Detection of simulated microcalcifications in a phantom with digital mammography: effect of pixel size

Abstract

Purpose: To evaluate the effect of pixel size on the detection of simulated microcalcifications in a phantom with digital mammography.

Materials and methods: A high-spatial-resolution prototype imager that yields variable pixel size (39 and 78 microm) and a clinical full-field digital mammography (FFDM) system that yields a 100-microm pixel size were used. Radiographic images of a contrast-detail (CD) phantom were obtained to perform four-alternative forced-choice observer experiments. Polymethylmethacrylate was added to obtain phantom thicknesses of 45 and 58 mm, which are typical breast thicknesses encountered in mammography. Phantom images were acquired with both systems under nearly identical exposure conditions by using an antiscatter grid. Twelve images were acquired for each phantom thickness and pixel size (for a total of 72 images), and six observers participated in this study. Observer responses were used to compute the fraction of correctly detected disks. A signal detection model was used to fit the recorded data from which CD characteristics were obtained. Repeated-measures analyses with mixed-effects linear models were performed for each of the six observers. All statistical tests were two sided and unadjusted for multiple comparisons. A P value of .05 or less was considered to indicate a significant difference.

Results: Statistical analysis revealed significantly better CD characteristics with 39- and 78-microm pixel sizes compared with 100-microm pixel size for all disk diameters and phantom thicknesses (P<.001). Increase in phantom thickness degraded CD characteristics regardless of pixel size (P<.001).

Conclusion: On the basis of the conditions of this study, reducing pixel size below 100 mum with low imaging system noise enhances the visual perception of small objects that correspond to typical microcalcifications.

Figure 1

Image ROI (a) before and (b) after automatic contrast enhancement. Improved visibility of the corner disk is observed after enhancement.

Figure 2

Examples of percent correct detection (Pc) characteristics obtained from a single observer for disk diameters (a) 0.31 and (b) 0.20 mm at various pixel sizes for a phantom thickness of 45 mm. Degradation in detection with increasing pixel size is observed. The lines are maximum likelihood estimated detection characteristics.

Figure 3

Contrast-detail (CD) characteristics obtained at 62.5% detection threshold after averaging data from six observers for (a) 45 mm and (b) 58 mm phantom thickness conditions at 39, 78, and 100 μm pixel sizes. Lower (better) threshold CD characteristics at 39 and 78 μm pixel sizes is observed. The error bars indicate 95% confidence interval.

Figure 4

Contrast-detail (CD) characteristics obtained at 62.5% detection threshold after averaging data from six observers for (a) 100 (b) 78, and (c) 39 μm pixel sizes at 45 and 58 mm phantom thickness conditions. Degradation in CD characteristics irrespective of pixel size is observed. The error bars indicate 95% confidence interval.