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DXA (Dual-energy X-ray absorptiometry) Bone Density Scan

Also known as "Bone Density Test"

We have selected the following expert medical opinion based on its clarity, reliability and accuracy. Credits: Sourced from the website Healthwise, authored by Healthwise Staff (see below). Please refer to your own medical practitioner for a final perspective, assessment or evaluation.

 

What is a DXA scan?

A bone density test is a kind of X-ray test. It measures the density of minerals (such as calcium) in your bones. This information helps your doctor estimate the strength of your bones.

We all lose some bone mass as we age. Bones naturally become thinner (called osteopenia) as you grow older. This is because existing bone tissue is broken down faster than new bone is made. As this occurs, our bones lose calcium and other minerals. They also become lighter and less dense. This makes the bones weaker and makes them more likely to break (fracture).

With further bone loss, osteopenia can lead to osteoporosis. So, the thicker your bones are, the longer it takes to get osteoporosis. Although osteoporosis can occur in men, it is most common in women older than age 65.

If your bone density is lower than normal, you can increase it and your strength. You can do things like exercising, lifting weights or using weight machines. You can also make sure to get enough calcium and vitamin D. And you may need to take certain medicines.

Osteoporosis

Osteoporosis is a progressive disease that causes bones to become thin and porous, significantly increasing the risk for spinal and hip fractures. Osteoporosis usually does not have an effect on people until they are 60 or older.

 

 

There are several different ways to measure bone density

Dual-energy X-ray absorptiometry (DXA) 

This is the most accurate way to measure bone density. It uses two different X-ray beams to estimate bone density in your spine and hip. Strong, dense bones allow less of the X-ray beam to pass through them. The amounts of each X-ray beam that are blocked by bone and soft tissue are compared to each other. DXA can measure as little as 2% of bone loss per year. It is fast and uses very low doses of radiation. Single-energy X-ray absorptiometry (SXA) may be used to measure bone density in the heel and forearm. But SXA is not used as often as DXA.

Peripheral dual-energy X-ray absorptiometry (P-DXA)

P-DXA is a type of DXA test. It measures the density of bones in the arms or legs, such as the wrist. It can't measure the density of the bones most likely to break, such as the hip and spine. P-DXA machines are portable units that can be used in a doctor's office. P-DXA also uses very low doses of radiation. The results are ready faster than standard DXA measurements. P-DXA is not as useful as DXA for finding out how well medicine used to treat osteoporosis is working.

Dual photon absorptiometry (DPA)

This test uses a radioactive substance to measure bone density. It can measure bone density in your hip and spine. DPA also uses very low doses of radiation. But the scan takes longer than the other methods.

Ultrasound is a screening test that is sometimes offered at events such as health fairs. If results from an ultrasound test find low bone density, DXA is advised to confirm the results. Ultrasound uses sound waves to measure bone density, usually in your heel. It is quick and painless. And it does not use potentially harmful radiation like X-rays. One downside of ultrasound is that it can't measure the density of the bones in the hip and spine. These are the bones most likely to fracture from osteoporosis. Ultrasound is not used to keep track of how well medicine for osteoporosis is working.

Before you are screened for osteoporosis, you may want to think about what you will do if the tests show that you have a high chance of getting osteoporosis.

Why a bone density test is done

Doctors may recommend bone density tests for:

• All women who are age 65 or older. It's also recommended for younger women who are at higher risk for broken bones caused by osteoporosis.
• Men and women who have hyperparathyroidism.
• Men and women who have been taking corticosteroids, such as prednisone, for a long time.
• Certain men at higher risk for osteoporosis.
• Follow-up of how well treatment for osteoporosis is working for men and women being treated for 2 years or longer.

How to prepare for a bone density test

Avoid wearing clothes with metal buttons or buckles. You also may want to remove any jewellery that might cause a problem with the scan. For instance, don't wear a bracelet if you are having the scan done on your wrist.

How a bone density test is done

In most cases, a bone density scan is done in a radiology department or clinic by a technologist. Peripheral dual-energy X-ray absorptiometry (P-DXA) machines are portable units that can be used in a doctor's office.

You will need to lie on your back on a padded table. You probably can leave your clothes on. You may need to lie with your legs straight. Or you may lie with your lower legs resting on a platform built into the table.

The machine will scan your bones and measure the amount of radiation they absorb. The DXA scan, which scans the hip and lower spine, takes about 20 minutes. Other kinds of scans may take 30 to 45 minutes.

