1. A medical imaging system comprising: an X-ray Talbot imaging apparatus comprising: an X-ray source which emits X-rays, an X-ray detector including conversion elements to generate an electrical signal according to the emitted X-rays, and reading the electrical signal generated by the conversion elements, as an image signal, a subject table to hold a subject for a joint of the subject to be imaged, and a controller which generates a reconstructed image from the image signal of the imaged subject, the reconstructed image including at least a differential phase image; and an image processing apparatus which measures a thickness of cartilage in the joint in the differential phase image or an image generated from the differential phase image, by reference to at least one of i) an edge of a bone in the joint identified in the reconstructed image or an image generated from the reconstructed image and ii) an edge of the cartilage identified in the differential phase image or the image generated from the differential phase image.