Size and shape of spherical objects on full-field digital mammography and digital breast tomosynthesis images

This study assessed the accuracy of shape and size representation of spherical objects on full-field digital mammography (FFDM) and digital breast tomosynthesis (DBT) images. Six 5-mm-thick polymethylmethacrylate slabs were positioned on the breast support table with 9 aluminum spherical objects of 30 (± 0.1) mm diameters between the first and second slabs. X-ray imaging was performed using FFDM and DBT (angular range 15°–40°, with correction of magnification), and repeated with the objects placed between the third and fourth slabs, and subsequently between the fifth and sixth slabs. The aspect ratio of the spherical objects and longer diameter were measured to evaluate the shape and size, respectively. A Steel-Dwass test was performed for comparative analysis. A P value <0.05 was considered significant. No significant differences in the aspect ratio of the spherical objects imaged using FFDM, DBT15°, or DBT40° images were observed (overall median: 1.02, overall range: 1.00–1.06). The longer diameter on the FFDM was increasingly magnified (median, range) with increasing distances of 20 mm (32.5, 31.8–33.5 mm) and 40 mm (33.6, 32.9–34.7 mm) between the breast support table and object center. However, in the case of DBT, the longer diameter was approximately the same as that of the actual object (overall, 30.4, 30.0–31.7 mm). At each height, the longer diameter was significantly different between the FFDM and DBT15° images and between the FFDM and DBT40° images (all P = 0.001), with no significant difference in that between the DBT15° and DBT40° images. The size on the FFDM images was magnified as compared to the size of the actual objects, and that on the DBT images was approximately the same as that of the actual objects. Thus, preoperative tumor size determination using FFDM images should be avoided.