Serial nuclear magnetic resonance imaging of acute myocardial infarction with and without reperfusion

T2 mapping in myocardial disease: a comprehensive review

Cardiovascular magnetic resonance (CMR) is considered the gold standard imaging modality for myocardial tissue characterization. Elevated transverse relaxation time (T2) is specific for increased myocardial water content, increased free water, and is used as an index of myocardial edema. The strengths of quantitative T2 mapping lie in the accurate characterization of myocardial edema, and the early detection of reversible myocardial disease without the use of contrast agents or ionizing radiation. Quantitative T2 mapping overcomes the limitations of T2-weighted imaging for reliable assessment of diffuse myocardial edema and can be used to diagnose, stage, and monitor myocardial injury. Strong evidence supports the clinical use of T2 mapping in acute myocardial infarction, myocarditis, heart transplant rejection, and dilated cardiomyopathy. Accumulating data support the utility of T2 mapping for the assessment of other cardiomyopathies, rheumatologic conditions with cardiac involvement, and monitoring for cancer therapy-related cardiac injury. Importantly, elevated T2 relaxation time may be the first sign of myocardial injury in many diseases and oftentimes precedes symptoms, changes in ejection fraction, and irreversible myocardial remodeling. This comprehensive review discusses the technical considerations and clinical roles of myocardial T2 mapping with an emphasis on expanding the impact of this unique, noninvasive tissue parameter.

Molecular imaging of cardiac remodelling after myocardial infarction

Myocardial infarction and subsequent heart failure is a major health burden associated with significant mortality and morbidity in western societies. The ability of cardiac tissue to recover after myocardial infarction is affected by numerous complex cellular and molecular pathways. Unbalance or failure of these pathways can lead to adverse remodelling of the heart and poor prognosis. Current clinical cardiac imaging modalities assess anatomy, perfusion, function, and viability of the myocardium, yet do not offer any insight into the specific molecular pathways involved in the repair process. Novel imaging techniques allow visualisation of these molecular processes and may have significant diagnostic and prognostic values, which could aid clinical management. Single photon-emission tomography, positron-emission tomography, and magnetic resonance imaging are used to visualise various aspects of these molecular processes. Imaging probes are usually attached to radioisotopes or paramagnetic nanoparticles to specifically target biological processes such as: apoptosis, necrosis, inflammation, angiogenesis, and scar formation. Although the results from preclinical studies are promising, translating this work to a clinical environment in a valuable and cost-effective way is extremely challenging. Extensive evaluation evidence of diagnostic and prognostic values in multi-centre clinical trials is still required.

Persistence of Infarct Zone T2 Hyperintensity at 6 Months After Acute ST-Segment-Elevation Myocardial Infarction: Incidence, Pathophysiology, and Prognostic Implications

BACKGROUND

The incidence and clinical significance of persistent T2 hyperintensity after acute ST-segment-elevation myocardial infarction (STEMI) is uncertain.

METHODS AND RESULTS

Patients who sustained an acute STEMI were enrolled in a cohort study (BHF MR-MI: NCT02072850). Two hundred eighty-three STEMI patients (mean age, 59±12 years; 75% male) had cardiac magnetic resonance with T2 mapping performed at 2 days and 6 months post-STEMI. Persisting T2 hyperintensity was defined as infarct T2 >2 SDs from remote T2 at 6 months. Infarct zone T2 was higher than remote zone T2 at 2 days (66.3±6.1 versus 49.7±2.1 ms; P<0.001) and 6 months (56.8±4.5 versus 49.7±2.3 ms; P<0.001). Remote zone T2 did not change over time (mean change, 0.0±2.7 ms; P=0.837), whereas infarct zone T2 decreased (-9.5±6.4 ms; P<0.001). At 6 months, T2 hyperintensity persisted in 189 (67%) patients, who were more likely to have Thrombus in Myocardial Infarction flow 0 or 1 in the culprit artery (P=0.020), incomplete ST-segment resolution (P=0.037), and higher troponin (P=0.024). Persistent T2 hyperintensity was associated with NT-proBNP (N-terminal pro-B-type natriuretic peptide) concentration (0.57 on a log scale [0.42-0.72]; P=0.004) and the likelihood of adverse left ventricular remodeling (>20% change in left ventricular end-diastolic volume; 21.91 [2.75-174.29]; P=0.004). Persistent T2 hyperintensity was associated with all-cause death and heart failure, but the result was not significant (P=0.051). ΔT2 was associated with all-cause death and heart failure (P=0.004) and major adverse cardiac events (P=0.013).

CONCLUSIONS

Persistent T2 hyperintensity occurs in two thirds of STEMI patients. Persistent T2 hyperintensity was associated with the initial STEMI severity, adverse remodeling, and long-term health outcome.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT02072850.

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.

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.

T1 mapping in ischaemic heart disease

A unique feature of cardiac magnetic resonance is its ability to characterize myocardium. Proton relaxation times, T1, T2, and T2* are a reflection of the composition of individual tissues, and change in the presence of disease. Research into T1 mapping has largely been focused in the study of cardiomyopathies, but T1 mapping also shows huge potential in the study of ischaemic heart disease. In fact, the first cardiac T1 maps were used to characterize myocardial infarction. Robust high-resolution myocardial T1 mapping is now available for use as a clinical tool. This quantitative technique is simple to perform and analyse, minimally subjective, and highly reproducible. This review aims to summarize the present state of research on the topic, and to show the clinical potential of this method to aid the diagnosis and treatment of patients with ischaemic heart disease.

Effect of physiological heart rate variability on quantitative T2 measurement with ECG-gated Fast Spin Echo (FSE) sequence and its retrospective correction

OBJECT

Quantitative T2 measurement is applied in cardiac Magnetic Resonance Imaging (MRI) for the diagnosis and follow-up of myocardial pathologies. Standard Electrocardiogram (ECG)-gated fast spin echo pulse sequences can be used clinically for T2 assessment, with multiple breath-holds. However, heart rate is subject to physiological variability, which causes repetition time variations and affects the recovery of longitudinal magnetization between TR periods.

MATERIALS AND METHODS

The bias caused by heart rate variability on quantitative T2 measurements is evaluated for fast spin echo pulse sequence. Its retrospective correction based on an effective TR is proposed. Heart rate variations during breath-holds are provided by the ECG recordings from healthy volunteers. T2 measurements were performed on a phantom with known T2 values, by synchronizing the sequence with the recorded ECG. Cardiac T2 measurements were performed twice on six volunteers. The impact of T1 on T2 is also studied.

RESULTS

Maximum error in T2 is 26% for phantoms and 18% for myocardial measurement. It is reduced by the proposed compensation method to 20% for phantoms and 10% for in vivo measurements. Only approximate knowledge of T1 is needed for T2 correction.

CONCLUSION

Heart rate variability may cause a bias in T2 measurement with ECG-gated FSE. It needs to be taken into account to avoid a misleading diagnosis from the measurements.

