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CT is a technique that creates cross-sectional images with the
use of x-rays and computerized image reconstruction. A large series
of two-dimensional x-ray images are taken around a single axis of
rotation as the patient passes through a gantry. The gantry contains
one or more x-ray tubes on one side and an array of detectors on
the other side. As the gantry rotates around the patient, obtained
information from the detectors is analyzed by computer and displayed
as an image. The image information can be manipulated by the computer
to display a greater spectrum of densities than can be displayed
on conventional x-ray film. These images can be manipulated, or “windowed,” to
display various tissue densities based on their ability to block
the x-ray beam. Initially, images were limited to the axial plane,
but modern scanners allow reconstruction of the images into both multiple
planes as well as three-dimensional (3D) (volumetric) depiction of
the structures. It is due to this ability to display images in multiple
planes that the term computed axial tomography has
fallen out of favor.
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While CT scanners use x-rays, MRI uses radio frequency to acquire
images. The ionizing radiation of CT scans can best be used to differentiate
calcified tissue from noncalcified tissue, whereas MRI acquires
its best images from noncalcified tissues and produces images with
greater soft tissue contrast. Both imaging modalities may use contrast
agents, with CT using high atomic weight agents, such as iodine
or barium, and MRI using agents with paramagnetic properties such
as gadolinium or manganese. See Chapter e299.3,
Magnetic Resonance Imaging, for a full discussion of MRI.
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The advantages of CT include availability, rapid acquisition
of images, and lower cost compared with MRI. The major limitations
of CT are significant radiation exposure and the inability to use
IV contrast in patients with renal insufficiency or significant allergy
to contrast. MRI, on the other hand, obtains exquisite images of
the central nervous system and stationary soft tissues such as joints,
does not use ionizing radiation, is generally thought to be safer
in pregnancy, and is associated with fewer significant allergic
reactions to gadolinium than to iodinated agents. The major limitations
of MRI are higher cost, relative unavailability (particularly during
off hours), claustrophobic issues for some patients, inability to
bring metallic objects near the magnet, and concern for nephrogenic
systemic fibrosis in patients receiving gadolinium with advanced
renal failure.1 In addition, the longer time required
to obtain images is a problem not only for hemodynamically unstable
or significantly ill patients, but it also leads to motion artifact.
New magnetic resonance technology with rapid imaging may change
the use of magnetic resonance in the ED in the future.
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Image Formation
and CT Numbers
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The various shades of gray that make up a CT image are determined
by the density of a structure and the amount of x-ray energy that
passes through it. This phenomenon is ...