A bone densitometry test, or scan, is designed to check for osteoporosis, a disease that occurs when the bones become thin and weak. Osteoporosis happens when the bones lose calcium and other minerals that keep them strong. Osteoporosis begins after menopause in many women, and worsens after age 65, often resulting in serious fractures. These fractures may not only bring disability, but may affect longevity. As many as one-fourth of women who fracture their hip after age 50 die within one year.
Most people today will get a bone density scan from a machine using a technology called dual energy x-ray absorptiometry, or DEXA. This machine takes a picture of the bones in the spine, hip, total body and wrist, and calculates their density. If a DEXA machine is not available, bone density scans can also be done with dual photon absorbtiometry (measuring the spine, hip and total body) and quantitative computed tomography scans (measuring the spine and hips). Bone density scanners that use DEXA technology to just measure bone density in the wrist (called pDEXA scans) provide scans at some drugstores. Yet these tests are not as accurate as those that measure density in the total body, spine or hip—where most fractures occur. The DEXA scanner is most commonly found in large medical groups and medical centers.
Two newer devices are also available to check bone density. The Sahara Clinical Bone Sonometer, introduced in 1998, uses the velocity of sound waves to determine bone density. The device is much smaller then the DEXA scanner and requires that the seated patient place the foot in the scanner for about one minute. The device sends ultrasound waves through the heel. Since the device is much smaller and cheaper than the DEXA machine, it is found most commonly in doctor's offices. The second device, introduced in 2000, is the Alara MetriScan, a tabletop scanner that allows clinicians to perform the test in a doctor's office. The patient puts the nondominant hand on the device's platform and the scan is done by x-raying the fingers.
A bone density scan measures the thickness of an individual's bones and determines the risk of fracture.
The test is not recommended for people who cannot lay flat for the test, and people with heart failure or back problems. People who are severely overweight may not be able to take the test if the machine cannot support their weight.
To take a DEXA bone density scan, the patient lies on a bed underneath the scanner, a curving plastic arm that emits x rays. These low-dose x rays form a beam that scans the patient. During the test, the scanner moves to capture images of the patient's spine, hip, arm, or entire body. A computer then compares the patient's bone thickness and risk of fracture to that of other people in the United States at the same age and to young people at peak bone density. Bones reach peak density at age 30 and then start to lose mass. The test takes about 20 minutes to do and is painless. The DEXA bone scan costs about $250. Some insurance companies and Medicare cover the cost. pDEXA wrist bone scans in drugstores are available for about $30.
A patient may need a bone density scan if he or she:
- is near menopause
- has broken a bone after a modest trauma
- has a family history of osteoporosis
- uses steroid or antiseizure medication
- has had a period of restricted mobility for more than six months
- has hyperthyroidism (high thyroid levels)
- has hypothyroidism (low thyroid levels)
Bone densitometry scans are used primarily in women, due to the relationship between bone density loss and menopause. However, there are some instances where a bone density scan may be in order for men with the following risk factors:
- low testosterone levels
- a family history of osteoporosis
- has been taking glucocorticoids (a group of steroids involved in carbohydrate, fat, and protein metabolism)
- has hyperparathyroidism (the presence of excess of the parathyroid hormone parathormone that is associated with calcium utilization)
- has suffered a fracture with a minimum amount of trauma
- has or has had prostate cancer that was treated with hormones or radiation
The patient wears comfortable but loose clothing, such as a sweatsuit, and lies on the table underneath the scanner. The patient must not be wearing any metal objects that would interfere with the scan, including zippers, fasteners, and jewelry.
No post-treatment procedures or care are required following a bone densitometry scan.
The DEXA bone scan exposes the patient to only a small amount of radiation—about one-fiftieth that of a chest x ray.
The patient's bone density is compared with the normal bone density (called a T-score.) A T-score above 1 means that a patient has a better bone mass than is typical. Scores from 0 to -1 mean that the patient has normal bone mass and should repeat the test in two to five years.
If patients' T-scores range from -1 to -2.5, they have low bone mass (osteopenia) and are at risk for osteoporosis. A T-score below -2.5 means osteoporosis is already evident. These patients should have a repeat bone density scan every year or two to assess the response to therapy.
Health care team roles
The DEXA test is done primarily by a radiology or nuclear medicine technician. The ultrasound and hand x-ray scans are administered mainly by registered nurses (RNs). The tests are reviewed and treatment prescribed by a physician. RNs also help educate the patient on the role nutrition and exercise play in bone health.
Calcium— A mineral that helps build bone. After menopause, when women start making less of the bone-protecting hormone estrogen, they may need to increase their intake of calcium.
DEXA bone density scan— A bone density scan that uses a rotating x-ray beam to measure the strength of an individual's bones and his or her fracture risk.
Osteoporosis— A disease that occurs when the bones lose the calcium and structure that keep them strong. It often occurs after menopause (around age 50) in women and in old age in men.
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National Osteoporosis Foundation. 1232 22nd St. NW, Washington, DC 20037. (800) 223-9994. 〈http://www.nof.org〉.