Human non-contrast T1 values and correlation with histology in diffuse fibrosis

Background

Aortic stenosis (AS) leads to diffuse fibrosis in the myocardium, which is linked to adverse outcome. Myocardial T1 values change with tissue composition.

Objective

To test the hypothesis that our recently developed non-contrast cardiac magnetic resonance (CMR) T1 mapping sequence could identify myocardial fibrosis without contrast agent.

Design, setting and patients

A prospective CMR non-contrast T1 mapping study of 109 patients with moderate and severe AS and 33 age- and gender-matched controls.

Methods

CMR at 1.5 T, including non-contrast T1 mapping using a shortened modified Look–Locker inversion recovery sequence, was carried out. Biopsy samples for histological assessment of collagen volume fraction (CVF%) were obtained in 19 patients undergoing aortic valve replacement.

Results

There was a significant correlation between T1 values and CVF% (r=0.65, p=0.002). Mean T1 values were significantly longer in all groups with severe AS (972±33 ms in severe asymptomatic, 1014±38 ms in severe symptomatic) than in normal controls (944±16 ms) (p<0.05). The strongest associations with T1 values were for aortic valve area (r=−0.40, p=0.001) and left ventricular mass index (LVMI) (r=0.36, p=0.008), and these were the only independent predictors on multivariate analysis.

Conclusions

Non-contrast T1 values are increased in patients with severe AS and further increase in symptomatic compared with asymptomatic patients. T1 values lengthened with greater LVMI and correlated with the degree of biopsy-quantified fibrosis. This may provide a useful clinical assessment of diffuse myocardial fibrosis in the future.

Cardiovascular magnetic resonance of myocardial edema using a short inversion time inversion recovery (STIR) black-blood technique: diagnostic accuracy of visual and semi-quantitative assessment

BACKGROUND

The short inversion time inversion recovery (STIR) black-blood technique has been used to visualize myocardial edema, and thus to differentiate acute from chronic myocardial lesions. However, some cardiovascular magnetic resonance (CMR) groups have reported variable image quality, and hence the diagnostic value of STIR in routine clinical practice has been put into question. The aim of our study was to analyze image quality and diagnostic performance of STIR using a set of pulse sequence parameters dedicated to edema detection, and to discuss possible factors that influence image quality. We hypothesized that STIR imaging is an accurate and robust way of detecting myocardial edema in non-selected patients with acute myocardial infarction.

METHODS

Forty-six consecutive patients with acute myocardial infarction underwent CMR (day 4.5, +/- 1.6) including STIR for the assessment of myocardial edema and late gadolinium enhancement (LGE) for quantification of myocardial necrosis. Thirty of these patients underwent a follow-up CMR at approximately six months (195 +/- 39 days). Both STIR and LGE images were evaluated separately on a segmental basis for image quality as well as for presence and extent of myocardial hyper-intensity, with both visual and semi-quantitative (threshold-based) analysis. LGE was used as a reference standard for localization and extent of myocardial necrosis (acute) or scar (chronic).

RESULTS

Image quality of STIR images was rated as diagnostic in 99.5% of cases. At the acute stage, the sensitivity and specificity of STIR to detect infarcted segments on visual assessment was 95% and 78% respectively, and on semi-quantitative assessment was 99% and 83%, respectively. STIR differentiated acutely from chronically infarcted segments with a sensitivity of 95% by both methods and with a specificity of 99% by visual assessment and 97% by semi-quantitative assessment. The extent of hyper-intense areas on acute STIR images was 85% larger than those on LGE images, with a larger myocardial salvage index in reperfused than in non-reperfused infarcts (p = 0.035).

CONCLUSIONS

STIR with appropriate pulse sequence settings is accurate in detecting acute myocardial infarction (MI) and distinguishing acute from chronic MI with both visual and semi-quantitative analysis. Due to its unique technical characteristics, STIR should be regarded as an edema-weighted rather than a purely T2-weighted technique.

Myocardial edema imaging in acute coronary syndromes

Acute coronary syndromes (ACS) continue to be the most common morbid condition of industrialized nations. The advent of and technical improvements in revascularization and medical therapy have led to a steady decline in mortality rates. However, many patients who suffer unstable angina or myocardial infarction require further testing and risk stratification to guide therapeutic selection and prognosis assignment. Myocardial edema imaging with cardiac magnetic resonance (CMR) affords the ability to define the amount of myocardium at risk, refine estimates of prognosis and provide guidance for therapies with excellent sensitivity compared with standard clinical markers. This review will discuss the rationale for edema imaging, how it is performed using CMR, and potential clinical applications.

Low b-value diffusion-weighted imaging: emerging applications in the body

Thanks to recent advances in magnetic resonance imaging technology, it has become possible to perform intravoxel incoherent motion (IVIM) diffusion-weighted imaging (DWI) in any part of the body. Extracranial applications of DWI are currently under active investigation, especially for oncological imaging. However, the use of non-quantitative low b-value (10-100 s/mm(2)) DWI in the body is still a relatively unexplored field, and its potential is not fully recognized. Non-quantitative low b-value DWI may especially be useful for the evaluation of structures that have an inherently low signal at high b-value DWI, including (but not limited to) the liver, heart, and small bowel. This article will review and discuss the basic principles and potential applications of nonquantitative low b-value DWI in the body.

Evaluation of myocardial viability with cardiac magnetic resonance imaging

Assessment of myocardial viability is of clinical and scientific significance. Traditionally, the detection of myocardial viability (either stunning or hibernation) has been used in aiding diagnosis before revascularization, especially in high-risk patients. There is a considerable body of observational evidence showing substantial improvement after revascularization in patients with significant left ventricular dysfunction and myocardial viability. Recent randomized evidence has questioned the benefit of viability testing but must be interpreted with caution. Dobutamine stress echocardiography, nuclear imaging, and cardiovascular magnetic resonance are the mainstays of viability testing and provide information on contractile function, cellular metabolism, and myocardial fibrosis, respectively. Larger, multicenter trials with outcome data are needed to define the nature of viability testing and, particularly, cardiovascular magnetic resonance in moderate-to-severe ischemic cardiomyopathy.

T₂ -weighted MRI of post-infarct myocardial edema in mice

T(2) -weighted, cardiac magnetic resonance imaging (T(2) w CMR) can be used to noninvasively detect and quantify the edematous region that corresponds to the area at risk (AAR) following myocardial infarction (MI). Previously, CMR has been used to examine structure and function in mice, expediting the study of genetic manipulations. To date, CMR has not been applied to imaging of post-MI AAR in mice. We developed a whole-heart, T(2) w CMR sequence to quantify the AAR in mouse models of ischemia and infarction. The ΔB(0) and ΔB(1) environment around the mouse heart at 7 T were measured, and a T(2) -preparation sequence suitable for these conditions was developed. Both in vivo T(2) w and late gadolinium enhanced CMR were performed in mice after 20-min coronary occlusions, resulting in measurements of AAR size of 32.5 ± 3.1 (mean ± SEM)% left ventricular mass, and MI size of 50.1 ± 6.4% AAR size. Excellent interobserver agreement and agreement with histology were also found. This T(2) w imaging method for mice may allow for future investigations of genetic manipulations and novel therapies affecting the AAR and salvaged myocardium following reperfused MI.

