Magnetic resonance imaging (MRI) involves the use of radio-frequency energy and magnetic fields to provide high-resolution three-dimensional images of subjects. Similar to CT scanning, MRI provides accurate measurements of muscle area and muscle volume (Heymsfield et al., 1995; Lukaski, 1997). MRI also demonstrates adequate sensitivity to detect small changes in muscle mass in response to resistive exercise (Ross et al., 1995). As a result, MRI is also commonly used in clinical sarcopenia research. It additionally carries the advantage of not involving radiation exposure (see Table 81.1).
The drawbacks of MRI are the size and expense of the required equipment. A trained operator is also required to operate the scanner. Additional limitation, especially in research involving older subjects, is that the strong magnetic field involved limits the use of MRI in subjects who have pacemakers, certain surgical clips, and certain metallic implants.
Bioelectric impedance Bioelectric impedance (BI) involves measuring the conductance of an electric current in the limb of a subject to estimate muscle mass. The technique relies upon the differing electric conducting properties of muscle, fat, and bone with muscle being the best conductor and bone the worst. Comparison of bioelectric impedance with measurements made by either CT scan or DEXA has shown good correlation with these techniques (see Table 81.1). BI requires the use of specialized equipment, but the required equipment is lower in cost and much more portable when compared to techniques like CT and MRI. It is not known if BI demonstrates sufficient sensitivity to measure small changes in muscle mass such as in response to resistive training. BI also has been less extensively studied and used in human sarcopenia studies.
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