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Measurement of CTDI in a CT scanner

CTDI measurement with a 10cm long CT chamber:

   I have been looking for the formula and equation in calculating the CTDI

when using a 10cm long CT chamber(the detector cert only specifies that it

is NIST calibrated). I have checked the detector calibration with Cs137

point sources that the 10cm long CT chamber should be 100% fully and

uniformly irradiated all along the detector in order that the dose monitor

unit would read the dose or dose rate accurately. In my understandings that

any sub-volume within the chamber should either have uniform charge or

charge concentration.

     But in the CTDI measurement, say a 5mm slice or 10mm slice, the

irradiation of the chamber volume is roughly 5% or 10% respectively which

means that the chamber is only partially exposed and the dose or dose rate

readout will not correspond to the actual calibration condition. Of course

the rough estimation is to multiply the measurement reading by 20 or 10

respectively, but I'm not sure what is the exact formula in the actual

calculation. Usually I come across with others who may irradiate the 10cm

long chamber sequentially all along the chamber with sufficient CT slices

irradiation to cover it up, however that becomes the MSAD (Multiple slice

average dose) which is no longer the actual definition of CTDI.

     I also got some unknown parameters known as Ks in CTDI calculation

which varies from 0.29 (1mm slice) to 0.87(5mm slice) in calculation within

a body/head phantom ?

     Thank for any helpful guidelines.

















John Lam, Physicist

Eastern Hospital, Hong Kong

email : lamhc@hkusau.hku.hk

Tel : 852-25954075



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