A bone scan will show inflammation or infection in the bones, while a bone density test will measure the amount of calcium in the bones. Your child may undergo these tests as part of their JIA diagnosis.
What is a bone scan? 
A bone scan is a series of pictures of the skeleton. Usually the entire body is scanned. The scan is typically done to show inflammation or infection, which might not show up on a regular X-ray.
How is a bone scan done?
A bone scan is done by a doctor called a radiologist, who specializes in doing imaging studies. The radiologist is assisted by a nuclear medicine technologist.
An intravenous (IV) tube will first be placed in your child’s vein. Through the IV, your child will be given a dose of a radioactive medicine which will travel to the bones (skeleton). This medicine is very safe. It will take two to three hours for the bones to absorb the medicine. If your child drinks lots of liquid, it will help their bones absorb the medicine.
After about two to four hours, your child will be asked to lie on an X-ray table. This is where the bone scan will be performed. A “gamma” camera will be placed very close to their body. This camera will record the gamma rays given off by the radioactive medication as flashes of light.
The gamma camera will capture an image or movie of their skeleton or bones in action. Very simply, it will create a living picture of their skeleton.
Once the scan starts, it will take about one to two hours. If your child has a hard time lying still, they may be given a sedative. This will help keep them still throughout the test.
What is a bone density test?
The bone density test helps to measure the amount of calcium in the bones. It can help your child’s doctor predict how strong your child’s bones are. It also helps the doctor predict your child’s risk of developing a fracture.
How is a bone density test done?
The most common method of doing a bone density test is by using low-dose X-rays. Your child will be asked to lie down on a table. The bone density scanner will pass over their body. Usually the machine will take X-rays of the lower spine and hips. The test is painless, but your child will need to stay still during the test. The test usually takes 10 to 15 minutes.