A technique optimization protocol and the potential for dose reduction in digital mammography

Abstract

Digital mammography requires revisiting techniques that have been optimized for prior screen/film mammography systems. The objective of the study was to determine optimized radiographic technique for a digital mammography system and demonstrate the potential for dose reduction in comparison to the clinically established techniques based on screen- film. An objective figure of merit (FOM) was employed to evaluate a direct-conversion amorphous selenium (a-Se) FFDM system (Siemens Mammomat Novation(DR), Siemens AG Medical Solutions, Erlangen, Germany) and was derived from the quotient of the squared signal-difference-to-noise ratio to mean glandular dose, for various combinations of technique factors and breast phantom configurations including kilovoltage settings (23-35 kVp), target/filter combinations (Mo-Mo and W-Rh), breast-equivalent plastic in various thicknesses (2-8 cm) and densities (100% adipose, 50% adipose/50% glandular, and 100% glandular), and simulated mass and calcification lesions. When using a W-Rh spectrum, the optimized FOM results for the simulated mass and calcification lesions showed highly consistent trends with kVp for each combination of breast density and thickness. The optimized kVp ranged from 26 kVp for 2 cm 100% adipose breasts to 30 kVp for 8 cm 100% glandular breasts. The use of the optimized W-Rh technique compared to standard Mo-Mo techniques provided dose savings ranging from 9% for 2 cm thick, 100% adipose breasts, to 63% for 6 cm thick, 100% glandular breasts, and for breasts with a 50% adipose/50% glandular composition, from 12% for 2 cm thick breasts up to 57% for 8 cm thick breasts.

Figure 1

(a) Photograph showing the FFDM imaging system used in this evaluation (Siemens Mammomat Novation^DR, Siemens Medical Systems) and the phantom configuration employed for evaluation of the FOM. Three breast compositions (100% adipose, 50% adipose/50% glandular, and 100% glandular) were imaged simultaneously in thicknesses of 2, 4, 6, and 8 cm. A large compression paddle was used to support petri dishes containing oil and simulated mass and calcium inclusions [see Fig. 1b]. (b) Overhead schematic of the relative position of mass and calcium simulated lesions with respect to the breast phantom.

Figure 2

[(a)–(f)] Graphs showing the relationship between the FOM and kVp for both Mo–Mo and W–Rh target∕filter combinations for 100% adipose [(a) and (d)], 50% adipose∕50% glandular [(b) and (e)], and 100% glandular [(c) and (f)]. The FOM, defined by the ratio of SdNR²∕MGD, was calculated for both mass [(a)–(c)] and calcification [(d)–(f)] lesions, where SdNR is the signal-difference-to-noise ratio and MGD is the mean glandular dose. The solid gray circles represent the previously established clinical technique based on Mo–Mo and screen∕film, shown here for comparison purposes.