Inflammation is important in both the pathogenesis and outcome of atherosclerosis. Plaques containing numerous inflammatory cells, particularly macrophages, have a high risk of rupture, whereas those with fewer inflammatory cells are at lower risk. The current ‘gold standard’ technique for imaging atherosclerosis is x-ray contrast angiography, which provides high-resolution definition of the site and severity of luminal stenoses, but no information about plaque inflammation. Quantification of plaque inflammation is desirable both to predict risk of plaque rupture and to monitor the effects of atheroma-modifying therapies. This is important since recent studies strongly suggest that HMG Co-A reductase inhibitors promote plaque stability by decreasing plaque macrophage content and activity without substantially reducing plaque size and therefore angiographic appearance. FDG is a glucose analogue that is taken up by cells in proportion to their metabolic activity. In this work, the central hypothesis was that plaque inflammation could be visualised and quantified non-invasively using FDG-PET. Initially, THP-1 monocytes and buffy-coat macrophages were stimulated with cellular activators, and the effect on deoxyglucose uptake was observed. It was demonstrated that both types of cell accumulated deoxyglucose in proportion to their metabolic activity. Next, FDG uptake was assessed in endarterectomy specimens from patients with symptomatic carotid disease. Autoradiography of excised plaques confirmed accumulation of deoxyglucose in macrophage-rich areas. Subsequently, co-registered FDG-PET imaging was performed in patients with transient ischaemic attack. FDG accumulated within carotid plaques, with significantly more FDG being taken up into symptomatic plaques than contralateral asymptomatic lesions. Finally, a rabbit model of atherosclerosis was established to investigate two related questions: firstly, whether an animal PET scanner (MicroPet) might detect atheroma, and secondly whether FDG-PET could image and perhaps quantify both atheroma progression and regression. Aortic atheroma was identified by FDG-PET, but full quantification was not possible, because the microPet system is currently unable to perform studies with attenuation correction. In summary, it has been shown, both in vitro and in vivo, that inflammation within atherosclerotic plaques can be successfully imaged by FDG-PET. In addition, pilot data from an experimental study of atherosclerosis in rabbits suggested that serial imaging with this technique might be useful for monitoring the effects of anti-atheroma drugs.

The non-invasive cardiovascular imaging modalities, cardiovascular magnetic resonance (CMR) and multi-detector computer tomography (MDCT) are playing an increasing role in both clinical and research settings. CMR is a unique imaging modality due to unsurpassed contrast between soft tissue structures that is non-invasive, does not use ionising radiation and is able to provide high-resolution information about cardiac anatomy, function, flow, perfusion, viability and metabolism. It has provided the gold standard in imaging in congenital heart disease. Recent advances in this technology have led to images of high spatial and temporal resolution that has made the characterisation of atheroma possible. While currently spatial resolution still limits its ability to characterise atheroma in native human coronary arteries in living patients, CMR imaging of the coronary arteries has future potential with further technological and sequence advances. MDCT has been used in clinical settings to measure of the amount of calcification in the coronary arteries with “coronary artery calcium scoring” of the coronary tree a surrogate marker of atherosclerosis. MDCT has also become the gold standard for angiographic imaging in most arterial beds such as the carotid and peripheral vascular systems. In the coronary arteries in particular, there have been major advances in the accuracy of coronary MDCT angiography, particularly with regards to its negative predictive value, although excessive calcification and blooming artefacts still limit the diagnostic accuracy of the technique for assessing stenotic severity. In this thesis, our aims were to address some specific novel areas advancing the utility of these imaging modalities in two major areas of interest, namely congenital heart disease and atheroma imaging. Our first step was to validate the accuracy and reproducibility of CMR, the main imaging modality we utilised. To achieve this, we assessed MR imaging of cardiac volumes and function in a normal adult Australian population with a specific focus on the reproducibility of the technique. In confirming that this technique in our hands is both accurate and reproducible, we would then be in a position to be able to confidently use this technique in our future chapters. However, more than this, we sought to establish some normal ranges for left and right atrial and ventricular parameters in our local population. This would be crucial background information for us to be able to make comparisons with future studies in patients with congenital heart disease. Having established our technique and reference ranges, we would then explore the two specific issues in the ensuing two chapters using CMR in one area of congenital heart disease, atrial septal defect. Atrial septal defect is the most common congenital heart defect first diagnosed in adults. The traditional method of assessment of these patients and for suitability for ASD closure involves semiinvasive investigation with transoesophageal