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Computed Tomography is based on the
x-ray principal: as x-rays pass through the body, they are absorbed or attenuated
(weakened) at differing levels creating a matrix or profile of x-ray beams of different
strength. This x-ray profile is registered on film, thus creating an image. In the case of
CT, the film is replaced by a banana shaped detector which measures the x-ray profile.
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| Outside
view of modern CT system showing the patient table and CT scanning patient aperture |
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Inside
view of modern CT system, the x-ray tube is on the top at the 1 o'clock position and the
arc-shaped CT detector is on the bottom at the 7 o'clock position. The frame holding the
x-ray tube and detector rotate around the patient as the data is gathered. |
A CT scanner looks like
a big, square doughnut. The patient aperture (opening) is 60 cm to 70 cm (24" to
28") in diameter. Inside the covers of the CT scanner is a rotating frame which has
an x-ray tube mounted on one side and the banana shaped detector mounted on the opposite
side. A fan beam of x-ray is created as the rotating frame spins the x-ray tube and
detector around the patient (see figure below). Each time the x-ray tube and detector make
a 360° rotation, an image or "slice" has been acquired. This "slice"
is collimated (focused) to a thickness between 1 mm and 10 mm using lead shutters in front
of the x-ray tube and x-ray detector.
As the x-ray tube and detector make
this 360° rotation, the detector takes numerous snapshots (called profiles) of the
attenuated x-ray beam. Typically, in one 360° lap, about 1,000 profiles are sampled. Each
profile is subdivided spatially (divided into partitions) by the detectors and fed into
about 700 individual channels. Each profile is then backwards reconstructed (or "back
projected") by a dedicated computer into a two-dimensional image of the
"slice" that was scanned.
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Diagram showing relationship of
x-ray tube, patient, detector, and image reconstruction computer and display monitor |
Multiple computers are used to control
the entire CT system. The main computer that orchestrates the operation of the entire
system is called the "host computer." There is also a dedicated computer that
reconstructs the "raw CT data" into an image. A workstation with a mouse,
keyboard and other dedicated controls allows the technologist to control and monitor the
exam. The CT gantry and table have multiple microprocessors that control the rotation of
the gantry, movement of the table (up/down and in/out), tilting of the gantry for angled
images, and other functions such as turning the x-ray beam on an off.
Updated: September 13, 2007
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