This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CT SCAN, Subject name: Radiology
CTDI represents 
Select and explain. 
A. CT densitometry index, B. CT dose index, C. CT duplication index, D. CT detection index
B: CT dose index
Answer: B: CT dose index 
Explanation: 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.