocardial T2-mapping (T2 gradient-spin-echo [GraSE]) that can be easily integrated in routine protocols.

METHODS

The study population comprised 25 pigs. Closed-chest 40 min ischemia/reperfusion was performed in 20 pigs. Pigs were sacrificed at 120 min (n = 5), 24 h (n = 5), 4 days (n = 5) and 7 days (n = 5) after reperfusion, and heart tissue extracted for quantification of myocardial water content. For the evaluation of T2 relaxation time, cardiovascular magnetic resonance (CMR) scans, including T2 turbo-spin-echo (T2-TSE, reference standard) mapping and T2-GraSE mapping, were performed at baseline and at every follow-up until sacrifice. Five additional pigs were sacrificed after baseline CMR study and served as controls.

RESULTS

Acquisition of T2-GraSE mapping was significantly (3-fold) faster than conventional T2-TSE mapping. Myocardial T2 relaxation measurements performed by T2-TSE and T2-GraSE mapping demonstrated an almost perfect correlation (R(2) = 0.99) and agreement with no systematic error between techniques. The two T2-mapping sequences showed similarly good correlations with myocardial water content: R(2) = 0.75 and R(2) = 0.73 for T2-TSE and T2-GraSE mapping, respectively.

CONCLUSIONS

We present the first in vivo validation of T2-mapping to assess myocardial edema. Given its shorter acquisition time and no requirement for specific software for data acquisition or post-processing, fast T2-GraSE mapping of the myocardium offers an attractive alternative to current CMR sequences for T2 quantification.

Pathophysiology Underlying the Bimodal Edema Phenomenon After Myocardial Ischemia/Reperfusion

BACKGROUND

Post-ischemia/reperfusion (I/R) myocardial edema was recently shown to follow a consistent bimodal pattern: an initial wave of edema appears on reperfusion and dissipates at 24 h, followed by a deferred wave that initiates days after infarction, peaking at 1 week.

OBJECTIVES

This study examined the pathophysiology underlying this post-I/R bimodal edematous reaction.

METHODS

Forty instrumented pigs were assigned to different myocardial infarction protocols. Edematous reaction was evaluated by water content quantification, serial cardiac magnetic resonance T2-mapping, and histology/immunohistochemistry. The association of reperfusion with the initial wave of edema was evaluated in pigs undergoing 40-min/80-min I/R and compared with pigs undergoing 120-min ischemia with no reperfusion. The role of tissue healing in the deferred wave of edema was evaluated by comparing pigs undergoing standard 40-min/7-day I/R with animals subjected to infarction without reperfusion (chronic 7-day coronary occlusion) or receiving post-I/R high-dose steroid therapy.

RESULTS

Characterization of post-I/R tissue changes revealed maximal interstitial edema early on reperfusion in the ischemic myocardium, with maximal content of neutrophils, macrophages, and collagen at 24 h, day 4, and day 7 post-I/R, respectively. Reperfused pigs had significantly higher myocardial water content at 120 min and T2 relaxation times on 120 min cardiac magnetic resonance than nonreperfused animals. Permanent coronary occlusion or high-dose steroid therapy significantly reduced myocardial water content on day 7 post-infarction. The dynamics of T2 relaxation times during the first post-infarction week were altered significantly in nonreperfused pigs compared with pigs undergoing regular I/R.

CONCLUSIONS

The 2 waves of the post-I/R edematous reaction are related to different pathophysiological phenomena. Although the first wave is secondary to reperfusion, the second wave occurs mainly because of tissue healing processes.

Quantitative T2 mapping after reperfusion therapy in patients with acute myocardial infarction: A comparison with late gadolinium enhancement and cine MR imaging

PURPOSE

This study evaluates myocardial edema by quantitative T2 mapping in patients with acute myocardial infarction (AMI) and compares the lateral extent of myocardial edema with those of infarcted and dysfunctional myocardium.

MATERIALS AND METHODS

Cardiac magnetic resonance images (MRIs) of 31 patients (M:F=29:2, mean age: 52.5±10.8years) with AMI were reviewed. On cine-MRI, all short axis images of the left ventricle (LV) were divided into 60 sectors. The regional wall motion of each sector was calculated as follows: systolic wall thickening (SWT, %)=[(LV wall thicknessES-LV wall thicknessED)/LV wall thicknessED]*100. Dysfunctional myocardium was defined as sectors with decreased SWT lower than 40%. On LGE-images, myocardial infarction was defined as an area of hyper-enhancement more than 5 SDs from the remote myocardium. On T2 map, myocardial edema was defined as an area in which T2 values were at least 2 SDs higher than those from remote myocardium. The lateral extents of infarcted myocardium, myocardial edema, and dysfunctional myocardium were calculated as the percentage of central angles ((central angle of the involved myocardium/360)*100 (%)) and then compared.

