CT dose index
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CT dose index (CTDI) (measured in mGy) is a standardized measure of radiation dose output of a CT scanner which allows the user to compare radiation output of different CT scanners. In the past CTDI100 (measured over a 100 mm long ionization chamber) and CTDIw (weighted average of dose across a single slice) were used; for helical scanners in current use, the parameter CTDIvol is the more commonly used index.
- CTDI100 (mGy) is a linear measure of dose distribution over a pencil ionization chamber and hence does not take into consideration the topographical variation of a human body and is therefore not in clinical use.
CTDIw (mGy) is closer to the human dose profile as compared with the CTDI100
- 2/3 CTDI100 (periphery) + 1/3 CTDI100 (center)
- CTDIvol (mGy) is obtained by dividing CTDIw by pitch factor.
Another commonly used index is the dose-length product (DLP) which factors in the length of the scan to show overall dose output
- DLP (mGy*cm) CTDIvol x scan length
It is important to remember that the CTDIvol or its derivative the DLP, as seen on consoles and outputted on the DICOM images, do not represent the actual absorbed or effective dose for the patient. They should be taken as an index of radiation output by the system for comparison purposes.
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