Digital mammography: Observer performance study of the effects of pixel size on the characterization of malignant and benign microcalcifications

Rationale and Objectives, The authors performed this study to evaluate the effects of pixel size on the characterization of mammographic microcalcifications by radiologists. Materials and Methods. Two-view mammograms of 112 microcalcification clusters were digitized with a laser scanner at a pixel size of 35 mum Images with pixel sizes of 70, 105, and 140 mum were derived from the 35-mum-pixel size images by averaging neighboring pixels. The malignancy or benignity of the microcalcifications had been determined with findings at biopsy or 2-year follow-up. Region-of-interest images containing the microcalcifications were printed with a laser imager. Seven radiologists participated in a receiver operating characteristic (ROC) study to estimate the likelihood of malignancy. The classification accuracy was quantified with the area under the ROC curve (A(z)). The statistical significance of the differences in the A(z). values for different pixel sizes was estimated with the Dorfman-Berbaum-Metz method and the Student paired t test. The variance components were analyzed with a bootstrap method. Results. The higher-resolution images did not result in better classification; the average A(z) with a pixel size of 35 mum was lower than that with pixel sizes of 70 and 105 mum The differences in A(z) between different pixel sizes did not achieve statistical significance. Conclusion. Pixel sizes in the range studied do not have a strong effect on radiologists' accuracy in the characterization of microcalcifications. The low specificity of the image features of microcalcifications and the large interobserver and intraobserver variabilities may have prevented small advantages in image resolution from being observed.