The Development of New Reference Standards for Digital Mammography

With the introduction and Food and Drug Administration (FDA) approval of mammography units using a tungsten (W) anode with either a silver (Ag) or rhodium (Rh) filter, there are new concerns about air kerma, dose and non-invasive kVp measurements.

The response of ionization chambers in the 20 kV to 40 kV mammography energy range is relatively flat.  However, the FDA is concerned that if a solid-state air-kerma measuring device is used that is not calibrated in the appropriate energy spectrum, dose errors will result.  The new NIST reference mammography beams using a tungsten target and silver, rhodium, molybdenum (Mo) and aluminum (Al) filters at 20 kV to 50 kV will provide FDA inspectors, chamber manufacturers and calibration facilities with the techniques that best represent the new beams used in clinics.  It may be possible in the near future for NIST to provide kV calibrations for non-invasive kVp meters, which are commonly used to measure the kV and half-value layer (HVL) on clinical units.  These new reference standards will be available for calibrations and the FDA-MQSA proficiency tests.  The 100 kV x-ray unit will be used for all the new techniques.  The kV has been fully calibrated using NIST-traceable voltage dividers.  The beam parameters will be the same as those techniques supported by the Physikalisch-Technische Bundesanstalt (PTB), the German standards laboratory, giving U.S. manufacturers the ability to get their chambers calibrated in the U.S. and distribute them within Europe.