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

Design, Synthesis, and Preclinical Evaluation of a High-Affinity 18F-Labeled Radioligand for Myocardial Growth Hormone Secretagogue Receptor Before and After Myocardial Infarction

The peptide hormone ghrelin is produced in cardiomyocytes and acts through the myocardial growth hormone secretagogue receptor (GHSR) to promote cardiomyocyte survival. Administration of ghrelin may have therapeutic effects on post-myocardial infarction (MI) outcomes. Therefore, there is a need to develop molecular imaging probes that can track the dynamics of GHSR in health and disease to better predict the effectiveness of ghrelin-based therapeutics. We designed a high-affinity GHSR ligand labeled with 18F for imaging by PET and characterized its in vivo properties in a canine model of MI. Methods: We rationally designed and radiolabeled with 18F a quinazolinone derivative ([18F]LCE470) with subnanomolar binding affinity to GHSR. We determined the sensitivity and in vivo and ex vivo specificity of [18F]LCE470 in a canine model of surgically induced MI using PET/MRI, which allowed for anatomic localization of tracer uptake and simultaneous determination of global cardiac function. Uptake of [18F]LCE470 was determined by time-activity curve and SUV analysis in 3 regions of the left ventricle-area of infarct, territory served by the left circumflex coronary artery, and remote myocardium-over a period of 1.5 y. Changes in cardiac perfusion were tracked by [13N]NH3 PET. Results: The receptor binding affinity of LCE470 was measured at 0.33 nM, the highest known receptor binding affinity for a radiolabeled GHSR ligand. In vivo blocking studies in healthy hounds and ex vivo blocking studies in myocardial tissue showed the specificity of [18F]LCE470, and sensitivity was demonstrated by a positive correlation between tracer uptake and GHSR abundance. Post-MI changes in [18F]LCE470 uptake occurred independently of perfusion tracer distributions and changes in global cardiac function. We found that the regional distribution of [18F]LCE470 within the left ventricle diverged significantly within 1 d after MI and remained that way throughout the 1.5-y duration of the study. Conclusion: [18F]LCE470 is a high-affinity PET tracer that can detect changes in the regional distribution of myocardial GHSR after MI. In vivo PET molecular imaging of the global dynamics of GHSR may lead to improved GHSR-based therapeutics in the treatment of post-MI remodeling.

68Ga-DOTA chelate, a novel imaging agent for assessment of myocardial perfusion and infarction detection in a rodent model

BACKGROUND

Magnetic resonance imaging (MRI) with Gadolinium 1,4,7,10-tetraazacyclododecane-N',N″,N''',N″″-tetraacetic acid (Gd-DOTA) enables assessment of myocardial perfusion during first-pass of the contrast agent, while increased retention can signify areas of myocardial infarction (MI). We studied whether Gallium-68-labeled analog, 68Ga-DOTA, can be used to assess myocardial perfusion on positron emission tomography/computed tomography (PET/CT) in rats, comparing it with 11C-acetate.

METHODS

Rats were studied with 11C-acetate and 68Ga-DOTA at 24 hours after permanent ligation of the left coronary artery or sham operation. One-tissue compartmental models were used to estimate myocardial perfusion in normal and infarcted myocardium. After the PET scan, hearts were sectioned for autoradiographic detection of 68Ga-DOTA distribution.

RESULTS

11C-acetate PET showed perfusion defects and histology showed myocardial necrosis in all animals after coronary ligation. Kinetic modeling of 68Ga-DOTA showed significantly higher k1 values in normal myocardium than in infarcted areas. There was a significant correlation (r = 0.82, P = 0.001) between k1 values obtained with 68Ga-DOTA and 11C-acetate. After 10 minutes of tracer distribution, the 68Ga-DOTA concentration was significantly higher in the infarcted than normal myocardium on PET imaging and autoradiography.

CONCLUSIONS

Our results indicate that acute MI can be detected as reduced perfusion, as well as increased late retention of 68Ga-DOTA.

