Bone densitometry, also called dual-energy x-ray absorptiometry, DEXA or DXA, uses a very small dose of ionizing radiation to produce pictures of the inside of the body to measure bone density and bone loss. It is commonly used to diagnose osteoporosis, to assess an individual's risk for developing osteoporotic fractures and usually consists of scanning the lower (or lumbar) spine and hips but scanning of the forearm and lateral spine may be required in some circumstances. In children and some adults, the whole body is also sometimes scanned. DXA is simple, quick and non-invasive. It is also the most commonly used and the most standard method for diagnosing osteoporosis.
Bone density scanning is an enhanced form of x-ray technology. An x-ray (radiograph) is a non-invasive medical test that helps physicians diagnose and treat medical conditions. Imaging with x-rays involves exposing a part of the body to a small dose of ionizing radiation to produce pictures of the inside of the body. X-rays are the oldest and most frequently used form of medical imaging.
This exam requires little to no special preparation. Tell your doctor and the technologist if there is a possibility you are pregnant or if you recently had a barium exam or received an injection of contrast material for a CT or radioisotope scan. You may have to wait 10 to 14 days before undergoing a DXA test.
The DXA machine sends a thin, invisible beam of low-dose x-rays with two distinct energy peaks through the bones being examined. One peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is a patient's bone mineral density.
DXA machines feature special software that compute and display the bone density measurements on a computer monitor.
This examination is usually done on an outpatient basis.
For a DXA examination of the hip and spine, the patient lies on a padded table. An x-ray generator is located below the patient and an imaging device, or detector, is positioned above.
To assess the spine, the patient's legs are supported on a padded box to flatten the pelvis and lower (lumbar) spine. To assess the hip, the patient's foot is placed in a brace that rotates the hip inward. In both cases, the detector is slowly passed over the area, generating images on a computer monitor.
You must hold very still and may be asked to keep from breathing for a few seconds while the x-ray picture is taken to reduce the possibility of a blurred image.
An additional procedure called Vertebral Fracture Assessment (VFA) is also done at our hospital. VFA is a low-dose x-ray examination of the spine to screen for vertebral fractures that is performed on the DXA machine.
The VFA test adds only a few minutes to the DXA procedure.
The DXA bone density test is usually completed within 10 to 30 minutes, depending on the parts of the body being examined.
You will be asked to fill out a questionnaire that will help the doctor and healthcare scientists determine if you have medical conditions or take certain medications that either increase or decrease your risk of a fracture.
Bone density tests are a quick and painless procedure.
Routine evaluations every two years may be needed to see a significant change in bone mineral density, decrease or increase. Few patients, such as patients on high dose steroid medication, may need follow-up scans.
A physician (rheumatologist, geriatrician or endocrinologist) or clinical scientist specifically trained to supervise and interpret DXA examinations, will analyse the images and send a signed report to your primary care or referring physician, who will discuss the results with you.
A clinician should review your DXA scan while assessing the presence of clinical risk factors such as:
…which is why it is important to answer your patient questionnaire carefully and with detail.
T score — Shows the amount of bone you have compared with a young adult of the same gender with peak bone mass. A score of -1 and above is considered normal. A score between -1.1 and -2.4 is classified as osteopenia (low bone mass). A score of -2.5 and below is defined as osteoporosis. The T score is used to estimate your risk of developing a fracture and also to determine if treatment is required.
Z score — Reflects the amount of bone you have compared with other people in your age group and of the same size and gender. If this score is unusually high or low, it may indicate a need for further medical tests.
FRAX — An additional score may or may not be used, depending on your individual situation. The FRAX score represents the risk of having an osteoporosis-related fracture in the next 10 years. This score will heavily depend on the answers you give us on your DXA Patient Questionnaire.
Small changes may normally be observed between scans due to differences in positioning and usually are not significant.
During x-ray examinations special care is taken to use the lowest radiation dose possible while producing the best images for evaluation. National and international radiology protection organizations continually review and update the technique standards used by radiology professionals and in our hospital, this is also done locally.
Modern x-ray systems have very controlled x-ray beams and dose control methods to minimize stray (scatter) radiation. This ensures that those parts of a patient's body not being imaged receive minimal radiation exposure.