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What is DEXA Scanning?
Dual Energy X-ray Absorptiometry, or DEXA scanning, is currently the most widely used method to measure bone mineral density. It is the most widely used for several reasons. When compared with radiographic absorptiometry or single energy x-ray absorptiometry, DEXA scanning more precisely documents small changes in bone mass and is also more flexible since it can be used to examine both the spine and the extremities. A scan of the spine, hip, or the total body requires only one, two or four minutes respectively. Qualitative computed tomography (QCT) is the only technique that can directly measure bone density and volume but can distinguish trabecular from cortical bone. DEXA scanning is less expensive than QCT, exposes the patient to less radiation, and is more sensitive and accurate at measuring subtle changes in bone density over time or in response to drug therapy.
How does DEXA work?
The patient lies down on an examination table, and the scanner rapidly directs x-ray energy from two different sources towards the bone being examined. The mineral density of the patient’s bone weakens or prolongs the transmission of these two sources of x-ray energy through a filter onto a counter in a degree related to the amount of bone mass present. The greater the bone mineral density, the greater the signal picked up by the photon counter. The use of the two different x-ray energy, sources rather than more traditional radioisotope studies (such that would be used for a bone scan), greatly improves the precision and accuracy of the measurements.
How are the results of DEXA scanning helpful?
Studies using DEXA scanning have shown that people with osteoporosis have substantially lower bone density measurements than normal, age-matched people. Bone mineral density is widely accepted as a good indicator of bone strength. Thus low values can be compared against standard bone density measurements and help predict a patient’s risk for fracture.
Who should have a DEXA scan?
- Gorham’s vanishing bone disease/Lymphangiomatosis
Estrogen deficiency in women at clinical risk for osteoporosis
Evidence of vertebral abnormalities
Long-term steroid use
The need for monitoring response to approved drug therapies for osteoporosis.
How do you interpret the test results and who is a candidate for treatment?
A DEXA scan report compares the patient’s bone mineral density values with those of a young normal patient (T score) and with an age-matched normal patient (Z score). In comparing patients’ bone density against that of their peers, a low score indicates there may be a reason other than age-related bone loss.
Patient risk factors for osteoporosis that should influence the decision to begin treatment include: a maternal history of hip fracture, any previous fracture after the age of fifty, tall height at age of 25, poor health, some sedatives and anticonvulsant drugs, and the inability to rise from a chair without the use of the arms. The current treatment recommendation is the start of drug therapy to reduce the risk of fracture for women.