The data collected during this study (ultrasonic backscatter and histologically processed carotid plaque) will provide the basis for a novel algorithm to add plaque composition information to the plaque size and location information that is currently provided by standard ultrasound imaging. The input parameters for the algorithm are derived from two types of non-invasive ultrasound data: quantitative ultrasound (QUS) and acoustic radiation force impulse (ARFI) imaging data. QUS analyzes the frequency content of the backscattered diagnostic ultrasound signals. These are the same signals currently used for imaging, however, imaging relies solely on the strength of the signal to form the image and ignores the spectral information. Thus QUS is an approach to use the spectral information which is sensitive to the number and nature of the scatterers. In contrast ARFI techniques are provide information on the stiffness and shear modulus of the tissue. Two ARFI techniques will be used to gather information on the carotid plaque and related tissue. ARFI is based on using a ultrasonic push pulse to slightly move the tissue (displacement on the order of microns). From this initial start two types of ARFI data are collected: image of the relative displacements of the tissue (ARFI image) and the measurement of the speed of the shear wave created by the tissue displacement (shear wave speed).