Dual-energy X-ray absorptiometry uses an X-ray source to produce photons of two different energies, which are attenuated as they pass through the tissues of the subject. The X-ray beam is attenuated depending upon the composition of the intervening tissue, with bone producing the strongest attenuation and fat the least attenuation (Heymsfield et al., 1995). Data derived from DEXA measurements can be used in two main ways. First, the measurements can be used as part of a four-component model to determine fat-free mass (Heymsfield et al., 1995). From this, muscle mass can be estimated as 49% of the fat-free mass. This approach may suffer from the same overestimation of muscle mass as noted for four-component models due to possible changes in the fat-free mass composition (Proctor et al., 1999). Alternatively, DEXA measurements can be focused on the extremities that comprise approximately 75% of total muscle mass (Heymsfield et al., 1995). Measuring extremities also carries the advantage that the lean mass of the extremities largely consists of skeletal muscle and this muscle is most important for daily activities including ambulation (see Table 81.1) (Heymsfield et al., 1995). DEXA measurements of extremity muscle mass have shown good correlation with those measured by CT scanning, though there does appear to be a slight overestimation of muscle mass (Visser et al., 1999; Heymsfield et al., 1995).
DEXA is widely used in sarcopenia research. The technique has been shown to have excellent repro-ducibility and interobserver variability (Visser et al., 1999). The main drawbacks to the technique are the need for specialized costly equipment, though much less expensive than CT or MRI, as well as an experienced operator. There is also a small radiation exposure associated with DEXA measurements.
Ultrasound uses sound waves to image and measure tissues. The obtained data can be used with anthropo-metric equations in an analogous fashion to skin-fold and circumference data (Heymsfield et al., 1995). More recently, improved ultrasound equipment has allowed the technique to be used directly for measurement of muscle cross-sectional area in a fashion analogous to CT or MRI (see Table 81.1) (Reeves et al., 2004). The cross-sectional areas measured by ultrasound compare favorably to those obtained from MRI and show consistent test-retest reproducibility (Reeves et al., 2004). Ultrasound offers the advantage of lower cost when compared with MRI and CT as well as portability.
Computerized tomography (CT) scanning uses collimated X-ray beams and computer manipulation to provide highresolution images of subjects. CT provides accurate measurements of muscle cross-sectional area and muscle volume (Heymsfield et al., 1995; Lukaski, 1997). The muscle volume can readily be converted to muscle weight as the density of muscle at body temperature is 1.04 grams per cubic centimeter (Heymsfield et al., 1995). CT scanning can be used to assess either total body or extremity muscle mass (see Table 81.1). The muscle masses obtained by CT have been validated through work using cadaver limbs for comparison with good agreement between CT results and anatomic dissection (Heymsfield et al., 1995). The technique has adequate sensitivity to detect even changes in muscle area seen in response to resistive exercise, which are on the order of 8 to 9% (Fiatarone et al., 1990). Additionally, studies in older subjects have noted that there are changes in muscle composition with aging that can be detected via CT and are correlated with declines in strength independent of decreases in muscle mass (Goodpaster et al., 2001). Consequently, CT scanning has become widely used in clinical sarcopenia research.
The drawbacks of CT are the expense of CT equipment and the need for a trained operator to produce accurate scans. The size of the equipment requires that subjects be able to come to the hospital where the CT scanner is housed. Also, due to the radiation exposure involved in CT scanning, there has been a trend toward the use of CT only on extremities (Heymsfield et al., 1995).
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