Hello there, I’m employed at Big Sky Medical Center situated in Big Sky, Montana. My current quest involves obtaining a CT Dose chart that outlines the DLP and CTDI ranges for the average patient dimensions across various body regions. My intention is to display this chart in our CT Tech section for their reference. Interestingly, the ACR didn’t have precisely what I needed and actually suggested reaching out to you. I greatly appreciate your time and assistance. Regards, Mike
CTDI (CT Dose Index) and DLP (Dose Length Product) are measures of the amount of radiation output from the x-ray tube during a CT scan. Larger patients require more radiation output than small patients to achieve the same level of image quality. Therefore, CTDI and DLP should increase with patient size when the CT scanning is performed properly. CTDI and DLP are not measures of patient dose (how much radiation the patient absorbed). Even though larger patients require more CTDI, the absorbed dose for patients of all sizes when averaged across their body thickness should be about the same when the CT scanning is done correctly.
The medical physics community uses "Diagnostic Reference Levels" (DRLs) to help imaging facilities determine if radiation doses are within a normal range as determined by national surveys. For the CT dose metrics CTDIvol and DLP, the DRLs are a function of patient size. The DRLs for 10 common adult and pediatric CT examinations in the United States have been published:
These values will eventually need to be updated to reflect changes in technology and practice.
It is worth noting that all CT scanners built since ~2005 have the ability to automatically adapt the radiation output to patient size using an approach called "automatic tube current modulation" which estimates the patients' size by analysis of the localizer images (aka scanned projection radiographs). Because of this, it is rarely necessary for CT technologists to choose the CTDI and DLP that will be used for a particular CT exam. It is done automatically to achieve consistent image quality.