Dual-energy x-ray absorptiometry

Dual-energy x-ray absorptiometry or dual-emission x-ray absorptiometry (DEXA) is a means of measuring body composition. Two x-ray beams with different energy levels, one high and one low energy beam, are aimed at the patient. The radiation that passes through the tissue is measured for each beam. Body composition can be determined from the absorption of each beam by different body tissues. When soft tissue absorption is subtracted out, the bone mineral DEXA is determined. DEXA is, by far, the most widely used technique for bone mineral density measurements. Although DEXA is used primarily to evaluate bone mineral density in clinical practice, it can also be used to measure or estimate total body mass, lean soft tissue mass, bone mineral content, and fat mass.

Regional DEXA

Regional DEXA applies the same concepts as whole-body DEXA but is limited to defined body regions. This technique is widely used for the assessment of bone mineral density and is most often performed on the lower spine and hips. However, regional DEXA can also be used to assess regional body composition of limbs or any other defined body region ( Fig. 1 ). This assessment can be helpful to determine if the changes in body composition are primarily appendicular, axial, or a complex combination. Another potential advantage of regional DEXA would be in the setting of a focal treatment such as the implantation of stem cells. Following the region of interest serially over time may identify an increase in muscle mass or lean tissue that would have been lost in whole-body evaluations.

Anatomic delineations used for regional body composition assessment.

Regional DEXA

Regional DEXA applies the same concepts as whole-body DEXA but is limited to defined body regions. This technique is widely used for the assessment of bone mineral density and is most often performed on the lower spine and hips. However, regional DEXA can also be used to assess regional body composition of limbs or any other defined body region ( Fig. 1 ). This assessment can be helpful to determine if the changes in body composition are primarily appendicular, axial, or a complex combination. Another potential advantage of regional DEXA would be in the setting of a focal treatment such as the implantation of stem cells. Following the region of interest serially over time may identify an increase in muscle mass or lean tissue that would have been lost in whole-body evaluations.

Anatomic delineations used for regional body composition assessment.

Advantages of DEXA

DEXA is relatively easy to perform, and the amount of radiation exposure is low. The technique is considered to be cheap, accessible, and able to provide accurate estimations of body composition. The radiation received by the patient is usually less than that of a transatlantic airline flight or similar to approximately one-twentieth that of a standard chest radiograph but varies from patient to patient. A regional and whole-body DEXA scan can be performed in less than 5 minutes, whereas it takes at least 30 to 60 minutes to acquire whole-body magnetic resonance imaging (MRI) data.

McDonald and colleagues found multifrequency bioelectric impedance analysis to be accurate in normal subjects, but it underestimated lean tissue mass and overestimated fat tissue mass in patients with Duchenne muscular dystrophy (DMD) compared with DEXA. Multifrequency bioelectric impedance analysis can only estimate whole-body composition. In comparison, DEXA has the capacity to estimate both whole-body and regional composition. Skinfold thickness measurements have also been used to estimate body composition. Mok and colleagues found that skinfold thickness measurements overestimate the fat-free mass in DMD, which results in an underestimation of the body fat percentage. This same study found bioelectric impedance analysis to be similar to DEXA. This differs from the findings of McDonald and colleagues. Bioelectric impedance has been shown to accurately predict fat-free mass and lean tissue mass in normal controls. The boys with DMD studied by Mok and colleagues were younger than the population evaluated by McDonald and colleagues, which would suggest less disease progression and less deviation from control body composition. This may account for the contradictory findings.

Disadvantages of DEXA

Although it very accurately measures mineral content and lean soft tissue mass, DEXA may skew the calculation of fat mass because of its method of indirectly calculating fat mass by subtracting lean soft tissue mass and mineral content, which are the elements DEXA actually measures. In addition, lean soft tissue mass as measured by DEXA is an overestimate of muscle mass. The reason for the overestimation is that lean tissue includes multiple nonmuscle compartments (ie, skin, connective tissue, and the fat-free portion of adipose tissue). Skin accounts for approximately 10% of lean soft tissue mass. The overestimation of muscle mass is magnified in dystrophic muscle because muscle is replaced by fibrous connective tissue. Both skeletal muscle and fibrous connective tissue are lean soft tissue mass.

Additionally, a traditional 2-dimensional DEXA scan is not able to account for anterior and posterior compartments despite its regional compartmentalization capability given that it is a 2-dimensional imaging modality. To more accurately assess 3-dimensional body composition, a 3-dimensional imaging modality, such as computed tomography (CT) or MRI, is needed. MRI provides the additional benefits of distinguishing between intramuscular and extramuscular lipid content using hydrogen-1 magnetic resonance spectroscopy as well as information regarding the quality of muscle tissues in vivo using sodium-23 MRI.

Bone mineral density assessed by DEXA is actually the measure of bone mineral content in a given 2-dimensional area (normally the outline of the osseous structure), thus calculating the apparent bone mineral density. This differs from the actual 3-dimensional volume of the bone being measured, so the values cannot be directly compared with other methods of assessing 3-dimensional bone mineral density such as quantitative CT (QCT). QCT is capable of measuring the bone’s volume and therefore is not susceptible to the confounding effect of apparent bone size in the way that DEXA is susceptible. Although a standard CT scanner can be used to assess bone mineral density, the radiation exposure is much higher and the cost is substantially greater than with DEXA. These reasons currently limit QCT from being used in general practice for the measurement of bone density and likely limit 3-dimensional technology in the assessment of whole-body or regional body composition. Despite the apparent limitations of a 2-dimensional imaging modality, DEXA technology has been applied to bone mineral density quite successfully. As with any imaging modality, severe joint contractures and scoliosis can distort body composition estimates.