is used to assess bone mass by measuring bone mineral density
(which is the amount of bone per unit of skeletal area). It
is the most sensitive screening tool to detect osteoporosisa
disorder characterized by fragile, weak bones due to a drop
in bone mass and increased risk of fracture. The following
techniques are used to measure bone mineral density.
x-ray absorptiometry (DEXA) is considered the gold standard for
quantifying bone mass in the spine and hip bone. In this technique,
an x-ray tube emits 2 x-ray beams, which pass through the bone
and are picked up by a detector. A computer is used to analyze
the resulting images and calculate bone density based on the
amount of radiation absorbed by the bonedenser bones absorb
DEXA (pDEXA) uses a small, portable device that works similar
to DEXA but is used to examine bone density in peripheral bones,
such as the wrist.
absorptiometry (SXA) also uses a small, portable device that
works similar to DEXA, but emits a single x-ray beam. It is used
to examine the heel bone or forearm.
ultrasound (QUS), the only technique for measuring bone density
that does not use x-rays, works by transmitting high-frequency
sound waves through the heel bone to a signal receiver.
computed tomography (QCT) uses a standard CT scannerfitted
with special equipment and software to measure bone massto
deliver x-rays to the spine at many different angles.
Purpose of the Test
To screen people
at high risk for osteoporosis, such as postmenopausal women or
on long-term therapy with corticosteroid drugs.
To detect suspected
osteoporosis in people who have experienced recent unexplained
DEXA is the
only method that may be used to make a definitive diagnosis of
osteoporosis and to monitor the response to treatment.
To detect and
evaluate other disorders affecting bone mass, such as the inherited
disorder osteogenesis imperfecta.
Who Performs It
or a technician.
DEXA is the
most accurate method for measuring bone density in the spine
and hip bone, the most common sites of fracture. It is of limited
use, however, in people with a spinal deformity or those who
have had previous spinal surgery. In addition, the presence of
vertebral compression fractures or degenerative disease such
as osteoarthritis may interfere with the accuracy of the test.
QCT is only
used in selected individuals due to its limited availability,
relatively high cost, and greater radiation exposure. It is the
only method that can evaluate spongy bone tissue in the spine,
which may be useful when assessing the response to therapy for
SXA, and QUS are performed by small, portable instruments and
are less expensive than DEXA or QCT, many physicians now use
these methods to screen patients in their offices.
should not undergo x-ray tests because exposure to ionizing radiation
may harm the fetus.
barium in the gastrointestinal tract can interfere with test
results, DEXA should not be performed until about 10 days after
an imaging study using this contrast agent.
blood vessels overlying the area being scannedsometimes
found in atherosclerotic plaquesmay falsely increase bone
the scan may alter the test results.
Before the Test
Do not take
calcium supplements for 24 hours before the test.
metallic objects (such as belt buckles, zippers, jewelry, coins,
and keys) that might obstruct the scanning path.
You may be
able to wear your street clothes, but for DEXA or QCT you may
be asked to disrobe and put on a hospital gown.
What You Experience
You will lie on an padded
table with an x-ray generator below and a detector camera above.
your spine, your legs will be supported on a padded box to flatten
your pelvis and lower (lumbar) spine. The detector device is
slowly passed over the lumbar area, producing images projected
onto a computer monitor.
To assess the
hip, your foot is placed in a brace that rotates the hip inward,
and the scanning procedure is repeated.
The scan may
take up to 30 minutes.
You place your finger or
hand in a small scanning device and a bone density reading
is obtained within a few minutes.
You immerse your heel or
forearm in a tub of water. A scanning device obtains a bone
density reading through the water within a few minutes.
An ultrasound device held
next to your heel obtains a reading in about 10 seconds.
You will lie on your back
on a narrow table that is then advanced into the CT scanner.
The scanner, which encircles you, rotates around you taking
pictures at different intervals and from various angles. You
will feel the table move during the test.
You must remain
as still as possible because any movement can distort the scan.
The test typically
takes 15 to 30 minutes.
Risks and Complications
imaging does not expose you to any radiation.
and SXA involve only minimal exposure to radiation. Although
radiation exposure is still minimal with QCT, you will receive
a higher dose (50 to 100 times greater than DEXA).
After the Test
You may resume
your normal activities immediately after the test.
reviews the test data to determine your bone mineral status.
If the findings
indicate bone loss due to osteoporosis, your doctor will prescribe
a treatment programwhich typically includes exercise, calcium
and vitamin D supplements, and sometimes hormone replacement
or other medicationsto help forestall further bone loss
From The Johns Hopkins Consumer Guide to Medical
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