Purpose
According to Radiology Info, DEXA stands for dual-energy X-ray absorptiometry, and it is sometimes called bone densitometry. DEXA scans have become the most common way of determining the density of the bones, which allows doctors to measure bone loss and diagnose conditions such as osteoporosis. The X-rays themselves are produced by a machine that uses small amounts of radiation to form the X-ray beams. X-rays are one of the oldest imaging techniques used in hospitals and are generally safe, and DEXA scans, Healthline notes, are actually much safer than standard X-rays because they use even lower amounts of radiation. Typically DEXA scans involve only looking at the hips or spine; bone loss there can cause immediate problems. Sometimes, however, the entire body is scanned.
Theory
DEXA scans use two kinds of X-ray beams. As Healthline notes, one of the beams is a higher energy beam, the other of a lower intensity. The lower intensity beam is only absorbed by soft tissue in the body such as muscle and fat, whereas the higher intensity beam gets absorbed both by bones and by soft tissue. The signal from the low intensity beam can then be subtracted from the signal for the high intensity beam, allowing technicians to measure the amount of the X-ray beam that is absorbed by the bone alone. The more dense the bone, the more X-ray radiation it absorbs.
Procedure and Analysis
The entire DEXA scan typically takes 10 to 30 minutes. For the most common form, the central DEXA scan, the patient will lie on a padded table. First the patient's legs will be supported to flatten the spine, and then the leg will be positioned to turn the hip inward, making it easier to examine. After the scan is done, the bone mineral density is compared to both that of a healthy young person (called the T score) and a healthy person of the same age as the patient (the Z score). T scores are a measure of absolute bone loss but, because most people lose some of their bone density as they age, the Z score is usually more relevant for the diagnosis of diseases such as osteoporosis.