RESULTS

The lateral extent of myocardial edema was slightly larger than that of infarcted myocardium (37.4±13.3% vs. 35±12.9%, p<0.01). The lateral extent of dysfunctional myocardium (50.6±15.3%) was significantly larger than that of infarcted myocardium or myocardial edema (p<0.001).

CONCLUSIONS

The lateral extent of myocardial edema beyond the infarcted myocardium might be narrow, but the dysfunctional myocardium could be significantly larger than myocardial edema, suggesting stunned myocardium without edema.

Integrated FDG PET/MR Imaging for the Assessment of Myocardial Salvage in Reperfused Acute Myocardial Infarction

PURPOSE

To compare the size of the area with reduced myocardial fluorodeoxygluose (FDG) uptake with the endocardial surface area (ESA) method as a marker for the area at risk in patients with reperfused acute myocardial infarction.

MATERIALS AND METHODS

The study was approved by the local institutional review board. All patients gave written informed consent prior to their examination. Twenty-five patients (mean age ± standard deviation, 54 years ± 14) underwent prospective cardiac positron emission tomography/magnetic resonance imaging after acute coronary occlusion and interventional reperfusion. On late gadolinium contrast enhancement images, the size of infarction and the area at risk, as determined with ESA, were assessed and compared with the area of reduced FDG uptake. Statistical analysis comprised paired t tests and Mann-Whitney U tests, as well as Pearson r and Spearman ρ for correlations.

RESULTS

In patients with infarcted myocardium and reduced FDG uptake (n = 18), a good correlation between the area of reduced FDG uptake and the area at risk according to ESA was observed (r = .70, P = .001). The area of reduced FDG uptake (31% ± 11 of left ventricular myocardial mass) was larger than the size of the infarct (10% ± 10, P < .0001) and the area at risk according to ESA (17% ± 13, P < .0001). In six patients, no late contrast enhancement was seen, whereas all patients had an area of reduced FDG uptake (29% ± 8) in the perfusion territory of the culprit artery.

CONCLUSION

In patients with reperfused acute myocardial infarction, the area of reduced FDG uptake correlates with the area at risk as determined with the ESA method and is localized in the perfusion territory of the culprit artery in the absence of necrosis, although the area of reduced FDG uptake largely overestimates the size of the infarct and the ESA-based area at risk.

Myocardial edema after ischemia/reperfusion is not stable and follows a bimodal pattern: imaging and histological tissue characterization

BACKGROUND

It is widely accepted that edema occurs early in the ischemic zone and persists in stable form for at least 1 week after myocardial ischemia/reperfusion. However, there are no longitudinal studies covering from very early (minutes) to late (1 week) reperfusion stages confirming this phenomenon.

OBJECTIVES

This study sought to perform a comprehensive longitudinal imaging and histological characterization of the edematous reaction after experimental myocardial ischemia/reperfusion.

METHODS

The study population consisted of 25 instrumented Large-White pigs (30 kg to 40 kg). Closed-chest 40-min ischemia/reperfusion was performed in 20 pigs, which were sacrificed at 120 min (n = 5), 24 h (n = 5), 4 days (n = 5), and 7 days (n = 5) after reperfusion and processed for histological quantification of myocardial water content. Cardiac magnetic resonance (CMR) scans with T2-weighted short-tau inversion recovery and T2-mapping sequences were performed at every follow-up stage until sacrifice. Five additional pigs sacrificed after baseline CMR served as controls.

RESULTS

In all pigs, reperfusion was associated with a significant increase in T2 relaxation times in the ischemic region. On 24-h CMR, ischemic myocardium T2 times returned to normal values (similar to those seen pre-infarction). Thereafter, ischemic myocardium-T2 times in CMR performed on days 4 and 7 after reperfusion progressively and systematically increased. On day 7 CMR, T2 relaxation times were as high as those observed at reperfusion. Myocardial water content analysis in the ischemic region showed a parallel bimodal pattern: 2 high water content peaks at reperfusion and at day 7, and a significant decrease at 24 h.

CONCLUSIONS

Contrary to the accepted view, myocardial edema during the first week after ischemia/reperfusion follows a bimodal pattern. The initial wave appears abruptly upon reperfusion and dissipates at 24 h. Conversely, the deferred wave of edema appears progressively days after ischemia/reperfusion and is maximal around day 7 after reperfusion.

