Background
Dual-energy X-ray absorptiometry (DEXA or DXA) is an imaging modality used to assess the density of bone. The density of bone is determined by the average concentration of minerals in an area of bone, also known as the bone mineral density (BMD). BMD gives us an estimate of bone strength, the lack of which can help us determine if an individual has osteopenia/osteoporosis.
Osteoporosis is one the most common metabolic bone disorders, with a wide prevalence in the elderly population. In America, in the age group 65 and over, approximately 25% of the women and 5.6% of the men have osteoporosis. Therefore, nearly 1.5 million American men aged >65 have osteoporosis, and 25% of these men will experience an osteoporotic fracture during their lifetimes, which is associated with high mortality rates. This highlights the need for early recognition of bone loss in older adults to decrease the risk for osteoporotic fractures. Dual energy X-ray absorptiometry (DEXA or DXA) is a simple, noninvasive diagnostic tool that evaluates bone mineral density (BMD) and is considered the gold standard for population screening of osteoporosis.
DXA scans are recommended for older men and women, especially those with high fracture risk factors. The scan is commonly performed on the lumbar spine or femoral neck to screen for osteoporosis. Alternatively, but less frequently DXA scan of the hands can also be performed. Inflammation-related bone damage in rheumatoid arthritis (RA) often presents as both erosions and osteoporosis. RA is a chronic systemic autoimmune disorder predominantly affecting women leading to high numbers of morbidity and mortality. The hands are one of the earliest sites to be affected in RA. DXA scans can be used to identify disease-related bone loss in these patients in the early phase of the disease process, but to date the clinical utility of these changes are yet to be clearly determined.1-4
Historical Overview
Until the mid-1980s, BMD measurements were used mainly for research, and it was only with the introduction of DEXA scanners that they entered routine clinical practice. DEXA, which is based on the method of X-ray spectrophotometry that was developed in the 1970s, was introduced commercially as the direct successor to dual-photon absorptiometry (DPA) in 1987. While DEXA uses the same principles as DPA, in DEXA the radionuclide source is replaced by an X-ray tube. The main advantages of an X-ray system over a DPA radionuclide system are shortened examination time due to an increased photon flux of the X-ray tube and greater accuracy and precision caused by higher resolution and removal of errors due to source decay.5,6
Description
Dual-energy X-ray absorptiometry utilizes two X-ray beams of two different intensities to determine BMD. DEXA relies on the difference in attenuation coefficient of soft tissue and bone to X-rays. Attenuation in this context means the reduction of the number/strength of X-rays after it pass through a medium, be it bone or soft tissue. This is not only dependent on the type of material or its thickness, but also on the initial energy/intensity of the X-ray beam.
For simplicity, let's consider soft tissue to be a mixture of water, fat, and muscle, whereas bone be a mixture of calcium and phosphorus. Thus, the density of soft tissue is less than bone as the atomic number is lower being primarily composed of carbon, hydrogen and oxygen.
The two different beam intensities help us calculate the bone mineral density by eliminating the attenuation caused by soft tissue. For simplicity, a low energy beam which is attenuated by soft tissue and bone is subtracted by a high energy beam which is attenuated by only bone. This helps us eliminate the attenuation of soft tissue regardless of volume and leaving us with only the attenuation caused by bone. Again, for simplicity, the initial beam intensity sent by the imaging source is subtracted from the final beam intensity detected by the detector giving us a value of attenuation caused by bone alone. Utilizing complex post processing and computer algorithms, a Bone Mineral Density is calculated.2,4
BMD is calculated in g/cm2, and is compared against two reference populations providing us with two scores. The one clinically used in postmenopausal women and men over the age of 50 is the T-score. A reference curve is generated matching for sex and ethnicity by the World Health Organization (WHO) categorizing based on standard deviation. In premenopausal women, men under the age of 50 and children the Z-score is used. A Z-score of less than 2.0 is considered below the range for age, and a cause for this low bone mineral density should be investigated. Utilizing this information one can calculate if the patient is of normal bone mineral density, has osteopenia or osteoporosis7.
Inflammatory disorders such as RA are known to cause to peri-articular osteopenia. Some studies have shown that the information obtained from performing a DEXA scan of the hands may provide clinicians with some useful information, but there is no clear correlation with disease severity yet.
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T-Score
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>-1.0
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Normal
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>-1.0 to >-2.5
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Osteopenia
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>-2.5
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Osteoporosis
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>-2.5 & Fragility Fracture
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Severe Osteoporosis
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