Quantitative molecular imaging of atherosclerotic endothelial dysfunction with perfluorocarbon (19F) nanoparticle magnetic resonance imaging and spectroscopy

Abstract

Methods Five NZW rabbits were fed a high fat diet for 9-12 months (cholesterol: 1200-1700 mg/dL). Fluorescently-labeled, nontargeted NP were injected (2 ml/kg) intravenously into rabbit ear vein. After circulation in vivo for 1, 6 or 24 hours, aortas were excised for 19F MRI and spectroscopy (Varian 11.7 T scanner); and whole mount fluorescence imaging (Xenogen IVIS system). Two human carotid endarterectomy tissues were collected from operation room. After pretreatment with plasmin to digest fibrin on the endothelial surface and incubation with nontargeted NP for 6 hours, tissues were rinsed and formalin fixed for 19F MRI and spectroscopy. A perfluorooctyl bromide standard enabled MRS-based quantification of NP concentration in each imaged voxel. Results In rabbit aortas, MRI (19F/1H overlay) revealed abundant 19F signal from intact NP that were localized heterogeneously in the plaque interstitium (Fig. 1A) but not in unaffected areas. The average tissue concentration of NP calculated from MR spectroscopy (Fig. 1B) was 2.36 ± 0.42 × 109 /g aorta. The accumulation of NP is distinct from macrophage uptake, as demonstrated by high resolution fluorescence microscopy (Fig. 1C). Fluorescence imaging (Fig. 1D) also confirmed the presence of NP. In human carotid arterectomy tissues, the detected 19F signals were primary located on the endothelial/luminal side (Fig. 2). Quantitative analysis showed that average NP concentration in carotid arterectomy tissue was 47.1 ± 18.3 × 109 /g tissue.