Feasibility of detecting myocardial infarction in the sheep fetus using late gadolinium enhancement CMR imaging

BACKGROUND

Late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR) imaging has enabled the accurate assessment of myocardial infarction (MI). However, LGE CMR has not been performed successfully in the fetus, where it could be useful for animal studies of interventions to promote cardiac regeneration. We believe that LGE imaging could allow us to document the presence, extent and effect of MI in utero and would thereby expand our capacity for conducting fetal sheep MI research. We therefore aimed to investigate the feasibility of using LGE to detect MI in sheep fetuses.

METHODS

Six sheep fetuses underwent a thoracotomy and ligation of a left anterior descending (LAD) coronary artery branch; while two fetuses underwent a sham surgery. LGE CMR was performed in a subset of fetuses immediately after the surgery and three days later. Early gadolinium enhancement (EGE) CMR was also performed in a subset of fetuses on both days. Cine imaging of the heart was performed to measure ventricular function.

RESULTS

The imaging performed immediately after LAD ligation revealed no evidence of infarct on LGE (n=3). Two of four infarcted fetuses (50%) showed hypoenhancement at the infarct site on the EGE images. Three days after the ligation, LGE images revealed a clear, hyper-enhanced infarct zone in four of the five infarcted fetuses (80%). No hyper-enhanced infarct zone was seen on the one sham fetus that underwent LGE CMR. No hypoenhancement could be seen in the EGE images in either the sham (n=1) or the infarcted fetus (n=1). No regional wall motion abnormalities were apparent in two of the five infarcted fetuses.

CONCLUSION

LGE CMR detected the MI three days after LAD ligation, but not immediately after. Using available methods, EGE imaging was less useful for detecting deficits in perfusion. Our study provides evidence for the ability of a non-invasive tool to monitor the progression of cardiac repair and damage in fetuses with MI. However, further investigation into the optimal timing of LGE and EGE scans and improvement of the sequences should be pursued with the aim of expanding our capacity to monitor cardiac regeneration after MI in fetal sheep.

Magnetic resonance imaging dynamic contrast enhancement (DCE) characteristics of healed myocardial infarction differ from viable myocardium

PURPOSE

To determine whether healed myocardial infarction alters dynamic contrast-enhancement (DCE) curve shapes as well as late gadolinium-enhancement (LGE).

MATERIALS AND METHODS

Twenty patients with chronic myocardial infarction underwent MR imaging at 1.5 T with blood and myocardial T1 measurements before and after contrast administration for forty minutes. Viable and infarcted myocardial partition coefficients were calculated using multipoint slope methods for ten different DCE sampling intervals and windows. Partition coefficients and coefficients of determination were compared with paired statistical tests to assess the linearity of DCE curve shapes over the 40 min time period.

RESULTS

Calculated partition coefficients did not vary significantly between methods (p=0.325) for viable myocardium but did differ for infarcted myocardium (p<0.001), indicating a difference in infarcted DCE. There was a significant difference between viable and infarcted myocardial partition coefficients estimates for all methods with the exception of methods that included measurements during the first 10 min after contrast agent administration.

CONCLUSION

Myocardial partition coefficients calculated from a slope calculation vary in healed myocardial infarction based on the selection of samples due to non-linear DCE curve shapes. Partition coefficient calculations are insensitive to data sampling effects in viable myocardium due to linear DCE curve shapes.

MRI study of cryoinjury infarction in pig hearts: i. Effects of intrapericardial delivery of bFGF/VEGF embedded in alginate beads

