Molecular imaging in cardiovascular diseases.

UNLABELLED: Cardiovascular diseases remain the leading cause of morbidity and mortality in industrialized and developing countries. In clinical practice, the in-vivo identification of atherosclerotic lesions, which can lead to complications such as heart attack or stroke, remains difficult. Imaging techniques provide the reference standard for the detection of clinically significant atherosclerotic changes in the coronary and carotid arteries. The assessment of the luminal narrowing is feasible, while the differentiation of stable and potentially unstable or vulnerable atherosclerotic plaques is currently not possible using non-invasive imaging. With high spatial resolution and high soft tissue contrast, magnetic resonance imaging (MRI) is a suitable method for the evaluation of the thin arterial wall. In clinical practice, native MRI of the vessel wall already allows the differentiation and characterization of components of atherosclerotic plaques in the carotid arteries and the aorta. Additional diagnostic information can be gained by the use of non-specific MRI contrast agents. With the development of targeted molecular probes, that highlight specific molecules or cells, pathological processes can be visualized at a molecular level with high spatial resolution. In this review article, the development of pathophysiological changes leading to the development of the arterial wall are introduced and discussed. Additionally, principles of contrast enhanced imaging with non-specific contrast agents and molecular probes will be discussed and latest developments in the field of molecular imaging of the vascular wall will be introduced. KEY POINTS: Molecular magnetic resonance imaging has great potential to improve the in vivo characterization of atherosclerotic plaques. Based on the molecular information is feasible to enable a better differentiation of stable and unstable (vulnerable) atherosclerotic plaques.

Molecular imaging in cardiovascular diseases.

Cardiovascular diseases remain the leading cause of morbidity and mortality in industrialized and developing countries. In clinical practice, the in-vivo identification of atherosclerotic lesions, which can lead to complications such as heart attack or stroke, remains difficult. Imaging techniques provide the reference standard for the detection of clinically significant atherosclerotic changes in the coronary and carotid arteries. The assessment of the luminal narrowing is feasible, while the differentiation of stable and potentially unstable or vulnerable atherosclerotic plaques is currently not possible using non-invasive imaging. With high spatial resolution and high soft tissue contrast, magnetic resonance imaging (MRI) is a suitable method for the evaluation of the thin arterial wall. In clinical practice, native MRI of the vessel wall already allows the differentiation and characterization of components of atherosclerotic plaques in the carotid arteries and the aorta. Additional diagnostic information can be gained by the use of non-specific MRI contrast agents. With the development of targeted molecular probes, that highlight specific molecules or cells, pathological processes can be visualized at a molecular level with high spatial resolution. In this review article, the development of pathophysiological changes leading to the development of the arterial wall are introduced and discussed. Additionally, principles of contrast enhanced imaging with non-specific contrast agents and molecular probes will be discussed and latest developments in the field of molecular imaging of the vascular wall will be introduced.

Detection of intracoronary thrombus by magnetic resonance imaging in patients with acute myocardial infarction.

BACKGROUND: Persistent intracoronary thrombus after plaque rupture is associated with an increased risk of subsequent myocardial infarction and mortality. Coronary thrombus is usually visualized invasively by x-ray coronary angiography. Non-contrast-enhanced T1-weighted magnetic resonance (MR) imaging has been useful for direct imaging of carotid thrombus and intraplaque hemorrhage by taking advantage of the short T1 of methemoglobin present in acute thrombus and intraplaque hemorrhage. The aim of this study was to investigate the use of non-contrast-enhanced MR for direct thrombus imaging (MRDTI) in patients with acute myocardial infarction. METHODS AND RESULTS: Eighteen patients (14 men; age, 61±9 years) underwent MRDTI within 24 to 72 hours of presenting with an acute coronary syndrome before invasive x-ray coronary angiography; MRDTI was performed with a T1-weighted, 3-dimensional, inversion-recovery black-blood gradient-echo sequence without contrast administration. Ten patients were found to have intracoronary thrombus on x-ray coronary angiography (left anterior descending, 4; left circumflex, 2; right coronary artery, 4; and right coronary artery-posterior descending artery, 1), and 8 had no visible thrombus. We found that MRDTI correctly identified thrombus in 9 of 10 patients (sensitivity, 91%; posterior descending artery thrombus not detected) and correctly classified the control group in 7 of 8 patients without thrombus formation (specificity, 88%). The contrast-to-noise ratio was significantly greater in coronary segments containing thrombus (n=10) compared with those without visible thrombus (n=131; mean contrast-to-noise ratio, 15.9 versus 2.6; P<0.001). CONCLUSION: Use of MRDTI allows selective visualization of coronary thrombus in a patient population with a high probability of intracoronary thrombosis.

