What is a Bone Density Scan?
Several methods are available to measure bone density, but currently the most widely used technique is DEXA (Dual Energy Xray Absorptiometry). This is the method used to determine efficacy in the recent large clinical trials, and to characterize fracture risk in large epidemiological studies. Older methods such as single photon absorptiometry do not predict hip fractures as well as DEXA.
Newer techniques such as Ultrasound appear to offer a more cost-effective method of screening bone mass. Ultrasound measurements are usually performed at the calcaneous and it is not possible to measure sites of osteoporotic fracture such as the hip or spine. Adding an ultrasound measurement to a DEXA does not improve the prediction of fractures. Although some have said that ultrasound measures the "quality" of bone, more careful studies suggest that it mainly measures the bone mass.
Yet another method, Quantitative Computed Tomography of the spine must be done following strict protocols in laboratories that do these tests frequently; in community settings the reproducibility is poor. The QCT measurements decrease more rapidly with aging, so the "T scores" in older individuals will be much lower than DEXA measurements.
How does a bone densitometry scan differ from an X-ray?
Prior to the bone densitometer, evaluating bone density using conventional x-ray systems which did not reveal a potential problem until a patient had lost 25-30 percent of her bone density. Now, in just 30 seconds, this highly sensitive densitometer helps us identify risk at a much earlier stage. It can also evaluate response to treatment so that we know whether our therapy is effective or if we need to modify our approach.
A bone densitometry scan measures the calcium content in the bones, which cannot be evaluated in an ordinary X-ray. Although an X-ray may appear to show that the bones have a low calcium content, the information is unreliable, so someone with osteoporosis may have perfectly normal X-rays. An X-ray is far more reliable when it comes to detecting a recent bone fracture.
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,
various anti-seizure medications 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 a parathyroid condition, such as hyperparathyroidism.
- Have experienced a fracture after only mild trauma.
- Have had x-ray evidence of vertebral fracture or other signs of osteoporosis.
The Lateral Vertebral Assessment (LVA), a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DEXA machine, may be recommended for older patients, especially if:
- They have lost more than an inch of height
- Have unexplained back pain
- If a DEXA scan gives borderline readings.
How is the procedure performed?
In the Central DEXA 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.
The patient 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 DEXA (pDEXA) test is even simpler. The patient's finger, hand, forearm or foot is placed 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 DEXA machine. The LVA test adds only a few minutes to the DEXA procedure.
The DEXA bone density test is usually completed within 10 to 30 minutes, depending on the equipment used and the parts of the body being examined.
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The information contained on this site is not intended to be a substitute for professional medical advice. Please visit Dr. Tapp prior to starting any new treatment or with any questions you may have regarding a medical condition or concern.