CT dose index

Dr Henry Knipe and Dr Usman Bashir et al.

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 CTDI(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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Article information

rID: 18981
Section: Physics
Tags: ctdi, dlp
Synonyms or Alternate Spellings:
  • CTDIvol
  • CT dose index (CTDI)
  • CTDI

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