Dual-energy X-ray absorptiometry
During DXA (also known as DEXA), the body or part of the body is scanned with X-rays of two distinct levels of energy. The attenuation of the tissues for the two different levels of radiation depends on its chemi-cal composition and is detected by photocells.
The instrument’s software generates a two-dimensional picture of the body or the body compartment under study. The software can calculate several body com-ponents: bone mineral content and bone mineral density, lean mass, and adipose tissue fat mass. These calculations are possible for each of the body parts, e.g., for legs, trunk, spine, femur, and arms. However, the method cannot distinguish between subcutane-ous adipose tissue and discrete adipose tissue sites such as perirenal adipose tissue. The reproducibility of DXA is very high, varying from about 0.5% for bone mineral density to about 2% for total body com-position. The reproducibility for regional body com-position is less. The method is quick and easy to perform and places very few demands on the subject. The radiation dose (0.02 mSv) is only a fraction of the radiation dose of a normal chest radiograph, and hardly higher than the normal background. Apart from repeated scanning, the radiation dose should not be a limiting factor in terms of volunteers being exposed to hazardous levels of radiation. A disadvan-tage of the method is that the attenuation of the X-rays depends on the thickness of the tissue. Therefore, correction for the body size has to be made. Compared with traditional methods, DXA scanning is easy and widely available which, in turn, leads to prediction equations for body composition based on DXA. However, as with other methods, DXA relies on certain assumptions and there are many publications showing that the error in body composi-tion measurements using DXA can be considerable (Figure 2.3). Moreover, identical machines, even using the same software versions, can give different results in scanning the same person.
Figure 2.3 Dual-energy X-ray absorptiometer (DXA) scan using a HOLOGIC whole-body DXA (QDR-4500). Subcutaneous body fat, bone, and muscle are distinguished by different colors.