| Bone Densitometry
What is Radiography-based (X-ray) Bone Densitometry? Back to Top
Every
day, physicians use radiography, or x-rays, to view and evaluate bone
fractures and other injuries of the musculoskeletal system. However, a
plain x-ray test is not the best way to assess bone density. To detect
osteoporosis accurately, doctors use an enhanced form of x-ray
technology called dual-energy x-ray absorptiometry (DXA or DEXA). DEXA
bone densitometry is today's established standard for measuring bone
mineral density (BMD). DEXA is a quick, painless procedure for
measuring bone loss. Measurement of the lower spine and hips are most
often done. More portable devices that measure the wrist, fingers or
heel are sometimes used for screening, including some that use
ultrasound waves rather than x-rays.
What are some common uses of the procedure? Back to Top
DEXA
bone densitometry is most often used to diagnose osteoporosis, a
condition that often affects women after menopause but may also be
found in men. Osteoporosis involves a gradual loss of calcium, causing
the bones to become thinner, more fragile and more likely to break. The
DEXA test can also assess your risk for developing fractures. If your
bone density is found to be low, you and your physician can work
together on a treatment plan to help prevent fractures before they
occur. DEXA is also effective in tracking the effects of treatment for
osteoporosis or for other conditions that cause bone loss. Bone density
testing is strongly recommended if you:
- are a post-menopausal woman and not taking estrogen.
- have a personal or maternal history of hip fracture or smoking.
- are a post-menopausal woman who is tall (over 5 feet 7 inches) or thin (less than 125 pounds).
- are a man with clinical conditions associated with bone loss.
- use
medications that are known to cause bone loss, including
corticosteroids such as Prednisone, various anti-seizure medications
such as Dilantin and certain barbiturates, or high-dose thyroid
replacement drugs.
- have type 1 (formerly called
juvenile or insulin-dependent) diabetes, liver disease, kidney disease
or a family history of osteoporosis.
- have high bone turnover, which shows up in the form of excessive collagen in urine samples.
- have a thyroid condition, such as hyperthyroidism.
- have experienced a fracture after only mild trauma.
- have had x-ray evidence of vertebral fracture or other signs of osteoporosis.
How should I prepare for the procedure? Back to Top
On the
day of the exam eat normally, but don't take calcium supplements for at
least 24 hours beforehand. Wear loose, comfortable clothing, avoiding
garments that have zippers, belts or buttons made of metal.
Inform your physician if you recently had a barium examination or
have been injected with a contrast material for a computed tomography
(CT) scan or radioisotope scan; you may have to wait 10 to 14 days
before undergoing a DEXA test. Women should always inform their
physician or x-ray technologist if there is a possibility they are
pregnant.
What does the DEXA equipment look like? Back to Top
There
are two types of DEXA equipment: the central device and the peripheral
device. Central DEXA devices measure bone density in the hip and spine,
while peripheral devices measure it in the wrist, heel or finger. The
central DEXA device is used in hospitals and medical offices, while the
smaller peripheral device is available in drugstores and on mobile
health vans in the community. CT scanners also can be used effectively
to evaluate the spine and hip for osteoporosis.
Central devices have a large, flat table and an "arm" suspended
overhead. The arm swings away so that the table can be used as a
treatment table or exam chair for routine patient examinations. The
peripheral DEXA (pDEXA) device weighs only about 60 pounds. It is a
portable box-like structure that includes a space to insert your foot
or forearm for imaging.
How does the procedure work? Back to Top
The DEXA machine
sends a thin, invisible beam of low-dose x-rays with two distinct
energy peaks through your bones. One peak is absorbed mainly by soft
tissue and the other by bone. The soft tissue amount can be subtracted
from the total and what remains is a patient's bone mineral density.
All devices feature special software to compute the data and display
them on a computer monitor, allowing your doctor to make an accurate
diagnosis. The amount of radiation used is extremely small—less than
one-tenth the dose of a standard chest x-ray.
How is the procedure performed? Back to Top
The DEXA bone
density test takes 10 to 30 minutes, depending on the equipment used
and the parts of the body being examined. You may be asked to undress
and put on a hospital gown. Then you'll lie on a padded table with an
x-ray generator below and a detector (an imaging device) above.
