Bone Mineral Density Testing: Helpful information about DEXA Scanning

Bone density scanning, also called dual energy x-ray absorptiometry (DXA or DEXA) or bone densitometry, is the most widely used method to measure bone mineral density and is the only method that can make a definitive diagnosis of osteoporosis and monitor a patient’s response to treatment. It is an enhanced form of x-ray technology that requires no injections, sedation, special diet or any other advance preparation and the test is performed by a physician or technician in about 30 minutes.

 

DEXA is most often performed on the lower spine and hips. Portable DEXA devices, including some that use ultrasound waves rather than x-rays, measure the wrist, fingers or heel and are sometimes used for screening purposes.

 

There is some controversy over which bones are best to use for bone density measurements. DEXA is most often performed on the lower spine and hip, which are the bones most commonly used during screening exams. In special cases, bones in the wrist, fingers or heel are sometimes used for screening purposes. For such cases, portable DEXA devices, including some that use ultrasound waves rather than x-rays can be used.

 

How does the procedure work?

The 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 the patient’s bone mineral density.

 

How is the procedure performed?

First, the patient lies on a padded table and usually can keep their clothes on for the procedure. An x-ray generator is located below the patient and an imaging device is positioned above.

 

To assess the spine, the patient’s legs are supported on a padded box to flatten the pelvis and lower spine. To assess the hip, the patient’s foot is placed in a brace that rotates the hip inward. In both cases, the imaging device is slowly passed over the area, generating images on a computer.

 

How are the results reported and what do they mean?

You test results can be reported in the form of two scores called a “T score” and a “Z score.”

 

The T score is used to estimate your risk of developing a fracture. It shows the amount of bone you have compared with a young adult of the same gender with peak bone mass. It is expressed as a standard deviation (SD), which is a statistical measure of how closely each person in a group is to the average of the group. A score above -1 is considered normal. A score between -1 and -2.5 is classified as osteopenia, the first stage of bone loss. A score below -2.5 is defined as osteoporosis.

 

The following table contains the World Health Organization’s definitions of osteoporosis based on T scores.

 

Bone Mineral Density

	T-score
Normal:	2.5 to 1 below the young adult reference range (2.5 to -1)
Low bone mass (osteopenia):	1 to 2.5 SDs below the young adult reference range (-1 to -2.5)
Osteoporosis:	2.5 or more SDs below the young adult reference range (-2.5 or less)
Severe osteoporosis:	2.5 or more SDs below the young adult reference range (-2.5 or less) and the presence of one or more bone fractures