How is a Bone Density Test Performed?

By Shayla Perkins

Bone density, commonly known as bone mineral density (BMD), refers to the level of matter per cubic centimeter in your bones. As we age, our bones naturally become thinner and our existing bones break down before our bodies can regenerate new bone mass. As a result, our bones become lighter and more porous due to a loss of calcium and other minerals. A bone density test is performed to diagnose possible osteoporosis, bone strength and risk of fracture. The two most common tests, currently used to measure bone mineral density (BMD), are the energy X-ray absorptiometry and qualitative computed tomography procedures. All bone density tests are non-invasive and painless and have very low levels of exposure to radiation.

Energy X-Ray Absorptiometry

Two types of energy X-ray absorptiometry are dual energy X-ray absorptiometry (DEXA) and single energy X-ray absorptiometry (SEXA). DEXA is considered the most accurate test for BMD measurement concerning risk of osteoporosis. The DEXA procedure can measure as little as a 3 percent loss of bone density in a patient. It is used to estimate bone density in your spine and hip. This test is performed by aiming two different energy source beams at the patient's bones as he lays on the examination table. The measurement of bone mass is determined through comparison of light beam amounts blocked by the bone and soft tissue. The stronger, or more dense bone allows fewer X-ray beams to pass through, and less dense bones permit more of the X-ray beams to pass through. Peripheral dual energy X-ray absorptiometry (pDEXA) is used to measure bone density in the arms, legs and wrists. pDEXA test is performed in a similar matter as the DEXA test. The pDEXA machine is smaller and portable, which enables the test to be performed right in the doctor's office. Single energy X-ray absorptiometry is not performed as often as DEXA and is used to measure bone density in the heel and forearm. SEXA is performed similar to the DEXA test. However, only one X-ray beam energy source is aimed at the bones as it passes over the patient.

Quantitative Computed Tomagraphy (QCT)

The QCT is a form of CT scan used to measure bone density in the spine. Peripheral QCT (pQCT) is performed to measure bone density in your arms, legs and wrists. These tests are conducted with a general CT machine, which rotates around the patient to generate 3-D images of his spine as he lays on the examination table. The 3-D images are used as a form of comparison to control bone density data based on subject age and weight. The QCT tests are not performed as often as other bone density test because of high costs, increased radiation exposure and less accuracy.

Diagnosis

BMD test results are generally reported as T-score and Z-score value. The T-score is used for diagnosis in post-menopausal women and men over the age of 50. The T-score is based on comparison of the patient results against the BMD control data of a healthy 30-year-old of the same sex and ethnicity. This comparison is useful in predicting the patient risk of future fractures. The Z-score is commonly used for diagnosis in pre-menopausal women, men under 50 years and children. The Z-score represents the number of standard deviations between the patient BMD data and the control BMD of their age, sex and ethnic group. In the case of the T-score and Z-score, lower bone density is indicated by a negative value.

Other testing procedures also used to test bone density include ultrasound, digital X-ray radiogrammetry and dual photon absorptiometry. Ultrasound is generally used to check for problems and possible fractures that may require further bone density examination through DEXA or QCT procedures. Dual photon absorptiometry is an older form of the DEXA procedure. Dual photon absorptiometry uses two energy level photon sources to determine BMD of the lower lumbar spine and proximal femur.

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