CT dose index
CT dose index (CTDI) 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 the most relevant one.
CTDI100 is a linear measure of dose distrubition 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 is closer to the human dose profile as compared with the CTDI100 . it is simply:
2/3 CTDI100 (periphery) + 1/3 CTDI100 (center)
CTDI vol is obtained by dividing CTDIw by pitch factor.
A point to remember is that CTDIvol or its derivative the DLP* , as seen on consoles and passed on into the DICOM image do not represent the actual dose administered to the patient; they should be taken as an index of radiation output by the system for comparison purposes.
* DLP (dose length product = CTDIvol x scan length)
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computed tomography (CT)
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CT dose
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- dose length product (DLP)
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CT artifacts
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physics-based artifacts
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