The first set of images show areas of low signal intensity in the superior weight-bearing areas of both femoral heads (marked in white above). This pattern is characteristic for osteonecrosis (avascular necrosis) of both femoral heads. There are many causes for osteonecrosis, including steroid use, lupus erythematosus, barotrauma (caisson disease), pancreatitis, alcoholism, marrow packing disorders such as Gaucher's syndrome, sickle cell disease, and others. Of these, steroid use is the most common cause by far.
Why does this patient have bilateral osteonecrosis? Well, notice the reniform structure in the right pelvis. The most common reason to have a pelvic kidney is following a renal transplant. The femoral artery and vein provide a convenient place for surgeons to attach the donor kidney. Although patients can occasionally have pelvic kidneys for congenital reasons, this is very rare. Why are patients with renal transplants predisposed to osteonecrosis? Generally because of the steroids given with the immunosuppressive therapy prior to and following surgery.