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					Computerised axial tomography (CAT or CT)

A ‘normal’ X ray gives only limited information because it
is rather like a shadow picture – fine detail within the
image may be invisible especially if one organ or bone
lies in front of the region of the body being studied.

In the 1970s a method was developed to give much
higher quality images including a 3D view of the patient.
This method of scanning is called computerised axial
tomography (CAT or CT). The name is derived from the
Greek for slice (tomos) and read (graphic)

                                       X ray detectors         Patient on couch




                       X ray beam
                                                             X ray source

             CT scanner with the X ray source and detectors shown in three positions

In the CT scanner there is one X ray source but a large number of detectors. The source and
the detectors are mounted in a large doughnut shaped machine (see diagram and photo)
and the patient is placed inside this on a couch. Each detector records an image and the
source and detectors are then rotated around the patient to give views from a variety of
direction. The image is called a tomogram. The couch and patient are then moved along the
axis of the machine and another set of images is taken. The complete process may take a
few minutes or up to half an hour.

This large number of images (many hundreds) are then combined by a computer to give a
composite detailed 3D image of the organs under investigation. The development of the CT
scanner has been of enormous help in the study of the tumours in cancer patients where
images of high quality are essential.

You can compare a ‘normal’ X ray with a CT scan where the X ray is like looking at a pack of
cards from the outside and the CT scan is like slowly removing each card and seeing the
different detail that is on each of them!

It has been said that the development of the CT scanner was possibly "the greatest legacy"
of the Beatles because EMI were able to use some of the massive profits resulting from their
record sales to fund scientific research.

A useful account of the CAT scanner can be found at: http://health.howstuffworks.com/cat-scan1.htm

				
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