Femoral osteonecrosis is a chronic disease in which the femoral head collapses and the hip joint becomes dysfunctional due to the destruction of the blood supply to the femoral head by different etiologies, while bone marrow edema is only one of the imaging signs seen by the imaging physician on the MRI. The joint. Femoral osteonecrosis and bone marrow edema are related but not identical conditions. The typical censored MRI presentation of simple femoral head necrosis is a focal signal change under the cartilage in the anterolateral weight-bearing area of the femoral head, with segmental hyposignal at T1WI and moderate intensity at T2W1. In contrast, the MRI presentation of bone marrow edema around femoral head necrosis is a combination of both necrosis and edema when the edema is mild. When the edema is mild, the edema is limited to the area around the femoral head lesion. With further development, the edema may extend to the femoral neck area. It is often accompanied by a moderate amount of joint effusion. In severe cases, the edema extends to the subtrochanteric region of the femur and there is a large amount of joint effusion. Bone marrow edema does not represent the ischemic changes of early necrosis, but is a secondary manifestation of femoral head collapse and stress changes following early femoral head necrosis. The scope of lesion is small, and edema does not occur in most patients. The scope of bone marrow edema in a few patients is limited to the area around the lesion of femoral head, and this small edema may be caused by tissue congestion and edema caused by inflammatory repair reaction in the early stage of ischemic necrosis. In the middle stage of femoral head necrosis, it is often accompanied by obvious bone marrow edema, which may be considered as increased intramedullary pressure, resulting in impaired femoral venous reflux and increased exudation, or may be caused by local collapse of the femoral head after necrosis and reactive edema of bone marrow tissue due to microfracture. Therefore, there is usually obvious bone marrow edema in femoral head necrosis, but there may not be any edema in the early stage of the disease, and the presence of bone marrow edema does not necessarily lead to femoral head necrosis.