The aim of the study was the testing of sustained intrapericardial delivery of vascular growth factors (GFs) from alginate beads on cryoinjury size and perfusion. In domestic pigs (15-20 kg, n = 21), the left ventricular (LV) anterolateral wall of exposed hearts was cryoinjured using an aluminum rod (25 mm o.d.) cooled in liquid nitrogen. Alginate beads (d = 3.2 ± 0.2 mm), containing human recombinant basic fibroblast GF (bFGF, 50 µg) and vascular endothelial GF (VEGF, 50 µg) + heparin (50 µg) or heparin alone (Con, n = 5), were sutured to the cryoinjured epicardium (GF, n = 5; Con, n = 3 ) or pericardium (GF, n = 3; Con, n = 2), or no beads were implanted (n = 4). Four pigs were sham-operated. Cine and T(1) -weighted MRI was performed in vivo at ~2.5 h and 1, 2, 3 and 4 weeks after injury in a 3T imager. A double bolus of GdDTPA was injected (0.05 and 0.15 mmol/kg) and first-pass and late enhancement kinetics were monitored. After 4-week cryoinjury, following the injection of 5 x 10(6) 15-µm NIR fluorescent microspheres (FMS, 645/680 nm), hearts were sliced and examined with fluorescence imaging. Triphenyltetrazolium chloride (TTC) staining was used to determine infarct areas. Epicardial GF-containing beads were encapsulated within the hypointense 3-4-week infarct tissue. This tissue had a 75% higher LV thickening index, a lower distribution volume for GdDTPA (0.44 ± 0.12 vs 0.68 ± 0.05, p = 0.02), and 25% faster first-pass Gd kinetics relative to control infarctions. TTC staining revealed TTC-positive islands in the core of treated infarcts, which showed higher FMS fluorescence relative to surrounding infarct tissue (0.64 ± 0.14 vs. 0.31 ± 0.14; p < 0.0001) and to control infarcts (0.37 ± 0.09, p < 0.05). GF-beads attached to the pericardium were not effective. We conclude that sustained intrapericardial release of bFGF + VEGF from alginate beads attached to the epicardium facilitated vascular growth in the cryoinjured area.

Contrast agents used in cardiovascular magnetic resonance imaging: current issues and future directions

Cardiovascular MRI is being increasingly used in the evaluation of ischemic heart disease, cardiac masses, complex congenital heart disease, and morphologic evaluation of the vascular anatomy throughout the body. Many and varied contrast media may be used to increase the sensitivity and specificity of detecting and evaluating various pathologies, and a knowledge of the different mechanisms of action, distributions and safety profiles of these agents is required for safe and effective imaging. This article reviews the currently available magnetic resonance (MR) contrast media, discusses the risks and benefits, and gives illustrated examples of current clinical applications in cardiovascular disease. A literature search covered the period 1990 to the present with the use of multiple databases including MEDLINE, PUBMED, SciSearch and Google Medical. All identified studies containing information relevant to the topic of cardiovascular MRI and cardiovascular MR contrast agents and their uses and properties were evaluated. Evaluation was limited to studies in English. The conclusions were that the use of contrast agents vastly increases the diagnostic yield, sensitivity and specificity of cardiovascular MRI in the non-invasive diagnosis of the full breadth of cardiovascular pathology. The use of contrast MRI for investigating ischemic heart disease, cardiac masses, and congenital heart disease and in angiography is now well established, and the referring physician, cardiologist, or radiologist requires an in-depth knowledge of the safety profiles and correct dosing of commonly prescribed contrast agents. As the number of MR contrast agents on the market continues to increase, knowledge of the basic mechanism of action is vital for keeping abreast of how new and emerging agents will affect clinical practice in the future.

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.

High-resolution dynamic imaging of contrast agent uptake in a beating heart

Dynamic contrast-enhanced (DCE) MR cardiac imaging has been recognized as a unique and powerful tool for assessing both cardiac functions and physiological conditions of the heart tissues (e.g., tissue rejection following heart transplantation). However, because of cardiac motion and the limited data acquisition speed of existing MRI techniques, it has been very difficult to acquire dynamic images of very high spatiotemporal resolution. This paper proposes a new generalized series (GS) based imaging technique to overcome this challenging problem. Specifically, the proposed technique collects two data sets: a) a sequence of highresolution reference images over several cardiac cycles using a gated cine acquisition scheme before the injection of a contrast agent (or a molecular probe), and b) a sequence of reduced data sets with very high frame rate during the transient wash-in/wash-out stage of the contrast agent. A GS model is then used to combine these two data sets to reconstruct a high-resolution image sequence, capturing both the cardiac motions and dynamic signal changes due to the interaction of the contrast agent with the cardiac tissues. The proposed technique has been validated using both simulated and experimental data, which show that high-resolution dynamic images can be acquired with as few as 8 encodings (in contrast to 256 encodings required in the traditional Fourier transform-based methods). The technique provides a very effective tool for physiological imaging of the beating heart with molecular probes.

