status of imaging methods; a final section provides suggestions for future research.

Students of human biology have expressed interest in body composition since the nineteenth century (Forbes, 1987; Lawes and Gilbert, 1859; von Bezold, 1857). Human shape, form, fatness, muscularity, and chemical composition were of interest to many investigators early in the twentieth century (Moulton, 1923; Spivak, 1915). Advances in radiographic methods led Stuart and colleagues (1940) to report the first use of standard x-ray films to capture fat and muscle "shadows." By the early 1960s, radiogrammetry was a well-developed method that provided investigators with estimates of subcutaneous adipose tissue (AT) layer thickness and muscle widths (Garn, 1957, 1961). Low image contrast and two-dimensional radiographs limited radiogrammetry to rough approximations of body composition components.

G. N. Hounsfield first introduced CT for brain imaging in 1971 and reported his findings in 1973. The theoretical framework and mathematical algorithms needed for image reconstruction were developed earlier by Raden (1917) and Cormack (1980). The historical roots for MRI appeared even earlier than for CT. Bloch et al. (1946) and Purcell et al. (1946) independently described the basic phenomena that ultimately gave rise to nuclear magnetic resonance spectroscopy in 1946. Differences between malignant tumors and normal tissue were first studied using magnetic resonance (MR) methods in 1971 by Damadian. That one could create images of phantoms using the signal from proton nuclear magnetic resonance was demonstrated in 1973 by Lauterber.

By the mid 1970s, CT scanners were being installed in many American medical centers, and by the early 1980s, MRI systems became available on a widespread basis. Unlike radiogrammetry, even early CT provided high-resolution cross-sectional images. Clear anatomic boundaries could be visualized between subcutaneous AT, skeletal muscle, visceral organs, brain, and skeleton. During this early era of CT technology, Heymsfield and colleagues were exploring the validity of anthropometric assessment methods as they applied to hospitalized patients. The installation of a CT scanner in the investigators' hospital provided an important opportunity to examine anthropometric assessment methods with CT as the "reference" standard. Their use of CT to quantify skeletal muscle mass was reported in 1979 (Heymsfield et al., 1979a), and subsequent reports by the group described methods of estimating visceral organ volumes (Heymsfield et al., 1979b) and visceral AT (Heymsfield and Noel, 1981).

The high radiation exposure of CT limited its use until the mid 1980s. Moreover, other methods with little or no radiation and lower cost appeared to provide adequate measures of components such as fat and fat-free body mass (Forbes, 1987; Lukaski, 1987). During this period, epidemiological studies began to appear that linked upper body AT distribution and visceral AT with obe-