Bone Densitometry (DEXA)

Osteoporosis involves a gradual loss of calcium causing the bones to become thinner, more fragile and more prone to fracture.  It often affects women, but may also be found in men.  Bone densitometry is a quick painless procedure for measuring bone loss.  To detect osteoporosis accurately an enhanced form of x-ray technology called dual energy x-ray absorptiometry (DEXA) is used.  DEXA is recognised as the established standard for measuring bone mineral density.

The DEXA test can assess your risk for developing fractures.  If your bone density is found to be low, you may work with your doctor on a treatment plan to help prevent fractures before they occur.  DEXA is also effective in tracking the effect of treatments for osteoporosis.

Preparation for the Bone Densitometry Examination

Wear loose comfortable clothing, avoiding garments that have metal belts zips, buttons or buckles.  Alternatively you will be asked to change into a loose fitting gown.  Women should always inform the technologist if there is any possibility they are pregnant. 

How the Procedure Works

The DEXA machine has a large flat table with an "arm" suspended overhead.  The machine sends a thin invisible stream of low dose x-rays through your bones via two energy streams.  One is absorbed mainly by soft tissue and the other by bone.  Most often the spine and hip regions are measured, as this is where most osteoporosis-related fractures occur.  During the examination of your spine, your legs are supported to flatten your pelvis and lower spine.  To assess the hip.  Your foot will be placed in a brace that rotates the hip inward.  In each case, the arm of the machine passes slowly over the area being measured. It is important you lie as still as possible to ensure a clear useful image.  The machine has specialised software to compute the data and displays it on a computer monitor, allowing the radiologist to make an accurate diagnosis.  The amount of radiation used is very small i.e. less than one tenth the dose of a standard chest x-ray.

Bone density testing may be recommended in the following circumstances:

• If you are a post-menopausal woman not taking oestrogen.
• If you are a post-menopausal woman who is very tall or thin.
• If you use medications known to be associated with bone loss, such as corticosteroids such as Prednisone, various anti-seizure medications, and high dose thyroid replacement drugs.
• If you have Type I diabetes, liver disease, kidney disease, a thyroid condition or a family history of osteoporosis.
• If you have experienced a fracture with only minimal trauma.
• Have had x-ray evidence of a vertebral fracture or other signs of osteoporosis.

The Bone Density Results

The results are interpreted by a Radiologist and a report is sent to your doctor.

The results are in the form of two scores:

1. T score -  this shows the amount of bone you have compared to a young adult of the same gender with peak bone mass.  A score above -1 is considered normal. A score between -1 and -2.5 is classified as osteopaenia, the first stage of bone loss.  A score below -2.5 is defined as osteoporosis.  It is used to estimate your risk of developing a fracture in the future.
2. Z score  - this reflects your bone density compared to other people in your age group and of the same size and gender.

What are the benefits of bone densitometry?

DEXA is the most accurate method available for the diagnosis of osteoporosis.  It is also considered and accurate estimator or fracture risk.  While it will not tell you whether you will or will not have a fracture, it gives the relative risk of sustaining one, just as cholesterol and blood pressure help determine risk for heart disease. 

As with other disease, early detection is the key to the prevention of further bone loss and eventual fractures.

Limitations of Bone Densitometry

DEXA is of limited use in people with a spinal deformity or those who have had previous spinal surgery.  The presence of vertebral compression fractures or osteoarthritis may also interfere with the accuracy of the test.