Magnetic Resonance Imaging of Acute Reperfused Myocardial Infarction: Intraindividual Comparison of ECIII-60 and Gd-DTPA in a Swine Model

PURPOSE

To compare a necrosis-avid contrast agent (NACA) bis-Gd-DTPA-pamoic acid derivative (ECIII-60) after intracoronary delivery with an extracellular agent Gd-DTPA after intravenous injection on magnetic resonance imaging (MRI) in a swine model of acute reperfused myocardial infarction (MI).

METHODS

Eight pigs underwent 90 min of transcatheter coronary balloon occlusion and 60 min of reperfusion. After intravenous injection of Gd-DTPA at a dose of 0.2 mmol/kg, all pigs were scanned with T1-weighted MRI until the delayed enhancement of MI disappeared. Then they were intracoronarily infused with ECIII-60 at 0.0025 mmol/kg and imaged for 5 hr. Signal intensity, infarct-over-normal contrast ratio and relative infarct size were quantified, compared, and correlated with the results of postmortem MRI and triphenyltetrazolium chloride (TTC) histochemical staining.

RESULTS

A contrast ratio over 3.0 was induced by both Gd-DTPA and ECIII-60. However, while the delayed enhancement with Gd-DTPA virtually vanished in 1 hr, ECIII-60 at an 80x smaller dose depicted the MI accurately over 5 hr as proven by ex vivo MRI and TTC staining.

CONCLUSION

Both Gd-DTPA and ECIII-60 strongly enhanced acute MI. Comparing with fading contrast in a narrow time window with intravenous Gd-DTPA, intracoronary ECIII-60 persistently demarcated the acute MI, indicating a potential method for postprocedural assessment of myocardial viability after coronary interventions.

Synergy of MDCT and Cine MRI for the Evaluation of Cardiac Motility

OBJECTIVE

The objective of our study was to validate the feasibility of the synergistic use of cardiac MR and CT data sets for ventricular motility analysis and to correlate measurement variability with underlying heart rate.

SUBJECTS AND METHODS

Twenty patients underwent concurrent ECG-gated MDCT and MRI for evaluation of ventricular motility, expressed as ventricular wall thickening and motion. Initially, individual measurement repetition series were analyzed by determining intraobserver variability and detecting intraobserver bias related to heart rates. Subsequently, absolute measurement differences of CT or MR data were statistically evaluated. Finally, absolute measurement differences were correlated with underlying heart rates by curve estimation regression.

RESULTS

Analysis of measurement reproducibility proved that data variability was dependent on only the anatomic localization of the analyzed ventricular segment, not on the imaging technique used or underlying heart rate, in normofrequent patients. Comparing MR and CT image data sets, no statistically significant differences were identified when ventricular motility was evaluated based on data sets of either imaging technique in normofrequent patients. Tachycardic frequencies, above 100 beats per minute, led to exponential error propagation due to insufficient temporal resolution of the current CT technology.

CONCLUSION

This study proved that cardiac motility assessment based on ECG-gated CT and MR data sets resulted in comparable ventricular function results for normofrequent patients; however, the high spatial resolution of cardiac MDCT cannot compensate for the lack of temporal resolution in patients with tachycardia, thus emphasizing the necessity of reporting ventricular motility analysis results in combination with heart rate to allow consideration of this possible cause for measurement variation.