T2-weighted cardiovascular magnetic resonance in acute cardiac disease

Cardiovascular magnetic resonance (CMR) using T2-weighted sequences can visualize myocardial edema. When compared to previous protocols, newer pulse sequences with substantially improved image quality have increased its clinical utility. The assessment of myocardial edema provides useful incremental diagnostic and prognostic information in a variety of clinical settings associated with acute myocardial injury. In patients with acute chest pain, T2-weighted CMR is able to identify acute or recent myocardial ischemic injury and has been employed to distinguish acute coronary syndrome (ACS) from non-ACS as well as acute from chronic myocardial infarction.T2-weighted CMR can also be used to determine the area at risk in reperfused and non-reperfused infarction. When combined with contrast-enhanced imaging, the salvaged area and thus the success of early coronary revascularization can be quantified. Strong evidence for the prognostic value of myocardial salvage has enabled its use as a primary endpoint in clinical trials. The present article reviews the current evidence and clinical applications for T2-weighted CMR in acute cardiac disease and gives an outlook on future developments."The principle of all things is water"Thales of Miletus (624 BC - 546 BC).

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.

Cardiovascular magnetic resonance T2 signal abnormalities in left ventricular ballooning syndrome

Left ventricular ballooning syndrome (LVBS), also known as Takotsubo cardiomyopathy, is characterized by regional left ventricular dysfunction associated with severe psychological stress. T2 weighted cardiac magnetic resonance (CMR) can identify myocardial edema due to ischemia or other insults. A standard clinical CMR scan with double inversion recovery fast spin echo T2 weighted sequences was performed on consecutive patients with LVBS. T2 signal was compared in myocardial segments with normal and impaired function based on systolic wall thickening (SWT). Eight LVBS patients were identified, all female, with a median age of 61 years and median left ventricular ejection fraction of 52%. Four patients had apical ballooning and four had mid-wall or basal ballooning. In severely dysfunctional segments (those with SWT < 25%), the median percentage of high T2 signal was 85 compared with 35 in those with SWT > 25% (P < 0.001). When the segments were categorized into tertiles based on SWT, the percentage of high T2 signal was greatest in segments with the worst function (68% vs. 43% vs. 31%, P = 0.005). In the five patients who returned for follow up, there was a significant reduction in high T2 signal compared with baseline in those segments that were initially severely dysfunctional (85% vs. 35%, P < 0.001). In conclusion, we describe elevated T2 signal consistent with myocardial edema in patients with LVBS. The T2 signal is highest in myocardium with the most impaired function and resolves over time.

Single-shot steady-state free precession can detect myocardial edema in patients: a feasibility study

PURPOSE

To demonstrate the ability of single-shot, T(2)/T(1) weighted steady-state free precession (SSFP) to detect myocardial edema in patients with an acute myocardial infarction.

MATERIALS AND METHODS

This study was performed in a series of patients (n = 10) referred for the assessment of acute myocardial infarcts (AMI). Localizers were used to obtain true short axis views of the left ventricle (LV). These views were used to plan and obtain T(2)-weighted STIR (short TI inversion recovery) images of the LV. These slices were then acquired using single-shot dark blood-prepared SSFP with a large (31) number of dummy pulses. Lastly, Contrast agent was injected, and late enhancement (LE) images were acquired. Images were analyzed using a multi-segment model of the heart. SSFP images were compared with STIR images, with STIR images used as the standard of truth for the presence of edema. LE images were used to identify segments which were positive for microvascular obstruction.

RESULTS

All techniques were successful in all patients. A total of 312 segments were analyzed. Excluding segments positive for microvascular obstruction, SSFP had a sensitivity/specificity of 80%/89%. Including segments positive for microvascular obstruction, sensitivity/specificity was 71%/88%. On a patient-based analysis, no AMI was missed using SSFP (sensitivity = 100%).

CONCLUSION

Using single-shot SSFP to detect myocardial edema in patients with AMI is feasible with a moderate sensitivity and high specificity.

Impact of heterogeneity of human peripheral blood monocyte subsets on myocardial salvage in patients with primary acute myocardial infarction

OBJECTIVES

We examined whether distinct monocyte subsets contribute in specific ways to myocardial salvage in patients with acute myocardial infarction (AMI).

BACKGROUND

Recent studies have shown that monocytes in human peripheral blood are heterogeneous.

METHODS

We studied 36 patients with primary AMI. Peripheral blood sampling was performed 1, 2, 3, 4, 5, 8, and 12 days after AMI onset. Two monocyte subsets (CD14(+)CD16(-) and CD14(+)CD16(+)) were measured by flow cytometry. The extent of myocardial salvage 7 days after AMI was evaluated by cardiovascular magnetic resonance imaging as the difference between myocardium at risk (T2-weighted hyperintense lesion) and myocardial necrosis (delayed gadolinium enhancement). Cardiovascular magnetic resonance imaging was also performed 6 months after AMI.

RESULTS

Circulating CD14(+)CD16(-) and CD14(+)CD16(+) monocytes increased in AMI patients, peaking on days 3 and 5 after onset, respectively. Importantly, the peak levels of CD14(+)CD16(-) monocytes, but not those of CD14(+)CD16(+) monocytes, were significantly negatively associated with the extent of myocardial salvage. We also found that the peak levels of CD14(+)CD16(-) monocytes, but not those of CD14(+)CD16(+) monocytes, were negatively correlated with recovery of left ventricular ejection fraction 6 months after infarction.

CONCLUSIONS

The peak levels of CD14(+)CD16(-) monocytes affect both the extent of myocardial salvage and the recovery of left ventricular function after AMI, indicating that the manipulation of monocyte heterogeneity could be a novel therapeutic target for salvaging ischemic damage.

Edema as a very early marker for acute myocardial ischemia: a cardiovascular magnetic resonance study

OBJECTIVES

This study was designed to determine whether imaging myocardial edema would identify acute myocardial ischemia before irreversible injury takes place.

BACKGROUND

Early identification of acute myocardial ischemia is a diagnostic challenge.

METHODS

We studied 15 dogs with serial T(2)-weighted and cine imaging at baseline, during transient coronary occlusion of up to 35 min, and after reperfusion in a 1.5-T magnetic resonance imaging system. Late gadolinium enhancement and troponin measurements were used to assess for the presence of irreversible injury. Myocardial water content was measured to assess myocardial edema.

RESULTS

We consistently observed a transmural area of high T(2) signal intensity matching areas with new onset regional akinesia 28 +/- 4 min after experimental coronary artery occlusion. At this time, the contrast-to-noise ratio between the ischemic and remote myocardium had significantly increased from 1.0 +/- 2.0 to 12.8 +/- 9.6 (p < 0.003), which further increased after reperfusion to 15.8 +/- 10.3 (p < 0.004 compared with baseline). Neither myocardial late gadolinium enhancement nor troponin elevation were noted at this time window. Myocardial water content of the ischemic segments was consistently higher (68.9 +/- 2% vs. 67.0 +/- 2%; p < 0.004) than in remote segments and the difference correlated significantly to the contrast-to-noise ratio in T(2) images (p < 0.04).

