Factors for conversion between human and automatic read-outs of CDMAM images

Abstract

According to the European protocol for the quality control of the physical and technical aspects of mammography screening (EPQCM) image quality of digital mammography devices has to be evaluated using the CDMAM (contrast-detail mammography) phantom. The evaluation of image quality is accomplished by the determination of threshold thicknesses of gold disks of different diameters (0.08 mm to 2 mm). The EPQCM requires this task to be performed by qualified human observers and revealed to be very time consuming.Therefore a software solution was provided by the European Reference Organisation for Quality Assured Breast Screening (Euref) known as 'cdcom'. The problem with this program is that it provides threshold thicknesses different from the results gained by human readers. Factors for the conversion from automatic to human read outs depend on the diameter of the gold disk and were estimated using a huge amount of data, both human and automatic read outs. But these factors provided by various groups differ from each other and are purely phenomenological. Our approach uses the Rose theory which gives a correlation between threshold contrast, diameter of the object and number of incident photons. To estimate the five conversion factors between the diameters of 0.2 mm and 0.5 mm we exposed with five different current-time products which resulted in 25 equations with six unknowns (5 factors and one constant). This optimization problem was then solved using the Excel built-in solver. The theoretical conversion factors amounted to be 1.62, 1.75, 2.04, 2.20, 2.39 for the diameters of 0.2, 0.25, 0.31, 0.4, and 0.5 mm. The corresponding phenomenological factors found in literature are 1.74, 1.78, 1.83, 1.88, and 1.93. The applied method reveals to be very robust and produces factors comparable to the phenomenological ones.