Establishing a clinical cardiac MRI service

After several years of research development cardiovascular MRI has evolved into a widely accepted clinical tool. It offers important diagnostic and prognostic information for a variety of clinical indications, which include ischaemic heart disease, cardiomyopathies, valvular dysfunction and congenital heart disorders. It is a safe non-invasive technique that employs a variety of imaging sequences optimized for temporal or spatial resolution, tissue-specific contrast, flow quantification or angiography. Cardiac MRI offers specific advantages over conventional imaging techniques for a significant number of patients. The demand for cardiac MRI studies from cardiothoracic surgeons, cardiologists and other referrers is likely to continue to rise with pressure for more widespread local service provision. Setting up a cardiac MRI service requires careful consideration regarding funding issues and how it will be integrated with existing service provision. The purchase of cardiac phased array coils, monitoring equipment and software upgrades must also be considered, as well as the training needs of those involved. The choice of appropriate imaging protocols will be guided by operator experience, clinical indication and equipment capability, and is likely to evolve as the service develops. Post-processing and offline analysis form a significant part of the time taken to report studies and an efficient method of providing quantitative reports is an important requirement. Collaboration between radiologists and cardiologists is needed to develop a successful service and multi-disciplinary meetings are key component of this. This review will explore these issues from our perspective of a new clinical cardiac MRI service operating over its first year in a teaching hospital imaging department.

A comparison of CH3-DTPA-GD (NMS60) and GD-DTPA for evaluation of acute myocardial ischemia

PURPOSE

Our objective was to evaluate the use of a new medium weight MRI contrast agent, NMS60 (a synthetic oligomeric Gd-complex containing three Gd(3+) atoms, molecular weight 2158 Da) compared to gadolinium-diethylene triamine pentaacetic acid (Gd-DTPA) in a pig myocardial ischemia model.

MATERIALS

We used 13 male white hybrid pigs. Animals were scanned in the acute phase 2-3 h after the onset of myocardial ischemia. Scans were acquired on a 1.5T GE Signa with dynamic T1-weighted imaging during a bolus injection of 0.1 mmol(gd)/kg of either NMS60 or Gd-DTPA, 2D CINE at 5 min after injection, and T1-weighted spin-echo imaging up to 60 min.

RESULTS

The postcontrast CINE scans showed improved contrast-to-noise ratio after NMS60 injection, compared to Gd-DTPA. There was significantly greater enhancement with NMS60 in both normal myocardium and in the ischemic lesion on T1-weighted spin-echo scans up to 60 min after injection. The dose ranging study shows a 24% greater enhancement with NMS60 compared to Gd-DTPA.

DISCUSSION

This new medium weighted contrast agent offers improved enhancement for cardiac MRI, compared to Gd-DTPA, with similar washout kinetics and lower toxicity, and may prove useful for better detection of myocardial ischemia as well as delayed or hyperenhancement after reperfusion.

Necrosis Avid Contrast Agents

Two categories of necrosis-avid contrast agents (NACAs), namely porphyrin- and nonporphyrin-based complexes, have thus far been discovered as necrosis-targeting markers for noninvasive magnetic resonance imaging (MRI) identification of acute myocardial infarction, assessment of tissue or organ viability, and therapeutic evaluation after interventional therapies. In addition to necrosis labeling, other less-specific functions, such as first-pass perfusion, blood pool contrast effect, hepatobiliary contrast enhancement (CE), adrenal and spleen CE, and renal functional imaging, also are demonstrated with NACAs. Despite various investigations with a collection of clues in favor of certain hypotheses, the mechanisms of such a unique targetability for NACAs still remain to be elucidated. However, a few things have become clear that porphyrin-like structures are not necessary for necrosis avidity and the albumin binding is not the supposed driving force but only a parallel nonspecific feature shared by both NACAs and non-NACA substances. Although the research and development of NACAs still remain in preclinical stage at a relatively small scale, their significance rests upon striking enhancement effects, which may warrant their eventual versatile clinical applications. The present review article is intended to summarize the cumulated facts about the evolving research on this topic, to demonstrate experimental observations for better understanding of the mechanisms, to trigger broader public interests and more intensive research activities, and to advocate, toward both academics and industries, further promotion of preclinical and clinical development of this unique and promising class of contrast agents.