CONCLUSIONS

We provide the first evidence that T(2)-weighted cardiovascular magnetic resonance imaging of edema detects acute ischemic myocyte injury before the onset of irreversible injury. T(2)-weighted cardiovascular magnetic resonance imaging may serve as a very useful diagnostic marker in clinical settings such as unstable angina or evolving infarction.

The salvaged area at risk in reperfused acute myocardial infarction as visualized by cardiovascular magnetic resonance

OBJECTIVES

We aimed to characterize the tissue changes within the perfusion bed of infarct-related vessels in patients with acutely reperfused myocardial infarction (MI) using cardiovascular magnetic resonance (CMR).

BACKGROUND

Even in successful early revascularization, intermittent coronary artery occlusion affects the entire perfusion bed, also referred to as the area at risk. The extent of the salvaged area at risk contains prognostic information and may serve as a therapeutic target. Cardiovascular magnetic resonance can visualize the area at risk; yet, clinical data have been lacking.

METHODS

We studied 92 patients with acute MI and successful reperfusion 3 +/- 3 days after the event and 18 healthy control subjects. Breath-hold T2-weighted and contrast-enhanced ("late enhancement") CMR were used to visualize the reversible and the irreversible myocardial injury, respectively.

RESULTS

All reperfused infarcts consistently revealed a pattern with both reversibly and irreversibly injured tissue. In contrast to the infarcted area, reversible damage was always transmural, exceeding the infarct in its maximal extent by 16 +/- 11% (absolute difference of the area of maximal infarct expansion 38 +/- 15% vs. 22 +/- 10%; p < 0.0001). None of the controls had significant T2 signal intensity abnormalities.

CONCLUSIONS

In patients with reperfused MI, CMR visualizes both reversible and irreversible injury. This allows for quantifying the extent of the salvaged area after revascularization as an important parameter for clinical decision-making and research.

MRI evaluation of microvascular obstruction in experimental reperfused acute myocardial infarction using a T1 and T2 preparation pulse sequence

PURPOSE

To investigate a T1 and T2 preparation pulse sequence to evaluate microvascular obstruction (MO) in a porcine model of reperfused acute myocardial infarction (AMI).

MATERIALS AND METHODS

A total of 14 pigs with reperfused AMI underwent MRI examinations at baseline and three to four hours after reperfusion. MRI scans included a left ventricular functional study, T1 and T2 measurement on a 1.5T MRI system. At reperfusion, first-pass myocardial perfusion (FPMP) images were obtained after bolus injection of gadopentetate dimeglumine followed by an intravenous drip. Delayed contrast-enhanced MRI (DE-MRI) and T1 measurements were performed 30 and 45 minutes, respectively, after the bolus, during a constant infusion of gadopentetate dimeglumine.

RESULTS

In 11 pigs MO was hypoenhanced in FPMP and DE-MRI. In later T1 preparation difference images postcontrast, MO was hyperenhanced while delayed hyperenhanced (DHE) regions appeared dark. MO areas on DE-MRI and T1 images were comparable. T1 reduction (%) postcontrast in MO was small compared to measurements from DHE regions (P < 0.0001) and similar to those from control segments (P = 0.66). Precontrast T1 and T2 values at reperfusion from MO and DHE regions were larger than in control regions.

CONCLUSION

Using T1 preparation under a constant gadopentetate dimeglumine (Gd-DTPA) infusion, delayed imaging at 30 to 45 minutes demonstrates MO as a positive contrast with larger T1 values. Elevated T1 and T2 values in MO precontrast may also help to differentiate them from both control and DHE regions.

Temporal DeltaB0 and relaxation in the rat heart

Field maps of the induced main magnetic field offset (DeltaB(0)) were measured in the rat heart at various points in the cardiac cycle for the purpose of identifying their effects on relaxation measurements. The mean DeltaB(0) of the left ventricle averaged across rats was found to be 0.11 +/- 0.35 ppm and 0.19 +/- 0.39 ppm at the onset of systole and diastole, respectively. The root mean square (RMS) variation in resonant frequency of the left ventricle averaged across rats was found to be 0.09 +/- 0.05 ppm and 0.06 +/- 0.04 ppm during systole and diastole, respectively. Temporal variations in DeltaB(0) could substantially affect quantitative MRI measurements. To assess this, transverse relaxation rates (R(2) and R(2)(*)) were measured at different points in the cardiac cycle, and the effects of DeltaB(0) were estimated using measured field map data. For a given region of the left ventricle, DeltaB(0) induced a mean error across rats of < or =3.9% for R(2) and < or =9.6% for R(2)(*). For R(2)(*) measurements, the static component of the field inhomogeneity was found to be responsible for most of the error induced.

Myocardial T1 mapping: application to patients with acute and chronic myocardial infarction

T(1) maps obtained with modified Look-Locker inversion recovery (MOLLI) can be used to measure myocardial T(1). We aimed to evaluate the potential of MOLLI T(1) mapping for the assessment of acute and chronic myocardial infarction (MI). A total of 24 patients with a first MI underwent MRI within 8 days and after 6 months. T(1) mapping was performed at baseline and at selected intervals between 2-20 min following administration of gadopentetate dimeglumine (Gd-DTPA). Delayed-enhancement (DE) imaging served as the reference standard for delineation of the infarct zone. On T(1) maps the myocardial T(1) relaxation time was assessed in hyperenhanced areas, hypoenhanced infarct cores, and remote myocardium. The planimetric size of myocardial areas with standardized T(1) threshold values was measured. Acute and chronic MI exhibited different T(1) changes. Precontrast threshold T(1) maps detected segmental abnormalities caused by acute MI with 96% sensitivity and 91% specificity. Agreement between measurements of infarct size from T(1) mapping and DE imaging was higher in chronic than in acute infarcts. Precontrast T(1) maps enable the detection of acute MI. Acute and chronic MI show different patterns of T(1) changes. Standardized T(1) thresholds provide the potential to dichotomously identify areas of infarction.

