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- What is a Bone Densitometry Scan (DEXA) or Bone Density Scan?
- Why it is being recommended?
- How should I prepare?
- How does the procedure work?
- How is the procedure performed?
- What will I experience during and after the procedure?
- Who interprets the results and how do I get them?
- What are the benefits vs. risks?
- What are the limitations of DEXA Bone Densitometry?
What is a Bone Densitometry Scan (DXA)?
A bone densitometry scan is a special type of X-ray test used to measure the calcium content of the bone.
The examination is also called a dual energy X-ray absorptiometry (DEXA) scan or QDR scan. The DEXA scan is the established standard for measuring bone mineral density (BMD).
This is a simple, quick and non-invasive medical test, which involves exposing particular parts of the body to very small amounts of ionisation radiation. This is then used to produce images of the insides of the body.
DEXA scans are used to measure and therefore diagnose low bone density and also to monitor patients where a diagnosis of low bone density has been made.
DEXA scans are usually performed on the lower spine and the hips. In children and also some adults, the whole body may be scanned. For very low bone densities, devices which target the extremities of the body and use either X-rays or ultrasound may be used instead of the traditional DEXA scan. Similarly there’s a CT scan with special software can also be used as an alternative method for measuring very low bone densities. This is known as a QCT, and although it can be very accurate, it’s much less commonly used that a DEXA scan,
Why it is being recommended?
A scan can help a doctor who is examining a patient for osteoporosis to assess their risk of having a bone fracture within the next few years. Osteoporosis is a condition which mainly affects women after the menopause, but it can also be found in man too.
The association between the post-menopause period and the development of osteoporosis is due to a change in the normal hormones, which usually help maintain a normal bone density. Because of this, bone mass density testing is recommended for all women over the age of 65.
Other risk factors associated with the development of osteoporosis include a family history of the disease, being of small and thin stature, an inactive lifestyle, smoking, excessive drinking, and the use of certain medications, such as steroids.
Bone density testing is also strongly recommended for patients with type 1 diabetes, liver disease, kidney disease, those who have a thyroid condition or male patients with clinical conditions associated with bone loss.
A lateral vertebral assessment, which is a low dose X-ray examination of the spine that screens for vertebral fractures and is also performed on a DEXA machine, may be recommended for older patient – particularly if there’s a history of unexplained back pain, if there has been a loss of more than an inch of height, or if the initial DEXA scan gives borderline readings.
In general, people with a bone mineral density significantly lower than that normal for their age and sex are more likely to break a bone, even with very minor trauma such as falling from a small height or even from violent sneezing and coughing.
Diagnosing patients with a low bone density can prevent the risk of those patients obtainging a fracture in the future.
If the patient receives medical treatment, their bone mineral density will increase and their risk of fracture will reduce.
Using DEXA scanning to monitor osteoporosis treatment is a controversial area, and there is still much debate surrounding how useful periodic DEXA scanning is for monitoring changes in bone density during treatment.
If DEXA scanning is recommended, this will usually be at one to two year intervals – although more frequent follow-up, approximately every 6 months, is recommended for those on steroid treatment.
How should I prepare?
On the day of the exam you may eat normally. You should not take calcium supplements for at least 24 hours before your exam.
You should wear loose, comfortable clothing, avoiding garments that have zippers, belts or buttons made of metal. Objects such as keys or wallets that would be in the area being scanned should be removed.
You may be asked to remove some or all of your clothes and to wear a gown during the exam. You may also be asked to remove jewelry, removable dental appliances, eye glasses and any metal objects or clothing that might interfere with the x-ray images. Inform your physician if you recently had a barium examination or have been injected with a contrast material for acomputed tomography (CT) scan or radioisotope scan. You may have to wait 10 to 14 days before undergoing a DXA test.
Women should always inform their physician and x-ray technologist if there is any possibility that they are pregnant. Many imaging tests are not performed during pregnancy so as not to expose the fetus to radiation. If an x-ray is necessary, precautions will be taken to minimize radiation exposure to the baby.
