BI-RADS: Mammo: A mass is 3-D and occupies space; it should be seen in two different mammographic projections. It has a completely or partially convex-outward contour and (if radiodense) is denser in the center than at the periphery. MRI: 3-D, space-occupying lesion, convex-outward contour. US: A mass is 3-D and occupies space. With 2-D it should be seen in two different planes; with volumetric acquisitions it should be seen in three planes. Masses can be distinguished from normal anatomic structure, such as ribs or fat lobules, using two or more projections and real-time scanning.