Mobile measurement setup according to IEC 62220-1-2 for DQE determination on digital mammography systems .

The international standard IEC 62220-1-2 defines the measurement procedure of the detective quantum efficiency (DQE) for digital x-ray imaging devices used in mammography. A mobile setup complying to this standard and adaptable to most current systems was constructed in the Helmholtz Zentrum München to allow for an objective technical comparison of current full field digital mammography units employed in mammography screening in Germany. The setup was first used in a DQE measurement for two radiation qualities, RQA-M 2 (Mo/Mo, 28 kV) and W/Rh, 28 kV, in a dose range of 87 – 550 µGy air kerma at detector surface. Irradiation geometry can be characterized independent of a system’s positioning utilities, enabling to optimize dosimetry and edge phantom placement in the radiation beam for measurement of the modulation transfer function (MTF). Relative measurement uncertainties calculated according to IEC recommendations were 1.6 % for air kerma, 0.14 - 1.4 % for MTF, increasing with spatial frequency from 0.5 – 9.5 mm-1, and 1.4 – 3.5 %  for the noise power spectrum above 1.5 mm-1 (coverage factor 1). The relative expanded uncertainty (coverage factor 2 as required in IEC 62220-1-2) of the DQE was 4 – 8 % for spatial frequencies of 2 – 9.5 mm-1; the corresponding absolute expanded uncertainties were between 0.007 and 0.06