Early workers in the field of clinical nutrition used
standard x-ray techniques to examine fat layer thickness.
The high radiation exposure and low image contrast limited
the research and clinical applicability of this approach.
In 1973, Geoffrey Hounsfield and his colleagues introduced
the first computerized axial tomography (CT) system,
and by the early 1980's, CT scanners were installed
in hospitals throughout the world. The early CT approach
provided cross-sectional images with high contrast and
by 1979, the first reports of skeletal muscle measurement
appeared. The first measurements of visceral organ volumes
were reported in 1979, as well as estimates of visceral
adipose tissue in 1981. Several contiguous image slices
were assembled into the complete three-dimensional adipose
tissue or organ compartment of interest. While CT was
a major breakthrough in quantifying the volumes of tissues
and organs, applicability was limited by radiation exposure.
Within a decade, the first reports of magnetic resonance
imaging (MRI) of humans appeared and Foster et al. in
1980s’reported the first MRI body composition
studies. As with CT, MRI provides the unique capability
of quantifying tissue and organ volumes in vivo but
without radiation hazard. Gradual advances in both CT
and MRI capabilities now make single or multiple slice
tissue, as well as organ analysis, a reference approach
against which other techniques can be compared to.
Both CT and MRI provide high-resolution cross-sectional
images through selected anatomic regions. At one extreme
the entire body can be imaged and the volume of all
major tissue-system level components estimated. Volume
estimates can be converted to mass values by assuming
specific tissue densities. Depending on the selected
slice number, whole-body evaluations require scan times
ranging from about 20 minutes to several hours. As CT
exposes subjects to radiation, there are several reports
of whole-body CT studies in humans. Moreover, CT and
MRI in phantom and cadaver studies provide similar tissue
volume estimates. The trend today is to apply MRI whenever
possible. Specific aspects of scanning protocols are
reported in earlier studies.
Once images are collected, analysis
can take one of several different pathways. For CT,
pixel intensities are designated in Hounsfield units
(HU), and calibrations are similar among all CT scanners.
Hounsfield unit ranges differ between tissues, notably
adipose tissue, lean soft tissues, and bone vary sufficiently
in pixel intensity to allow component separation using
designated HU ranges. Hounsfield unit ranges for adipose
tissue, muscle/organs, and bone are: -190 to –30
HU; -30 to +100 HU; >100 HU.
Selected organs and tissues can also be traced directly
on the CT scanner console and related areas within each
slice established. As CT imaging time is usually rapid
(several seconds per slice), images of moving objects
such as the viscera, secondary to peristalsis, and respiration,
are still relatively sharp and boundaries are clear.
Analysis of MRI scans is
more complex as pixel intensity varies according to
the selected imaging sequence and other factors. Standard
pixel “ranges” cannot therefore be set, and
analysis is on a scan-by-scan basis. Dedicated image
analysis software is usually applied rather than standard
system radiology software. Image acquisition times for
MRI are usually longer than they are for CT, and patients
should maximize their breath holds and maintain a stable
position during the scan. Images of the viscera tend
to be less sharp than they are for CT, although new
MRI scanning sequences are improving image clarity.
Gated MRI scans allow development of high contrast images
of the myocardium. For both CT and MRI, training is
required for image analysis and reading times can vary
from several minutes for a single slice to several days
for a whole body.
Imaging methods, both CT
and MRI, are uniquely capable of acquiring tissue-organ
level volume estimates including all major organs and
tissues, visceral adipose tissue, and regional estimates.
CT and MRI estimates of visceral adipose tissue, either
a single slice or multiple slices, are considered the
reference against which other techniques are compared.
Cost, instrument access, and the need for trained image
analysis technicians may limit routine imaging method
use to specialized research studies and centers. CT
and MRI are not appropriate for use in field studies
of body composition, although both methods can be used
to “calibrate” or validate other simpler
less costly methods.
While today we still focus
mainly on produced images, rapid growth in magnetic
resonance spectroscopy and functional MRI offer great
promise in the study of human physiology and metabolism.
It is likely that these new developments will not only
add to our ability to quantify body composition, but
to enhance of knowledge of closely-related metabolic
processes as well