Answer to Question #13565 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:


Please explain the displayed CTDI-vol and DLP in a CT scan.


All modern computed tomography (CT) scanners display a value called CT dose index-volume (CTDI-vol), in units of milliGray (mGy). CTDI-vol is calculated by the scanner based on the radiation output for the particular scan. Since most modern scanners automatically adjust output based on the size and density of the patient, this will vary patient to patient.

The CTDI-vol is based on the radiation dose which would be absorbed by a specific size phantom, either 16 or 32 cm. It is not the dose to the specific patient, but it is meant to be a comparison metric for different scans or scanners. Accreditation requires sites to measure the accuracy of the calculated CTDI-vol, by actually measuring it with an ion chamber and phantom.

The dose-linear product (DLP) is calculated by multiplying the CTDI-vol, which is given for a centimeter (cm), by the number of centimeters scanned. It is given in units of mGy-cm.


Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 4 August 2020. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.