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Bone Densitometry: Why and How is it Done?

Bone Densitometry: Why and How is it Done?

Bone Densitometry utilizes specialized X-rays and techniques to analyze the Bone Mineral Density (BMD), that is, the strength and thickness of bones. The bones of the hip and spine, and in some cases the forearm, are most commonly scanned through this test. There are various techniques that are applied for this test. The most commonly used techniques are dual-energy X-ray absorptiometry (DXA) and quantitative ultrasound.

This test is also called as bone density test and DEXA or DEXA scan.

Why is Bone Densitometry Done?

As mentioned previously in the article, the purpose of the DEXA scan is to measure the strength and thickness of bones. To be more specific, it diagnoses the presence of osteoporosis, a condition of thin and weak bones, and osteopenia, a condition related to reduced bone mass.

The earlier the diagnosis and treatment, the lesser the risk of aggravations to the medical conditions. Lack of treatment in the advanced stages of these medical conditions can also lead to bone fractures.

This is especially true in the case of osteoporosis amongst the elderly. Effective treatment does not reverse age-related bone degeneration like osteoporosis, but can slow down the pace of further degeneration.

In case of reduced bone mass, early treatment can also improve bone mass, which can be monitored through the DEXA scan.

The thickness of bones usually weakens in both sexes after the age of 50 years. However, the strength and thickness of bones in women degenerate more rapidly post-menopause as a result of hormonal changes.

Therefore, women are at a greater risk of osteoporosis and osteopenia before the age of 70 years.

Other purposes of the DEXA scan are:

  • Diagnose osteoporosis when there is a bone fracture
  • Predict the risk of future bone fractures
  • Evaluate the rate of bone loss

Prerequisites for Bone Densitometry

The following are the preparations that are usually advised to patients who have been recommended the DEXA scan:

  • Make sure to let your doctor know if you are pregnant or feel that you might be pregnant. This is because the intensity of the X-ray rays is known to have adverse effects on the growth and development of the fetus. In such cases, the intensity of the waves is kept under control to a certain degree around the abdominal area. A protective covering may also be spread on the abdomen to further protect the growing fetus from developing birth defects.
  • Calcium may interact with the accuracy of results of this test. Therefore, it is generally recommended that you do not take a calcium supplement within a day before undergoing DEXA. Discuss this with your doctor for necessary guidance.
  • Barium or nuclear medicine exam, undertaken within 72 hours before the DEXA scan, may interact with the results of this scan. Remember to talk about this with your doctor for their advice.
  • You may be asked to remove metallic objects like jewellery, belt, keys, etc. These may interfere with the X-ray waves. Hence, it is often advised that you wear loose-fitted clothes that are devoid of hooks, buttons, and buckles. You may also be asked to change into a medical gown.

Procedure

The procedure for DEXA is as below:

  • You will be asked to lie flat on your back on a table with legs placed on a padded box. This helps in maintaining a flattened position of the pelvis and lumbar spine.
  • A photon generator, installed below the table, passes slowly beneath you. This recreates images of the pelvis and lumbar spine onto a computer monitor that is connected to the generator.
  • Next, your foot is inserted into a brace for rotating the side of the hip that is comparatively less dominant than the other.
  • The photon generator repeats the imaging procedure for one or two bones of the lower arm. This is applicable unless there is a medical history of a fracture on this arm.
  • The monitor measures the amount of photons that have not been absorbed by the bones. This helps in calculating the Bone Mineral Content (BMC). The BMD is then calculated by the radiologist overseeing the bone densitometry test.

Understanding the Results of Bone Densitometry

A bone densitometry is analyzed against two types of scores:

  • T-score: Young and healthy adults between the ages of 25 years and 35 years, and
  • Z-score: Age-matched adults

The score of your DEXA scan is first compared with young and healthy adults between the ages of 25 years and 35 years belonging to the same sex and ethnicity.

Your unique T-score is the difference between the T-score of young, healthy adults and your Bone Mineral Density (BMD). This difference is also referred to as the Standard Deviation (SD).

Positive T-score: When you get a positive T-score, it implies that your bones are stronger than usual.

Negative T-score: When you get a negative T-score, it indicates that your bones are not as strong as normal.

The following are the implications of the results for bone densitometry levels:

Normal result: A T-score within 1 SD (+1 or -1) of the young adult mean of the same sex and ethnicity within 25 years and 35 years.

Low bone mass: A T-score of 1 to 2.5 SD below the young adult mean (-1 to -2.5 SD) indicates low bone mass.

Poor bone mass: A T-score of 2.5 SD or more below the young adult mean (more than -2.5 SD) confirms the presence of osteoporosis.

With every 1 SD below normal (T-score of -1), there is twice the risk of bone fractures in comparison to an individual with a normal BMD score.

On the other hand, the Z-score is calculated in the same way as the T-score. The only difference being that the comparison here is with individuals of the same age group, height, sex, ethnicity, and weight.

When Should a Bone Densitometry be Repeated?

A repeat DEXA scan may be advised in the following instances:

  • Your doctor is most likely to advise you to undergo a repeat test after completion of the prescribed line of treatment as per the BMD of the first scan. In case a repeat DEXA scan shows normal results, treatment may be discontinued. If the results have not improved or have been reduced further, the dosage of medications, if any, is likely to be reviewed. An alternative course of treatment may be prescribed. Under such conditions, more repeat tests are likely to be advised in the future to monitor the BMD closely.
  • Your doctor may recommend that you undergo this scan at regular intervals. This enables them to monitor the recurrence of the condition and start the required treatment at the earliest possible time, if necessary.

Hello Health Group does not provide medical advice, diagnosis or treatment.

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Written by Nikita Bhalla Updated Oct 20
Fact Checked by Bianchi Mendoza, R.N.