Cine and tagged magnetic resonance imaging in short-term stunned versus necrotic myocardium

We investigated the potential of Cine and 2D Tagged Cardiac Magnetic Resonance (CMR) Imaging to distinguish stunned from necrotic left ventricular (LV) myocardium in the early postischemic phase in an open-chest animal model (N = 12). Reversible and permanent occlusion of the LAD coronary artery resulted in global LV dysfunction in both groups without significant differences. LAD perfused segments revealed significant higher values for end systolic wall thickening (ESWT) and percentual systolic wall thickening in animals with stunned myocardium. Analysis of strain parameters showed significant regional differences (maximal principal strain lambda1, deviation angle beta) between postischemic and remote myocardium within both groups, however results were not significantly different comparing animals with stunned myocardium to animals with myocardial necrosis. In conclusion, at rest neither global LV functional nor regional strain parameters derived from Cine and 2D Tagged CMR Imaging can distinguish animals with short-term stunned myocardium from respective animals with necrotic myocardium. Diagnostic value of ESWT is limited due to the spatial resolution of the gradient-echo sequence used.

Functional Cardiac CT and MR

This study sought to validate different software applications for cardiac function analysis using ECG-gated CT and MR datasets in correlation with underlying heart rate. Ten patients and a set of ventricular phantoms underwent concurrent multislice-CT and cine-MR imaging for evaluation of cardiac function. Datasets from both imaging modalities were evaluated utilizing 2 volumetric analysis tools to determine left ventricular volume and mass. Initially, intraobserver measurement variability was assessed. Detected measurement variability was correlated with underlying absolute magnitude of cardiac volumes and masses. Subsequently, results were statistically evaluated by determining significant data variability depending on imaging modality and choice of evaluation software. Finally, the data variability was correlated with underlying heart rates. This study showed that all analyzed datasets uniformly presented intraobserver variations below 2%, and variability was not related to the magnitude of measurement. Significant measurement accuracy was proven in all calculated parameters obtained from the cardiac phantoms. Acquired patient datasets and calculated functional parameters showed significant data homogeneity, with measurement variability coefficients ranging from 0.935-0.955. CT datasets showed maximal data variability at heart rates below 60 BpM. MR datasets showed maximal data variability at heart rates above 90 BpM. In conclusion, CT and MR datasets allowed an interchangeable utilization of volumetric analysis tools. However, reliable volumetric analysis was limited to an optimal range of cardiac rates for each modality, thus emphasizing the necessity of reporting volumetric measurement results in combination with heart rate to allow for consideration of this possible cause for measurement variation.

Experimental application of thermosensitive paramagnetic liposomes for monitoring magnetic resonance imaging guided thermal ablation

The use of a liposomal paramagnetic agent with a T(1)-relaxivity that increases markedly at temperatures above the phase transition temperature (T(m)) of the liposomal membrane was evaluated during magnetic resonance imaging (MRI) guided hyperthermia ablation. A neodymium-yttrium aluminum garnet (Nd-YAG) laser unit and a radiofrequency ablation system were used for tissue ablation in eight rabbit livers in vivo. One ablation was made in each animal prior to administration of the liposomal agent. Liposomes with a T(m) of 57 degrees C containing gadodiamide (GdDTPA-BMA) were injected iv, and two additional ablations were performed. T(1)-weighted scans were performed in heated tissue, after tissue temperature had normalized, and 15-20 min after normalization of tissue temperature. Increase in signal intensity (DeltaSI) for ablations prior to injection of the agent was 13.0% (SD = 5.7) for the laser group and 9.1% (SD = 7.9) for the radiofrequency group. Signal intensity after administration of the agent unrelated to heating was not statistically significant (DeltaSI = 1.4%, P = 0.35). For ablations made after injection of the agent, a significant increase was found in the laser (DeltaSI = 34.5%, SD = 11.9) and radiofrequency group (DeltaSI = 21.6%, SD = 22.7). The persistent signal enhancement found in areas exposed to a temperature above the threshold temperature above T(m) allows thermal monitoring of MRI guided thermal ablation.

