Signal-to-noise ratio

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). A lower signal-to-noise ratio generally results in a grainy appearance to images. 

Each modality has its own source(s) of noise and therefore techniques for signal-to-noise ratio maximization are modality-specific.

Radiographic interpretation

Plain radiograph/fluoroscopy

In radiography, the signal-to-noise ratio, and thus the apparent noisiness of the image, is proportional to the amount of contrast and the square root of the number of photons transmitted. Measures to increase the number of photons, such as increasing the mAs (tube current-exposure time product), therefore increase the signal-to-noise ratio. Contrast can be increased by administering contrast material or lowering the photon energy by reducing the kVp (tube potential).


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

In MRI the signal-to-noise ratio is measured frequently by calculating the difference in signal intensity between the area of interest and the background (usually chosen from the air surrounding the object). In air, any signal present should be noise. The difference between the signal and the background noise is divided by the standard deviation of the signal from the background - an indication of the variability of the background noise. 

Signal-to-noise ratio is proportional to the volume of the voxel and to the square root of the number of averages and phase steps (assuming constant-sized voxels). Since averaging and increasing the phase steps takes time, SNR is related closely to the acquisition time. 

In MRI, the signal-to-noise ratio can be improved by:

Additionally, SNR can be improved by tweaking scan parameters. Assuming all other factors remain the same, SNR can be improved by:  

Physics and Imaging Technology: MRI
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Article information

rID: 14045
Tag: pending
Synonyms or Alternate Spellings:
  • Signal to noise ratio (SNR)
  • SNR
  • Signal-noise ratio
  • Signal/noise ratio

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