Contrast in NMR imaging and microscopy

Nuclear magnetic resonance ( N M R ) imaging is increasingly being used for medical diagnosis and in scientific research and applications. In practice, it is often difficult to quantitatively relate the image intensity to the nuclear spin density, because other factors complicate a proportional interpretation. These factors include chemical shift, magnetic susceptibility variation, nuclear spin relaxation. and molecular motion (ordered flow and random Brownian motion) However, this kind of information is valuable in its own right and it can be quantitatively and accurately extracted from imaging experiments. This paper defines an image contrast as the image intensity change or modulation due to the contribution of molecular information other than the nuclear spin density. It is this kind of quantitative measurement that makes NMR imaging a useful tool for scientific research and practical applications. This article uses the concept of image contrast to summarize and describe various contrast mechanisms in NMR-imaging experiments.