echocardiography (TOE) for measurement of the defect size and atrial septal margins. MRI assessment of patients prior to percutaneous device closure compared to TOE assessment would provide information on the accuracy of TOE assessment and provide information of the utility of cardiac MRI as an alternative to TOE for the work-up of these patients prior to ASD closure. In our third original research chapter, we utilised CMR to understand the effects of percutaneous ASD closure on cardiac chamber volumes. We achieved this by assessing with cardiac MRI pre-closure and post-closure atrial and ventricular cardiac volumes. Longstanding right heart dilatation in the setting of an ASD may lead to complications including right heart failure, pulmonary hypertension and arrhythmia. Closure of the ASD should reduce right heart volumes by removing left-to-right shunting and lead to normalisation of ventricular volumes. The assessment of atrial volume changes with ASD closure may be important in furthering our understanding in its contribution to arrhythmia. Having assessed the ability of CMR to assess both the ASD dimensions, and therefore suitability for percutaneous closure, as well as the effects of ASD closure on cardiac chamber size, we look in the final two original research chapters to move to another area of research development with these highresolution imaging technologies, atherosclerosis imaging. Two particular areas we wished to focus on included the potential of high-resolution MR imaging to monitor effects of HDL infusion on atherosclerosis, and secondly to explore mechanisms behind limitations in MDCT imaging of atherosclerosis, specifically calcification and blooming artifacts. For assessing the effects of HDL infusion on atherosclerosis, we utilised a cholesterol-fed rabbit model of atherosclerosis. The abdominal aorta of the rabbit is comparable in size to the human coronary artery. Previous work with the rabbit model of atherosclerosis and magnetic resonance imaging of the aortic wall has shown that it can provide information about atherosclerotic composition as well as provide serial data of the arterial wall. While high intensity lipid-lowering with statins remains the first line management of at risk individuals, modest manipulations of serum HDL levels are associated with a significant impact on cardiovascular risk. Thus, we assessed the effect of HDL infusion and atorvastatin in a rabbit model of using MRI aortic atherosclerosis as the endpoint. In our fifth and final original research chapter, we assessed the accuracy of quantification of atherosclerotic calcification with MDCT in the carotid arteries of patients undergoing carotid endarterectomy, and sought to identify algorithms or techniques that may improve quantification of calcification. This would potentially lead to an improvement in the ability of MDCT techniques to quantify stenotic severity in coronary arteries that were calcified. To achieve these we utilised MDCT in vivo and in comparison with carotid endarterectomy specimen micro-CT. Importantly, as part of this study, we undertook a thorough assessment of reproducibility of these techniques. Thus, in summary, we have been able to confirm the accuracy and reproducibility of CMR and MDCT in the areas of a specific congenital defect (ASD) and atherosclerosis imaging, and utilised these techniques to advance our understanding of these disease states. This thesis identifies strengths and weaknesses of these techniques that will allow us to more appropriately use them for future purposes in cardiovascular disease. Future work directly stemming from this thesis has already begun, and now looks to address issues of whether CMR and MDCT may provide complimentary information about atherosclerotic lesions that may benefit outcomes in certain conditions. Specifically the work in this thesis has led to studies commencing in carotid atherosclerosis and saphenous vein graft atherosclerosis and using these imaging techniques to potentially predict adverse future outcomes. / Thesis (Ph.D.) -- University of Adelaide, School of Medical Sciences, 2008

The non-invasive cardiovascular imaging modalities, cardiovascular magnetic resonance (CMR) and multi-detector computer tomography (MDCT) are playing an increasing role in both clinical and research settings. CMR is a unique imaging modality due to unsurpassed contrast between soft tissue structures that is non-invasive, does not use ionising radiation and is able to provide high-resolution information about cardiac anatomy, function, flow, perfusion, viability and metabolism. It has provided the gold standard in imaging in congenital heart disease. Recent advances in this technology have led to images of high spatial and temporal resolution that has made the characterisation of atheroma possible. While currently spatial resolution still limits its ability to characterise atheroma in native human coronary arteries in living patients, CMR imaging of the coronary arteries has future potential with further technological and sequence advances. MDCT has been used in clinical settings to measure of the amount of calcification in the coronary arteries with “coronary artery calcium scoring” of the coronary tree a surrogate marker of atherosclerosis. MDCT has also become the gold standard for angiographic imaging in most arterial beds such as the carotid and peripheral vascular systems. In the coronary arteries in particular, there have been major advances in the accuracy of coronary MDCT angiography, particularly with regards to its negative predictive value, although excessive calcification