Bone mineral density (BMD) is a measure of the amount of calcified tissue in bones. BMD is best measured using DXA scanning.
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. DXA does involve a small amount of radiation, it is usually administered by a department qualified to make use of radiation for medical imaging.
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 30 in both males and females. This BMD is used as the standard to which all DXA results are compared. 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 30 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 of -2.5 or less is indicative of osteoporosis.
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