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Dual-energy X-ray absorptiometry (DXA) is a means of measuring bone mineral density (BMD) using spectral imaging.
Two X-ray beams, each with a different energy level, are aimed at the patient’s bones.
The soft tissue absorption is subtracted out, and the bone mineral density (BMD) can be determined from the absorption of each beam by bone.
Dual-energy X-ray absorptiometry is the most widely used bone density measurement technology.
DXA is typically used to diagnose and follow osteoporosis.
One cannot reliably compare DXA measurements overtime from repeat testing unless the measurements are performed on the same the x-ray machine use for the earlier test.
Differences between DXA machines can eexceed 10%
A single X-ray beam passing through the body will be attenuated by both soft tissue and bone.
The attenuation coefficients vary with the energy of the X-rays, and, the ratio of the attenuation coefficients also varies.
DXA uses two energies of X-ray, and the difference in total absorption between the two can be used to subtract out the absorption by soft tissue, leaving just the absorption by bone, which is related to bone density.
The combination of dual X-ray absorptiometry and laser measures the thickness of the region scanned, allowing for varying proportions of lean soft tissue and adipose tissue within the soft tissue improving the accuracy.
The U.S. Preventive Services Task Force recommends that women over the age of 65 should get a DXA scan.
Men should be tested at age age 70.
Osteoporosis risk can be measured using the FRAX calculator, which includes: risk factors including prior fragility fracture, use of glucocorticoids, heavy smoking, excess alcohol intake, rheumatoid arthritis, history of parental hip fracture, chronic renal and liver disease, chronic respiratory disease, long-term use of phenobarbital or phenytoin, celiac disease, inflammatory bowel disease, and other risks.
DEXA assessment of bone mineral density of the femoral neck and the lumbar spine are performed.
Bone densities are often given to patients as a T score or a Z score.
A T score tells what the patient‘s bone mineral density is in comparison to a young adult of the same gender with peak bone mineral density.
A normal T score is -1.0 and above, low bone density is between -1.0 and -2.5, and osteoporosis is -2.5 and lower.
A Z score is just a comparison of what a patient’s bone mineral density is in comparison to the average bone mineral density of a male or female of their age and weight.
DXA overestimated the bone mineral density of taller subjects and underestimate the bone mineral density of smaller subjects.
Because DXA calculates BMD using area it is not an accurate measurement of true bone mineral density, which is mass divided by a volume.
When interpreting the BMD results, clinicians should refer to the total hip, femoral neck, and lumbar spine.
Local factors affecting bone density such as weight bearing, muscle usage and co-existing osteoarthritis may result in differences in BMD categories between bones.
Quantitative computed tomography (QCT) can measure the bone’s volume, and are, therefore, not susceptible to the confounding effect of bone-size in the way that DXA results are susceptible.
Repeat BMD measurements should be done on the same machine each time.
Error between machines, can introduce errors large enough to wipe out the sensitivity of the measurements.
WHO classification of osteoporosis and osteopenia in adults cannot be applied to children.
DXA scans can also be used to measure total body composition and fat content with a high degree of accuracy.
The DXA scans, a low resolution fat shadow image can also be generated, which gives an overall impression of fat distribution throughout the body.
DXA uses radiation: dose of current DEXA systems is small, as low as 0.001 mSv, much less than a standard chest or dental x-ray.
Uses less radiation than a standard chest x–ray.