The overall goal of this project is to improve the predictive power of imaging studies that are performed to select patients for endarterectomy surgery. Endarterectomy surgery has been shown to be more beneficial than using medical intervention in treating patients with carotid artery stenosis determined on the basis of catheter injected X-ray angiography criteria alone. However, the predictive power of those studies is relatively poor. This project is intended to determine which of the current standard imaging modalities predicts the degree and extent of stenosis that best correlates with the true extent of atherosclerotic disease. The primary aim of this project is therefore to determine the accuracy of the three major Imaging methods currently used to assess stenosis at the carotid bifurcation, namely Doppler Ultrasound Catheter Angiography, and Magnetic Resonance Angiography. The gold standard by which these modalities will be evaluated will be the endarterectomy specimen removed en bloc at surgery. All patients will be examined with each of the three modalities prior to surgery. High resolution 3-D Magnetic Resonance images of the excised specimen will be obtained to determine the actual degree of stenosis for each patient. The high resolution data will be analyzed to determine the reduction of lumenal cross-sectional area at the location of the most severe stenosis. The agreement of each Imaging study with measurement from the specimen will be determined. The trial plans to accrue 300 cases in which all three imaging studies have been acquired within 2 months prior to surgery. Several institutions will participate in this trial in order to ensure that the requisite number of subjects can be recruited to the trial, and data analysis can be completed within three years of the study initiation. The results of this study will determine which descriptors extracted from the imaging studies provide, in combination or alone, the most accurate prediction of carotid artery stenosis. A more accurate assessment of carotid artery stenosis will provide improved selection of patients for carotid endarterectomy surgery. This will reduce the number of surgeries that are performed on patients who would otherwise be incorrectly assessed as being at increased risk for stroke. At the same time, it will ensure that disease in patients who will benefit from endarterectomy surgery is not underestimated. Improvements in the predictive value of diagnostic imaging will result in significant savings to the health care system by reducing unnecessary procedures, and by more appropriately selecting patients for surgery.