10.5.6 The CT Scanner

• Computerised axial tomography (CT) scanning is an imaging technique which uses X-rays to produce very high-resolution images of the internal structures of the body

• A CT scan can produce:
  • Sharp, focused 2D images of thin slices of the body
  • Detailed 3D images of sections of the body

• The main features of a CT scanner are
  • A ring-shaped structure which allows for rotation of the components
  • An X-ray tube mounted on one side of the ring
  • An array of detectors mounted on the other side of the ring (opposite the X-ray tube)
  • A computer which processes the images

Features of a CT scanner

During a CT scan, the patient lies in the centre of a ring while the X-ray tube and detectors are rotated around the organ being examined

• The main principles of the operation of a CT scan are as follows:
  • The patient lies stationary at the centre of a ring while the X-ray tube and array of detectors are rotated around them in opposite directions
  • The X-ray tube produces a narrow, monochromatic beam of X-rays as short pulses
  • The X-ray beam passes through the patient and arrives at the array of detectors on the opposite side of the ring
  • The X-ray tube rotates and sends beams through the same slice of the body in different directions
  • Signals from the detectors are fed into a computer and are combined to generate a 2D image of the slice
  • This process is repeated to build up images of successive slices
  • A computer combines the images to produce a 3D image which can be rotated and viewed from different angles

Building an image using CT scanning

CT scans take several 2D images from multiple positions to create a 3D image

• How the image is built up:
  • The detectors are arranged around the outside of the path of the X-rays
  • The X-rays pass through the patient and arrive at the detector on the opposite side
  • The detectors register a lower intensity than the initial intensity of the transmitted beam
  • The detectors relay this information to a computer which produces a cross sectional image over time

Image of a slice produced by a CT scan

A CT scan produces detailed images of slices of the body. This diagram shows a CT scan through a patient's abdomen, which can be combined with many slices to allow a comprehensive 3D image to be built up

• A simple X-ray image can provide useful, but limited, information about internal structures in a 2D image
• When a more comprehensive image is needed, a computerised axial tomography (CT) scan is needed

Advantages of CT scans:

• Produces high resolution and high contrast images (software can add colour and sharpen images, and parts of the image can be edited out)
• Can distinguish between tissues with similar attenuation coefficients
• Soft tissue and bone can be imaged in a single process
• Produces a 3D image of the body by combining the images in each direction
• No overlapping images (e.g. bones obscuring organs)

Disadvantages of CT scans:

• The patient receives a much higher radiation dose compared to a normal X-ray
• CT scans are time-consuming and expensive
• Possible side effects from the contrast media