How does the procedure work?
The DXA machine sends a thin, invisible beam of low-dose x-rays with two distinct energy peaks through the bones being examined. 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.
DXA machines feature special software that compute and display the bone density measurements on a computer monitor.
How is the procedure performed?
This examination is usually done on an outpatient basis.
In the Central DXA examination, which measures bone density in the hip and spine, the patient lies on a padded table. An x-ray generator is located below the patient and an imaging device, or detector, is positioned above.
To assess the spine, the patient’s legs are supported on a padded box to flatten the pelvis and lower (lumbar) spine. To assess the hip, the patient’s foot is placed 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.
You must hold very still and may be asked to keep from breathing for a few seconds while the x-ray picture is taken to reduce the possibility of a blurred image. The technologist will walk behind a wall or into the next room to activate the x-ray machine.
The peripheral tests are simpler. The finger, hand, forearm or foot is placed in a small device that obtains a bone density reading within a few minutes.
An additional procedure called Lateral Vertebral Assessment (LVA) is now being done at many centers. LVA is a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine.
The LVA test adds only a few minutes to the DXA procedure.
The DXA bone density test is usually completed within 10 to 30 minutes, depending on the equipment used and the parts of the body being examined. You will probably be asked to fill out a questionnaire that will help the doctor determine if you have medical conditions or take certain medications that either increase or decrease your risk of a fracture. The World Health Organization has recently released an online survey that combines the DXA results and a few basic questions and can be used to predict 10-year hip fracture risk for post-menopausal women. This will be coming into more use in the next few years.
What will I experience during and after the procedure?
Bone density tests are a quick and painless procedure.
Routine evaluations every two years may be needed to see a significant change in bone mineral density, decrease or increase. Few patients, such as patients on high dose steroid medication, may need follow-up at six months.
Who interprets the results and how will I get them?
A radiologist, a physician specifically trained to supervise and interpret radiology examinations, will analyze the images and send a signed report to your primary care or referring physician, who will discuss the results with you.
DXA scans are also interpreted by other physicians such as rheumatologists and endocrinologists. A clinician should review your DXA scan while assessing the presence of clinical risk factors such as:
- rheumatoid arthritis
- chronic renal and liver disease
- respiratory disease
- inflammatory bowel disease
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 (low bone mass). A score below -2.5 is defined as osteoporosis. The T score 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 this score is unusually high or low, it may indicate a need for further medical tests. Small changes may normally be observed between scans due to differences in positioning and usually are not significant.
What are the benefits vs. risks?
- DXA bone densitometry is a simple, quick and noninvasive procedure.
- No anesthesia is required.
- The amount of radiation used is extremely small—less than one-tenth the dose of a standard chest x-ray, and less than a day’s exposure to natural radiation.
- DXA bone density testing is the most accurate method available for the diagnosis of osteoporosis and is also considered an accurate estimator of fracture risk.
- DXA equipment is widely available making DXA bone densitometry testing convenient for patients and physicians alike.
- No radiation remains in a patient’s body after an x-ray examination.
- X-rays usually have no side effects in the typical diagnostic range for this exam.
- There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk.
- Women should always inform their physician or x-ray technologist if there is any possibility that they are pregnant. See the Safety page for more information about pregnancy and x-rays.
- The effective radiation dose for this procedure varies.
- No complications are expected with the DXA procedure.
What are the limitations of DXA Bone Densitometry?
- A DXA test cannot predict who will experience a fracture but can provide indications of relative risk.
- Despite its effectiveness as a method of measuring bone density, DXA 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; in such instances, CT scans may be more useful.
- Central DXA devices are more sensitive than pDXA devices but they are also somewhat more expensive.
- A test done on a peripheral location, such as the heel or wrist, may help predict the risk of fracture in the spine or hip. These tests are not helpful in following response to treatment, however, and if they indicate that drug therapy is needed, a baseline central DXA scan should be obtained.