The Leeds Teaching Hospitals NHS Trust


Bone Density Scan (DEXA)

dexa scanner

Introduction

A DEXA scan is a special type of X-ray that measures bone mineral density (BMD).

DEXA stands for "dual energy X-ray absorptiometry". This type of scan is also often known as DXA, or "dual X-ray absorptiometry". It's also sometimes referred to as a bone density scan or a bone densitometry scan.

DEXA scans are often used to help diagnose bone-related conditions such as osteoporosis (or the risk of developing it).
They can also be used to assess the risk of osteoporosis developing in women aged over 50 and in men over 60.

As well as being quick and painless, a DEXA scan is more effective than normal X-rays in identifying low bone mineral density.

Measuring Bone

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.

Your bone density measurements will be compared with the bone density of a young healthy adult or an adult of your own age, gender and ethnicity.

When a DEXA (DXA) scan is used

DEXA scans are often used to diagnose osteoporosis but a total body DEXA scans can also be used to measure body composition (the amount of bone, fat and muscle in the body). This type of scan is routinely used in children, but is still a research application in adults.

When is it recommended?
  • A DEXA scan may be recommended if you have an increased risk of developing a bone condition such as osteoporosis. Your risk is increased if you:
  • have had a fracture after a minor fall or injury
  • are a woman who has had an early menopause, or you've had your ovaries removed at a young age (before 45) and haven't had hormone replacement therapy (HRT)
  • are a post-menopausal woman and you smoke or drink heavily, have a family history of hip fractures or you have a body mass index (BMI) of less than 21
  • are a man or a woman with a condition that leads to low bone density, such as rheumatoid arthritis
  • are a woman who has large gaps between periods (more than a year)
  • are a man or a woman taking oral glucocorticoids for three months or more (glucocorticoids are used to help treat inflammation, but can also cause weakened bones)
How a DEXA scan is performed

A DEXA (DXA) scan is a quick and painless procedure that involves lying on your back on an X-ray table so that an area of your body can be scanned.

When you arrive for your DXA scan, no special preparations are needed. You may be asked to wear a gown but in most cases you will be able to remain fully clothed. Your height and weight will be measured. Two scans are taken , one of your lower spine and one of your left hip. When scanning the spine, a cushioned box will be placed under your knees. The cushioned box allows the small of your back or lower spine to lie flat on the table. To scan the hip, this box is removed and atriangular frame will be placed between your feet. The frame allows the leg being scanned to be positioned accurately. The foot is strapped to the triangle by Velcro, and the knee can also be held in place by a Velcro strap to keep the leg still.

The scanning process will need you to lie down on your back on a flat and open X-ray table. To stop blurred images you must lie very still.

During the scan a large scanning arm will be passed over your body to measure bone density. As the scanning arm moves slowly over your body, a narrow beam of low-dose X-rays will be passed through the part of the body being examined.
A DEXA scan usually takes around five minutes, although it depends what part of the body is being scanned. You will be able to go home after you have had it done.

Safety

DEXA scans use a much lower level of radiation than standard X-ray examinations, which means that the radiographer (the technical specialist carrying out the scan) can stay in the scanning room with you during the scan.

The amount of radiation used during a DEXA scan will vary depending on the area of the body being examined, but is very low and less than two days' exposure to natural background radiation (NBR).

Despite being very safe procedures, DEXA scans and X-rays aren't recommended for pregnant women, as X-rays can damage an unborn child.

Results

A DEXA scan compares your bone density with the bone density expected for a young healthy adult or a healthy adult of your own age, gender and ethnicity.

The difference is then calculated as a standard deviation (SD) score. This measures the difference between your bone density and the expected value in terms of the natural spread of values in the healthy population.

The difference between your measurement and that of a young healthy adult is known as a T score, and the difference between your measurement and that of someone of the same age is known as a Z score.