Introduction

As we all know, real life is far from perfect and MRI is just as bad! MR scanners do not have absolutely uniform magnetic fields, the gradients don't produce exactly the pulse shapes programmed by the pulse sequence and patients don't keep still. These problems, and many others, produce artefacts in MR images. An artefact can be defined as any feature in an image which misrepresents the object in the field of view. This could be a bright signal lying outside the body, or lack of signal where there should be something. It might also be a distortion in the image, so that a straight line appears curved, or a certain area is artificially magnified or reduced. A large group of MR artefacts appear as ‘ghost’ images, where a faint copy of the anatomy appears in the image displaced in one direction or another. In general artefacts are most serious when they degrade the rest of the image.

In this chapter we will describe the most common artefacts encountered in MRI, along with ways to avoid or minimize them. The causes of artefacts can be broadly divided into three groups: motion, inhomogeneity and digital imaging artefacts:

• Motion artefacts tend to appear as ghosts along the phase-encode direction, and may be produced by physiological motion or involuntary movement by the patient.

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