What is bone mineral densitometry (BMD)?

BMD is a test done to measure a person's bone mineral density (BMD). The measurements are used for the diagnosis of osteoporosis, estimation of the fracture risk and assessment of response to treatment.

There are three ways of doing BMD, but the most commonly used method is DEXA.

Dual Energy X-Ray Absorbitary (DEXA)

DEXA is a simple, low-radiation (about a quarter of a chest x-ray dosage) test that measures the mineral content of the bones.

A detector detects the energy emitted after an X-ray is passed through the patient. The more the bone mass, the greater is the obstruction to energy passing through. Thus the more energy emitted, the weaker is the bone.

The computer processes the data, tabulates the results and calculates the bone mineral density (BMD value). Two scores generate from this scan. One is the T-score, which indicates the amount of bone loss by comparing the patient's BMD at his/her current age to the peak bone mass achieved by young individuals.

The other, the Z-score, retakes the patient's bone loss to his/her age- matched peers' expected bone loss to assess whether the patient's BMD is normal.

Why DEXA is used (in comparison with other methods of BMD)

DEXA Ultrasound Quantitative CT Scan
  • Cheap. Short time scanning
  • Not operator dependent so results can be compare across time.
  • Relatively safe, low dose of radiation
  • Safe, no x-ray.
  • Operator dependent. If the operator holds the probe differently, it can generate different values. Thus results are not comparable.
  • Very accurate as it can calculate at every level.
  • Time consuming.
  • Not cost effective.

 

Osteoporosis and BMD

Osteoporosis is a condition where bones become thin and weak as bone tissue is lost faster than it can be replaced.

It is diagnosed when the bone mineral density (BMD) measured by bone densitometer falls below a critical threshold of 2.5 SD(standard deviation) from the average BMD of young healthy people.

Osteoporosis results in brittle bones. Bones in the wrist, spine and hip have the highest risk of breaking. As the condition is painless, people with osteoporosis are often unaware of it until their bone collapses or breaks. BMD is thus carried out as a predictive and preventive measure.

Who should do the scan?

Osteoporosis has no symptoms. To detect osteoporosis before your bones fracture, you should have a BMD scan done, especially if you are in the high risk group for osteoporosis.

Bone loss is a natural process for both men and women. However women have a higher risk of getting osteoporosis. The following checklist should help you assess your risk factors.

If you:

  • have menopause or your ovaries have been removed (oestrogen level decline);
  • are over 50 years old;
  • have lost height over the years;
  • are small framed or have smart bones;
  • are lactating (breast-feeding);
  • seldom take milk or milk products (too tittle calcium in the diet);
  • are not physically active and seldom exercise;
  • smoke;
  • drink coffee, tea or alcohol regularly;
  • have family members with osteoporosis;
  • have fair skin;
  • your risk of getting/having osteoporosis increases with the number of checks.

Preparation and expectation

There are no special preparations required for BMD. You may eat and drink as usual and take your regular medicine where necessary.

On the day of your appointment, you will be asked to change into our hospital robe and your height and weight measurements wit! be taken. The radiographer will then assist you to lie down on the examination table.

Usually, the scan will be performed at your spine and hip. You will be required to remain still while the x-ray is being taken. For the spine examination, your legs will be propped up on a box to straighten the lumbar arch.

Your feet will be aligned to a Perspex immobaliser for the hip examination so as to maintain the position of the hip.

The whole procedure takes about 20 minutes to half an hour. You should not feel any discomfort during the examination. When it is done, a report will be generated and the findings either explained or sent to your referring doctor