Biology Reference
In-Depth Information
Until the 1960s, pregnant women were radiographed for diagnostic purposes, which was
later linked to fetal/infant deaths and cancer (
Szabo, 2005
). Ultrasounds have replaced this
usage and are now standard in living subjects because they carry the lowest risk to a patient
(and a fetus in utero). Like DEXA, ultrasounds provide both T- and Z-scores for quickly diag-
nosing osteoporosis.
Ultrasonography has not caught on in skeletal biology, due to the lower quality of images
compared to the other imaging methods discussed, but the image quality has now improved.
Two disadvantages are that it loses resolution at greater depths within the body and it
requires a high level of skill to interpret the images (
Szabo, 2005
). Existing ultrasounds of
living subjects, however, could provide useful information on bone density, comparing the
bone density to average young individuals in a population or to sex and age-matched indi-
viduals, providing T- and Z-scores, respectively. One study on living subjects used ultra-
sounds to determine bone density in males and females from Nigeria compared to those
of European and Asian ancestry (
VanderJagt et al., 2004
).
Computed Tomography
Computed tomography is superior to both MRI and ultrasound for imaging the skeleton
because it produces superior detail of the dense skeletal tissues. CT performs multiple two-
dimensional slices of three-dimensional objects and mathematically reconstructs the cross-
sectional image from the X-ray measurement of thin slices (
Brant and Helms, 2007
). In
essence, the CT can create three-dimensional radiographs, but there is an intermediate
process called segmentation necessary to interpolate the information from one slice to the
next to create the 3-D model of a bone.
The advantages of CT are numerous: it (1) allow for rapid data acquisition, (2) is relatively
nondestructive (although some DNA degradation is possible as it generates extremely high
radiation), (3) provides high-resolution, three-dimensional data of both internal and external
bone surfaces, and (4) provides information on bone density. The potential for CT data has
been recognized for several decades in anthropology and skeletal biology (
Lovejoy et al.,
1976
;
Ruff and Leo, 1986
), although its use has mainly been limited to cross-sectional geom-
etry and costs are often prohibitive for large-scale application.
How CT Works
For CT, narrow X-ray beams pass through the subject in a helical fashion as they rotate
around the subject. Variation in the tissue densities causes changes in the X-rays' absorption
and scatter, which are received by banana-shaped detectors on the opposite side of the
subject. The subject is strapped to a table that moves through a doughnut-shaped device
in which the X-ray is rotating, enabling the process to be repeated many times automati-
cally. Each time the X-ray makes it 360 degrees around the subject, a new transverse radio-
graph is created, called a DICOM image. As the image is being interpreted in three
dimensions digitally, the data are recorded as voxels, which are volumetric (i.e. three
dimensional) pixels. The dimensions of a voxel are determined by a chosen algorithm
between 1 and 10 mm in resolution. The computer then mathematically reconstructs the
cross-sectional DICOM image from the X-ray measurement of the thin slices.
