DXA (Dual X-ray Absorptiometry)

A DXA scan, or Dual X-ray Absorptiometry, is a fast and painless test performed to measure bone density - the strength of your bones. Not only can the test diagnose osteoporosis, but is used to detect early stage bone loss (osteopenia). A DXA scan, also called bone densitometry, may be repeated to measure the success of treatment to prevent osteoporosis. Whether you have osteoporosis or low bone mass, your doctor needs to know your T-score so a treatment plan can be implemented to reduce your risk for spinal fracture.

A DEXA scan is a special type of X-ray that measures bone density. DEXA stands for dual energy X-ray absorptiometry.

DEXA scans, also known as DXA scans, bone density scans or bone densitometry scans are most commonly used to diagnose osteoporosis (where the bones become weak and fragile and more likely to break). They can also be used to assess the risk of osteoporosis developing.

A DEXA bone scan can also help detect other bone-related conditions, such as osteopenia (very low bone mineral density) and osteomalacia (softening of the bones caused by a vitamin D deficiency). In children, osteomalacia is known as rickets.

There are two types of DEXA scan that are described below.

  • Axial or central DEXA scan – a large scanning arm is passed over the body to measure bone density in the centre of the skeleton, such as in the hip and lower spine.
  • Peripheral DEXA scan (pDEXA) – a large scanning arm or smaller portable device is used to measure bone density in the outer regions of the skeleton, such as the wrist, heel or hand.

Central DEXA devices are more commonly used to diagnose osteoporosis. Peripheral devices have the advantage of being small and portable.

 

Osteoporosis

You may need to have a DEXA scan if you are at risk of developing osteoporosis.

Osteoporosis can affect people of both sexes and all ages, although older, post-menopausal women are particularly at risk. This is because after the menopause the level of the hormone oestrogen falls, resulting in a decrease in bone density.

The denser your bones, the stronger and less likely they are to fracture (break). Osteoporosis does not cause any symptoms until a bone is broken.

In the past it was difficult to measure bone density and identify those at risk of developing osteoporosis, until a fracture occurred.

However, by using bone densitometry techniques, such as bone density scans, it is now possible to measure bone density before fractures occur.

Read more about when DEXA scans are used.

Measuring bone density

During a DEXA scan, X-rays will be passed through your body. Some radiation will be absorbed by the bone and soft tissue, and some will travel through your body.

Special detectors in the DEXA scanner measure how much radiation passes through your bones and this information is sent to a computer.

The measurements will be compared to the normal range for bone density in a healthy adult and someone of the same gender and ethnicity.

Safety

A DEXA scan is a quick and painless bone scan and is more effective than a normal X-ray in identifying low bone density. It also uses a much lower level of radiation than a standard X-ray.

A DEXA scan uses the equivalent of less than 10% of one day’s exposure to natural background radiation. By comparison, a chest X-ray uses the equivalent of about five days' exposure to natural background radiation.

Despite being very safe procedures, DEXA scans and X-rays are not recommended for women who are pregnant. X-rays are not considered to be safe to use during pregnancy because they can cause damage to an unborn child.

 

Who may need DXA?
Both men and women are at risk for low bone density and osteoporosis. Below is a short list of who may need to be tested. Of course, your doctor is the final authority.

Women

  • Post-menopausal, not taking estrogen
  • Thin and/or thin boned

Men and Women

  • Family history of osteoporosis
  • Smoker
  • Corticosteroid use
  • Certain anti-seizure drug use
  • Have Type 1 diabetes, liver or kidney disease, thyroid/parathyroid disorder
  • Previous fracture
  • Sustained a fracture with mild trauma

What is a DXA machine?
DXA is the newer technology previously known as DEXA (or dual energy x-ray absorptiometry). DXA and Quantitative CT (Computerized Tomography) are used to measure bone density in the spine and hip. This type of testing is preferred over peripheral or similar machines (pDEXA) that only measure bone density of the finger, wrist, or heel.

Test Preparation
Other than delaying calcium supplementation until after the test, no special preparation is necessary. You can remain fully clothed as long as there is nothing metallic in your clothing from the waist down.

What happens during the test?
You lie flat on a well-padded table. The technician may place a special pillow under your knees or between your ankles during part of the test. Radiation exposure is minimal and the test is over within 10-minutes.

What is a T-score?
Your T-score is determined by comparing your bone density to that of an average young adult. The score is further computed by determining the number of standard deviations of your bone density above or below the standard.