The goal of this observational study is to validate the diagnostic performance of photon counting/spectral CT to distinguish tissue components in ischemic stroke with MRI, semi-automated segmentation software or histology as the reference standard. The main question\[s\] it aims to answer are: 1. Presence or absence of calcium, hemorrhage, and lipid plaque components as determined on CT will be confirmed with carotid MRI, segmentation quantitative (e.g., volume) output from segmentation software, or histologic staining. 2. Volumes of hemorrhage, lipid, and calcium components will be measured to determine plaque phenotype and correlated with ipsilateral intracranial stroke burden. Participants will be asked to do the following: Aim 1: * 1 Photon counting/spectral CT angiography of the neck * 1 Research Carotid/Neck MRI of the carotid arteries (if eligible) Aim 2: * 1 Research Carotid/Neck MRI of the carotid arteries (if eligible) * 1 Photon counting/spectral CT angiography of the neck * 1 Carotid plaque tissue from participants surgery will be retrieved, imaged, and stained (if eligible)