Most often, doctors focus on bone loss in the spine and hip where
most osteoporosis-related fractures happen. During an examination of
the spine, your legs will be supported on a padded box to flatten your
pelvis and lower (lumbar) spine. To assess your hip, the technologist
will place your foot in a brace that rotates the hip inward. In both
cases the detector is slowly passed over the area, generating images on
a computer monitor.
The peripheral DEXA (pDEXA) test is even simpler. You place your
finger, hand, forearm or foot in a small device, and a bone density
reading is obtained within a few minutes. These tests may not be as
sensitive—especially in younger people—and cannot be used to monitor
response to treatment.
Lateral Vertebral Assessment (LVA)
An additional
procedure called Lateral Vertebral Assessment (LVA) is now being done
at many centers. This is a low dose x-ray examination of the spine to
screen for vertebral fractures. It is performed on the DEXA machine.
Vertebral fractures are common in older individuals and may indicate
increased risk for additional fractures if they are present. They are
often asymptomatic. The test is painless and adds only a few minutes to
the DEXA procedure. It may be recommended for older patients,
especially if they have lost more than an inch of height, have
unexplained back pain, or if the DEXA scan gives borderline readings
and the increased risk if the patient has evidence of fracture would
influence the decisions in regard to treatment.
What will I experience during the x-ray procedure? Back to Top
DEXA
bone densitometry is a simple, non-invasive procedure. Once on the
table you may be asked to hold an awkward position for a short time
while the arm of the machine passes over your body taking measurements.
It is important that you stay as still as possible during the procedure
to ensure a clear, useful image. No anesthesia is required. The
procedure is painless and radiation exposure is minimal.
Who interprets the results and how do I get them? Back to Top
The
results of a DEXA bone density exam are interpreted by a radiologist*,
who is a physician specially trained to diagnose conditions and
diseases by obtaining and interpreting medical images. The radiologist
will send an interpretation of your results and a signed report to your
primary care physician who will work with you to develop a treatment
plan. Usually available within a few days, your test results will be in
the form of two scores:
T score — This number shows the amount of bone you have compared
with a young adult of the same gender with peak bone mass. A score
above -1 is considered normal. A score between -1 and -2.5 is
classified as osteopenia, the first stage of bone loss. A score below
-2.5 is defined as osteoporosis. It is used to estimate your risk of
developing a fracture.
Z score — This number reflects the amount of bone you have compared
with other people in your age group and of the same size and gender. If
it is unusually high or low, it may indicate a need for further medical
tests.
* Many DEXA scans are interpreted by other physicians such as rheumatologists and endocrinologists.
What are the benefits vs. risks? Back to Top
Benefits
- DEXA bone density testing is the most accurate method
available for the diagnosis of osteoporosis. It is also considered an
accurate estimator of fracture risk. It will not tell whether you will
or will not have a fracture, but gives relative risk of suffering a
fracture, just as cholesterol and blood pressure help determine risk
for heart disease. A low reading should not cause you to be anxious but
may help you set healthy goals. As with other diseases and conditions,
early detection is the key to prevention of further bone loss and
eventual fractures. DEXA equipment is widely available making DEXA bone
densitometry testing convenient for patients and doctors alike.
Risks
- No complications are expected with the DEXA procedure.
What are the limitations of DEXA Bone Densitometry? Back to Top
Despite
its effectiveness as a method of measuring bone density DEXA is of
limited use in people with a spinal deformity or those who have had
previous spinal surgery. The presence of vertebral compression
fractures or osteoarthritis may interfere with the accuracy of the
test. CT scans may be more useful in such instances. DEXA cannot
predict who will experience a fracture but can provide indications of
relative risk.
Central DEXA devices are more sensitive than pDEXA devices but they
are also somewhat more expensive. The peripheral devices don't
accurately follow changes in your bones during therapy. A test done on
a peripheral location, such as your heel or wrist, may help predict the
risk of fracture in your spine or hip. But because bone mass tends to
vary from one location to the other, measuring the heel is not as
accurate as measuring the spine or hip. Small changes may normally be
observed between scans due to differences in positioning and may not be
significant. As with mammograms, the examination must be done with
great care to maximize accuracy. Back to Top |
