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Bone Density ChamberHow Bone Densitometry Works

A bone density test (bone densitometry) measures the strength of a bone. The test uses X-rays to measure how many grams of calcium and other bone minerals are packed into a segment of bone.

Bone density tests differ from bone scans. Bone scans require an injection beforehand and are usually used to detect fractures, cancer, infections and other abnormalities in the bone.

Why It Is Used

A bone density test is most often used to detect osteoporosis — a disorder that makes bones more fragile and more likely to break — or osteopenia, decrease in bone mass that is slightly less severe than osteoporosis.

A physician may order a bone density test to:

  • Confirm a diagnosis of osteoporosis if you have already had a bone fracture
  • Determine your rate of bone loss and your risk of broken bones
  • Identify decreases in bone density before you break a bone
  • Monitor osteoporosis treatment

What to Expect

Bone density tests are fast and painless. No preparation is needed, though it is important to tell your doctor beforehand if you recently had a barium exam or had contrast material injected for a CT scan or a nuclear medicine test.

Before the test

Be sure to:

  • Avoid taking calcium supplements for 24 hours before your test
  • Avoid wearing clothes with zippers, belts or buttons – wear loose, comfortable clothing
  • Don’t wear jewelry and remove all metal objects from your pockets, such as keys

During the test

Bone density tests are usually done on bones that are most likely to break because of osteoporosis, including:

  • Bones in the forearm
  • The narrow neck of the thighbone, near the hip joint
  • Vertebrae in the lower spine

The test itself usually takes between 10 and 30 minutes. You may be asked to change into an examination gown. You’ll lie on a padded platform while a mechanical arm passes over your body to take X-rays of the bone. The amount of radiation you're exposed to is very low, much less than the amount emitted during a chest X-ray.

The Benefits of Bone Densitometry 

Bone densitometry testing can find decreasing bone density and strength at a much earlier stage than conventional X-rays. Early detection of these problems makes treatment more effective.