High spatial resolution and high contrast visualization of brain arteries and veins: impact of blood pool contrast agent and water-selective excitation imaging at 3T.

PURPOSE: To investigate a blood pool contrast agent and water-selective excitation imaging at 3 T for high spatial and high contrast imaging of brain vessels including the veins. METHODS AND RESULTS: 48 clinical patients (47 ± 18 years old) were included. Based on clinical findings, twenty-four patients received a single dose of standard extracellular Gadoterate-meglumine (Dotarem®) and 24 received the blood pool contrast agent Gadofosveset (Vasovist®). After finishing routine MR protocols, all patients were investigated with two high spatial resolution (0.15 mm (3) voxel size) gradient echo sequences in random order in the equilibrium phase (steady-state) as approved by the review board: A standard RF-spoiled gradient-echo sequence (HR-SS, TR/TE 5.1/2.3 msec, FA 30°) and a fat-suppressed gradient-echo sequence with water-selective excitation (HR-FS, 1331 binominal-pulse, TR/TE 8.8/3.8 msec, FA 30°). The images were subjectively assessed (image quality with vessel contrast, artifacts, depiction of lesions) by two investigators and contrast-to-noise ratios (CNR) were compared using the Student's t-test. The image quality and CNR in the HR-FS were significantly superior compared to the HR-SS for both contrast agents (p < 0.05). The CNR was also improved when using the blood pool agent but only to a minor extent while the subjective image quality was similar for both contrast agents. CONCLUSION: The utilized sequence with water-selective excitation improved image quality and CNR properties in high spatial resolution imaging of brain arteries and veins. The used blood pool contrast agent improved the CNR only to a minor extent over the extracellular contrast agent.

[MR imaging of lymph nodes using Gadofluorine M: feasibility in a swine model at 1.5 and 3T]. / MR-Bildgebung von Lymphknoten mit Gadofluorine M in einem Schweinemodell bei 1,5 and 3T.

PURPOSE: To investigate the potential of Gadofluorine M for targeted lymph node imaging in a human size animal model and on a clinical MR scanner at 1.5 and 3 T. MATERIALS AND METHODS: Pelvic and cervical lymph nodes in a swine model were investigated prior to and 24 hours after intravenous administration of 50 micromol/kg body weight Gadofluorine M, an experimental contrast agent. MR imaging was carried out on clinical 1.5 T and 3 T whole-body MR systems using clinically available coils and T 1-weighted sequences. The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) with respect to the surrounding tissue were assessed and compared using the Student's t-test. The Gd concentration in the lymph nodes (n = 43) was measured post mortem by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES). RESULTS: Gadofluorine M allowed for high signal and high contrast visualization of lymph nodes in all stations on post-contrast images with a significantly increased SNR and CNR (SNR pelvic lymph nodes post vs. pre: 46 +/- 7 vs.14 +/- 3, SNR cervical lymph nodes post vs. pre: 105 +/- 64 vs. 32 +/- 21; CNR pelvic lymph node vs. muscle post vs. pre 28 +/- 5 vs. 0.2 +/- 0.5, CNR cervical lymph node vs. muscle post vs. pre 76 +/- 53 vs. 11 +/- 15, p < 0.05 for all comparisons). The SNR and CNR in the pelvis were further improved using 3 T compared to 1.5 T scanners (SNR lymph nodes 3 T vs. 1.5 T 84 +/- 6 vs. 46 +/- 7, CNR lymph node vs. muscle 3 T vs. 1.5 T 53 +/- 9 vs. 28 +/- 5 respectively, p < 0.05). A high concentration of Gd in the lymph nodes was found (149 +/- 25 mmol Gd/L). CONCLUSION: Gadofluorine M accumulates in the lymph nodes and allows for selective targeted high contrast MR imaging of lymph node tissue in a large animal model using clinically available MR imaging techniques. 3 T further improves SNR and CNR compared to 1.5 T.

High spatial resolution magnetic resonance imaging of experimental cerebral venous thrombosis with a blood pool contrast agent.

