Dual-energy X-ray absorptiometry ( DXA , formerly DEXA ) is a tool for measuring bone mineral density (BMD). Two X-ray beams, with different energy levels, are aimed at the patient's bones. When the soft tissue absorption is reduced, bone mineral density (BMD) can be determined from the absorption of each block by the bone. Dual-energy X-ray absorptiometry is the most widely used and most studied bone density measurement technology.
DXA scans are commonly used to diagnose and follow osteoporosis, in contrast to nuclear bone scanning, which is sensitive to certain metabolic diseases of bone where bone seeks to heal infections, fractures, or tumors.
Video Dual-energy X-ray absorptiometry
Physics
One type of DXA scanner uses a cerium filter with an 80 kV tube voltage, producing an effective photon energy of about 40 and 70 keV. There is also a type of DXA scanner using a samarium filter with a 100 kV tube voltage, producing an effective energy of 47 and 80 keV. Also, the tube voltage can be continuously diverted between low values ​​(eg 70 kV) and high (eg 140 kV) in sync with the main electrical frequency, producing effective energy alternating between 45 and 100 keV.
Combination of dual absorptiometry of X-ray and laser using laser to measure the thickness of scanned area.
Maps Dual-energy X-ray absorptiometry
Bone density measurements
Indication
The US Preventive Services Task Force recommends that women over the age of 65 should get a DXA scan. The date on which the men should be tested is uncertain but some sources recommend age 70. In women at risk should consider getting a scan when their risk is similar to that of a normal 65-year-old woman.
One's risk can be measured using the FRAX World Health Organization calculator, which includes many different clinical risk factors including fracture fractures, glucocorticoid use, heavy smoking, excess alcohol intake, rheumatoid arthritis, history of hip fractures, kidney and chronic liver. diseases, chronic respiratory diseases, long-term use of phenobarbital or phenytoin, celiac disease, inflammatory bowel disease, and other risks.
Scoring
The World Health Organization has defined the following categories based on bone density in white women:
The WHO committee does not have enough data to make definitions for men or other ethnic groups.
Special considerations are involved in the use of DXA to assess bone mass in children. In particular, comparing bone mineral density of children with adult reference data (to calculate the T-score) would underestimate the BMD of children, since children had less bone mass than fully developed adults. This will lead to excessive diagnosis of osteopenia in children. To avoid overestimation of bone mineral deficiency, BMD scores are generally compared with reference data for the same gender and age (by computing a Z-score).
Also, there are other variables other than age that are recommended to confuse BMD interpretation as measured by DXA. One important confounding variable is bone size. DXA has been shown to exaggerate higher mineral bone mineral density and underestimate the mineral density of the smaller subject bones. This error is caused by the way DXA calculates the BMD. In DXA, the bone mineral content (measured as X-ray attenuation by the bone being scanned) is divided by the area (also measured by the machine) from the site being scanned.
Since DXA calculates BMD using the area (aBMD: Bone Mineral Density area), it is not an accurate measurement of actual bone mineral density, whose mass is divided by volume. To differentiate DXA BMD from volumetric bone mineral density, researchers sometimes refer to DXA BMD as bone mineral density (aBMD). The confounding effect of bone size difference is due to the depth value lost in calculating bone mineral density. Despite the DXA technology problem by estimating volume, it is still a fairly accurate measure of bone mineral content. Methods to correct this deficiency include calculating the estimated volume of the size of the area projected by DXA. DXD DXD results adjusted in this way is called the apparent bone mineral density (BMAD) and is the ratio of bone mineral content versus estimated cuboidal bone volume. As a result for BMD, BMAD results do not accurately represent actual bone mineral density, as they use estimated bone volume. BMAD is used primarily for research purposes and has not been used in clinical settings.
Other imaging technologies such as Computed Quantitative Computer Tomography (QCT) are capable of measuring bone volume, and therefore, are not susceptible to bone-size disruption effects in the way that DXA results are susceptible.
DXA uses X-rays to assess bone mineral density. The radiation dose of the older original DEXA system is low by about 1/10 of standard chest X-rays, causing little interest from radiation dose specialists. However, the dose of the second generation DEXA system can reach 35 mGy.
The quality of DXA carriers varies greatly. DXA is not regulated like other radiation-based imaging techniques because of its low dose. Each US state has a different policy regarding what certification is required to operate a DXA machine. California, for example, requires state-run courses and tests, while Maryland does not have the requirements for DXA technicians. Many states require training courses and certificates from the International Society of Clinical Densitometry (ISCD).
It is important for patients to get repeatable BMD measurements performed on the same machine each time, or at least the machines of the same manufacturer. Errors between machines, or attempting to convert measurements from one manufacturer standard to another can cause errors large enough to erase the measurement sensitivity.
DXA results need to be adjusted if the patient is taking strontium supplements.
Current clinical practice in pediatrics
DXA, by far, is the most widely used technique for bone measurement, as it is considered cheap, accessible, easy to use, and capable of providing accurate estimates of bone mineral density in adults.
The official position of the International Society for Clinical Densitometry (ISCD) is that a patient may be tested for BMD if he is suffering from a condition that can trigger bone loss, will be prescribed drugs known to cause bone loss, or are being treated and need to be monitored. The ISCD states that there is no clearly understood correlation between BMD and the risk of a child suffering from a fracture; the diagnosis of osteoporosis in children can not be done using densitometric criteria. T-Scores are forbidden for children and may not even appear on DXA reports. Thus, the WHO classification of osteoporosis and osteopenia in adults can not be applied to children, but a Z-score can be used to assist the diagnosis.
Some clinics may routinely scan for DXA in pediatric patients with conditions such as nutritional renal, lupus, and Turner Syndrome. DXA has been demonstrated to measure skeletal maturity and body fat composition and has been used to evaluate the effects of pharmaceutical therapy. It may also assist pediatricians in diagnosing and monitoring the treatment of bone mass acquisition disorders in childhood.
However, it seems that DXA is still in its early days in pediatrics, and there are many widely recognized limitations and disadvantages with DXA. There is the view that DXA scans for diagnostic purposes should not be done outside of specialist centers, and, if scanning is done outside one of these centers, it should not be interpreted without consultation with experts in the field. In addition, most drugs given to adults with low bone mass can be given to children only in closely monitored clinical trials.
The entire body of calcium measured by DXA has been validated in adults using in-vivo activation of total body calcium neutrons but this is not suitable for pediatric subjects and studies have been conducted in child-sized animals.
Radiation exposure
Radiation received by patients during scanning is less than airline flights from California to New York and back.
Body composition measurement
DXA scans can also be used to measure total body composition and fat content with a high degree of accuracy compared to hydrostatic weighing with some important alerts. However, it has been suggested that, while very accurately measuring minerals and slender soft tissues (LSTs), DXA can provide skewed results because its method indirectly calculates fat mass by subtracting it from LST and/or body cell mass (BCM). that DXA actually measures. DXA scans are also used to assess adiposity in children, especially for conducting clinical research.
References
External links
- The non-invasive bone density test is described
- Information for patients, from RSNA
- Bone Densitometry is described
Source of the article : Wikipedia
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