Rapid MR image acquisition in the presence of background gradients

Image acquisition in magnetic resonance imaging (MRI) is based on the application of external gradients to traverse the 2D Fourier domain of the object, known as k-space. This model holds up very well provided the underlying magnetic field is highly homogeneous, however, this is often not the case, particularly for higher field MRI. The constant component of magnetic field inhomogeneity can lead to blurring or geometric shifting in rapid imaging methods, such as spiral and echo planar imaging. The gradient of this background magnetic field variation leads to a local distortion of the acquisition trajectory. This distortion can lead to local sample density variations and shifts in the k-space trajectory that can lead to signal loss. This paper presents an analysis of background gradients, their impact on image quality, and some acquisition and image reconstruction approaches to reduce their effects.