The recent posts on osteoporosis have mentioned the changes in bone mineral density that show on DXA without discussing what DXA is. The DXA scan is presently the most readily available, and relatively accurate, estimate of osteoporosis today. Some would argue that a bone biopsy can give more information. But that is limited to experimental animals, and is not practical for patients.
Other similar radiological tests evaluate bone for osteoporosis. The main two are Quantitative Computed Tomography and Quantitative Ultrasound. Their use is less than the use of DXA. And they are not yet standard like DXA.
Other clinical tests for osteoporosis include some blood and urine tests. They are good for determining if an intervention or treatment may be working. But they do not give the bone mineral density.
The DXA scan has more uses than just bone density. It also measures body composition. It can measure lean tissue or fat. But the most common use is to measure bone mineral density. The letters in its name stand for Dual energy x-ray Absorptiometry. The machine uses small amounts of x-rays to calculate the bone mineral density. It also calculates lean tissue, with the remainder considered fat.
Types Of DXA Scan
The two basic types of DXA scan are central and peripheral. The central scan measures the bone density at the hip and spine. The peripheral scan usually measures the bone density at the wrist or heel. Because the peripheral scan has had little evaluation, the central DXA remains the standard.
For the central DXA scan the patient lies on a horizontal table with a small imaging arm suspended overhead. The scan measures density in the hip and spine. Density in the hip and spine has a good correlation with the risk of fracture.
Another type of scan is the peripheral DXA scan. It uses a smaller portable box type of unit. The peripheral scan measures the density in the wrist or heel. But the peripheral scan does not yet correlate well with the central scan. So it is not as helpful for figuring fracture risk or response to treatment.
The peripheral scan use is for screening. Thus it determines the need for a further test. Sometimes the peripheral scan, and also the Quantitative Ultrasound, are the only practical tests available. So another use of the peripheral scan is to study indigenous groups or others at remotely located places. Usually the DXA scan is impractical in such situations.
How DXA Scan Is Done
The technique of the DXA involves a beam of x-rays with 2 energy peaks. The beam passes from the table below through the bone to the imaging arm above as it passes over the bone. Soft tissues absorb one energy peak and bone absorbs the other. Subtracting the soft tissue absorption from the total absorption yields the bone density. The bone mineral density is measured as g/cm2.
What The Scores Mean
The scan report gives two scores – the Z score and the T score. The Z score compares your density to others of your age, gender and size. It is usually used for younger people. The T score compares your bone density to someone of your gender and ethnic group at the age of peak bone density. It is usually used for older patients. A T score less than -2.5SD indicates osteoporosis. A T score between -1 and -2.5SD indicates osteopenia.
Mineral deposits into the bone slowly. So usually a patient will schedule a follow up scan a year after the patient starts corrective therapy. If the scan shows improvement, the scans usually spread out over longer and longer intervals. There have not been enough studies to determine ideal intervals or when to stop screening.
The U.S. Preventive Services Task Force recommends screening women over 65 years of age. Multiple other government agencies have their own individual guidelines. So these guidelines also factor into determining the scanning schedule.
Although DXA does use radiation, the dose is quite small. The amount received is less than that received from a round-trip airline flight from California to New York.
There is a good article at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2600106/ about the technical aspects for clinical physicians if you would like more details about DXA.