CAT Scans (OCR A Level Physics)

Revision Note

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Katie M


Katie M



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

Operation of a CT Scanner, downloadable AS & A Level Physics revision notes

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


Exam Tip

Don’t confuse CAT scans with MRI scans.  The machines both look like large doughnuts but MRI uses magnetic fields not X-rays!

CAT Scan vs X-ray Image

  • Advantages of CAT scans:
    • Produces much more detailed 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 giving a higher resolution image
    • Soft tissue and bone can be imaged in a single process
    • Produces a 3D image of the body by combining the images at each direction
    • No overlapping images (for example bones obscuring organs)


  • Disadvantages of CAT scans:
    • The patient receives a much higher dose than a normal X-ray
    • Possible side effects from the contrast media

Worked example

An X-ray image is taken of the skull of a patient. Another patient has a CT scan of his head.

By reference to the formation of the image in each case, suggest why the exposure to radiation differs between the two imaging techniques.


  • 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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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.