In bone scans osteoblasts take up radioactive marked (technetium-99m) biphosponates. So you can detect areas of high bone turnover, which could be a fracture, a bone metastasis, and more including inflammation of joints, which affect the bone around the joint, or mechanical stress to the bone around a joint caused by OA. The pictures of OA and RA usually differ in bone scans, though OA joint might also be activated and show a similar picture, but then you don’t treat bone scans, so you can compare your physical examination of the patient with the bone scan, something which radiologists can’t do. The pattern of affected joints in RA, OA, PsoA, peripheral arthritis in spondyloarthritis differs.
My initial point was: the radiologist is usually not trained in joint patterns, which the rheumatologist sees every day (and works with). The radiologist looks for what he is trained for: bone metastasises, that’s what most bone scans are done for. He often misinterprets scans done in the field of rheumatology (at least that’s the case in Germany). Or you have one radiologist working closely with your center and the radiologist learns, what you want him to do.