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.
Joe Och, MS, DABMP, MRSE
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