Bone densitometry, which is known as dual energy X-ray absorptiometry, also called DXA or DEXA, is a test that calculates bone density quickly and accurately. Used to discover osteopenia or osteoporosis, the test uses a very small dose of ionizing radiation to take pictures inside the body, normally the lower spine and hip, to measure bone loss. Osteopenia and osteoporosis cause the bone to lose minerals and density, which increases the possibility of fractures. While the bone densitometry test is often recommended after age 60, it can be prescribed earlier in cases where reduced bone density is presenting or suspected. DEXA is the most accurate way to diagnose osteoporosis and osteopenia.
There is no special preparation for a bone density test. As is the case when any X-ray is taken, contraindications include pregnancy and a recent contrast CT or radioisotope examination. The patient should leave jewelry at home, wear loose clothing, and avoid taking a calcium supplement in the 24 hours before the test. The patient must remain very still and hold the breath for several seconds, while the technician conducts the test from behind a protective wall. Other parts of the body may be shielded from the X-rays with protective equipment.
The bone density test is an enhanced X-ray, and DEXA is the standard for calculating bone mineral density. The machine used to conduct the test is called a bone densitometer. X-rays are the oldest method for medical imaging. Typically conducted on the lower spine and hips, there are cases with children and adults where the whole body is scanned. You will also find peripheral X-ray and ultrasound machines that are used in conjunction with the test to detect low bone mass. Another method of diagnosing low bone density is a CT scan with special software, which is as accurate as DXA scanning, but less commonly available and used.
The main bone densitometer equipment usually consists of a central device, and occasionally an additional peripheral device (pDXA). Both are referred to as DXA or DEXA equipment. The central device measures bone density in the hip and spine and is usually found in a hospital or medical practice setting. It has a large, flat table with a suspended arm over head. For smaller bones like the wrist, heel, or finger, a peripheral device is often sufficient. It is much smaller, weighing only about 60 pounds, and is a portable box that can accommodate a foot or hand. Less common are other portable technologies that use ultrasound to scan and screen for bone density.
The bone densitometer utilizes a thin, invisible, low-dose X-ray beam. There are two energy peaks that penetrate the bones in the scan process. One is for soft tissue absorption, and the other is for bone. The soft tissue absorption quantity is subtracted from the bone absorption quantity, and the remaining amount is the patient’s bone density. The bone densitometer has special software that automatically analyzes the bone density measurements and displays the results on a computer monitor.
During an exam using the central bone densitometer to measure hip and spine bone density, the patient will remain still and prone on a padded table. The X-ray generator is below the patient and the imaging device, also known as a detector is located above the patient. The legs are supported on a padded box when scanning the spine, and for scanning the hip, the foot is placed in a brace to rotate the hip inward. For both the hip and spine scan, the detector moves slowly over the area to create images that are displayed on the computer monitor. Peripheral bone density exams are faster, with results available in a few minutes. The central bone densitometer device test usually takes up to 30 minutes, depending on the type and extent of exam.
The bone density exam is a good predictor of risk of fracture or bone-related medical conditions and can guide preventive medicine prescriptions. The results can also help predict a future hip fracture for postmenopausal women. It is forecasted that bone densitometer tests will be used increasingly in the years to come.