## 3 Cardiovascular Disease
### Atherosclerotic Arterial Disease
#### 3.2.3 Imaging Atherosclerosis  
Elastin  
Taking advantage of the increased abundance of elastin within the extracellular matrix of atherosclerotic plaque compared to the normal arterial wall, a novel elastin-specific magnetic resonance contrast agent (ESMA, BMS753951) was used for the non-invasive quantification of plaque burden and arterial remodelling in a mouse and swine model of atherosclerosis [57; 58]. ESMA is a paramagnetic Gd-labelled C\({}_{32}\)H\({}_{40}\)N\({}_{7}\)O\({}_{11}\)Gd low-molecular-weight contrast agent (855.95 Da), which has high vessel wall and plaque uptake (peaking at 30 min post injection in an apolipoprotein E-deficient ApoE-/- mouse) and relatively low uptake within other tissues (e.g., heart, liver, lung and muscle). ApoE-/- mice fed a high fat diet have become a popular animal model of atherosclerosis because they reproducibly develop plaques in the aorta, aortic root and brachioccephalic arteries with little variability between animals. It is also the quickest animal model that can achieve advanced atherosclerosis within 3 months of high-fat feeding in comparison, for example, to LDLR-/- mice that require around an additional month to display the same plaque burden and features as ApoE-/- mice [59]. In an ApoE-/- mouse model fed a high fat diet (HFD), the plaque contrast to noise ratio (CNR) post ESMA injection was significantly higher at 8 weeks and 12 weeks compared with control mice. Furthermore, there was a significant reduction in the post ESMA CNR at 12 weeks in mice who were treated with pravastatin compared with mice on a high fat diet alone; demonstrating the potential for non-invasive quantitative detection of plaque elastin content and size after therapeutic treatment (Figure 6) [57]. In a swine model of arterial remodelling post stenting, significantly increased CNR was observed within the coronary arteries post ESMA injection compared with a non-targeted gadolinium agent [58]. Furthermore in a rabbit model of plaque rupture, ESMA enabled a more accurate assessment of vascular remodelling and identification of unstable plaque that rupture and form thrombus compared with gold-standard native T1 weighted black blood imaging (Figure 7) [60]. More recently, [PERSON] et al. introduced tropoelastin as a new and attractive biomarker for plaque progression and instability [61]. As elastogenesis and elastolysis favour the accumulation of tropoelastin, rather than mature cross-linked elastin, tropoelastin may serve as a more sensitive imaging biomarker of active, yet incomplete, elastogenesis to detect plaque progression and instability: MRI imaging of tropoelastin using a gadolinium labelled tropoelastin-binding contrast agent (TESMA) detected increased pathologic elastogenesis during atherosclerosis progression in ApoE\({}^{-/-}\) mice that was reduced with statin treatment. Moreover, tropoelastin MRI was more sensitive at detecting unstable plaque in a rabbit model of plaque rupture compared with ESMA [61]. This is because TESMA binds only to tropoelastin that accumulates in higher proportion in unstable plaques as a result of a dysfunctional cross-linking or elastolysis in the presence of disease, whereas ESMA binds equally to both tropoelastin and endogenously present polymeric elastin. Thus, ESMA could be used as an imaging probe to assess the net elastin increase and plaque burden, whilst TESMA could be used to selectively visualise dysfunctional elastogenesis or elastolysis (Figure 8) [61]. Figure 6: In vivo assessment of plaque burden by morphometric measurements. (**a**) Cross-sectional views of brachicephalic arteries by MRI of control and ApoE\({}^{-/-}\) mice 4, 8 and 12 weeks after the onset of HFD (n = 8 per group). High-resolution delayed-enhancement images overlaid on TOF images with corresponding sections from histology (H&E and EvG stain). (**b**) Comparison of average PAMV, calculated from morphometric measurement on high-resolution DE images after the injection of ESMA (n = 8 per group).