An overview on development and application of an experimental platform for quantitative cardiac imaging research in rabbit models of myocardial infarction

To exploit the advantages of using rabbits for cardiac imaging research and to tackle the technical obstacles, efforts have been made under the framework of a doctoral research program. In this overview article, by cross-referencing the current literature, we summarize how we have developed a preclinical cardiac research platform based on modified models of reperfused myocardial infarction (MI) in rabbits; how the in vivo manifestations of cardiac imaging could be closely matched with those ex vivo macro- and microscopic findings; how these imaging outcomes could be quantitatively analyzed, validated and demonstrated; and how we could apply this cardiac imaging platform to provide possible solutions to certain lingering diagnostic and therapeutic problems in experimental cardiology. In particular, tissue components in acute cardiac ischemia have been stratified and characterized, post-infarct lipomatous metaplasia (LM) as a common but hardly illuminated clinical pathology has been identified in rabbit models, and a necrosis avid tracer as well as an anti-ischemic drug have been successfully assessed for their potential utilities in clinical cardiology. These outcomes may interest the researchers in the related fields and help strengthen translational research in cardiovascular diseases.

Three-dimensional whole-heart T2 mapping at 3T

PURPOSE

Detecting variations in myocardial water content with T2 mapping is superior to conventional T2 -weighted MRI since quantification enables direct observation of complicated pathology. Most commonly used T2 mapping techniques are limited in achievable spatial and/or temporal resolution, both of which reduce accuracy due to partial-volume averaging and misregistration between images. The goal of this study was to validate a novel free breathing T2 mapping sequence that overcomes these limitations.

METHODS

The proposed technique was made insensitive to heart rate variability through the use of a saturation prepulse to reset magnetization every heartbeat. Respiratory navigator-gated, differentially T2 -weighted volumes were interleaved per heartbeat, guaranteeing registered images and robust voxel-by-voxel T2 maps. Free breathing acquisitions removed limits on spatial resolution and allowed short diastolic windows. Accuracy was quantified with simulations and phantoms.

RESULTS

Homogeneous three-dimensional (3D) T2 maps were obtained from normal human subjects and swine. Normal human and swine left ventricular T2 values were 42.3 ± 4.0 and 43.5 ± 4.3 ms, respectively. The T2 value for edematous myocardium obtained from a swine model of acute myocardial infarction was 59.1 ± 7.1 ms.

CONCLUSION

Free-breathing accurate 3D T2 mapping is feasible and may be applicable in myocardial assessment in lieu of current clinical black blood, T2 -weighted techniques.

Remote ischemic conditioning reduces myocardial infarct size and edema in patients with ST-segment elevation myocardial infarction

OBJECTIVES

This study aimed to determine whether remote ischemic conditioning (RIC) initiated prior to primary percutaneous coronary intervention (PPCI) could reduce myocardial infarct (MI) size in patients presenting with ST-segment elevation myocardial infarction.

BACKGROUND

RIC, using transient limb ischemia and reperfusion, can protect the heart against acute ischemia-reperfusion injury. Whether RIC can reduce MI size, assessed by cardiac magnetic resonance (CMR), is unknown.

METHODS

We randomly assigned 197 ST-segment elevation myocardial infarction patients with TIMI (Thrombolysis In Myocardial Infarction) flow grade 0 to receive RIC (four 5-min cycles of upper arm cuff inflation/deflation) or control (uninflated cuff placed on upper arm for 40 min) protocols prior to PPCI. The primary study endpoint was MI size, measured by CMR in 83 subjects on days 3 to 6 after admission.

RESULTS

RIC reduced MI size by 27%, when compared with the MI size of control subjects (18.0 ± 10% [n = 40] vs. 24.5 ± 12.0% [n = 43]; p = 0.009). At 24 h, high-sensitivity troponin T was lower with RIC (2,296 ± 263 ng/l [n = 89] vs. 2,736 ± 325 ng/l [n = 84]; p = 0.037). RIC also reduced the extent of myocardial edema measured by T2-mapping CMR (28.5 ± 9.0% vs. 35.1 ± 10.0%; p = 0.003) and lowered mean T2 values (68.7 ± 5.8 ms vs. 73.1 ± 6.1 ms; p = 0.001), precluding the use of CMR edema imaging to correctly estimate the area at risk. Using CMR-independent coronary angiography jeopardy scores to estimate the area at risk, RIC, when compared with the control protocol, was found to significantly improve the myocardial salvage index (0.42 ± 0.29 vs. 0.28 ± 0.29; p = 0.03).