T2-prepared SSFP improves diagnostic confidence in edema imaging in acute myocardial infarction compared to turbo spin echo

T2-weighted MRI of edema in acute myocardial infarction (MI) provides a means of differentiating acute and chronic MI, and assessing the area at risk of infarction. Conventional T2-weighted imaging of edema uses a turbo spin-echo (TSE) readout with dark-blood preparation. Clinical applications of dark-blood TSE methods can be limited by artifacts such as posterior wall signal loss due to through-plane motion, and bright subendocardial artifacts due to stagnant blood. Single-shot imaging with a T2-prepared SSFP readout provides an alternative to dark-blood TSE and may be conducted during free breathing. We hypothesized that T2-prepared SSFP would be a more reliable method than dark-blood TSE for imaging of edema in patients with MI. In patients with MI (22 acute and nine chronic MI cases), T2-weighted imaging with both methods was performed prior to contrast administration and delayed-enhancement imaging. The T2-weighted images using TSE were nondiagnostic in three of 31 cases, while six additional cases rated as being of diagnostic quality yielded incorrect diagnoses. In all 31 cases the T2-prepared SSFP images were rated as diagnostic quality, correctly differentiated acute or chronic MI, and correctly determined the coronary territory. Free-breathing T2 prepared SSFP provides T2-weighted images of acute MI with fewer artifacts and better diagnostic accuracy than conventional dark-blood TSE.

Magnetic resonance imaging and its role in myocardial regenerative therapy

There has been extensive interest recently in cardiac stem cell therapy. Current research has been hampered by differences in cell type, methods of delivery and efficacy evaluation. In this article we review the use of magnetic resonance imaging in this growing area and argue that it is well suited to all areas of myocardial regeneration: from patient identification, through cell delivery and tracking of appropriately labeled cells, to evaluation of therapeutic effect. Potential future advances are discussed including magnetic resonance imaging-guided intervention suites and the use of higher field strength magnets for cell tracking.

Multi-contrast delayed enhancement provides improved contrast between myocardial infarction and blood pool

PURPOSE

To develop and test a delayed-enhancement imaging method for improving the contrast between myocardial infarction (MI) and blood pool.

MATERIALS AND METHODS

The T(2) of blood is significantly longer than that of acute or chronic MI. The proposed multi-contrast delayed-enhancement (MCODE) imaging method produces a series of images with both T(1) and T(2) weightings, which provides both excellent contrast between normal and infarcted myocardium, and between blood and MI.

RESULTS

The subendocardial border between MI and blood pool was easily discriminated in the T(2)-weighted image. The measured MI-to-blood contrast-to-noise ratio (CNR) was better in the T(2)-weighted image than in the T(1)-weighted image (22.5+/-8.7 vs. 2.9+/-3.1, mean+/-SD, N=11, P<0.001, for True FISP, and 19.4+/-10.8 vs. 3.9+/-2.3, N=11, P<0.001, for Turbo FLASH).

CONCLUSION

The MCODE method provides a significant improvement in the ability to easily discriminate subendocardial MI by providing a T(2)-weighted image with high contrast between blood and MI. MCODE should improve both the detection and accurate sizing of MI.

MRI relaxation fluctuations in acute reperfused hemorrhagic infarction

MRI evaluations of intramyocardial hemorrhage in acute infarction have relied on T(2) and T(2)(*) shortening only. We propose a more comprehensive evaluation of hemorrhagic infarction based on the concept that fluctuations in T(2) and T(1) relaxation in acute reperfused infarction will reflect transient edema and hemoglobin oxidative denaturation to uncompartmentalized methemoglobin. Anteroapical infarction was created via percutaneous balloon in young swine (22-25 kg, N = 12). T(2), T(1), diastolic wall thickness (DWT), and the Gd-DTPA partition coefficient (lambda) were measured on days 0, 2, and 7. DWT was elevated at 1 hr postreperfusion (128% +/- 53%, P = 0.0001), and alleviated on days 2 and 7 (48% +/- 10%, P = 0.008; 53% +/- 24%, P = 0.003). T(2) and T(1) elevations were coincident with early edema (DeltaT(2) = 55% +/- 24%, P < 0.0001; DeltaT(1) = 27% +/- 18%, P < 0.04). T(2) and T(1) were nearly normal on day 2 (DeltaT(2) = 8% +/- 8%, P = 0.27; DeltaT(1) = 0% +/- 1%, P = 0.65). On day 7, T(2) increased while T(1) decreased (DeltaT(2) = 27% +/- 16%, P = 0.005; DeltaT(1) = -14% +/- 10%, P = 0.02). Lambda was elevated by >150% at all time points (P < or = 0.002). Histology verified hemorrhagic injury. T(1) and T(2) fluctuations are consistent with transient edema, as well as hemoglobin oxidative denaturation to decompartmentalized methemoglobin. This methodological development may broaden our understanding of hemorrhagic microvascular injury and improve its detection in clinical populations.

ECG-gated multi-detector row spiral CT in the assessment of myocardial infarction: correlation with non-invasive angiographic findings

Our objective was to retrospectively evaluate the ability of multidetector-row computed tomography (MDCT) to detect previous myocardial infarctions (MIs) and to correlate necrosis with the status of coronary arteries supplying the infarcted territory. After having clinically evaluated 187 patients referred for ECG-gated MDCT of the coronary arteries, 30 previous MIs were identified in 29 patients (9 recent and 21 chronic). MDCT data were evaluated qualitatively and quantitatively by measuring attenuation values and wall thickness within the infarcted region and normal adjacent myocardium. Each MI was also assigned to the distribution territory of a coronary vessel, and morphological data were combined with MDCT angiographic findings. MDCT was able to detect 25/30 MIs showing an overall sensitivity and specificity of 83 and 91%, respectively. Quantitative analysis revealed a statistically significant difference in attenuation values between normal and infarcted regions (38.9+/-14 HU vs. 104.0+/-16 HU). Regional wall thinning was observed in chronic MIs (4.1+/-2 mm vs. 10.5+/-3.8 mm), and not in patients with recent event (7.9+/-1.6 mm vs 9.1+/-4 mm). In 22/25 cases, MDCT angiographic findings showed the presence of suspicious critical lumen narrowing (n=3), previous coronary stenting (n=14) and surgical revascularization (n=5) in the infarct-related coronary. During a single examination, MDCT might provide comprehensive imaging of MI offering a combined morphological and angiographic assessment.

Delayed enhancement and T2-weighted cardiovascular magnetic resonance imaging differentiate acute from chronic myocardial infarction

BACKGROUND

Delayed enhancement (DE) cardiovascular magnetic resonance (CMR) detects acute and chronic myocardial infarction (MI) by visualizing contrast media accumulation in infarcted segments. T2-weighted CMR depicts infarct-related myocardial edema as a marker of acute but not chronic myocardial injury. We investigated the clinical utility of an approach combining both techniques to differentiate acute from chronic MI.

METHODS AND RESULTS

Seventy-three MI patients were studied in 2 groups. Group A consisted of 15 acute MI patients who were studied twice, on day 1 and 3 months after MI. In group B, 58 patients with acute or chronic MI underwent 1 CMR scan. T2-weighted and DE images of matched slices were acquired on a 1.5-T system. In group A, quantitative segmental and region of interest-based analyses were performed to observe signal changes between the acute and chronic phases. In group B, T2-weighted and DE images were examined visually by 2 blinded observers for the presence or absence of hyperintense areas in corresponding segments. For infarct localization, coronary angiography and/or ECG changes served as the reference standard. In group A, the contrast-to-noise ratio on T2-weighted images dropped in the infarcted segments from 2.7+/-1.1 on day 1 to 0.1+/-1.2 after 3 months (P<0.0001). There was no significant change in contrast-to-noise ratio in DE images (1.9+/-1.5 versus 1.3+/-1.0; P=NS). The qualitative assessment of T2-weighted and DE images in group B yielded a specificity of 96% to differentiate acute from chronic lesions.

