DEXA and Bone Density Tests vs. Bone Density Scans

Neil Gonter, MD Health Pro April 26, 2007
  • What is a Bone Density Test? How does the doctor choose where to scan you (wrist, ankle, hip)?  What is the difference between this and a bone scan?

     

    What is a Bone Density Test?

     

    A bone density test is an examination by either special x-rays or ultrasound to determine how much bone mineral content (calcium and other minerals) is present in any section of bone.  The higher the mineral content, the denser the bone.  This is one of the ways to determine the risk of fracture. 

     

    What are the different types of bone density tests and what are the differences between them?

     

    There are various types of tests available, peripheral and central.  The peripheral devices measure the finger or ankle, and these central devices measure the spine and hips.

     

    The peripheral devices are significantly cheaper and are easier to use; however, they are often used for screening for they do not measure the actual areas where fractures occur.  This is due to the different types of bone in the body and its varying density in different areas.

     

    Central devices include the Quantitative CT (QCT) and the DXA scan (Dual Energy X-ray Absorptiometry).  QCT uses a computerized tomography (CT) scanner together with computer software to determine the bone density, usually at the spine.  QCT is the only commercially available technique to measure 3-dimensional bone images.  It is the most sensitive scan to detect bone disease and can take into account other diseases such as arthritis that may affect the bone.

     

    A DXA scan measures the spine and often one or both hips.  The test emits minimal radiation, about 1/30th of what one would receive from a chest x-ray.  During the study, the patient lies down on an examining table and the scanner rapidly directs x-ray energy to determine bone mineralization.  The test often takes 5-10 minutes and is totally comfortable and painless, unless the patient has difficulty lying down.  DXA scanning is more sensitive and accurate than the QCT at measuring small changes in bone density over time or in response to drug therapy. 

     

    Due to the inferior ability to monitor change, the extra expense, and  the significantly increased radiation that QCT requires, DXA scanning has become the gold standard and the most widely used method to measure bone mineralization.

     

    What areas should be evaluated?

     

    It important to evaluate the areas where most fractures occur.  These include the lumbar spine and the hip.  Both of these areas are important as they are composed of different types of bone (trabecular and cortical) and therefore can have different mineralization and fracture rates.  f these areas are not available for evaluation, the wrist may be scanned as well.  This can be done in patients with severe obesity, difficulty with positioning, hip or vertebral fractures or severe arthritis.

     

    What is the difference between this and a bone scan?

     

    Finally, it is important to note that bone density tests are not the same as bone scans.  Bone scans are nuclear medicine tests that require an injection and are used to detect fractures, cancer, infections and other abnormalities in the bone.  They are not useful for evaluating  evaluate bone density.