If you have your bone density test done at a hospital, it will most likely be performed on a central device, where you lie on a padded platform while a mechanical arm passes over your body. The amount of radiation exposure levels is very low, significantly lower than the amount emitted during a chest X-ray. The test typically takes about 10 minutes to half an hour. Such as those in your finger, wrist or heel, a small, portable machine can measure bone density in the bones at the far ends of your skeleton. Peripheral devices are the instruments used for these tests and are often found in pharmacies. Tests done on central devices are more expensive than those tests of peripheral bone density. A measurement taken at your heel usually isn't as accurate a predictor of fracture risk as a measurement taken at your spine or hip because bone density can vary from one location in your body to another. Consequently, your doctor might recommend a follow-up scan at your spine or hip to confirm your diagnosis if your test on a peripheral device is positive.