Imaging Characteristics of Carotid Plaques by Doppler Ultrasonography and Magnetic Resonance Imaging in Ischemic Stroke: A Comparative Analysis

ABSTRACT

Background: Carotid plaques contribute a significant cause of stroke and transient ischemic attacks together with long term disability worldwide. About 20-30% of cerebral infarction has been correlated with carotid atherosclerotic plaque and artery stenosis. The characterization of carotid artery plaque presents an opportunity to quantify patients with risk of cerebrovascular events and may be used to improve the therapeutic decision-making process such as carotid endarterectomy or angioplasty or stent placement. This study attempted to evaluate the imaging characteristics of carotid plaques using ultrasonography with Magnetic Resonance Imaging correlation and predict the risk of plaque rupture based on plaque enhancement characteristics as a surrogate marker. Materials and methods: Aim of the study was to evaluate and compare morphological characteristics of carotid plaques with Doppler ultrasonography and MRI and to predict plaque enhancement characteristics on MRI as a potential surrogate marker for plaque rupture leading to recurrent strokes. The present study evaluated 113 patients (65 males and 48 females) with stroke and transient ischemic attacks with ultrasound documented carotid plaques. These cases underwent Doppler evaluation of the carotid plaques and contrast enhanced MRI on the same day. The plaque morphology was evaluated both in the longitudinal as well as the transverse axis, and the stenosis was calculated based on the pulsed- wave Doppler evaluation of blood flow velocity as well as the area and diameter of the stenosis together with the signal and enhancement characteristics of the carotid plaques on MRI. The carotid artery at the region of maximal intimal thickness was evaluated for maximal area stenosis and diameter stenosis. The area stenosis was calculated as percentage stenosis in axial sections at the site of maximal thickness of the plaque. Results: The maximum incidence of carotid plaques was noted in the 61-80 years age group. The incidence was maximal at 71-80 years of age. Out of the 113 lesions, 65 (57.5%) were in men and 48 (42.4%) were in women. 44(38.9%) lesions were in patients who presented with TIA, while 69 (61.06%) presented with Ischemic stroke. 67 (59.29%) lesions were found on the left side and 46 (40.7%) on the right side. 67(59.3 %) lesions were found in the carotid bulb. The intimal thickness ranged from 1.1 mm to 5.6 mm with a mean of 2.97 mm. The range of stenosis was from 0% to 100%. Heterogeneous plaques were associated with symptomatic lesions in 76.2 % and homogeneous plaques were seen in 23.8%. The incidence of plaque calcification was inversely proportional to symptomatic plaques. Only 47.2 % of patients showed concordance between US and MRI with regard to homogeneity of the plaque. T2 weighted MR imaging of exvivo atherosclerotic plaques aided in the detection and evaluation of fibrous caps. 39 (34.1%) patients were given a gadolinium-based contrast agents, out of which 19 (48.7 %) did not show enhancement and 20 (51.2%) showed enhancement of the carotid plaque tissue. Doppler showed abnormalities in 29 patients (61.7%), whereas in MRA it is 18 patients (38.2 %), p value = < 0.004* in cases with less than 50% stenosis (n=47). In cases with 50-69% stenosis (n=29), doppler showed abnormalities in 19 patients (65.5%) whereas in MRA it was (34.4 %); P value was 0.431, The difference was not found to be statistically significant. While as in cases with 70-90% stenosis (n=37), MRA showed abnormalities in 25 patients (67.5%) whereas Doppler showed it in 12 (32.4%); P value was 0.017. Calcification was seen in 22 patients which appeared as hypointense focus on T1, T2, and TOF images. The heterogeneous lesions were better visualized on ultrasound compared to MRI. Conclusion: Ultrasound is a more sensitive modality than MRI for plaque size < 1.5 mm and a better tool in assessing the plaque characteristics when the area of stenosis is less than 50%. Hence, ultrasound is better as a screening tool. The homogenous lesions on ultrasound appeared homogenous on MRI with the plaque content of fat. The heterogeneous plaque on ultrasound, however, did not correlate with MRI. Enhancement of carotid plaque tissue, which implies vascular wall inflammation, is a marker of vulnerable plaque. MRA has a better discriminatory power compared with duplex ultrasonography in detecting 70-90% stenosis