The determinants of intra-plaque neovascularisation: a study by contrast-enhanced carotid ultrasonography

Abstract

Atherosclerosis is a chronic inflammatory disorder, initiated by arterial wall injury, mediated by well-recognised cardiovascular risk factors and culminating in formation of plaques, the patho-biological substrate that precedes events such as stroke and myocardial infarction.

Intraplaque neovascularisation (IPN) is one of several defence mechanisms in response to atherosclerosis. With development of an atherosclerotic plaque within the intima, the distance between the deeper intimal layers and the luminal surface increases, producing hypoxia within the arterial wall. This stimulates release of pro-angiogenic factors that induces neoangiogenesis in an attempt to normalise oxygen tension. However, these neo-vessels are fragile, immature and leaky and thought to be the primary cause of intraplaque haemorrhage, now appreciated to be a key risk factor for plaque rupture. Therefore, the presence of IPN is now widely recognised as a precursor of the “vulnerable plaque”.

Contrast-enhanced ultrasound (CEUS) is a non-invasive method of imaging carotid plaques and, as contrast bubbles travel wherever erythrocytes travel, they permit visualization of IPN. Prior research studies have demonstrated that CEUS can detect IPN with a high degree of accuracy (on comparison with histological plaque specimens) and have shown a relationship between extent of plaque neovessels and plaque echogenicity and between plaque neovascularization and prior cardiovascular events. However, CEUS is a relatively recently described imaging technique and there were a number of unanswered questions in this field, some of which formed the basis for study in this research Thesis. In this Thesis, research studies were conducted on human subjects using CEUS imaging to identify IPN and its determinants. The incidence and determinants of IPN in healthy asymptomatic individuals was unknown and was studied in subjects from the London Life Sciences Population (LOLIPOP) study, a large study exploring mechanisms for differences in cardiovascular disease (CVD) between South Asian and European White individuals. The study found that approximately half of all plaques contain IPN. The only variable associated with IPN presence in an adjusted analysis was Asian ethnicity. This finding potentially has significant implications as it may help explain, in part, the greater CVD burden observed in Asian populations. A study comparing visualization of the carotid tree during B-mode and CEUS imaging was also conducted. Both IMT visualization and plaque detection were significantly improved by CEUS, implying that CEUS is superior to B-mode imaging for detection of sub-clinical atherosclerosis.

Radiotherapy (RT) damages arterial walls and promotes atherosclerosis. The carotid arteries frequently receive significant incidental doses of radiation during RT treatment of head and neck cancers. The effect of RT on plaque composition – specifically IPN – had not been studied and thus a collaborative cardio-oncological study was conducted to assess the effects of RT upon IPN in cancer survivors who had previously received RT. A significant association between RT and IPN was found which may provide insights into the mechanisms underlying the increased stroke risk amongst cancer survivors treated by RT.

Finally, a collaboration with biophysicists was formed to develop and validate a novel algorithm for quantitative analysis of IPN. Patients clinically scheduled to undergo carotid endarterectomy were recruited and underwent CEUS imaging prior to surgery. This study did not achieve its principal aims due to challenges with patient recruitment, challenges in image quality and with the quantification software also. Future directions of study in this promising field have been addressed in the thesis summary.