and blooming artefacts still limit the diagnostic accuracy of the technique for assessing stenotic severity. In this thesis, our aims were to address some specific novel areas advancing the utility of these imaging modalities in two major areas of interest, namely congenital heart disease and atheroma imaging. Our first step was to validate the accuracy and reproducibility of CMR, the main imaging modality we utilised. To achieve this, we assessed MR imaging of cardiac volumes and function in a normal adult Australian population with a specific focus on the reproducibility of the technique. In confirming that this technique in our hands is both accurate and reproducible, we would then be in a position to be able to confidently use this technique in our future chapters. However, more than this, we sought to establish some normal ranges for left and right atrial and ventricular parameters in our local population. This would be crucial background information for us to be able to make comparisons with future studies in patients with congenital heart disease. Having established our technique and reference ranges, we would then explore the two specific issues in the ensuing two chapters using CMR in one area of congenital heart disease, atrial septal defect. Atrial septal defect is the most common congenital heart defect first diagnosed in adults. The traditional method of assessment of these patients and for suitability for ASD closure involves semiinvasive investigation with transoesophageal echocardiography (TOE) for measurement of the defect size and atrial septal margins. MRI assessment of patients prior to percutaneous device closure compared to TOE assessment would provide information on the accuracy of TOE assessment and provide information of the utility of cardiac MRI as an alternative to TOE for the work-up of these patients prior to ASD closure. In our third original research chapter, we utilised CMR to understand the effects of percutaneous ASD closure on cardiac chamber volumes. We achieved this by assessing with cardiac MRI pre-closure and post-closure atrial and ventricular cardiac volumes. Longstanding right heart dilatation in the setting of an ASD may lead to complications including right heart failure, pulmonary hypertension and arrhythmia. Closure of the ASD should reduce right heart volumes by removing left-to-right shunting and lead to normalisation of ventricular volumes. The assessment of atrial volume changes with ASD closure may be important in furthering our understanding in its contribution to arrhythmia. Having assessed the ability of CMR to assess both the ASD dimensions, and therefore suitability for percutaneous closure, as well as the effects of ASD closure on cardiac chamber size, we look in the final two original research chapters to move to another area of research development with these highresolution imaging technologies, atherosclerosis imaging. Two particular areas we wished to focus on included the potential of high-resolution MR imaging to monitor effects of HDL infusion on atherosclerosis, and secondly to explore mechanisms behind limitations in MDCT imaging of atherosclerosis, specifically calcification and blooming artifacts. For assessing the effects of HDL infusion on atherosclerosis, we utilised a cholesterol-fed rabbit model of atherosclerosis. The abdominal aorta of the rabbit is comparable in size to the human coronary artery. Previous work with the rabbit model of atherosclerosis and magnetic resonance imaging of the aortic wall has shown that it can provide information about atherosclerotic composition as well as provide serial data of the arterial wall. While high intensity lipid-lowering with statins remains the first line management of at risk individuals, modest manipulations of serum HDL levels are associated with a significant impact on cardiovascular risk. Thus, we assessed the effect of HDL infusion and atorvastatin in a rabbit model of using MRI aortic atherosclerosis as the endpoint. In our fifth and final original research chapter, we assessed the accuracy of quantification of atherosclerotic calcification with MDCT in the carotid arteries of patients undergoing carotid endarterectomy, and sought to identify algorithms or techniques that may improve quantification of calcification. This would potentially lead to an improvement in the ability of MDCT techniques to quantify stenotic severity in coronary arteries that were calcified. To achieve these we utilised MDCT in vivo and in comparison with carotid endarterectomy specimen micro-CT. Importantly, as part of this study, we undertook a thorough assessment of reproducibility of these techniques. Thus, in summary, we have been able to confirm the accuracy and reproducibility of CMR and MDCT in the areas of a specific congenital defect (ASD) and atherosclerosis imaging, and utilised these techniques to advance our understanding of these disease states. This thesis identifies strengths and weaknesses of these techniques that will allow us to more appropriately use them for future purposes in cardiovascular disease. Future work directly stemming from this thesis has already begun, and now looks to address issues of whether CMR and MDCT may provide complimentary information about atherosclerotic lesions that may benefit outcomes in certain conditions. Specifically the work in this thesis has led to studies commencing in carotid atherosclerosis and saphenous vein graft atherosclerosis and using these imaging techniques to potentially predict adverse future outcomes. / Thesis (Ph.D.) -- University of Adelaide, School of Medical Sciences, 2008