1. An apparatus for positioning a small animal subject, the apparatus comprising: a first structure including a mount for the subject to positioned, the first structure having a cylindrical region with an inner surface and an outer surface and having an opening on at least one end; a second structure coupled to the first structure and including a mount for the subject to be positioned, the second structure having a cylindrical region with an inner surface and an outer surface and having an opening on at least one end, wherein an outer radius of curvature of at least a portion of the second structure is less than an inner radius of curvature of at least a portion of the first structure and wherein at least a portion of the second structure forms a platform for positioning the small animal subject; and a mechanism for coupling the second structure to the first structure such that the second structure is configured to rotate with respect to a first axis of rotation parallel to an axis of its cylinder region.