CONCLUSIONS

This randomized study demonstrated that in ST-segment elevation myocardial infarction patients treated by PPCI, RIC, initiated prior to PPCI, reduced MI size, increased myocardial salvage, and reduced myocardial edema.

Correlation of electrocardiogram and regional cardiac magnetic resonance imaging findings in ST-elevation myocardial infarction: a literature review

BACKGROUND

Patients with acute ST-elevation myocardial infarction (STEMI) benefit substantially from emergent coronary reperfusion. The principal mechanism is to open the occluded coronary artery to minimize myocardial injury. Thus the size of the area at risk is a critical determinant of the patient outcome, although other factors, such as reperfusion injury, have major impact on the final infarct size. Acute coronary occlusion almost immediately induces metabolic changes within the myocardium, which can be assessed with both the electrocardiogram (ECG) and cardiac magnetic resonance (CMR) imaging.

METHODS

The 12-lead ECG is the principal diagnostic method to detect and risk-stratify acute STEMI. However, to achieve a correct diagnosis, it is paramount to compare different ECG parameters with golden standards in imaging, such as CMR. In this review, we discuss aspects of ECG and CMR in the assessment of acute regional ischemic changes in the myocardium using the 17 segment model of the left ventricle presented by American Heart Association (AHA), and their relation to coronary artery anatomy.

RESULTS

Using the 17 segment model of AHA, the segments 12 and 16 remain controversial. There is an important overlap in myocardial blood supply at the antero-lateral region between LAD and LCx territories concerning these two segments.

CONCLUSION

No all-encompassing correlation can be found between ECG and CMR findings in acute ischemia with respect to coronary anatomy.

Distinction of salvaged and infarcted myocardium within the ischaemic area-at-risk with T2 mapping

AIM

Area-at-risk (AAR) measurements often rely on T2-weighted images, but subtle differences in T2 may be overlooked with this method. To determine the differences in oedema between salvaged and infarcted myocardium, we performed quantitative T2 mapping of the AAR. We also aimed to determine the impact of reperfusion time on T2 in the AAR.

METHODS

Twenty-two dogs underwent 2 h of coronary occlusion followed by 4 or 48 h of reperfusion before cardiac magnetic resonance imaging at 1.5 T. Late gadolinium enhancement images were used to define the infarcted, salvaged, and remote myocardium. T2 values from T2 maps and signal intensities on T2-weighted images were measured in the corresponding areas.

RESULTS

At both imaging time points, the T2 of the salvaged myocardium was longer than of remote (66.0 ± 6.9 vs. 51.4 ± 3.5 ms, P < 0.001 at 4 h, and 56.7 ± 7.3 vs. 48.1 ± 3.5 ms, P < 0.001 at 48 h). The T2 was also longer in the infarcted myocardium compared with remote at both 4 and 48 h (71.4 ± 7.6 ms, P < 0.01 vs. salvage and 64.0 ± 6.9 ms, P = 0.03 vs. salvage, both P < 0.001 vs. remote). The increase in T2 in the salvaged myocardium compared with remote was greater after 4 h than after 48 h (14.7 ± 5.6 vs. 8.7 ± 5.1 ms, P = 0.02).

CONCLUSIONS

T2 relaxation parameters are different in the infarcted and salvaged myocardium, and both are significantly longer than remote. Furthermore, the magnitude of increase in T2 was less in the salvaged myocardium after longer reperfusion, indicating partial resolution of oedema in the first 48 h after reperfusion.

Cardiac magnetic resonance imaging for the investigation of cardiovascular disorders. Part 2: emerging applications

Cardiac magnetic resonance imaging has emerged as a robust noninvasive technique for the investigation of cardiovascular disorders. The coming-of-age of cardiac magnetic resonance-and especially its widening span of applications-has generated both excitement and uncertainty in regard to its potential clinical use and its role vis-à-vis conventional imaging techniques. The purpose of this evidence-based review is to discuss some of these issues by highlighting the current (Part 1, previously published) and emerging (Part 2) applications of cardiac magnetic resonance. Familiarity with the versatile uses of cardiac magnetic resonance will facilitate its wider clinical acceptance for improving the management of patients with cardiovascular disorders.

Towards stratifying ischemic components by cardiac MRI and multifunctional stainings in a rabbit model of myocardial infarction

OBJECTIVES

We sought to identify critical components of myocardial infarction (MI) including area at risk (AAR), MI-core and salvageable zone (SZ) by using cardiac magnetic resonance imaging (cMRI) and multifunctional stainings in rabbits.

