CIE A Level Physics (9702) 2019-2021

Revision Notes

27.4.4 Computed Tomography Scanning

Computed Tomography Scanning

  • 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 (CAT or CT) scan is used
  • The main features of the operation of a CT scan are as follows:
    • An X-ray tube rotates around the stationary patient
    • A CT scanner takes X-ray images of the same slice, at many different angles
    • This process is repeated, then images of successive slices are combined together
    • A computer pieces the images together to build a 3D image
    • This 3D image can be rotated and viewed from different angles

Advantages & Disadvantages of CAT Scans

  • Advantages:
    • Produces much more detailed images
    • Can distinguish between tissues with similar attenuation coefficients
    • Produces a 3D image of the body by combining the images at each direction
  • Disadvantages:
    • The patient receives a much higher dose than a normal X-ray
    • Possible side effects from the contrast media

Worked example

Worked_Example_-_Comparing_X-rays_and_CT_scans_Question, downloadable AS & A Level Physics revision notes


The simple X-ray image involves taking a single exposure

This produces a single 2D image

CT scan

The CT scan requires taking several exposures of a slice from many different angles

This is then repeated for different slices before being combined together to build a 3D image

This involves taking a much greater exposure than the simple X-ray

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