February 20, 2017
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Bone Density Scan (cont.)

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How is BMD measured?

Dual energy X-ray absorptiometry, or DXA, is the most common method to measure a patient's BMD. DXA, or densitometry, is relatively easy to perform and the amount of radiation exposure is low. A DXA 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 X-ray beams, the bone density can be measured. The radiation exposure from a DXA scan is actually much less than that from a traditional chest X-ray.

At present, DXA scanning gives information on the BMD two main areas, the hip and spine. Another bone that is often evaluated is the bone of the forearm. 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 lie still on the table during the testing. There is no IV or other injection needed for this test. In preparation for a DXA, 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 DXA scan, making result less reliable. These include a lumbar spinal deformity (scoliosis), extensive degenerative arthritis, a large amount of calcium in the blood vessels (atherosclerosis), or multiple fractures. These conditions can falsely elevate the measured BMD with the DXA scan.

What are other methods of measuring BMD?

There are small DXA scanners called peripheral DXA machines. These machines often measure BMD at the heel (calcaneus), shin bone (distal tibia), or kneecap (patella). Regular DXA machines have a standard reference (called NHANES III) that can be used for all machines, no matter the manufacturer. However, peripheral DXA 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 DXA to be ready for more widespread use. Efforts are in progress to make the peripheral DXA technique more standardized. At present, it is best used as a screening test to consider whether or not a patient would benefit from further bone density testing.

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 DXA 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 DXA scan to confirm the results.

New techniques that are being developed to measure both the BMD and even the quality of the bone are micro CT and MR, which use technologies related to CT and MRI scans. These are not yet available for clinical use.

Medically Reviewed by a Doctor on 7/22/2016

Source: MedicineNet.com

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