CONCLUSIONS

An imaging approach combining DE and T2-weighted CMR accurately differentiates acute from chronic MI.

Contrast-enhanced cardiovascular magnetic resonance imaging

Since its introduction in the early 1990s, contrast-enhanced (CE) cardiac magnetic resonance imaging (MRI) has evolved rapidly for the assessment of cardiac pathologies, including in particular ischemic heart disease and inflammatory conditions. Likewise, CE-magnetic resonance angiography (MRA) is now used routinely to evaluate the thoracic vasculature. This article reviews the current use of extracellular gadolinium-based agents in CE cardiovascular imaging, focusing on ischemic heart disease, inflammatory myocardial conditions, and the use of CE-MRA in imaging of the pulmonary and aortic vasculature. Recent advances in fast and ultrafast MRI combined with the use of extracellular contrast media allow noninvasive measurements of multiple parameters of the cardiovascular system in less than 40 minutes. Beyond the assessment of left ventricular wall motion and morphology, CE cardiac MRI allows depiction of myocardial perfusion and thereby provides information regarding microvascular integrity and myocardial viability. The excellent spatial resolution of MRI, especially for the distinction of nontransmural versus transmural extent of pathology, has been shown to be superior to other modalities that are often nonlocalizing, nonspecific, or more invasive. Additional advantages of CE-MRA, particularly for the thoracic vasculature, include safety, its noninvasive character, large field of view, and the ability to demonstrate complicated three-dimensional relationships without the need for iodinated, nephrotoxic contrast media.

Optimized spiral imaging for measurement of myocardial T2 relaxation

Microcirculation oxygen levels and blood volumes should be reflected in measurements of myocardial T(2) relaxation. This work describes the optimization of a spiral imaging strategy for robust myocardial T(2) measurement to minimize the standard deviation of T(2) measurement (sigmaT(2)). Theoretical and experimental studies of blurring at muscle/blood interfaces enabled the derivation of parameter sets which reduce sigma T(2) to the level of 5%. T(2) relaxation mapping within healthy volunteers provided estimation of residual sigmaT(2) within the optimized technique. The standard deviation in T(2) measurement across regions of interest (ROIs) in different locations is about 9%. The standard deviation in T(2) measurement in an ROI across different time points is about 5%.

The use of Gd-DTPA as a marker of myocardial viability in reperfused acute myocardial infarction

At present, accurate assessment of the extent of myocardial viability after acute myocardial infarction is limited due to the spatial resolution of currently available imaging modalities. MR cardiac imaging, with its superior spatial resolution, would be used if viable and infarcted tissue could be separated based on signal intensity. In infarcted tissue, cell membrane breakdown allows the entry of the MR contrast agent Gd-DTPA which is normally extracellular. The increased space for Gd-DTPA distribution (partition coefficient, lambda) in this infarcted tissue results in increased Gd-DTPA concentration and hence increased signal intensity on T1-weighted MR images. In a canine model of ischemia/reperfusion injury, the partition coefficient in infarcted tissue increased as early as 1 min post reperfusion. lambda in infarcted tissue stayed increased over that in normal tissue for at least 8 weeks. The accuracy of contrast-enhanced MRI was confirmed by results of 201Tl SPECT and a cine MRI dobutamine wall motion study in a patient 1 week after an acute myocardial infarction. Thus, contrast-enhanced MRI shows great promise for the non-invasive determination of myocardial viability after acute myocardial infarction.

Relationship between function and perfusion early after acute myocardial infarction

To assess the relationship between baseline left ventricle function, functional reserve and resting myocardial perfusion in patients with acute myocardial infarction (AMI). After AMI the presence of dysfunctioning but viable myocardium plays a determinant role in clinical outcome. Regional ventricular function was evaluated by echocardiography both in resting conditions and during dobutamine infusion (10 microg/kg/min). Perfusion was assessed by magnetic resonance imaging in a single slice approach where the first pass of an intravenously injected bolus of gadolinium-based contrast agent was followed through six regions of interest within the myocardium. In each patient a region with normal function was used as reference and the cross-correlation coefficient (CCC), which described the myocardial perfusion relatively to the reference region (CCC = 1 means equivalent perfusion), was obtained for the other five myocardial regions. Twenty-two patients were enrolled into the study. Sixty-one segments had normal function and normal perfusion (CCC = 0.92+/-0.23). The perfusion deficit was more marked in the 29 regions with resting akinesia-dyskinesia than in the 20 hypokinetic regions (CCC = 0.71+/-0.45 vs. 0.84+/-0.23; p < 0.05). Out of the 29 regions with resting akinesia-dyskinesia the 13 segments which showed functional improvement following dobutamine had a higher resting perfusion than the 16 segments which were unresponsive to dobutamine (CCC = 0.83+/-0.32 vs. 0.61+/-0.52, p < 0.05). Similarly, out of the 20 regions with resting hypokinesia the 11 segments having functional reserve showed an higher resting perfusion than the segments which did not (0.96+/-0.21 vs. 0.69+/-0.19; p < 0.05). Early after AMI, the perfusion deficit reflects the severity of the mechanical dysfunction. In regions with baseline dyssynergy resting perfusion is, in general, higher when contractile reserve can be elicited by stress-echo.

Subacute myocardial infarction: assessment by STIR T2-weighted MR imaging in comparison to regional function

PURPOSE

Increased T2 signal intensity (SI) can be regularly observed in myocardial infarction. However, there are controversial reports about the relationship of elevated T2 SI to myocardial viability and some authors propose that high T2 SI serves as a sign of irreversible myocardial injury. This study investigates increased T2 SI compared to myocardial function in patients with reperfused subacute myocardial infarction. Preserved function was used as criterion for viability.

METHODS

Ten healthy volunteers and 17 patients with myocardial infarction and patent infarct related coronary artery were examined on a 1.5 T Magnetom Vision system (Siemens). For T2-weighted MR imaging a breath-hold STIR sequence with dark-blood preparation was used. Cine FLASH 2D imaging was applied to assess myocardial function. Signal-to-noise (S/N) in STIR T2 images was measured in normal and infarcted regions and subsequently identified by two independent observers. Based on a 20 segment model of the left ventricle findings were compared to regional myocardial function.

RESULTS

Elevated STIR T2 SI was found in all 17 patients and observed in 27% (204/754) of segments. S/N of normal myocardium was 5.1 +/- 0.7 in volunteers and 4.9 +/- 0.8 in patients (P = NS). Infarcted myocardium presented with significantly increased S/N 12.8 +/- 1.9 (P < 0.0001). Significant transmural elevation of T2 SI was noted in 32% of segments with preserved systolic function.

