Spectrally Neutral X-Ray Attenuators
R Jennings*, FDA Center for Devices & Radiological Health, Silver Spring, MDTU-A-218-8 Tuesday 8:00:00 AM - 9:55:00 AM Room: 218
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.