PURPOSE: The purpose of this study was to investigate the feasibility of clot visualization in small sinus and cortical veins with contrast enhanced MRA in a cerebral venous thrombosis animal model using a blood pool contrast agent, Gadofosveset, and high spatial resolution imaging. MATERIAL AND METHODS: For induction of cerebral venous thrombosis a recently developed combined interventional and microsurgical model was used. Cerebral sinus and cortical vein thrombosis was induced in six pigs. Two further pigs died during the procedure. Standard structural, time-of-flight- and phase contrast-angiograms were followed by fast time resolved high resolution 3D MRA (4D MRA) and subsequent high spatial resolution 3D MRA in the equilibrium phase with and without addition of parallel imaging. Visualization of the clots using the different sequences was subjectively compared and contrast-to-noise ratio (CNR) was assessed. RESULTS: In the remaining six animals the procedure and MR-imaging protocol including administration of Gadofosveset was successfully completed. The 3D high resolution MRA in the equilibrium phase without the addition of parallel imaging was superior to all the other applied MR measurement techniques in terms of visualization of the clots. Only applying this sequence bridging vein thromboses were also seen as a small filling defect with a high CNR of >18. CONCLUSION: Only the non-accelerated high spatial resolution 3D MRA in the equilibrium in conjunction with the blood pool agent Gadofosveset allows for high-contrast visualization of very small clots in the cerebral sinus and cortical veins. STATEMENT CLINICAL IMPACT: Detection of cortical vein thrombosis is of high clinical impact. Conventional MRI sequences often fail to visualize the clot. We could demonstrate that, in contrast to conventional sequences, with high spatial resolution 3D MRA in the equilibrium in conjunction with the blood pool agent Gadofosveset very small clots in the cerebral sinus and cortical veins could be successfully visualized. We think that with the presented approach cortical vein thrombosis might also be sufficiently visualized in patients.

Molecular coronary MR imaging of human thrombi using EP-2104R, a fibrin-targeted contrast agent: experimental study in a swine model.

PURPOSE: The aim of this study was to investigate the use of a fibrin-specific contrast agent (EP-2104R, EPIX Pharmaceuticals, Lexington, Massachusetts, USA) for targeted molecular magnetic resonance (MR) imaging of human clot material removed from patients in a model of coronary thrombosis in swine. MATERIALS AND METHODS: Freshly ex vivo engineered clots from human blood and human in situ developed clots removed from patients were delivered into the coronary arteries of nine domestic swine. For MR imaging a navigator-gated, free-breathing, cardiac-triggered 3D inversion recovery black-blood gradient echo sequence was performed prior to clot delivery (baseline), after clot delivery but prior to contrast media administration, and two hours after systemic (i.v.) injection of 4 micromol/kg EP-2104R. MR images were analyzed by two investigators and the contrast-to-noise ratio and Gadolinium (Gd) concentration in the clots were assessed. RESULTS: On baseline images and prior to contrast media application no thrombi were visible. Post contrast administration all 10 coronary emboli (five ex vivo engineered clots and five human clots removed from patients) were selectively visualized as white spots with a mean contrast-to-noise ratio to the blood pool and the surrounding tissue of >12 and a mean Gd concentration of >100 microM. CONCLUSION: Molecular MR imaging using the fibrin-targeted contrast agent EP-2104R allows selective visualization of human clot material in a model of coronary thrombosis in swine.

Identification of mucosal injury in the murine nasal airways by magnetic resonance imaging: site-specific lesions induced by 3-methylindole.

A magnetic resonance imaging (MRI) technique was developed to identify mucosal damage to the nasal passages of mice resulting from exposure to respiratory toxicants. 3-Methylindole (3-MI) was chosen as a model nasal toxicant because systemic administration of this compound in mice results in a well-characterized necrotizing nasal lesion that is restricted to the olfactory mucosa. MRI technology allows imaging of the same mice before and at time points after injection. In addition, morphological alterations and increases in the area of sinus cavity airspace can be followed as a function of dose and time following exposure. For 3-MI, the cross-sectional area of the sinus airspaces increased by 1.7-fold in mice injected with 200 mg/kg and 2.6-fold in mice injected with 300 mg/kg at 3 days after injection. Alterations in the nasal turbinates lined by olfactory mucosa were identified 1, 3, and 6 days postadministration of 3-MI using MRI. Postmortem histological examination of the nasal tissue confirmed the intranasal location and distribution of the 3-MI-induced lesions observed by MRI. MRI can be a useful technique to identify toxicant-induced mucosal injury in the nasal passages at an in-plane resolution less than 60 microm.

Effects of aminooxyacetate on glutamate compartmentation and TCA cycle kinetics in rat hearts.

