This application discusses, among other things, curved plates for immobilizing a breast of a subject for medical imaging. In an example, an immobilization assembly Can include a curved compression plate assembly, a curved support plate assembly, an assembly coupling configured to position a first concave major surface of the curved compression plate assembly opposite a second concave major surface of the curved support plate assembly, wherein a cross section view through the curved compression assembly and the curved support plate assembly includes two portions of a single ellipse separated by a distance, and wherein the curved compression plate assembly and the curved support plate assembly are configured to immobilize the biological specimen between the first concave major surface and the second concave major surface.