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 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.
There are other similar radiological tests for osteoporosis. The main two are Quantitative Computed Tomography and Quantitative Ultrasound. They are not used as much as DXA and have not become standard like DXA has.
Other clinical tests for osteoporosis include some blood and urine tests. They are good for distinguishing if an intervention or treatment may be working, but do not give the bone mineral density. They also do not tell if the results of the test indicate an improvement in bone mineral density until a DXA scan is done.
The DXA scan can be used to measure body composition. It can be used for measuring 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. Small amounts of x-rays are used to calculate the bone mineral density as well as lean tissue, with the remainder being considered fat.
Types Of DXA Scan
There are two basic types of DXA scan – 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 not been evaluated as much, the central DXA remains the standard.
The central DXA scan is performed with the patient on a horizontal table with a small imaging arm suspended overhead. This is used to measure density in the hip and spine. It has a good correlation with the risk of fracture.
Another type of scan is the peripheral DXA scan done with a smaller portable box type of unit that can be used to measure the density in the wrist or heel. 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 can be used to screen and determine if a further test is needed. It, and the Quantitative Ultrasound, also are sometimes the only practical tests available, such as when studying indigenous groups or others at remotely located places.
How DXA Scan Is Done
The technique of the DXA involves a beam of x-rays with 2 energy peaks that is aimed at the bone from the table below and is received by the imaging arm passing over the bone. One energy peak is absorbed by the lean tissues and the other by the bone. By subtracting the soft tissue from the total the bone density can be calculated. The bone mineral density is measured as g/cm2.
What The Scores Mean
Two scores are reported from the scan. The Z score and the T score. The Z score compares your density to others of your age, gender and size and 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. Osteoporosis is present if the T score is less than -2.5SD. Osteopenia is present if the T score is between -1 and -2.5SD.
It takes a while for mineral to deposit in the bone, so usually a follow up scan is scheduled a year after a patient has been started on a corrective course. If there is improvement, the scans are usually spread out at longer intervals. There have not been enough studies to determine ideal intervals or when to stop screening.
The recommendation of the U.S. Preventive Services Task Force is to start screening women over 65 years of age. Various government guidelines are in place so the scanning schedule is often determined by those factors also.
Although radiation is used, 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.