Undesired gradients in low-field magnetic resonance imaging

Abstract

Magnetic resonance imaging (MRI) is a beautiful application of the phenomenon of nuclear magnetic resonance (NMR). Classically, the field strength is regarded as one measure of its quality because high field strength gives higher signal to noise ratios, better resolution and reduced scan times. So high field MRI has historically drawn a lot of attention. However, high-field MRI has some disadvantages like reduced relaxation times and high susceptibility gradients. Furthermore, high-field MRI systems are bulky, immovable, and very expensive. These reasons have motivated interest in the subject of low field MRI. The downside is that in the low field regime, we encounter the problem of undesired gradients appearing along with the desired ones. The presence of these additional gradients, generally known as concomitant gradients, directly follows from the fundamental Maxwell equations. The undesired gradients cause strong image distortions. In this article we discuss undesired gradients and work out their quantitative contribution towards the resulting image distortion. The gradient field is expressed in terms of a consistent tensorial description, including terms to arbitrary order. It is explored that the exact form of the undesired gradients can have an impact even in certain high-field MRI cases. Finally, we also appreciate the effects of these gradients in other physical systems such as the Stern-Gerlach experiment and the steering of magnetic species in solution. © 2009 Wiley Periodicals, Inc.