Verification of AAPM TG 116 X-Ray Beam Quality by CdTe Spectroscopy
J Chung1, C Willis2*, (1) University of Houston, Houston, Texas, (2) MD Anderson Cancer Center, Houston, TXSU-E-I-95 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose:X-rays produced by an x-ray tube comprise a polychromatic spectrum. The American Association of Physicists in Medicine Task Group 116 (AAPM TG116) defined a method for producing a beam of standard quality for the purpose of calibrating digital x-ray image receptors. The standard beam calibration condition is based on simulations of the International Electrotechnical Commission (IEC) RQA5. Our project was to measure the spectrum produced under TG 116 conditions and compare to the RQA5 spectrum predicted by simulation.
Methods:X-ray spectra were acquired using a CdTe solid state detector with a spectrum analyzer for three different x-ray beams conforming to TG116 conditions. Acquired spectra were corrected for Cd and Te escape events using XRF-XP software and for CdTe energy dependent efficiency. Spectra were simulated using the Tung1 computer program. Inherent filtration and tube potential were varied to obtain the best simulation of the RQA5 half-value layer (HVL).
Results:HVLs calculated from the acquired spectra are lower than HVLs measured. Under same kVp and inherent filtration, Al spectra and Cu spectra are very similar. Comparison of direct measurements with simulations reveals the strong influence of kVp on the resulting spectra. Acquired spectra are more similar to each other than to any of the simulated RQA5 spectra.
Conclusion:Direct measurements of x-ray spectra confirm the similarity of beams produced under TG116 conditions with either Cu or Al filtration. However, contrary to TG 116, the selection of kVp makes a significant difference in the spectra. Further work is needed to acquire spectra at the highest kVp allowed by TG 116 to fully assess the effect of kVp and HVL tolerances on the x-ray spectrum.