Signal, Noise, Signal-to-Noise, and Contrast-to-Noise Ratios

In previous chapters we have discussed signal values for various pulse sequences as a function of user-selectable parameters. Formal definitions of signal, noise, signal-to-noise ratios (SNR), and contrast-to-noise ratios (CNR) appear below. This chapter then presents general SNR and CNR considerations, including the effect of user-selectable imaging parameters on SNR and CNR. SNR is important, as it is a good measure of image quality. In detecting lesions in the body, however, high SNR alone will not guarantee that sufficient contrast exists to make the lesion detectable. CNR between lesion and background is important, as it serves as a quantitative metric for low-contrast lesion detection: the higher the CNR between lesion and background, the more likely the lesion's detection.¹