Measurement of absolute energy spectra from a clinical CT machine under working conditions using a Compton spectrometer

Absolute measurements of photon energy spectra (keV-1 sr-1 mA-1 s-1) from a clinical CT machine have been performed under normal working conditions (140 mA tube current) using a Compton spectrometer. The inaccuracy of the measured spectra is estimated to be +/- 6%, determined by uncertainties in dead-time corrections and in the parameters of the geometrical set-up. Absorbed doses measured in thermoluminescent LiF dosimeters agree within this uncertainty with calculated ones derived from measured spectra (80 kVp, 120 kVp and 140 kVp) and tabulated mass energy absorption coefficients for LiF. Comparison with tabulated energy spectra from the literature clearly shows the effect of the extremely small anode angle (7 degrees) in the CT machine (15 degrees and 17 degrees for the tabulated energy spectra).