Bone Mineral Density
How It Is Done continued...
You will need to lie
on your back on a padded table. You can usually leave your clothes on. You may
need to lie with your legs straight or with your lower legs resting on a
platform built into the table.
The machine will scan your bones
and measure the amount of radiation they absorb. The DEXA technique, which
scans the hip and lower spine, takes about 20 minutes to perform. Other
techniques may take 30 to 45 minutes.
Portable machines (P-DEXA)
can measure bone density in the wrist or forearm.
Testing at least two different bones
(preferably the hip and spine) each time is the most reliable way of measuring
BMD. It is best to test the same bones and to use the same measurement
technique and BMD equipment each time.
How It Feels
A bone mineral density test does not cause
pain. If you have back pain, it may be uncomfortable to lie still on a table
during the scan.
During a bone mineral density (BMD) scan, you are
exposed to a very low dose of radiation. A BMD scan is not recommended for
pregnant women because of the radiation exposure to the unborn baby.
A bone mineral density (BMD) test measures
the density of minerals (such as
calcium) in your bones using a special
computed tomography (CT) scan. Results are usually available in 2 to 3
Results of bone mineral density tests can be reported in
Your T-score is your BMD compared to the
average score of a healthy 30-year-old. It is expressed as a standard deviation
(SD), which is a statistical measure of how closely each person in a group is
to the average (mean) of the group. The average BMD is determined by measuring
the bone density of a large group of healthy 30-year-olds (young adult
reference range). BMD values are then reported as a standard deviation from the
mean of this reference group. Almost all 30-year-old people have a BMD value
within 2 standard deviations of this mean.
- A negative (–) value means that you have
thinner bones (lower bone density) than an average 30-year-old. The more
negative the number is, the less bone density you have compared with an average
- A positive (+) value means that your bones are
thicker and stronger than an average 30-year-old.