CNR (imaging)

Contrast to Noise Ratio, written also as CNR, refers to the ability of an imaging modality such as MRI or fluoroscopy to distinguish between various contrasts of an acquired image and the inherent noise in the image.

In MRI, it is defined as:

CNR = (SA-SB) / noise

where SA and SB are signal intensities for tissues A and B. Noise is usually measured from an ROI (region of interest) in the background, i.e. the measured standard deviation of the ROI. Noise is inherent in the image and can arise from imaging hardware as well as from the tissue itself. This definition may also apply to other diagnostic imaging modalities in general as well.