While the above regulation requires that all air kerma measuring instruments be calibrated to a standard that is traceable to the National Institute of Standards and Technology (NIST), the regulation does not specify the target filter combination to be used. Currently, all air kerma measuring instruments are typically calibrated in the mammography kVp range using only one target filter combination, such as molybdenum/molybdenum (Mo/Mo), even if the instrument is used with other combinations such as tungsten/silver (W/Ag) or tungsten/rhodium (W/Rh). In addition, NIST currently does not have the ability to provide calibration standards for W/Ag or W/Rh target filter combinations in the mammography kVp range. Consequently, while FDA continues to require that all air kerma measuring instruments be calibrated to a NIST-traceable standard for at least one target filter combination in the mammography x-ray range, FDA does not intend to enforce such calibration requirement with respect to W/Ag or W/Rh beams.
It is well known that the energy response for ionization chamber-based air kerma measuring instruments is typically flat in the 20-40 kVp range. For solid-state air kerma measuring instruments however, the energy response is not flat and because of this, the air kerma readings from these instruments may need to be adjusted by an appropriate correction factor. The correction may already be handled internally by the instrument or you may need to contact the instrument manufacturer for the correction factor.