Noise is an undesirable background interference or disturbance that affects image quality. The image noise is commonly characterized by the standard deviation of CT numbers in the image of a uniform object (phantom - generally water) relative to the difference in CT numbers between water and air.
Noise is an important limiting factor of CT image quality and is characterized by a grainy appearance, sometimes described as a salt and pepper pattern. Noise as a result of low radiation dose requires a higher tube current (mA) in combination with narrow slices to maintain image noise at an acceptable level.

Noise distributed in a normal (Gaussian) pattern. In such a distribution, approximately 65% of all points fall within one standard deviation (s) of the mean.

See also Full Width at Tenth Maximum.

The distribution associated with the magnitude of the noise amplitude following a Gaussian distribution. The mean value of this distribution is roughly 1.25 s0, where s0 is the standard deviation of the original Gaussian distribution.

Electronic noise decreases the image quality. There are various noise sources in any electronic system, including Johnson noise, shot noise, or thermal noise. Electronic noise can be caused by vibrations of any of the hardware components, especially the rotational components of a CT scanner or power fluctuations.

Radiographic noise is a fluctuation in optical density on radiographic or mammographic images, often as a result of low radiation dose.

See also Electronic Noise.