Improving the diagnosis and treatment of atherosclerosis: targeting calcification as a basis for patient management

The presence of atherosclerotic calcification serves as a surrogate marker for plaque burden and a prognostic marker of cardiovascular risk. Additionally, calcification plays a critical role in plaque stability and heavily calcified lesions are associated with increased transcatheter therapy failures. The purpose of this thesis is to advance calcification-based cardiovascular patient management. Specifically, an improved understanding of the capabilities of clinical image- and blood-based biomarkers to distinguish between high risk patients and high-risk plaques based on calcified content is required. Additionally, more accurate stiffness properties for calcified and non-calcified tissue constituents will optimise computational predictions of plaque rupture and device-tissue interactions. Calcifications within ex vivo atherosclerotic lesions were quantified using micro computed tomography (micro-CT), clinical CT, magnetic resonance (MR), intravascular ultrasound (IVUS) and optical coherence tomography (OCT). Coronary artery calcium (CAC) scores were obtained from non-contrast chest CT scans. Circulating blood biomarkers of vascular calcification were measured using commercial immunoassays. Nanomechanical techniques were employed to characterise the stiffness of calcified and non-calcified tissue portions. These biomechanical techniques were coupled micro-CT, scanning electron microscopy, energy dispersive x-ray spectroscopy and histological analyses to confirm the biological content of the regions of interest being examined. Agatston calcium scores correlate well with calcification volumes and are therefore good markers of atherosclerotic burden. However, calcified particle distributions are not estimated and larger calcified particles have higher maximum x-ray attenuation densities. An assessment into the effect of decreasing CT resolution on measures of calcification revealed the inaccuracies acquired for calcification volume, density and particle measurements. Additionally, a heterogeneous distribution of calcium density was identified. The efficacy of coronary-derived calcium scores or the fraction of low- or high-density calcium to differentiate between symptomatic or asymptomatic carotid plaques was investigated. Neither the Agatston, Volume or Density-Volume coronary calcium scores could differentiate between carotid plaques based on patient preoperative cerebrovascular symptoms. However, asymptomatic plaques contained significantly lower levels of low-density calcification and higher levels of high-density calcification. Clinical CT also exhibited the closest correlation to micro-CT for measures of calcified content. No differences were observed between circulating blood-biomarkers of vascular calcification or bone formation with patient endarterectomy or number of diagnosed atherosclerotic locations groups. Moderate and weak negative associations were observed between dephospho-uncarboxylated Matrix Gla Protein and coronarty artery calcium (CAC) density scores, and between percent undercarboxylated osteocalcin and CAC scores or total volume of calcification for certain participant subgroups only. There is a clear distinction between the elastic modulus of calcification with respect to radiographic density. Furthermore, there is no difference in the behaviours of carotid versus lower extremity calcification. This study confirms the hypothesis that the mechanical properties of calcification are similar to that of human bone tissues (17–25 GPa). Moreover, greater than 6 orders of magnitude compliance mismatch exists between the calcified and non-calcified portions of carotid atherosclerotic plaques. Collectively, this research demonstrates an improved understanding of the use of atherosclerotic calcification measurements in clinical diagnostics and the biomechanics of advanced calcified atherosclerotic plaques.