Chapter 6 - Image Artifacts

Abstract

This chapter focuses on several major artifacts that are caused by factors predominantly associated with the imaged object: magnetic field nonuniformity, fat/water chemical shift, flow, and motion. Magnetic resonance (MR) images degrade frequently by different artifacts that occur for a variety of reasons. The quality of MR images is often low because of the artifacts caused by heterogeneity of the main field, eddy currents, nonlinearity of imaging gradients, and other scanner-related problems. The frequency of occurrence and severity of these artifacts in MR images have been reduced significantly due to technical improvements in magnetic resonance imaging (MRI) hardware. During MR imaging the magnetic field in the object can be nonuniform for various reasons including nonuniformity of the external magnetic field, presence of metal implants and spatial variations in magnetic susceptibility of the object. Magnetic field nonuniformity causes two major types of artifacts in MR images: geometric distortion and signal loss due to intravoxel dephasing. A chemical shift between fat and water causes two different types of artifacts: misregistration of fat or water in MR images; and visible reduction in image intensity as a result of destructive interference between the fat and water signals. Motion-related changes in the specimen during different phase-encoding cycles create a potential source of artifacts in MR images. Motion-induced artifacts, such as blurring and image ghosts represent a major problem in diagnostic MRI because they impede visualization of small structures in images. Several techniques used to reduce motion artifacts in MRI are also discussed in the chapter.