Validation of a statistical method for low contrast detectability as a simple tool for QC in digital mammography

摘要

The aim of this work was to validate an innovative and simplified method for threshold contrast evaluation in digital mammography based on a single exposure of a homemade test tool. A homogeneous region was used to calculate threshold contrast for details of varying size with a statistical approach. An included aluminium step wedge permitted to express the threshold contrast also as an absolute quantity in terms of mm Al.The method has been applied to 9 mammography systems from 5 vendors with different acquisition geometries and detector characteristics in a variety of exposure setups. MTF data were used to correct statistical threshold contrast results for different spatial resolution properties.Validation of the statistical method was carried out as comparison with the widespread method of CDMAM image analysis. Threshold contrasts LCth obtained with the two methods showed good correspondence. For all systems the ratio, averaged over all exposure conditions, laid within ± 15% from the mean. With respect to the overall figure of merit IQF_(inv) a linear correlation between the two methods was demonstrated in every unit (r> 0.94 units 1-8, r = 0.89 unit 9). The ratio between the IQF_(inv) values determined with CDMAM analysis and the statistical analysis was calculated for every exposure condition and averaged for each system. Theses average ratios laid within ± 10% from the mean IQF_(inv) determined from all units. This is equivalent to a universal angular coefficient between the IQF_(inv) determined with the two methods, valid for all systems.Finally, mean LCth ratio values calculated from all data were used to propose reference values for acceptable and achievable threshold contrasts LCth expressed in terms of mm of aluminium, in analogy to threshold contrasts in terms of gold thickness for CDMAM evaluation.