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Bone densitometry is a radiologic study used for evaluation of bone density. Bone density naturally decreases with age, and the rate of bone loss is most pronounced in post-menopausal women. This increased bone fragility, also known as osteoporosis, makes bones more likely to break. Fractures of the spine and hip are most common, and at the same time most concerning, as they result in severe pain and disability.
X-ray technology used by radiologists to measure bone density is called dual-energy X-ray absorptiometry (DEXA). DEXA bone densitometry is the most accurate method available to diagnose osteoporosis and estimate fracture risk. It helps physicians develop
an appropriate treatment plan to prevent fractures before they occur. DEXA is also useful in monitoring the effects of the treatment of osteoporosis and other diseases that result in bone loss.
Patients who are particularly predisposed to osteoporosis include:
post-menopausal women, especially those not taking estrogen and those who weigh less than 127 pounds;
patients with a family history of osteoporosis;
patients with a history of tobacco smoking;
patients using medications that are known to cause bone loss, such as steroids, anti-seizure medications and high-dose thyroid replacement medications;
patients with a history of hyperthyroidism;
patients with a history of spine (vertebral) fractures, or other fractures after age of 50 that occurred after only mild trauma.
There are two types of DEXA bone densitometry devices: central and peripheral. Central DEXA devices measure bone density in the spine and hips, while peripheral devices measure bone density in the wrist and forearm. Central devices have a large, flat table (similar to an X-ray table) with an arm suspended overhead. The peripheral DEXA device is a portable box-like structure
that includes a space to insert the forearm for imaging.
The DEXA machine sends an invisible beam of very low-dose X-rays through the patient’s bones. The amount of radiation used is extremely small–less than one-tenth the dose of a standard chest X-ray. These low-dose X-rays have two energy peaks: one peak is absorbed mainly by soft tissue and the other by bone. The soft tissue absorption value is then subtracted from the
total, and what remains is the patient's bone mineral density. The data is calculated and displayed on a computer monitor, allowing the radiologist to make an accurate assessment of bone density.
The DEXA bone density test takes 10 to 30 minutes, depending on the equipment used and the parts of the body being examined. Patients may be asked to undress and put on a hospital gown. During an examination of the spine and hips, the patient is placed on a flat table while a detector slowly passes over the area to be examined. When the spine is examined, the patient’s legs are supported on a padded box to flatten the pelvis and lower the spine. When densitometry of the hip is performed, the patient’s foot is placed in a brace that rotates the hip inward. Densitometry of the wrist/forearm is even simpler–the forearm is placed in a small device and the reading is obtained within minutes.
On the day of the exam, patients can eat normally but are advised not to take calcium supplements for at least 24 hours before the study. Patients should inform the technologist if they recently had a barium examination or were recently injected with a contrast material for a computed tomography (CT) scan or radioisotope scan. In these cases, the DEXA scan should not be performed until 10 to 14 days later. Women should always inform their physician or the X-ray technologist if there is a possibility they may be pregnant.
