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15 March 2010

Answer to Question #8455 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

Which modality has the highest leakage radiation—fluoroscopy (100 mR h^-1 at 1 meter) or CT?
Is CT dose measured by CTDI, CTDIvol, or effective dose?

All x-ray tubes are restricted to a leakage radiation limit of <100 mR h^-1 at 1 meter from the tube housing.
With the advent of MDCT (multidetector CT), adding table translation into the equation, we now have to consider "pitch" as a dose contributing factor, either overscanning (pitch <1) or underscanning (pitch >1). Without pitch, the CT dose is CTDI(w), which is just a combination of the (peripheral exposure)/3 + 2(central exposure)/3. But with helical scanning the CT dose is CTDI(w)/pitch.

Kishor M. Patel, PhD, DABR

Answer posted on 18 August 2009. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.