MR imaging artifacts that simulate disease: how to recognize and eliminate them.

Abstract

Occasionally, artifacts may simulate pathologic conditions on magnetic resonance (MR) images. Motion artifacts especially affect images of the chest and abdomen. There are a number of techniques for reducing motion artifacts, including respiratory and cardiac gating, k-space phase reordering, gradient moment nulling, even echo rephasing, and physical restraints. Aliasing occurs when the field of view does not include all of the anatomic structures present in the imaged section. Aliasing artifacts can be eliminated by increasing the field of view, oversampling, and use of saturation pulses or surface coils. Truncation artifacts represent the difference between the original and the reconstructed image and can be reduced with data extrapolation algorithms or image filtering. Chemical shift artifacts and magnetic susceptibility artifacts are due to a local deformity of the magnetic field, resulting in spatial misregistration. Chemical shift artifacts are more severe in images acquired with a narrow-bandwidth technique; magnetic susceptibility artifacts are more severe in images acquired with a long echo time. Pitfalls in the interpretation of MR images can be avoided by becoming familiar with the appearances and causes of common MR imaging artifacts.

High Signal Intensity on T1-Weighted MR Image Related to Vacuum Cleft in the Intervertebral Disk: Clinical and Phantom Study

“…These differences in susceptibility lead to distortions in the local magnetic environment, causing altered spin relaxation properties which may generate signal loss, misregistration and accelerated T1 relaxation (7). Because there is more time for protons to dephase and a lack of refocusing of the 180 pulse, magnetic susceptibility artifacts are more prominent in images acquired with longer TE or a gradient-echo sequence with increased flip angle (8).…”

Motion-Compensation Techniques in Neonatal and Fetal MR Imaging

“…24 Motion artifacts are actually artifactual images of the source image, resulting from uncorrected data inconsistencies due to source motion. 25 These can be quite widespread and overt and degrade image quality, ultimately rendering examinations nondiagnostic, or they may be more localized and inconspicuous, leaving much of the useful imaging data unaffected.…”

“…The final MR image and the raw data, or k-space data, are related by the Fourier transform so that each pixel of the image is composed of a weighted sum of every k-space point and each k-space point contains a weighted sum of signals from every point in the region of the object being imaged; therefore, any inconsistency in k-space data sampling has the potential to affect every pixel in the resulting MR image. 26,27 There are essentially 2 ways in which data inconsistencies due to motion may produce artifacts 24 : 1) intra-view (or within view) effects are caused by motion occurring between each RF pulse excitation and echo formation, and 2) inter-view (or view-toview) effects are caused by motion occurring between the acquisition of successive phase-encoding steps. Interview effects result in phase errors due to the inconsistent location and signal intensity of the moving spins during phase-encoding, while intra-view effects result in phase incoherence among the moving spins at the time of echo formation.…”

“…The appearance of ghost artifacts resulting from strictly periodic sinusoidal motion is governed by different factors summarized in the following simple formula 24,28 : Distance (in pixels) ϭ TR ϫ Phase-Encoding Steps ϫ Number of Signal Averages ϫ Motion Frequency. Therefore, the location of ghost artifacts is directly proportional to the TR, the matrix size in the phase-encoding direction, the NSA (assuming parallel averaging, where each k-space line is acquired NSA times before moving to the next), and the frequency (rate) of motion; the higher the rate, the bigger is the distance between the ghosts (Fig 7).…”

Artefatos em ressonância magnética do abdome: ensaio iconográfico

“…1 -Artefato de "retroprojeção" ou "dobradura" (em inglês, "aliasing" ou "wraparound" ou "fold-over") (1)(2)(3)(4) Este artefato ocorre quando uma ou mais dimensões do objeto de estudo são maiores do que o campo de visão (FOV) para aquela imagem. Dessa forma, as regiões fora do campo de visão são erroneamente codificadas e aparecem "dobradas" e em cima da estrutura examinada, sobrepondo-se a esta última (Figuras 1 e 2).…”