Signal-to-noise ratio (CT)

Last revised by Andrew Murphy on 23 Mar 2023

Signal-to-noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (i.e. reflecting actual anatomy) to noise (e.g. random quantum mottle).

Signal-to-noise ratio in CT roughly follows the same principles as those of plain radiographs. It is calculated by comparing the level of the desired signal to the background deviation from normal pixel values. In general, the larger the number of photons transmitted, the greater the SNR.

In CT the signal-to-noise ratio is determined by:

  • mAs
    • greater mAs increases SNR
  • slice thickness
    • thicker slices increase SNR
  • patient size
    • larger patients reduce SNR

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