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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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