Molecular imaging of coronary atherosclerosis and myocardial infarction: considerations for the bench and perspectives for the clinic

Abstract

Motivated by the promise to transform preclinical research and clinical care, cardiovascular molecular imaging has made advances toward targeting coronary atherosclerosis and heart failure. Here, we discuss recent progress in the field, highlight how molecular imaging may facilitate preventive patient care, and review specific challenges associated with coronary and heart failure imaging. Practical considerations stress the potential of fluorescence imaging for basic research and discuss hybrid protocols such as FMT-CT and PET-MRI.

Figure 1. Selected molecular imaging targets in atherosclerosis

VCAM-1: PET-CT imaging of adhesion molecule expression in the root of apoE^−/− mice with the tetrameric peptide ¹⁸F-4V. Adapted with permission . oxLDL: Gd-loaded microvesicles targeted to oxLDL result in increased T1 MRI contrast. Adapted with permission . Mono/Mac: Quantitation of myeloid cells in atherosclerotic plaque using iron oxide nanoparticles that increase T2* contrast on MRI (inset) and can be used for PET-CT imaging due to labeling with ⁶⁴Cu. Adapted with permission . FDG: PET-CT imaging of metabolic activity in a patient with coronary heart disease using the glucose analogon ¹⁸F-FDG. Adapted with permission . Proteases: FMT-CT imaging of cathepsin protease activity in the root of apoE^−/− mice. Adapted with permission . MPO: MR imaging of myeloperoxidase activity in a rabbit model of atherosclerosis. Image courtesy of Dr. John Chen. Integrins: PET-CT imaging using ¹⁸F-Galacto-RGD in LDLr^−/− mice. Adapted with permission . Fibrin: MR imaging of fibrin within coronary artery clots in a swine model. Adapted with permission . Mφ: Macrophage.

Figure 2. Molecular imaging targets in myocardial infarction

Myocyte death: Use of annexin-V decorated iron oxide nanoparticles in a mouse model of myocardial ischemia results in signal decay (arrow). Adapted with permission . Myofibroblasts: Fused SPECT-MR image shows uptake of an integrin targeted SPECT tracer in the infarct of a patient, delineated by delayed enhancement MRI. Adapted with permission . Collagen: Postinfarction myocardial scarring in mice imaged by MRI with the use of a collagen-targeting agent. The inset shows histological collagen localization in the infarct. Adapted with permission . Transglutaminase: SPECT-CT imaging of factor XIII activity in a mouse with coronary ligation predicts infarct healing and remodeling. Adapted with permission . Angiogenesis: Integrin PET-CT imaging in a patient with MI. Adapted with permission . Proteases: FMT-CT imaging in a murine infarct shows protease activity in the healing myocardium. Adapted with permission . Myeloperoxidase: Use of MPO-Gd in T1-weighted MRI in a mouse with coronary ligation to measure inflammation non-invasively. Adapted with permission . Monocytes: Iron oxide nanoparticles accumulate in monocytes and macrophages in the ischemic myocardium of a mouse. T2* weighted imaging at 7 Tesla. Own, previously unpublished data. Mφ: Macrophage, PMN: neutrophil.

Figure 3. Dual channel MRI of myocyte death

Molecular MRI of apoptotic myocardium (yellow arrows in left column, T2* weighted imaging using an echo time of 4 ms) and simultaneous delayed enhancement MRI of Gd-DTPA (right, T1 weighted imaging with an echo time of 1ms and myocardial signal suppression). At the midventricular level, only a small area in the subendocardium of the lateral wall showed delayed enhancement (red arrows). The extent of delayed enhancement increased progressively in the more apical slices (red arrows) and was fairly extensive at the apex. Adapted with permission .

Figure 4. Imaging of monocyte/macrophages in myocardial infarction

(A) T2* weighted MRI of iron oxide nanoparticles in a murine infarct model. Using a long echo time, imaging was done at 7 Tesla and shows the typical proton signal decay caused by interaction of protons with iron oxide (arrows). Own, previously unpublished data. (B) F19 fluorine MRI merged with proton MR image for anatomic information. Nanoemulsions of perfluorocarbons are taken up by myeloid cells in the infarct. Adapted with permission . (C) ¹⁸F-FDG PET image in a mouse 7 days after coronary ligation. In this study, PET signal was higher in the subacute MI (arrows) compared to the remote myocardium, likely due to the enhanced presence of metabolically active monocytes/ macrophages in the infarct. The inset shows the corresponding macrophage stain. Adapted with permission . (D) Molecular ultrasound imaging in a dog model of myocardial ischemia with leukocyte targeted microbubbles. The inset shows the location of the infarct in the corresponding TTC stain. Adapted with permission . (E) FMT-CT reconstruction showing the distribution of a fluorescent sensor in the infarct of a mouse. Own, previously unpublished data, image reconstruction courtesy of Dr. Claudio Vinegoni.

