The contrast-to-noise in relaxation time, synthetic, and weighted-sum MR images.

The contrast-to-noise ratio (CNR) in three types of computed MR images is compared in a computer simulation. The original data consist of two spin-echo or two saturation-recovery images. Each pair of images is used to generate a relaxation time image, a synthetic image at arbitrary echo or repetition time, and an image which is a weighted sum of the original images. The CNR produced by these three methods is compared for signals spanning a wide range of relaxation times. In every comparison an optimally weighted sum produces the highest CNR that is statistically attainable. The CNR in the optimum synthetic image equals this bound only if contrast reversal does not occur in the original images. The CNR in relaxation time images is always less than the statistical bound and can be less than the CNR in the original images.