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Computed tomography principles
X-ray slice data is generated using an X-ray source that rotates
around the object; X-ray sensors are positioned on the opposite
side of the circle from the X-ray source. Many data scans are progressively
taken as the object is gradually passed through the gantry. They
are combined together by the mathematical procedure known as tomographic
reconstruction.
Newer machines with faster computer systems and newer
software strategies can process not only individual cross sections
but continuously changing cross sections as the gantry, with the
object to be imaged, is slowly and smoothly slid through the X-ray
circle. These are called helical or spiral CT machines. Their computer
systems integrate the data of the moving individual slices to generate
three dimensional volumetric information, in turn viewable from
multiple different perspectives on attached CT workstation monitors.
EBT Machine
This image has an uncertain copyright status and is pending deletion.
You can comment on the removal.In conventional CT machines, an X-Ray
tube is physically rotated behind a circular shroud (see the image
above right); in the less used electron beam tomography (EBT) the
tube is far larger, note the internal funnel shape in the photo,
with a hollow cross-section and only the electron current is rotated.
The data stream representing the varying radiographic
intensity sensed reaching the detectors on the opposite side of
the circle during each sweep—360 degree in conventional machines,
220 degree in EBT—is then computer processed to calculate
cross-sectional estimations of the radiographic density, expressed
in Hounsfield units.
CT is used in medicine as a diagnostic tool and as
a guide for interventional procedures. Sometimes contrast materials
such as intravenous iodinated contrast is used. This is useful to
highlight structures such as blood vessels that otherwise would
be difficult to delineate from their surroundings. Using contrast
material can also help to obtain functional information about tissues.
Pixels in an image obtained by CT scanning are displayed
in terms of relative radiodensity. The pixel itself is displayed
according to the mean attenuation of the tissue that it corresponds
to on a scale from -1024 to +3071 on the Hounsfield scale. Water
has an attenuation of 0 Hounsfield units (HU) while air is -1000
HU, bone is typically +400 HU or greater and metallic implants are
usually +1000 HU.
Improvements in CT technology have meant that the
overall radiation dose has decreased, scan times have decreased
and the ability to reconstruct images (for example, to look at the
same location from a different angle) has increased over time. Still,
the radiation dose from CT scans is several times higher than conventional
X-ray scans.
Presently, the cost of an average CT scanner is US$1.3
million.
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