CONCLUSION

Increased STIR T2 SI can be observed transmurally in post-ischemic myocardial regions with preserved function. It therefore cannot be used as an exclusive marker for the non-viable region.

Sustained postinfarction myocardial oedema in humans visualised by magnetic resonance imaging

OBJECTIVE

To demonstrate postinfarction myocardial oedema in humans with particular reference to the longitudinal course, using magnetic resonance imaging (MRI).

DESIGN

Prospective observational study. Subjects were studied one week, one month, three months, six months, and one year after presenting with a myocardial infarct.

SETTING

Cardiology and magnetic resonance departments in a Danish university hospital.

PATIENTS

10 patients (three women, seven men), mean (SEM) age 58.2 (3.20) years, with a first transmural myocardial infarct.

MAIN OUTCOME MEASURES

Location and duration of postinfarction myocardial oedema.

RESULTS

All patients had signs of postinfarction myocardial oedema. The magnetic resonance images were evaluated by two blinded procedures, employing two MRI and two ECG observers: (1) MRI determined oedema location was compared with the ECG determined site of infarction and almost complete agreement was found; (2) the time course of postinfarction myocardial oedema was explored semiquantitatively, using an image ranking procedure. Myocardial oedema was greatest at the initial examination one week after the infarction, with a gradual decline during the following months (Spearman's rank correlation analysis: rho(observer 1) = 0.94 (p < 0.0001) and rho(observer 2) = 0.97 (p < 0.0001)). The median duration of oedema was six months.

CONCLUSIONS

Postinfarction myocardial oedema seems surprisingly long lasting. This observation is of potential clinical interest because the oedema may have prognostic significance.

Magnetic resonance imaging of myocardial infarct

Magnetic resonance imaging offers the unique opportunity to directly visualize the size and location of myocardial infarcts (MIs) with excellent spatial resolution. Because infarct size is the most important determinant of postinfarct outcome, precise determination of infarct size may be valuable to risk stratify patients after acute MI. In addition, infarct imaging may provide direct information on the amount of irreversibly injured myocardium and thus can be used to identify myocardial viability in dysfunctional regions. Acute infarcts can be recognized as hyperintense signal on T2-weighted spin-echo images. This technique, however, does not identify chronic infarcts and may overestimate infarct size by including area at risk. Also, T2-weighted images often have a low signal-to-noise ratio. Contrast-enhanced perfusion imaging provides better-quality images. Extravascular contrast agents such as (Gd-DTPA) gadolinium diethyletriamine-pentaacetic acid identify infarcts as hyperenhanced regions on images acquired late after contrast injection. In addition, these tracers can examine the integrity and permeability of infarct microvasculature on first-pass perfusion images. Necrosis avid tracers and 23Na imaging are other new exciting approaches to identify infarcted myocardium acutely after MI. These techniques, are still investigational, and their value for clinical imaging remains to be established.

MRI for the evaluation of regional myocardial perfusion in an experimental animal model

Myocardial perfusion was assessed in nine pigs using ultrafast gradient-echo MRI (.5 T, 15-mT/m gradients) at different levels of myocardial blood flow (range, .005-1.84 ml/min/g), generated either by adenosine infusion or by a mechanical occluder, and measured independently using radiolabeled microspheres. Sixty-four consecutive, ECG-triggered, diastolic, short axis images of the left ventricle were obtained during intravenous bolus injections (n = 30) of .05 mmol/kg of gadopentetate dimeglumine. Relative changes in peak intensity, time to peak intensity, washin slope, correlation coefficient, and cross-correlation coefficient were computed from the time-intensity curves obtained from four regions of interest, namely septal, anterior, lateral, and inferior walls. The values from the inferior wall acted as reference for evaluating relative changes in the other three regions. The cross-correlation coefficient (P < .001, rho = .60) and the peak intensity (P < .001, r = .72) showed the best correlation with myocardial blood flow. The washin slope showed a weak positive trend (P < .05), but the low value of r (r = .28) indicated that the use of this parameter to predict flow was invalid; the correlation coefficient and time to peak intensity were not correlated (P = ns). In conclusion, this study shows that it is possible to evaluate relative myocardial perfusion after the first pass of a an intravenously injected bolus of gadopentetate dimeglumine, using dynamic MRI on a conventional medium field MRI system. The cross-correlation coefficient and the peak intensity resulted in more efficient parameters to evaluate relative inhomogeneity of regional myocardial perfusion.

Magnetic resonance imaging in coronary artery disease

The cardiovascular applications of nuclear magnetic resonance (MR) techniques in coronary artery disease have increased considerably in recent years. Technical advantages of MR imaging in comparison with other techniques are the excellent spatial resolution, the characterization of myocardial tissue, and the potential for three-dimensional imaging. This allows the accurate assessment of left ventricular mass and volume, the differentiation of infarcted tissue from normal myocardial tissue, and the determination of systolic wall thickening and regional wall motion abnormalities. Myocardial perfusion, metabolism, and inducible myocardial ischemia with the use of pharmacological stress also can be assessed by MR techniques. Future technical improvements in real-time imaging and development of noninvasive visualization of the coronary arteries and coronary artery bypasses will constitute a tremendous progress in clinical cardiology. Early detection and flow assessment of stenosed coronary arteries by MR angiography with the use of flow velocity measurements may outweigh the cost inherent to the MR imaging procedure. A particular strength of the MR technique is the potential to encompass cardiac anatomy, perfusion, function, metabolism, and coronary angiography in a single test. The replacement of multiple diagnostic tests with one MR test may have major effects on cardiovascular healthcare economics.

Reperfused myocardial infarctions on T1- and susceptibility-enhanced MRI: evidence for loss of compartmentalization of contrast media

The purpose of this study was to characterize the contrast caused by a susceptibility MRI contrast agents, on spin echo T2-weighted imaging of reperfused myocardial infarction. Our interest in this model focused on the expected requirement that such agents be compartmentalized in the tissue to cause signal loss on spin echo images, a condition which may not be present in reperfused infarcted myocardium. Accordingly, nine rats were subjected to 2 h of left coronary artery occlusion followed by 3 +/- 0.5 h of reperfusion prior to administration of contrast media. Three sets of MR images were acquired: (a) baseline axial images at the midventricle, both T1-weighted (TR/TE = 300/20) and T2-weighted (TR/TE = 1500/60); (b) T1-weighted images after administering a T1-enhancing agent, Gd-DTPA-BMA (0.2 mmol/kg), to document that contrast media is delivered to the reperfused infarction; and (c) T2-weighted images after administering the susceptibility agent, Dy-DTPA-BMA (1.0 mmol/kg). Gadolinium-enhanced T1 images depicted reperfused infarction as regions with greatly enhanced signal intensity compared with uninfarcted myocardium, indicating that contrast agent was delivered to the infarcted zone. Dysprosium-enhanced T2 images depicted the injury as a region of persistent signal intensity relative to depletion of signal in normal myocardium, consistent with failure of the contrast agent to cause signal loss. Similar infarction sizes were observed for unenhanced T2-weighted images (33 +/- 5%), gadolinium-enhanced T1-weighted images (36 +/- 5%) and postmortem staining (30 +/- 6%); strong correlations (r > 0.9) were noted in comparisons of these data.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphonate-modified Gd-DTPA complexes. III: The detection of myocardial infarction by MRI

