Concomitant Gradients in Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a beautiful application of the phenomenon of Nuclear Magnetic Resonance (NMR). MRI’s foremost identity lies in being a non-invasive diagnostic technique, but in fact, it has many other very important applications in biology, engineering and materials science. Classically, the field strength is regarded as one major 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 inhomogeneous fields (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 concomitant gradients cause strong image distortions. This is one of the most crucial handicaps of low field MRI.

In this manuscript we discuss concomitant gradients and work out their quantitative contribution towards the resulting image distortion. An introduction to the basics of NMR is outlined in the first chapter. Extending the basics of NMR, a brief account of MRI is presented in chapter 2. We have introduced and demonstrated a new method of MRI simulations with significantly reduced processing times in chapter 3. In chapter 4, we address concomitant gradients. We have computed the contribution of concomitant gradients analytically and simulated results for various arrangements of the gradient fields.