Vulnerable atherosclerotic plaques are characterized by distinct attributes regarding morphology, stenosis, inflammation, thrombogenicity, injury, and enhanced proteinase activity.

Based upon the increasing molecular knowledge regarding atherogenesis, different principles have been successfully used to image atherosclerotic plaques. One major complex is the molecular imaging of inflammation, which includes enhanced metabolic activity, chemotaxis, cell recruitment, and lipoprotein accumulation. Furthermore, mediators of angiogenesis, apoptosis, and matrix metalloproteinase (MMP) activity have been successfully applied. Another promising approach to detect vulnerable atherosclerotic plaque is the visualization of plaque thrombogenicity, including thrombosis and exposure of thrombogenic subendothelial matrix proteins. ECM = extracellular matrix; FCH = fluorocholine; FDG = fluorodeoxyglucose; GP = glycoprotein; LDL = low-density lipoprotein; L19 = antibody against the extra-domain B of fibronectin; MCP = monocyte chemoattractant protein; MDA2 = malondialdehyde epitope on oxidized low-density lipoprotein; ox-LDL = oxidized low-density lipoprotein; RGD = protein sequence "arginine-glycine-aspartic acid."

(Top) (A) Coronal image section from a ¹⁸F-Galacto-RGD positron emission tomography of a patient with a malignant melanoma and a lymph node metastasis at the right axilla obtained 60 min after tracer injection. The image shows a clearly contrasting tumor and rapid predominately renal elimination with low background activity in almost all areas of the body. (B and C) Patient with a soft tissue sarcoma dorsal of the right knee joint. (B) The image fusion of the ¹⁸F-Galacto-RGD positron emission tomography and the corresponding computed tomography scan shows that the regions of intense tracer uptake correspond with the enhancing tumor wall, whereas the nonenhancing hypodense center shows no tracer uptake. (C) Immunohistochemistry of a peripheral tumor section using the anti-_vβ₃ monoclonal antibody LM609 demonstrates intense staining predominantly of tumor vasculature. (Bottom) (D) Immunohistochemical staining of tumor sections using the anti-_vβ₃ mAb LM609 demonstrates that the squamous cell carcinoma of human origin do not express the _vβ₃ integrin. (E) In contrast, staining of sections with an antibody against the murine β₃-subunit indicates that the tumor vasculature is _vβ₃-positive. (F and G) Transaxial images of nude mice bearing a human squamous cell carcinoma at the right shoulder acquired 90 min after ¹⁸F-Galacto-RGD injection show a clearly contrasting tumor. The signal reflects tracer accumulation due to _vβ₃ expression exclusively in the tumor vasculature. Tracer accumulation can be blocked by injecting 18-mg cyclo(-Arg-Gly-Asp-DPhe-Val-) per kilogram mouse 10 min before tracer injection indicates receptor-specific accumulation. Adapted from Haubner et al. (43).

(A) (Oxidized) lipoproteins initiate and sustain the inflammation process in atherosclerotic lesions and are essential for foam cell generation. Scavenger receptors mediate the uptake of lipoproteins into macrophages and, thus, contribute substantially to foam cell formation. (B) A soluble dimeric form of the scavenger receptor CD68 conjugated to an Fc-fragment was radiolabeled with ¹²⁴I and used to detect atherosclerosis in vivo. (C) Apolipoprotein E ^–/– (ApoE^–/–) or wild type mice were fed a high cholesterol diet for 17 weeks. Subsequently, mice were injected intravenously with ¹²⁴ICD68-Fc and sacrificed after 48 h. Ex-vivo nuclear imaging was performed to evaluate tracer activity in the aortic arch. Activity of ¹²⁴ICD68-FC was enhanced in ApoE^–/– compared with wild type mice (bottom) and correlated well with plaque extension as shown by sudan red staining in the aortic arch specimen (top). LDL = low-density lipoprotein.

(A) Pathophysiology of platelet adhesion, secretion, and thrombus formation at sites of injured vascular endothelium with exposed extracellular matrix. 1) Within the intact vessel, platelets do not adhere to the endothelial monolayer under physiological conditions. 2) At site of atherosclerotic lesions, subendothelial proteins such as von Willebrand factor (vWF) and collagen are exposed to blood flow. Platelet adhesion receptors glycoprotein Ib (GPIb) and glycoprotein VI (GPVI) mediate tethering of platelets. 3) After activation of the integrins ₂β₁ (collagen receptor) and _IIbβ₃ (fibrinogen receptor), platelets firmly adhere via interaction of these receptors with extracellular matrix proteins. 4) Subsequently, platelets get activated and secrete distinct mediators resulting in: 5) platelet aggregation via fibrinogen bridges between 2 _IIbβ₃ receptors and thrombus formation. (B) A soluble dimeric form of human platelet GPVI conjugated to an Fc-fragment was used, which was radiolabeled with ¹²⁴I. Glycoprotein VI is essential to establish the first interaction of platelets with an exposed collagen surface. Therefore, we made use of this natural mechanism to detect thrombogenic and, thus, vulnerable plaques. (C and D) Apolipoprotein E^–/– mice were analyzed with ¹²⁴IGPVI-Fc using an animal microPET scanner (MicroPET Focus 120, Siemens, Knoxville, Tennessee) (left). Images were acquired 24 h after administration of the tracer and imaging time was 20 min. Positron emission tomography (PET) images were correlated with computed tomographic (CT) data (right) to verify anatomical structures. (C) Shows transverse sections of these experiments. (D) Ex vivo nuclear imaging was performed to evaluate tracer activity in the aortic arch (top). The bottom shows the specimen after staining with sudan III. Activity of ¹²⁴IGPVI-Fc correlated well with plaque extension (left). The signal could be nearly abolished, when nonlabeled GPVI-Fc was injected before application of ¹²⁴IGPVI-Fc (right).

Technetium-99m labeled annexin A5 accumulates in unstable atherosclerotic plaques. (A) Single-photon emission computed tomography images of a patient with a recent transient ischemic attack. (B) Annexin A5 in sections of the resected plaque stains strongly in a macrophage-rich area. In contrast, a minimal annexin A5 signal (C) appears in the carotid artery of a patient with a transient ischemic attack 3 months before imaging. (D) Annexin A5 immunoreactivity is at a background level in the corresponding smooth-muscle-cell-rich lesion. Images provided courtesy of Dr. Bas Kietselaer and Dr. Leonard Hofstra, University Hospital of Maastricht, the Netherlands. Reprinted, with permission, from Kietselaer et al. (70).