Bone Density Scan (cont.)
How is BMD measured?
Dual energy X-ray absorptiometry, or DEXA, is the most common method to measure a
patient's BMD. DEXA is relatively easy to perform and the amount of radiation exposure is low. A DEXA scanner is a machine that produces
two X-ray beams, each with different energy levels. One beam is high energy while the other is low energy. The amount of
X-rays that pass through the bone is measured for each beam. This will vary depending on the thickness of the bone. Based on the difference between the
two beams, the bone density can be measured. The radiation exposure from a DEXA scan is actually much less than that from a traditional chest
X-ray.
At present, DEXA scanning focuses on two main areas, the hip and spine. Although osteoporosis involves the whole body, measurements of BMD at one site can be predictive of fractures at other sites. Scanning generally takes 10 to 20 minutes to complete and is painless. The patient needs to be able to lay still on the table during the testing. There is no IV or other injection needed for this test. On the day
of the test, you may eat a normal meal, but you should not take any calcium supplements for 24 hours prior to the test.
Certain conditions can alter the results of the DEXA scan, making result less reliable. These include a lumbar spinal deformity (scoliosis), a large amount of calcium in the blood vessels (atherosclerosis), or multiple fractures. These conditions can falsely elevate the measured BMD with the DEXA scan.
What are other methods of measuring BMD?
There are small DEXA scanners called peripheral DEXA machines. These machines often measure BMD at the heel (calcaneus), shin bone (distal tibia), or kneecap (patella). Regular DEXA machines have a standard reference (called NHANES III) that can be used for all machines, no matter the manufacturer. However, peripheral DEXA machines do not yet have a uniform reference standard for the normal peak young adult bone mass that can apply to all machines and all manufacturers. This is necessary for peripheral DEXA to be ready for more widespread use. Efforts are in progress to make the peripheral DEXA technique more standardized.
Quantitative computed tomography (QCT) can be used to assess BMD. A standard CT scanner is used in this method. However, the amount of radiation exposure is higher than with DEXA and the cost is greater. For these reasons, QCT is not in general clinical use.
Ultrasound is a relatively new diagnostic tool to measure BMD. There is no radiation source with this procedure. An ultrasound beam is directed at the area being analyzed. The scattering and absorption of the waves allow for an assessment of bone density. The results are not as precise as with the other methods mentioned. This technique is relatively new, and there is considerable research being conducted in this area. Since ultrasounds can easily be performed in a physician's office, this method may become valuable for screening larger populations if its accuracy becomes more refined. If the BMD is low on the ultrasound test, you might be asked to have a DEXA scan to confirm the results.
Next: How often should DEXA scans be repeated to monitor
treatment? »
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