Portable machines (P-DXA) can measure bone density in the wrist or forearm.

Testing at least two different bones each time is the most reliable way of measuring bone density. The hip and spine bones are preferred. It is best to test the same bones each time. The same type of scan and bone density equipment should also be used.

How a bone density test feels

A bone density test does not cause pain. But if you have back pain, it may bother you to lie still on a table during the scan.

 

Risks

During a bone density scan, you are exposed to a very low dose of radiation. A bone density scan is not advised for pregnant women because it exposes the unborn baby to radiation.

Bone density test results

A bone density test is a kind of X-ray test. It measures the density of minerals (such as calcium) in your bones. Results are usually ready in 2 to 3 days.

Results of bone density tests can be reported in several ways.

T-score

Your T-score is your bone density compared to the average score of a healthy 30-year-old. (This is called the young adult reference range). It is expressed as a standard deviation (SD).

A negative (–) value means that you have thinner bones (lower bone density) than an average 30-year-old. The more negative the number is, the less bone density you have compared with an average 30-year-old.

A positive (+) value means that your bones are thicker and stronger than an average 30-year-old's.

The following table contains the World Health Organization's definitions of osteoporosis based on bone density T-scores.

• Normal: T-score - Less than 1 standard deviation (SD) below the young adult reference range (more than –1)
• Low bone mass (osteopenia): T-score - 1 to 2.5 SDs below the young adult reference range (–1 to –2.5)
• Osteoporosis: T-score - More than 2.5 SDs below the young adult reference range (–2.5 or less)

 

If your bone density test result is low:

You may have osteoporosis. Doctors usually use the lowest T-score to diagnose osteoporosis. For example, if your T-score at your spine is –3 and your T-score at your hip is –2, the spine T-score would be used to diagnosis osteoporosis.

Your chance of breaking a bone is higher than average. The more negative your T-score, the greater your chances of breaking a bone during a fall or from a minor injury. Every change of 1 SD means you have double the chance of a break at that site. For example, if you have a T-score of –1, your chances of having a broken bone are 2 times greater than if your T-score was 0.

Low bone density values may be caused by other problems, such as:

• Taking certain medicines.
• Cancer, such as multiple myeloma.
• Cushing's syndrome, hyperthyroidism, and hyperparathyroidism.
• Diseases of the spine, such as ankylosing spondylitis.
• Early menopause.
• Low levels of vitamin D.
• Heavy alcohol use.

 

Z-score

Your bone density value may also be compared to other people of your age, sex, and race. This is called your Z-score. It is given in standard deviations (SD) from the average value for your age group.

A negative (–) value means that your bones are thinner (lower bone density) and weaker than most people in your age group. The more negative the number is, the less bone density you have compared with others in your age group.

A positive (+) value means that your bones are thicker and stronger than most people in your age group.

What affects the bone density test?

You may not be able to have the test, or the results may not be helpful, if:

• You aren't able to be put in the correct position during the test.
• You had a broken bone in the past. This can cause falsely high bone density results.
• You have arthritis of your spine. The changes caused by arthritis in the spine may not make the spine the best place to measure for osteoporosis.
• You have metal implants from hip replacement surgery or hip fracture.
• You had an X-ray test that uses barium within 10 days of the bone density test.

What to think about

Experts disagree about which bones are best to use for bone density measurements. Bones in the lower spine and hip are tested most often. These bones generally have the most bone loss and are more likely to break. Sometimes bones in the wrist are measured. Ultrasound screening is done on the bone in the heel.

A bone density measurement should be done only when the results of the test will affect treatment decisions. Bone density does not need to be measured more often than every 2 years to find out how well treatment is working.

Using DXA to measure bone density is replacing older methods, such as dual photon absorptiometry (DPA).

Regular X-rays can't detect mild bone loss. A bone must lose at least a quarter of its weight before a regular X-ray can detect the problem.

If your bone density is lower than normal, you can increase it and your strength. You can do things like exercising, lifting weights or using weight machines. You also need to make sure you are getting enough calcium and vitamin D. And you may need to take some medicines. 

 

 

About the source

Healthwise Staff

Medical Reviewers:

• Anne C. Poinier, MD - Internal Medicine
• Martin J. Gabica, MD - Family Medicine
• Kathleen Romito, MD - Family Medicine
• Carla J. Herman, MD, MPH - Geriatric Medicine

Current as of March 29, 2018

 

Recommended websites

For further reading go to:

• To learn more about how you can increase your bone strength and density, see the topic Osteoporosis.

 

 

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