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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.

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.

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.

Image Formation and CT Numbers

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 ...

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