TU-A-218-08: Spectrally Neutral X-Ray Attenuators

Purpose: To develop a step wedge with constant beam quality for all steps. This would allow single‐shot sensitometry with image receptors like CR that vary their response adaptively, depending on exposure conditions. Methods: Appropriately chosen thicknesses of two different filter materials can produce x‐ray spectra with nearly identical shapes but different fluence rates. To demonstrate, a 70 kVp x‐ray spectrum from IPEM Report 78 was numerically filtered with 29.8 mm of Al to yield a half‐value layer of 7.1 mm, which is spectrum RQA5 from IEC Standard 62220–1 (2003). A second spectrum was calculated with the previously determined thickness of aluminum reduced by 7 mm. The resulting spectrum was filtered with 112 mm of PMMA, which gave the best‐fit match to the shape of the RQA5 spectrum. Seven additional spectra were generated starting from the RQA5 spectrum with paired decrements in Al thickness of 1 mm and increments in PMMA thickness of 16 mm. The shapes of the resulting spectra were compared, and their HVLs and ratios of fluence rates were calculated. Results: The shapes of the resulting spectra are nearly identical. The HVLs of the resulting spectra vary monotonically from 7.100 mm of Al for the 7 mm Al filter to 7.111 mm of Al for the 112 mm PMMA filter. The fluence rate decrement per step is 0.74592 ± 0.00002. Fluence rates normalized to the value for the 7 mm Al filter range from 1.0 to 0.1285. Conclusions: Combining a range of filter pairs like the ones described above into a step wedge would allow exposure of a detector with a range of intensities at the same beam quality in a single shot. The concept of a constant beam quality step wedge has been presented. Practical implementation of the concept remains to be demonstrated.