Dual-energy X-ray absorptiometry
Dual energy X-ray absorptiometry (DXA, previously DEXA) is a means of measuring bone mineral density (BMD). Two X-ray beams with differing energy levels are aimed at the patient's bones. When soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone. DXA is the most widely used and most thoroughly studied bone density measurement technology.
Commonly known as a bone density scan or bone densitometry, DXA scans are used as a screening and diagnostic test for osteoporosis. The bones that are most commonly fractured in humans with osteoporosis are scanned for screening purposes, although osteoporosis can occur in any bone and is not necessarily uniformly distributed in the skeleton. These include the proximal femur, and the lumbar spine. Under some circumstances, the distal radius and ulna are also scanned, usually in obese patients, or those whose orthopedic impairments make scanning of the spine and hips impossible.
In patients who have already sustained a fracture, the DXA scan is used to diagnose osteoporosis if it is suspected. For example, a 50 year old man falls and fractures his hip. The fall is minor enough to suspect some disease of bone may be present. A DXA scan would be used under these circumstances to determine the presence of osteoporosis.
Maximal BMD occurs at age 20 in both males and females. This BMD is used as the standard to which all DXA results are compared. Standards exist for all bones, for both males and females, and for ages 20 to 90. A DXA scan report shows the measured BMD, the difference between the measured BMD and the age-sex matched average, known as the Z score, and the difference between the measured BMD and the sex matched average 20 year old standard, known as the T score. A T score of -1.0 to -2.4 is diagnostic of osteopenia, which confers a modest fracture risk. A T score or -2.5 or less is indicative of osteoporosis.
DEXA scans can also be used to measure total body fat content, which is useful for athletes, models and overly health-conscious people.
Special considerations are involved in the use of DXA to assess bone mass in children. Specifically, comparing the bone mineral density of children to the reference data of adults (to calculate a T-score) will underestimate the BMD of children, because children have less bone mass than fully developed adults. This would lead to an overdiagnosis of osteopenia for children. To avoid an overestimation of bone mineral deficits, BMD scores are commonly compared to reference data for the same gender and age (by calculating a z-score).
Also, there are other variables in addition to age which are suggested to confound the interpretation of BMD as measured by DXA. One important confounding variable is bone size. DXA has been shown to overestimate the bone mineral density of taller subjects and underestimate the bone mineral densit of smaller subjects. This error is due to the way in which DXA calculates BMD. In DXA, two X-ray beams with differing energy levels scan a subject's bones. One energy level is absorbed mainly by soft tissue, while the other energy level is absorbed by bone. The subject's bone mineral content (BMC), measured in grams, can be determined by subtracting the amount of ionizing radiation absorption in soft-tissue from the total amount of absorption. Dividing this value by the projected area of the subject's skeleton or measured bone site, which the DXA machine also measures, gives the subject's BMD.
Because DXA calculates BMD using area, it is not an accurate measurement of true bone mineral density which is mass divided by a volume. Researchers sometimes refer to DXA BMD as an areal bone mineral density (aBMD) to distinguish it from volumetric bone mineral density. The confounding effect of differences in bone size is due to the missing depth value in the calculation of bone mineral density. (It should be noted that despite DXA technology's problems with estimating volume, it is still a fairly accurate measure of bone mineral content.) Methods to correct for this shortcoming include the calculation of a volume which is approximated from the projected area measure by DXA. DXA BMD results adjusted in this manner, are referred to as the bone mineral apparent density and are a ratio of the bone mineral content verus a cuboidal estimation of the volume of bone. Like aBMD, BMAD results does not accurately represent true bone mineral density, since bone is hollow and not cuboidal, but BMAD offers a value that controls for size discrepencies and can be compared to reference material for meaningful assessments of bone mass.
BMAD is used primarily for research purposes and is not used in clinical settings, yet.