When a patient comes to the clinic and tells us about his hip and knee pain, the doctor usually conducts a physical examination, and if he suspects that it is femoral head necrosis, in addition to the regular front and side X-rays, he usually has to do MRI of both hips. We know that in the early stage of femoral head necrosis, X-rays and CT generally do not change, only in the head necrosis to a certain extent, that is, cystic degeneration and collapse. It is only when the head necrosis reaches a certain level, i.e., cystic degeneration and collapse, that can be shown on X-ray and CT. In contrast, magnetic resonance imaging (MRI) is a resonance formed by the alignment of atomic nuclei in a magnetic field, so it can detect lesions at an early stage. MRI is necessary for early detection of femoral head necrosis. In the knee joint, we sometimes look at the patient’s film and see that there is no fracture, dislocation, or joint space, but the patient is in pain, so we recommend MRI because there is cartilage, meniscus, and ligaments on the surface of the knee joint. These tissues do not contain calcium, so they are not visible on X-rays or CT. Only on MRI can they be seen, so we must do MRI to diagnose cartilage, meniscus and ligament damage. Therefore, it is necessary to do MRI if there is no change in X-ray and CT examination.