Effects of exposure equalization on image signal-to-noise ratios in digital mammography: a simulation study with an anthropomorphic breast phantom

Abstract

Purpose: The scan equalization digital mammography (SEDM) technique combines slot scanning and exposure equalization to improve low-contrast performance of digital mammography in dense tissue areas. In this study, full-field digital mammography (FFDM) images of an anthropomorphic breast phantom acquired with an anti-scatter grid at various exposure levels were superimposed to simulate SEDM images and investigate the improvement of low-contrast performance as quantified by primary signal-to-noise ratios (PSNRs).

Methods: We imaged an anthropomorphic breast phantom (Gammex 169 "Rachel," Gammex RMI, Middleton, WI) at various exposure levels using a FFDM system (Senographe 2000D, GE Medical Systems, Milwaukee, WI). The exposure equalization factors were computed based on a standard FFDM image acquired in the automatic exposure control (AEC) mode. The equalized image was simulated and constructed by superimposing a selected set of FFDM images acquired at 2, 1, 1/2, 1/4, 1/8, 1/16, and 1/32 times of exposure levels to the standard AEC timed technique (125 mAs) using the equalization factors computed for each region. Finally, the equalized image was renormalized regionally with the exposure equalization factors to result in an appearance similar to that with standard digital mammography. Two sets of FFDM images were acquired to allow for two identically, but independently, formed equalized images to be subtracted from each other to estimate the noise levels. Similarly, two identically but independently acquired standard FFDM images were subtracted to estimate the noise levels. Corrections were applied to remove the excess system noise accumulated during image superimposition in forming the equalized image. PSNRs over the compressed area of breast phantom were computed and used to quantitatively study the effects of exposure equalization on low-contrast performance in digital mammography.

Results: We found that the highest achievable PSNR improvement factor was 1.89 for the anthropomorphic breast phantom used in this study. The overall PSNRs were measured to be 79.6 for the FFDM imaging and 107.6 for the simulated SEDM imaging on average in the compressed area of breast phantom, resulting in an average improvement of PSNR by ∼35% with exposure equalization. We also found that the PSNRs appeared to be largely uniform with exposure equalization, and the standard deviations of PSNRs were estimated to be 10.3 and 7.9 for the FFDM imaging and the simulated SEDM imaging, respectively. The average glandular dose for SEDM was estimated to be 212.5 mrad, ∼34% lower than that of standard AEC-timed FFDM (323.8 mrad) as a result of exposure equalization for the entire breast phantom.

Conclusions: Exposure equalization was found to substantially improve image PSNRs in dense tissue regions and result in more uniform image PSNRs. This improvement may lead to better low-contrast performance in detecting and visualizing soft tissue masses and micro-calcifications in dense tissue areas for breast imaging tasks.

Figure 1

The anthropomorphic breast phantom “Rachel” was imaged using a GE Senographe 2000D FFDM system.

Figure 2

Schematic drawing of the proposed scanning equalization digital mammography (SEDM) system.

Figure 3

(a) Exposure equalization factors computed from a pre-scan FFDM image of the anthropomorphic breast phantom; (b) simulated SEDM image; (c) restored SEDM image.

Figure 4

Flow chart to illustrate the process of simulating exposure equalization.

Figure 5

System linearity plotted as mean pixel value versus exposure (mAs) for images acquired with and without the anti-scatter grid. Exposures made at 28 kVp using a 50-mm-thick PMMA slab (50% adipose/50% glandular composition).

Figure 6

(a) Variances of total noise and individual noise components plotted as a function of mean pixel value; (b) fraction of noise component variance plotted as a function of pixel value. Measurements made at 28 kVp with various exposure levels using a 50-mm-thick PMMA slab (50% adipose/50% glandular composition).

Figure 7

SPRs measured for slot-scan images acquired with various slot widths (5, 10, 15, and 20 mm) and for FFDM images with and without the anti-scatter grid.

Figure 8

(a) PSNRs measured in the FFDM image; (b) PSNRs measured in the SEDM image; and (c) PSNR improvement factors (PSNRIFs) measured as a ratio of PSNR for the SEDM image over that for the FFDM image.

Figure 9

Spatial distribution of glandular dose estimated in the entire breast phantom for (a) FFDM and (b) SEDM. (c) Spatial distribution of RD_g estimated in the entire breast phantom.