MATERIALS AND METHODS

Fifteen rabbits received 90-min coronary artery (CA) ligation and reopening to induce reperfused MI. First-pass perfusion weighted imaging (PWI(90')) was performed immediately before CA reperfusion. Necrosis avid dye Evans blue (EB) was intravenously injected for later MI-core detection. One-day later, cMRI with T2-weighted imaging (T2WI), PWI(24h) and delayed enhancement (DE) T1WI was performed at a 3.0T clinical scanner. The heart was excised and CA was re-ligated with aorta infused by red-iodized-oil (RIO). The heart was sliced into 3-mm sections for digital radiography (DR), histology and planimetry with myocardial salvage index (MSI) and perfusion density rate (PDR) calculated.

RESULTS

There was no significant difference between MI-cores defined by DE-T1WI and EB-staining (31.13±8.55% vs 29.80±7.97%; p=0.74). The AAR was defined similarly by PWI90' (39.93±9.51%), RIO (38.82±14.41%) and DR (38.17±15.98%), underestimated by PWI(24h) (36.44±5.31%), but overestimated (p<0.01) by T2WI (56.93±8.87%). Corresponding MSI turned out to be 24.17±9.5% (PWI(90')), 21.97±9.41% (DR) and 22.68±9.65% (RIO), which were significantly (p<0.01) higher and lower than that with PWI(24h) (15.15±7.34%) and T2WI (45.52±7.5%) respectively. The PDR differed significantly (p<0.001) between normal myocardium (350.6±33.1%) and the AAR (31.2±15%), suggesting 11-times greater blood perfusion in normal myocardium over the AAR.

CONCLUSION

The introduced rabbit platform and new staining techniques together with the use of a 3.0T clinical scanner for cMRI enabled visualization of MI components and may contribute to translational cardiac imaging research for improved theranostic management of ischemic heart disease.

Bifunctional staining for ex vivo determination of area at risk in rabbits with reperfused myocardial infarction

AIM: To develop a method for studying myocardial area at risk (AAR) in ischemic heart disease in correlation with cardiac magnetic resonance imaging (cMRI).

METHODS: Nine rabbits were anesthetized, intubated and subjected to occlusion and reperfusion of the left circumflex coronary artery (LCx) to induce myocardial infarction (MI). ECG-triggered cMRI with delayed enhancement was performed at 3.0 T. After euthanasia, the heart was excised with the LCx re-ligated. Bifunctional staining was performed by perfusing the aorta with a homemade red-iodized-oil (RIO) dye. The heart was then agar-embedded for ex vivo magnetic resonance imaging and sliced into 3 mm-sections. The AAR was defined by RIO-staining and digital radiography (DR). The perfusion density rate (PDR) was derived from DR for the AAR and normal myocardium. The MI was measured by in vivo delayed enhancement (iDE) and ex vivo delayed enhancement (eDE) cMRI. The AAR and MI were compared to validate the bifunctional straining for cardiac imaging research. Linear regression with Bland-Altman agreement, one way-ANOVA with Bonferroni’s multiple comparison, and paired t tests were applied for statistics.

RESULTS: All rabbits tolerated well the surgical procedure and subsequent cMRI sessions. The open-chest occlusion and close-chest reperfusion of the LCx, double suture method and bifunctional staining were successfully applied in all animals. The percentage MI volumes globally (n = 6) and by slice (n = 25) were 36.59% ± 13.68% and 32.88% ± 12.38% on iDE, and 35.41% ± 12.25% and 32.40% ± 12.34% on eDE. There were no significant differences for MI determination with excellent linear regression correspondence (r_global = 0.89; r_slice = 0.9) between iDE and eDE. The percentage AAR volumes globally (n = 6) and by slice (n = 25) were 44.82% ± 15.18% and 40.04% ± 13.64% with RIO-staining, and 44.74% ± 15.98% and 40.48% ± 13.26% by DR showing high correlation in linear regression analysis (r_global = 0.99; r_slice = 1.0). The mean differences of the two AAR measurements on Bland-Altman were almost zero, indicating RIO-staining and DR were essentially equivalent or inter-replaceable. The AAR was significantly larger than MI both globally and slice-by-slice (P < 0.01). After correction with the background and the blank heart without bifunctional staining (n = 3), the PDR for the AAR and normal myocardium was 32% ± 15% and 35.5% ± 35%, respectively, which is significantly different (P < 0.001), suggesting that blood perfusion to the AAR probably by collateral circulation was only less than 10% of that in the normal myocardium.

CONCLUSION: The myocardial area at risk in ischemic heart disease could be accurately determined postmortem by this novel bifunctional staining, which may substantially contribute to translational cardiac imaging research.