フルデジタルマンモグラフィシステムの撮影線量と信号検出能―Ｃ‐Ｄファントムによる研究―

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  • Incident Dose and Detection Capability of a Full Digital Mammography System:A Contrast-detail Phantom Study

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Purpose : The purpose of this study was to investigate the relationship between incident exposure and detectability of the low-contrast signal obtained with full-field digital mammography (FFDM), based on amorphous silicon technology, and to determine the optimal radiographic conditions for obtaining comparable image quality by FFDM.<BR>Materials and Methods : A contrast-detail (CDMAM) phantom was used to compare the detection capabilities of FFDM and screen-film systems. The CDMAM phantom consists of a polymerized methyl methacrylate base with circular gold disks of various thicknesses and diameters. Within a row the disk diameter is constant with logarithmically increasing thickness (0.03μm-2.00μm), and within a column the disk thickness is constant, with logarithmically increasing diameter (0.06mm-2.00mm). To simulate the mammography, the CDMAM phantom was exposed together with an acrylic phantom 4 cm thick. To compare the effect of incident exposure on detectability, we varied the incident X-ray exposure by changing the exposure time. The incident exposure that yielded a radiographic density of 1.40 with a MinR2000/MinR2000 screen-film system was defined as the relative incident exposure of I. The exposure relative to the detector was varied from 1 to 0.2 (×1, ×0.8, ×0.4, ×0.2). For each combination of the relative incident exposure level, three image samples were read. Observer performance tests were conducted with eight participating observers.<BR>Each observer was required to identify a randomly located disk in phantom images with a 50% confidence level. The average contrast-detail (C-D) curves for FFDM and screen-film images were obtained from the observation results. Image quality figures (IQF) were also calculated from the results of the individual observer performance tests.<BR>Results : C-D curves and IQFs indicated that the FFDM system can offer improved detection capability over that obtainable with a screen-film system under the same exposure conditions. The C-D curve for the FFDM system under 80% exposure conditions provided a detection capability comparable to that of a screen-film system.<BR>Conclusion : It is concluded that FFDM curves exhibit threshold contrast characteristics that are better than those of a screen-film system.

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