Osteonecrosis is usually asymptomatic in the early stages, and later on there will be groin pain when walking. Anteroposterior and frog radiographs of the hip should be taken. The x-ray presentation of osteonecrosis is related to the stage the lesion is in. In the early stage, the X-ray plain film can be normal. As the disease progresses, the X-ray changes will gradually become obvious, such as the appearance of increased density and translucent areas in the femoral head. If the disease progresses further, the crescentic sign (most obvious in the frog position), which is typical of the disease, will appear. In the advanced stage of the disease, the femoral head collapses and severe arthritic changes appear on both sides of the joint space. Bone scan can be helpful in the diagnosis of osteonecrosis, especially when determining polyarticular disease. However, there is no correlation between the findings of a nuclear scan of the femoral head and hip function and pain if clinical symptoms are already present. Magnetic resonance can diagnose early femoral head necrosis and also allow accurate staging and determination of the extent of necrosis by this non-invasive means. It can also differentiate transient osteoporosis from ischemic necrosis. MRI is also useful for tracking disease progression and evaluating treatment outcomes. When x-rays show lesions on only one side of the joint, a pelvic MRI is necessary to determine not only the extent of the lesion in the symptomatic hip, but also to examine the asymptomatic hip on the other side, allowing for early detection of the lesion. In the early stages of the lesion, most treatments are very effective.