Ischaemic stroke is a major cause of death and disability worldwide. In patients with recent stroke, the 18F-fluoride positron emission tomography-computed tomography highlights high-risk culprit carotid plaque and is more discriminatory than 18F-fluorodeoxyglucose. Using hybrid positron emission tomography-magnetic resonance imaging investigators propose to build upon these findings by prospectively assessing 18F-fluoride uptake in a broad range of patients with acute transient ischaemic attack or ischaemic stroke. Investigators will specifically examine the association of 18F-fluoride uptake with multiparametric magnetic resonance imaging assessments of atherosclerotic plaque, especially the role of thrombus and lipid. Finally, using transcranial Doppler and diffusion-weighted magnetic resonance brain imaging, an assessment of the functional consequences of 18F-fluoride-positive atherosclerotic plaque will be performed. If successful, this technique has a number of valuable translational applications including the better selection of patients for carotid intervention.

The ability to identify the culprit carotid plaque represents a key goal in carotid artery imaging. Although an array of non-invasive imaging techniques can detect a wide spectrum of complementary high-risk characteristics, no single modality can reliably identify vulnerable plaques associated with future stroke development. Substantial histological data suggests that specific plaque components identify patients at high-risk for future ipsilateral stroke and cardiovascular events. This implies that investigators need to look beyond the traditional paradigm where the basis for carotid endarterectomy were formulated by an invasive imaging modality that provided no information on the arterial wall composition. Alternative imaging strategies are therefore required targeting not only in vivo carotid morphology but also plaque biology and disease activity. This is fundamental to optimal risk-stratification and appropriate selection of patients for high-risk vascular intervention. One new approach is to use non-invasive molecular imaging targeted at plaque biology using hybrid systems such as positron emission tomography-magnetic resonance imaging.

Inclusion Criteria:

1. Patients above 40 years of age with carotid artery stenosis that do not achieve criteria for CEA (<50% for men, <70% for women, by NASCET criteria) or the patient has declined CEA.
2. Patients above 40 years of age with atherosclerotic disease of aortic arch and its branches.

Exclusion Criteria:

1. Patients with new stroke and a modified Rankin score >3
2. Chronic kidney disease with an estimated Glomerular Filtration Rate (eGFR) of <30 ml/min/1.73 m2
3. Atrial fibrillation
4. Pregnant women
5. Prior ipsilateral carotid intervention
6. Prior neck radiotherapy
7. Inability to tolerate the supine position
8. Participation in the study would result in delay to surgery
9. Psychiatric illness/social situations that would limit compliance with study requirements
10. History of allergic reaction attributed to 18F-Fluoride
11. History of allergic reaction to gadolinium contrast media
12. Metal implants or devices including pacemakers and defibrillators

Two cohorts of patients with evidence of an acute stroke, transient ischaemic attack or amaurosis fugax will be recruited as early as possible, but within 14 days of symptom onset. All patients will undergo careful clinical evaluation including carotid Doppler ultrasound assessment and magnetic resonance imaging of the brain.