The nonspecific transaminase inhibitor aminooxyacetate (AOA) has multiple influences on the dynamics of 13C appearance in glutamate in rat hearts as measured by 13C nuclear magnetic resonance (NMR) without altering O2 consumption or tricarboxylic acid (TCA) cycle flux. These include the following: 1) a reduced rate of 13C enrichment at glutamate C3 and C4; 2) a near coalescence of the C3 and C4 fractional enrichment curves; 3) a dramatic alteration in the time-dependent evolution of the glutamate C4 multiplets, C4S and C4D34; and 4) a decrease in the NMR visibility of glutamate. A fit of the 13C fractional enrichment curves of glutamate C4 and C3 in the absence of inhibitor to a kinetic model of the TCA cycle gave values for transaminase flux of 7.5 mumol.min-1.g dry wt-1 and TCA cycle flux of 7.5 mumol.min-1.g dry wt-1, thereby confirming reports by others that the kinetics of 13C enrichment of glutamate C3 and C4 in heart tissue is significantly affected by flux through reactions other than TCA cycle. The 13C fractional enrichment data collected in the presence of 0.5 mM AOA could not be fitted using this same kinetic model. However, kinetic simulations demonstrated that the time-dependent changes in C4S and C4D34 are only consistent with a 10-fold reduction in the size of intermediate pools undergoing rapid turnover in the TCA cycle. We conclude that inhibition of glutamic-oxalacetic transaminase by AOA effectively reduces the size of the alpha-ketoglutarate pool in rapid exchange with the TCA cycle. Our data indicate that changes in glutamate multiplet areas in the 13C NMR spectra of heart (as demonstrated by glutamate C4S and C4D34) are more sensitive to alterations in metabolic pool sizes in exchange with the TCA cycle than are measurements of 13C fractional enrichment at glutamate C3 and C4.

Cardioprotective effects of 70-kDa heat shock protein in transgenic mice.

Heat shock proteins are proposed to limit injury resulting from diverse environmental stresses, but direct metabolic evidence for such a cytoprotective function in vertebrates has been largely limited to studies of cultured cells. We generated lines of transgenic mice to express human 70-kDa heat shock protein constitutively in the myocardium. Hearts isolated from these animals demonstrated enhanced recovery of high energy phosphate stores and correction of metabolic acidosis following brief periods of global ischemia sufficient to induce sustained abnormalities of these variables in hearts from nontransgenic littermates. These data demonstrate a direct cardioprotective effect of 70-kDa heat shock protein to enhance postischemic recovery of the intact heart.

A 13C isotopomer n.m.r. method for monitoring incomplete beta-oxidation of fatty acids in intact tissue.

An n.m.r. method is presented for monitoring the extent to which fatty acids undergo beta-oxidation without release of shorter-chain intermediates. It is based upon a 13C isotopomer analysis of glutamate from tissue presented with a mixture of [2,4,6,8-13C]octanoate and [1,2,3,4-13C]octanoate. The method does not require steady-state metabolic or isotopic conditions, so it may be applied during a variety of metabolic circumstances, including perfused tissue under stress and in vivo. We have tested the method in perfused rat hearts during anoxia, a model where previous work has shown that beta-oxidation of palmitate is incomplete and shorter-chain intermediates are released [Rabinowitz and Hercker (1974) Arch. Biochem. Biophys. 161, 621-627]. Indeed, n.m.r. spectra of freeze-clamped, acid-extracted tissue show that octanoate undergoes complete beta-oxidation in control normoxic rat hearts, but not in anoxic hearts. Complete beta-oxidation of octanoate was observed under a number of other metabolic conditions in perfused rat hearts, including low-pressure-induced ischaemia, KCl arrest and in the presence of high concentrations of competing substrates. We also demonstrate that the technique is applicable in intact tissue by taking direct measurements in perfused rat hearts using a recently published [13C]homonuclear decoupling technique and in in vivo heart and liver removed from rats after an intravenous infusion of a mixture of [2,4,6,8-13C]octanoate and [1,2,3,4-13C]octanoate.

13C isotopomer analyses in intact tissue using [13C]homonuclear decoupling.

Entry of 13C-enriched acetyl-CoA into the citric acid cycle results in scrambling of 13C into the various carbon positions of all intermediate pools. The eventual result is that the 13C resonances of all detectable intermediates or molecules exchanging with those intermediates appear as multiplets due to nearest neighbor spin-spin couplings. We have previously shown that an isotopomer analysis of the glutamate 13C multiplets provides a history of 13C flow through the cycle pools and that relative substrate utilization and relative anaplerotic flux can be quantitated (C.R. Malloy, A.D. Sherry, and F.M.H. Jeffrey, Am. J. Physiol. 259, H987-H995 (1990)). A major limitation of the method for in vivo applications is spectral resolution of multiline resonances required for a complete isotopomer analysis. We now show that [13C]homonuclear decoupling of the glutamate C3 resonance collapses nine-line C4 and C2 resonances into three-line multiplets. We demonstrate that these three-line 13C multiplets are well resolved in isolated, perfused rat hearts and present steady-state equations that allow an isotopomer analysis from data obtained in intact tissue. This advancement offers for the first time the possibility of extending 13C isotopomer methods to complex metabolic conditions in vivo.