Basic MR physics

Abstract

In magnetic resonance, the properties of protons in tissue giving rise to so-called magnetic moments are exploited. The sum of many magnetic moments yields what is referred to as net magnetization, which can be seen as similar to the magnetization a bar magnet produces. The relation and interaction between magnetic moments, net magnetization, the static magnetic field, and radiofrequency fields are discussed. It is shown that radiofrequency excitation can be used to manipulate the net magnetization, such that it can be detected using radiofrequency antennae or coils. Upon excitation, the net magnetization will recover back to its equilibrium orientation with tissue-specific time constants for the transverse and longitudinal components, which, in turn, are important sources of image contrast in cardiac imaging. The discussion concludes with a foray into susceptibility and chemical shift effects resulting from different molecular environments in which protons can reside and which provide additional image contrast mechanisms.