Figure 5. Integrative fluorescence imaging

Non-invasive imaging (FMT): The upper panel shows imaging with FMT-CT in an apoE^−/− mouse after injection of an activatable protease reporter. Adapted with permission . The lower panel illustrates spectrally resolved 4-channel FMT-CT imaging in a mouse that was subcutaneously injected with 4 fluorochromes of different excitation/emission wavelengths. For reconstruction, the 4 channels were color-coded and merged with anatomic CT. Adapted with permission . Intravital microscopy: Dual channel imaging of a fluorescent blood pool marker (red) and monocytes expressing green fluorescent protein in the spleen ^, . The time series shows a monocyte that departs from the organ in a mouse with MI. Adapted with permission . Intravital microscopy imaging in the vessels and the heart is complicated by motion; however, future strategies will suppress motion with tissue stabilizing during acquisition and post processing compensation algorithms. Ex-vivo imaging: The upper panel shows fluorescence reflectance imaging of a myocardial short axis ring after injection of a protease reporter (inset: TTC stain). Adapted with permission . The lower panel illustrates optical projection tomography (OPT) in a mouse heart . OPT allows to study molecular events in anatomical context and with high resolution. The panel shows a reconstructed image for the absorption coefficient acquired in a mouse heart with 360 projections. Previously unpublished, image courtesy of Dr. Claudio Vinegoni. FACS: After imaging, target tissue can be minced, digested and filtered to produce cell suspensions which are analyzed by multicolor flow cytometry. The specific example shows leukocyte populations in digested infarct tissue. Gate ii comprises CD11b⁺ lineage^- myeloid cells, including monocytes and macrophages. The histogram on he right shows fluorescence in these cells, which reports on uptake of a fluorescent molecular agent. Image courtesy of Dr. Filip Swirski. Fluorescence microscopy: Alternatively, tissue can be embedded to study signal distribution by fluorescence microscopy. The panel shows distribution of an integrin targeted probe (blue), a protease reporter (red) and nanoparticles targeted to myeloid cells (green) in a tumor model. Adapted with permission .

Figure 6. Clinical multimodal imaging

(A) Hybrid X-ray CT angiography fused to ¹⁸F-FDG PET for myocardial viability. This patient presented with new onset of heart failure. Severe left ventricular dysfunction with akinesia in the apex, anterior septum, and anterior free wall as well as posterolateral wall was detected in echocardiography. Coronary CT angiography showed chronic occlusion in the proximal LCX, severe stenosis in the proximal LAD and total occlusion of the distal LAD. ¹⁸F-FDG PET images demonstrated severe defects in the apex and lateral wall consistent with infarct scar (blue). However, there was preserved ¹⁸F-FDG uptake in the septum and most of the anterior wall as well as in the inferior wall consistent with large areas of viable myocardium (green and red colors). Subsequently, the patient underwent successful bypass surgery with venous grafts implanted in the distal LAD, diagonal branch and LCX, which relieved heart failure symptoms. Images and case report are previously unpublished and courtesy of Dr. Juhani Knuuti. (B) SPECT imaging of integrin expression (3 weeks after ischemia) combined with delayed enhancement MRI (1 year after ischemia) in a patient with myocardial infarction. The first row demonstrates delayed enhancement MR image of scar tissue in the LAD region (white arrows) in short and long axis views. The second row shows slices with technetium-99 uptake (blue arrows). The last row shows SPECT/MR fusion images. Adapted with permission.

Figure 7. Achievements and opportunities

While the last decade has seen profound progress of cardiovascular imaging technology, which resulted in convincing proof of principle molecular imaging studies in animal models and in patients, our rapidly evolving understanding of biological systems will likely foster discovery of improved imaging targets.