Contrast resolution

[1] Image contrast can be expressed mathematically as: where SA and SB are signal intensities for signal-producing structures A and B in the region of interest.

An alternative definition is: where Sref is a reference signal intensity, which is independent of the type of signal-producing structure under investigation.

Unlike most other imaging modalities, such as x-ray CT in which the Hounsfield units value for water is set to zero, there is no standard reference signal for MRI.

Thus the contrast-to-noise ratio is often employed as an index for contrast because this metric does not require a reference signal.

The picture below shows one such set of images produced using the low contrast detectability inserts of the phantom employed in the MRI accreditation program of the American College of Radiology.