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CT - How it works

CT Introduction CT History CT how it works CT patient info

CT uses x-rays to produce cross sectional images of the body. The CT scanner is a large square machine with a hole in the centre.

The circular structure contains a rotating frame with an x-ray tube mounted on one side and a detector mounted on the other side. During a CT scan beams of x-rays are emitted from the x-ray tube as it rotates in a circle around the body. In one rotation of the body 1000 beams of x-ray are emitted form the x-ray tube.

                                                                                A CT scanner

As the x-rays pass through the body, different tissues absorb different amounts of x-rays. The more dense tissues such as bone absorb more x-rays, whereas the less dense tissues such as lung absorb less x-rays. The x-rays detectors measure the amount of x-rays not absorbed by the tissues.  They send this information to the computer, which processes the information and produces a cross-sectional picture detailing the organs and tissues. On the CT scan denser tissues such  as  bone appear white, less dense tissues appear darker and air appears black.

 

Figure showing how CT scanning works

 

Conventional CT

In a conventional CT scanner the X-ray tube rotates once around the patient to produce one cross-sectional image slice. The x-ray tube cannot rotate more than 3600 and thus has to stop and rotate backwards before a second image slice can be produced. During this time the patient then moves a set distance further through the machine before a second cross sectional image is taken. In this way a number of cross sectional images of the patient can be taken. It takes the scanner 1-2 seconds to produce each image slice. The technique can be compared to cutting a loaf of bread into slices to see what it is inside the loaf.

Spiral / helical CT

Newer CT scanners have emerged with better mechanics. These scanners known as helical or spiral CT scanners are able to rotate the frame supporting the x-ray tube continuously. While the tube is rotating, the table supporting the patient also moves continuously through the scanner. In this way it is possible to scan a volume of tissue without gaps rather than scanning individual slices of the body.  The advantages of spiral CT are that it can be used produce three dimensional images for example of blood vessels (CT angiography). Spiral CT is faster than conventional CT; it can image an entire body region like the chest in 30 seconds, which as advantageous when examining children and critically ill patients. Additionally faster scan times give more accurate images as they help eliminate inaccuracy caused by of a patient motion such as breathing and bowel movement. 

A CT examination often requires the use of dyes or contrast materials to make it easier to distinguish between different types of tissues.

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