The potential of a phosphonate-modified-Gd-DTPA for MR image enhancement of myocardial infarction has been demonstrated in imaging experiments on rats. The agent, 1-hydroxy-3-aminopropane-1,1-diphosphonate-modified-Gd-DTPA (Gd-DTPA-HPDP) accumulates in two models of myocardial infarction, (i.e., drug-induced diffusely infarcted whole hearts and in focal acute myocardial infarction from a left coronary artery ligation). The time course of the accumulation of the agent in the focal model of infarction and subsequent washout has also been followed in vitro. Results of this kinetics demonstrate that the agent first perfuses all normal fluid spaces and then slowly diffuses into the occluded zone where it is retained for a prolonged period, in sufficient quantities to be useful as an MRI contrast agent. Wash-out of the agent from normal myocardium is fast and complete with MR signal returning to background in minutes. The specificity of Gd-DTPA-HPDP for soft-tissue calcification and its retention within the infarcts permitted imaging at 1 to 2 h postinjection, (after unbound material has cleared the normal tissues). Infarcted tissue appeared as regions of increased signal intensity in T1-weighted images (> 200% enhancement), and correlated with histopathology. Unmodified Gd-DTPA was not retained under identical conditions. Gd-DTPA-HPDP permits a more accurate infarct delineation than is possible with the unmodified agent.

Sequential analysis of infarcted and normal myocardium in piglets using in vivo gadolinium-enhanced MR images

Gadolinium-enhanced magnetic resonance (MR) imaging was performed before, and 1 and 3 wk after coronary occlusion in domestic piglets. After administration of the contrast agent gadopentetate dimeglumine, two different enhancement patterns within the infarcted region were observed. The first pattern, showing peripheral enhancement of the infarcted region with absence of contrast in the center, was seen at 1 wk after occlusion at 5 min after administration of the contrast agent. The second pattern showed signal enhancement of the center of the infarcted region and was observed at 1 wk after occlusion, 30 min following contrast administration, and at 3 wk after occlusion, both 5 and 30 min following contrast administration. Infarct size and left ventricular (LV) mass by MR imaging, measured 3 wk after infarction, corresponded well with pathologic assessment. LV mass, measured by static and dynamic MR imaging, increased during the period of investigation. It is concluded that gadolinium-enhanced MR imaging clearly identifies infarcted myocardium early and late after coronary occlusion in the piglet. Combined results of infarct size and LV mass can be obtained simultaneously during one imaging procedure.

Assessment of postreperfusion myocardial hemorrhage using proton NMR imaging at 1.5 T

BACKGROUND

Intramyocardial hemorrhage occurs frequently after reperfusion of acute myocardial infarction. However, its significance has not yet been established, mainly because of the lack of methods for detecting such hemorrhage. The following ex vivo study was carried out to assess the potential of nuclear magnetic resonance (NMR) imaging to detect and quantitate postreperfusion intramyocardial hemorrhage.

METHODS AND RESULTS

Sixteen adult mongrel dogs underwent 3 hours of coronary occlusion followed by 1 hour of reperfusion, and three dogs underwent 4 hours of occlusion without reperfusion. Radiolabeled microspheres and 51Cr-labeled red blood cells were used to assess flow and evaluate the extent of hemorrhage. These results were later compared with both NMR and histology. Spin-echo NMR imaging was performed on the excised hearts using a 1.5-T system. Macroscopic assessment of the sliced myocardium revealed the existence of hemorrhage in 14 of the 16 dogs that underwent reperfusion but in none of those with occlusion only. In all 16 dogs with reperfusion, zones of increased signal intensity (SI) ratio (1.68 +/- 0.41 compared with control, p less than 0.05) were seen in regions relating to the distribution of the occluded coronary artery, whereas in 13 of the 16 dogs, areas of decreased SI within the zone of increased SI ratio (0.81 +/- 0.16 compared with control, p less than 0.05) were also seen, corresponding to regions with macroscopic hemorrhage. In contrast, in the three dogs without reperfusion, no macroscopic hemorrhage was observed, and likewise, no NMR zones of reduced SI were detected. Hemorrhage size by NMR (decreased SI zones), correlated well with hemorrhage size calculated from tissue slices (r = 0.96, SEE = 0.92%, p less than 0.01), or by 51Cr labeling (r = 0.78, SEE = 1.5, p = 0.1). In the reperfusion group, T2 relaxation times in the infarcted hemorrhagic zone (58 +/- 9 msec) were significantly lower than the infarcted zones without hemorrhage (98 +/- 13 msec, p less than 0.001). In contrast, when compared with control (964 +/- 72 msec), T1 relaxation times were significantly increased in both infarct zones, either with (1,284 +/- 176 msec) or without (1,266 +/- 103 msec) hemorrhage. The selective shortening of T2 relaxation times in the hemorrhagic regions is consistent with the paramagnetic effects of deoxyhemoglobin.

CONCLUSIONS

NMR imaging may provide a noninvasive approach for the detection and quantitation of intramyocardial hemorrhage. This observation may provide a means to further characterize pathological processes associated with acute myocardial infarction and assess the role of myocardial hemorrhage after reperfusion therapy.

Characterization of acute myocardial infarction by magnetic resonance imaging

The T2-weighted spin-echo technique is currently the most frequently used magnetic resonance imaging (MRI) method to visualize acute myocardial infarction. However, image quality is often degraded by ghost artifacts from blood flow, and respiratory and cardiac contractile motion. To enhance the usefulness of this technique for detailed characterization of infarction, a velocity-compensated spin-echo pulse sequence was tested by imaging a flow phantom, 6 normal subjects and 17 patients with acute myocardial infarction. After preliminary studies were performed in 7 patients to determine optimal imaging parameters, a standardized imaging protocol was used in the next 10. The location of myocardial infarction identified by the electrocardiogram and coronary anatomy was correctly identified in 10 of 10 patients. Distribution of the injury within the left ventricle was clearly visualized, and showed that patients often had a mixture of transmural and nontransmural injury. Heterogenous distribution of signal intensity within the infarction suggested the presence of hemorrhage. Papillary muscle involvement was readily apparent. Signal intensity of the infarction (brightest segment) was increased by 89 +/- 31% compared with the mean of the remote segments. The myocardial/skeletal muscle ratio was significantly (p less than 0.001) increased for the infarction segments compared with that for remote myocardium, allowing quantitative analysis of segmental signal intensity. The MRI wall motion study obtained as part of this protocol demonstrated wall thickening in 58% of the infarction segments and in 6 of 10 patients. This finding suggested the presence of reversibly injured myocardium. In conclusion, the results demonstrate the potential of MRI for detailed tissue characterization after acute myocardial infarction.