Arterial stiffness and haemorrhage in the vulnerable carotid plaque

Background Magnetic resonance imaging of carotid plaque haemorrhage (MRIPH) has recently been shown to be more superior than the degree of stenosis in predicting ischaemic stroke. Recently, mechanical factors such as arterial stiffness have also been suggested to be associated with stroke. Pulse wave velocity (PWV) is a non-invasive imaging technique to assess arterial stiffness and studies have shown that aortic PWV is an independent predictor of cardiovascular and cerebrovascular morbidity as well as mortality. The aims of this thesis were to test the feasibility of assessing carotid PWV using magnetic resonance imaging (MRI), to examine the association between carotid PWV and the degree of stenosis as well as the association between the status of MRIPH and carotid PWV. Methods 29 patients (55 carotid arteries) with at least 50% unilateral carotid artery stenosis and 8 healthy volunteers (16 carotid arteries) were included in the study. PWV was derived from cine phase contrast using MRI at 3 Tesla. Carotid PWV was compared between the two groups, the relationship between carotid PWV and the degree of carotid stenosis as well as the presence of MRIPH were examined. All multiple linear and logistic regression analyses were adjusted for age and blood pressure. Results Repeatability for the measurement of carotid PWV was good (Cronbach's Alpha=0.703; Bland-Altman plot bias=0.25, upper and lower limits of agreement=+4.23, -3.74; n=16). Similarly, there was good intra-observer consistency (Cronbach's Alpha=0.747; Bland-Altman plot bias=0.06, upper and lower limits of agreement=+1.90, -1,78; n=13). The mean difference in transit time with and without static phantom correction for background velocity was 0.12 ms, P=0.4. There was no significant difference in carotid PWV in healthy volunteers (median [IQR]: 4.1 m/s [2.8-5.60]) and patients with carotid artery stenosis (70-99%: 3.5 m/s [1.4-3.5], 50-69%: 4.9 m/s [2.5-5.3], <30%: 4.5 m/s [2.3-5.8]). There was also no association between carotid PWV and the degree of carotid stenosis (R2=0.055, P=0.943). However, carotid PWV was significantly associated with increasing age group (R2=0.35, P=0.001). None of the blood pressure parameters were significantly associated with carotid PWV (systolic blood pressure, R2=0.047, P=0.07; diastolic blood pressure, R2=0.007, P=0.5; pulse pressure, R2=0.038, P=0.1; and mean arterial blood pressure, R2=0.028, P=0.1). There was a trend for higher PWV in carotids with MRIPH+ (4.14 m/s [1.9-4.7] versus 3.0 m/s [1.9-4.7], P=0.09), but it was insignificant. However, MRIPH+ carotid arteries were significantly associated with higher carotid PWV after adjusting for age, blood pressure and the degree of carotid stenosis (OR 1.22, 95% CI 1.02-1.47, P=0.03). This association was maintained even after exclusion of high grade carotid stenosis due to potential error in image acquisition in this group (OR 1.21, 95% CI 0.99-1.48, P=0.05). Furthermore, symptomatic carotid stenosis was found to have higher value of carotid PWV compared to asymptomatic ones (OR 1.39, 95% CI 1.0-1.8, P=0.007). Conclusion This study has demonstrated the feasibility of assessing PWV in the carotid arteries with MRI. However, more work is needed to optimise the temporal and spatial resolution of the MRI sequence and to validate the technique. The association between MRIPH and carotid PWV requires further study with a larger cohort of symptomatic patients.