What is DEXA Scanning?
Dual Energy X-ray Absortiometry, or DEXA scanning, is currently the most widely used method to measure bone mineral density. For the test, a patient lies down on an examining table, and the scanner rapidly directs x-ray energy from two different sources towards the bone being examined in an alternating fashion at a set frequency. 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.
DEXA scanning has become the most widely used method for measuring bone mineral density for several reasons. When compared with radiographic absortiometry or single energy x-ray absortiometry, 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, 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 than is QCT.
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 based upon the DEXA scan measurements.
Who should have a DEXA scan?
People with Gorhams vanishing bone disease / Lymphangiomatosis
estrogen deficiency in women at clinical risk for osteoporosis
evidence of vertebral abnormalities
long term steroid use
patients with primary hyperparathyroidism
the need for monitoring to assess 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 young normal patient (T score) and with age matched normal patient (Z score). By comparing a patient's bone density against there peers, a low score indicates there may be a reason other than age related bone loss.
Patients risk factors for osteoporosis that should play a part in the decision to begin treatment include: a maternal history of a 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 recommendations are the start of drug therapy to reduce the risk fracture for all women with a bone mineral density T score of less than -2 without other risk factors and for those with a T score of less than -1.5 if other risk factors are present.