CTDI represents

CT Dose Index (CTDI): It is a standardized measure of radiation dose output of a CT scanner which allows the user to compare the radiation output of different CT scanners. In CT the computed tomography dose index (CTDI), and the dose-length product (DLP) are used. These operational dose qualities are often stored together with patient information in the digital archives like PACS and thus the radiation exposure of individual patients may be retrieved afterward.