Hip synovitis in children is a common outpatient disease, also known as temporary synovitis, mostly seen in children aged 3-10 years old. The cause is still unclear and may be related to viral infection, trauma, bacterial infection and allergic reaction. Unilateral hip or groin pain is the most common clinical symptom, and some patients may present with mid-thigh or knee pain. In very young children, nocturnal cries can be manifested, and careful examination can reveal painful claudication. A recent history of upper respiratory tract infection, pharyngitis, bronchitis, and otitis media should also be noted, as these can be present in nearly half of patients with hip synovitis. In children with hip synovitis, there is usually no fever or mildly elevated body temperature, and hyperthermia is very rare. Examination 1. Physical examination (1) The affected limb is flexed at the hip, with mild abduction and external rotation. In 1/3 of patients with hip synovitis, there is no obstacle to hip movement, but they can still feel mild resistance to movement, especially when abducting and internally rotating the hip joint. (2) Pain occurs when the hip joint is moved passively. (3) When the patient is kept in a lying position and the examiner rolls the patient’s lower extremity, involuntary protective contraction of the affected muscles can be felt. (4) In patients with symptoms in the knee joint, the knee joint should be examined to exclude other diseases. (2) Imaging examination (1) X-ray examination generally has no abnormal bone performance, sometimes it may show mild pelvic tilt, swelling of the hip capsule, widening of the joint space, without bone destruction. (2) MRI examination shows widening of the hip joint space and joint cavity fluid on the affected side, and it is clearer than the line plain film. MRI shows that the synovial tissue between the acetabulum and the femoral cartilage has a medium signal in T1W1 and a high signal in T2W1. (3) B-mode ultrasound examination of the anterior femoral neck neck gap of the affected hip was significantly wider than that of the healthy side, with a bilateral difference of >1 mm. anterior femoral neck neck gap, i.e. the maximum distance between the periosteal surface of the femoral neck and the outer edge of the joint capsule. 3. Laboratory tests (1) Total blood leukocyte count is normal, or slightly elevated. (2) Blood sedimentation is normal or slightly elevated. If the blood sedimentation is significantly elevated, exceeding 20 mm/h, combined with an elevated body temperature exceeding 37.5°C and an increased white blood cell count, it suggests infectious arthritis. (3) Significantly elevated C-reactive protein is a sign of infectious arthritis. (4) Negative bacterial culture. For the first attack of hip synovitis, treatment is based on treatment of possible causes and bed rest for 2-3 weeks, and the recovery status is reviewed by ultrasound. However, for children with multiple recurrent episodes, the possibility of childhood femoral head necrosis perthes disease needs to be highly alerted. Although this disease has lameness and hip pain, it has a long history, and once it develops to the middle stage, when deformation and compression of the femoral head epiphysis is visible on X-ray, it often causes more serious hip deformity and requires major surgery for intervention, with residual deformity affecting the possible function of the hip joint in adulthood. Foreign studies have shown that MRI can detect early childhood femoral head necrosis, and recent studies have even suggested that angiography of the hip by MRI or CT can detect vasospasm or blood supply lesions to the femoral head much earlier, thus providing the possibility of early intervention and satisfactory treatment results at minimal economic and medical costs. Magnetic resonance angiography is a test that uses electromagnetic waves to produce images of two- or three-dimensional structures of the body. It is also sometimes called “magnetic resonance imaging (MRI)”. It is a type of tomographic imaging that uses the magnetic resonance phenomenon to obtain electromagnetic signals from the body and reconstruct information about the body. Since it is completely free from the damage caused by ionizing radiation, it has many parameters, a large amount of information, multi-directional imaging, and high resolution of soft tissues, etc., it has attracted the attention of scholars from all aspects since its introduction. It is widely used in clinical diagnosis of diseases, and has become an essential examination method for some lesions. The amount of information provided by MR is not only greater than many other imaging techniques in medical imaging, but also different from existing imaging techniques, so it has great potential superiority for the diagnosis of diseases. MR is very effective in detecting intracerebral hematomas, extracerebral hematomas, brain tumors, intracranial aneurysms, arteriovenous malformations, cerebral ischemia, intravertebral tumors, spinal cord cavitation and spinal fluid, and other common cranio-cerebral diseases. It is also effective in the diagnosis of posterior lumbar disc protrusion, primary liver cancer and other diseases.