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According to the 'European protocol for the quality control of the physical and technical aspects of mammography
screening' (EPQC) image quality digital mammography units has to be evaluated at different breast thicknesses.
At the standard thickness of 50 mm polymethyl methacrylate (PMMA) image quality is determined by the
analysis of CDMAM contrast detail phantom images where threshold contrasts are calculated for different gold
disc diameters. To extend these results to other breast thicknesses contrast-to-noise ratios (CNR) and threshold
contrast (TC) visibilities have to be calculated for all required thicknesses. To calculate the latter the mass
attenuation coefficient (MAC) of gold has to be known for all possible beam qualities in the tube voltage range
between 26 and 32 kV.
In this paper we first determined the threshold contrast visibility using the CDMAM phantom with the same
beam quality at different current-time products (mAs). We can derive from Rose theory that CNR • CT • α =
const, where α is the diameter of the gold cylinder. From this the corresponding attenuation coefficients can be
calculated. This procedure was repeated for four different beam qualities (Mo/Mo 27kV, Rh/Rh 29kV, Rh/Rh
31 kV, and W/Rh 29 kV)).
Next, we measured the aluminium half value layer (HVL) of all x-ray spectra relevant for mammography.
Using a first order Taylor expansion of MAC as a function of HVL, all other desired MAC can be calculated.
The MAC as a function of the HVL was derived to MAChvl = -286.97 * hvl+186.03 with R2 = 0.997, where
MAChvl indicates the MAC for all specific x-ray spectrum defined by its aluminium half value layer. Based on
this function all necessary MACs needed for quality assurance (QA) were calculated. The results were in good
agreement with the data found in the protocol.
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