Assessing contrast on MR images

Magnetic resonance imaging pulse sequences are frequently judged by their ability to facilitate discrimination between pathologic and normal tissue. Objective analysis is usually based on signal intensity measurements. However, the literature shows disagreement as to how this analysis should be performed. The ability to visually differentiate two objects on the basis of signal intensity depends on the contrast-to-noise ratio (CNR). This parameter, however, can be calculated only by measuring the intensity of photons reaching the eye from two distinct objects and, hence, is display dependent. The signal difference-to-noise ratio (SDNR) is a display-independent parameter that reflects the contrast-generating ability of a pulse sequence. When comparing two imaging sequences, the SDNR is proportional to the CNR, assuming the images being compared are displayed so that corresponding regions have the same intensity (i.e., photon fluxes). Because the SDNR is display independent, it should be the preferred parameter for assessing the contrast-generating ability of a pulse sequence. The value and limitations of these parameters are discussed.