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An x-ray image is produced when a small
amount of radiation passes through the body to expose
sensitive film on the other side. The ability of x-rays
to penetrate tissues and bones depends on the tissue's
composition and mass. The difference between these two
elements creates the images. The chest x-ray is the most
common radiologic examination. Contrast agents, such as
barium, can be swallowed to highlight the esophagus,
stomach, and intestine and are used to help visualize an
organ or film.
CT
Computed tomography or CT, shows organs of interest at
selected levels of the body. They are visual equivalent
of bloodless slices of anatomy, with each scan being a
single slice. CT examinations produce detailed organ
studies by stacking individual image slices. CT can
image the internal portion of organs and separate
overlapping structures precisely. The scans are produced
by having the source of the x-ray beam encircle or
rotate around the patient. X-rays passing through the
body are detected by an array of sensors. Information
from the sensors is computer processed and then
displayed as an image on a video screen.
MRI
Like CT, MRI produces images, which are the visual
equivalent of a slice of anatomy. MRI, however, is also
capable of producing those images in an infinite number
of projections through the body. MRI uses a large magnet
that surrounds the patient, radio frequencies, and a
computer to produce its images. As the patient enters a
MRI scanner, his body is surrounded by a magnetic field
up to 8,000 times stronger than that of the earth. The
scanner subjects nuclei of the body's atoms to a radio
signal, temporarily knocking select ones out of
alignment. When the signal stops, the nuclei return to
the aligned position, releasing their own faint radio
frequencies from which the scanner and computer produce
detailed images of the human anatomy. Patients who
cannot undergo a MRI examination include those people
dependent upon cardiac pacemakers and those with
metallic foreign bodies in the brain or around the eye.
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