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Reference radiation beams using a tungsten target and silver, rhodium, molybdenum and aluminum filters at 20 kVp to 50 kVp were developed to model digital mammography x rays.
These beams will provide FDA inspectors, ionization chamber manufacturers, and calibration facilities with the spectra that best represent what is used in clinics. These new reference radiation techniques will be available for calibrations and the FDA-MQSA proficiency tests. The beam parameters are similar to those supported by the German standards laboratory, PTB, allowing U.S. manufacturers the ability to get their ionization chambers calibrated in the U.S. and distribute them in Europe. The NIST 100 kVp tungsten anode x-ray unit and the Ritz primary standard free-air ionization chamber are being used for the digital mammography techniques.
The correction factors for the primary standard have been established and are in the evaluation process which involves calibrating and comparing the results of a reference class transfer ionization chamber to the tungsten anode digital mammography beams and the Mo/Mo radiation beams at NIST and eventually at another NMI. Once the uncertainty has been established, dissemination of the NIST standard using the new digital mammography techniques can begin. Completion of the digital mammography standard will allow the end user to receive calibrations of their instrumentation under conditions which accurately represent those found in a clinic.