Fig 1. Maximum intensity projection (A) and sagittal reformat (B) of a CT angiogram reveal left internal carotid artery (ICA) stenosis with severe calcification. The entire circumference of the ICA (C) and the circumference of the calcified plaque (D) on the axial source image are measured at the narrowest portion of the ICA. The arc (degree) of the calcification is calculated as: arc = external circumference of the calcified plaque/external circumference of the carotid artery x360°.