Contrast-enhanced MR imaging of the heart: overview of the literature

The use of magnetic resonance (MR) imaging for cardiac diagnosis is expanding, aided by the administration of paramagnetic contrast agents for a growing number of clinical applications. This overview of the literature considers the principles and applications of cardiac MR imaging with an emphasis on the use of contrast media. Clinical applications of contrast material-enhanced MR imaging include the detection and characterization of intracardiac masses, thrombi, myocarditis, and sarcoidosis. Suspected myocardial ischemia and infarction, respectively, are diagnosed by using dynamic first-pass and delayed contrast enhancement. Promising new developments include blood pool contrast media, labeling of myocardial precursor cells, and contrast-enhanced imaging at very high fields.

Diagnostic value of routine clinical parameters in acute myocardial infarction: a comparison to delayed contrast enhanced magnetic resonance imaging. Delayed enhancement and routine clinical parameters after myocardial infarction

AIMS

Contrast enhanced magnetic resonance imaging (ceMRI) has been shown to reliably identify irreversible myocardial injury. The aim of this study was to compare the findings on ceMRI with routine clinical markers of myocardial injury in patients with acute myocardial infarction (MI).

METHODS AND RESULTS

Twenty-four patients with acute MI were investigated at 1.5 T. The global myocardial function was analysed with a standard cine MR protocol and a stack of short axis slices encompassing the entire left ventricle. Corresponding short axis slices were acquired for delayed ceMRI 15-20 min after the administration of 0.2 mmol gadolinium-DTPA/kg body weight. Mass of hyperenhancement and peak creatine kinase release (peak CK) was determined for each patient. The presenting 12-lead ECG was analysed for ST-elevation on admission and later development of Q-waves. Mass of hyperenhancement correlated moderately well to peak CK (r = 0.65, p < 0.01) and endsystolic volume index (r = 0.55, p < 0.01). Mass of hyperenhancement was inversely correlated to ejection fraction (r = -0.50, p = 0.02). Neither the presence of ST elevation on the admission ECG nor the later development of Q-waves did relate to the transmural extent of hyperenhancement and to the mass of hyperenhancement.

CONCLUSION

Mass of hyperenhancement significantly correlates to global myocardial function and to peak CK. However, there is no relationship between the findings in ceMRI and 12-lead ECG abnormalities on admission suggesting an advantage of ceMRI in defining transmural extent and depicting small areas of necrosis.

High field magnetic resonance imaging evaluation of superparamagnetic iron oxide nanoparticles in a permanent rat myocardial infarction

RATIONALE AND OBJECTIVES

The purpose of this study was to evaluate superparamagnetic iron oxide (SPIO) nanoparticles to discriminate infarcted from normal tissue after myocardial infarction using high field MR imaging (7 tesla).

MATERIALS AND METHODS

Permanent myocardial infarction was induced in rats. SPIO nanoparticles (1 mg Fe/kg) were assessed with T1-weighted gradient echo sequence to visualize the myocardial infarction 48 hours after ligature (n = 6). Furthermore, MR Imaging was performed using a T2-weighted RARE sequence and nanoparticles were injected (5 or 10 mg Fe/kg) on 36 rats 5, 24 or 48 hours after infarction.

RESULTS

No changes in contrast between normal and infarcted myocardium was observed after nanoparticle injection on T1-weighted images. However, nanoparticles induced a significant contrast increase between normal and infarcted myocardium on T2-weighted images whatever the delay between infarction and imaging (2.99 +/- 1.66 preinjection vs. 7.82 +/- 1.96 after SPIO injection at a dose of 5 mg Fe/kg 5 hours postinfarction, P = 0.0001).

CONCLUSIONS

Nanoparticle injection made it possible to discriminate normal from infarcted myocardium on T2-weighted images. However, the high magnetic field prevented the visualization of the T1 effect of SPIO nanoparticles.