MRI Artifacts: Mechanism and Control

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Figure 8.The dark bands visible on the T1-weighted Spin Echo axial image are due to the intersection of other axial images through the image. These are acquired in a multi-planar fashion and thus cause pre-excitation (saturation) of the protons in the area where the slices intersect. The sagittal image illustrates where the slices were obtained and how they intersect posteriorly. M. Shading Shading artifacts manifest as foci of relatively reduced signal intensity involving a portion of the image. Abnormalities contained in the shaded portion of the MR image may be obscured. 9 There are many potential causes for this artifact, including partial volume averaging malfunction of the RF transmitter, amplifier, or receiver, excessive RF absorption, etc. 10, 11 To minimize shading artifacts, the anatomy of interest should be centered within the magnet, within the coil, and within the group of sections to be acquired. III. Conclusions Artifacts are common in magnetic resonance imaging. Most occur as a result of interactions of multiple factors, especially motion. MR artifacts primarily cause image degradation, although they can occasionally mimic pathological lesions. Some MR artifacts, such as those caused by periodic respiratory or vascular motion, may obscure important diagnostic findings. These and other MR artifacts may be minimized in some instances by proper selection of the directions of the phase and frequency encoding gradients. The ability to select and vary the direction of these gradients is a useful option in MR imaging. Investigations in the origins of MRI artifacts can not only lead to further understanding of the imaging process itself, but can also improve the quality and diagnostic accuracy of MR examination. 1 Saloner D, “Flow and motion,” Magn Reson Imaging Clin N Am 1999 Nov;7(4):699-715. 2 Barish MA and Jara H, “Motion artifact control in body MR imaging,” Magn Reson Imaging Clin N Am 1999 May;7(2):289-301. 3 Hedley M, Yan H, “ Motion artifact suppression: a review of post-processing techniques,” Magn Reson Imaging 1992;10(4):627-35. 4 Huber ME, Hengesbach D and Botnar RM, “ Motion artifact reduction and vessel enhancement for free-breathing navigator-gated coronary MRA using 3D k-space reordering,” Magn Reson Med 2001 Apr;45(4):645-52. 5 Altbach MI, Trouard TP and Van de Walle R, “Chemical-shift imaging utilizing the positional shifts along the readout gradient direction,” IEEE Trans Med Imaging 2001 Nov;20(11):1156-66. 6 Tsai CM and Nishimura DG, “ Reduced aliasing artifacts using variable-density k-space sampling trajectories,” Magn Reson Med 2000 Mar;43(3):452-8. 7 Mugler JP 3rd, “Overview of MR imaging pulse sequences,” Magn Reson Imaging Clin N Am 1999 Nov;7(4):661-97. 8
8 Clark JA 2nd and Kelly WM, “Common artifacts encountered in magnetic resonance imaging,” Radiol Clin North Am 1988 Sep;26(5):893-920. 9 Jones RW and Witte RJ, “Signal intensity artifacts in clinical MR imaging,” Radiographics 2000 May- Jun;20(3):893-901. 10 Mirowitz SA, “MR imaging artifacts. Challenges and solutions,” Magn Reson Imaging Clin N Am 1999 Nov;7(4):717-32. 11 Mirowitz SA, “MR imaging artifacts. Challenges and solutions,” Magn Reson Imaging Clin N Am 1999 Nov;7(4):717-32. 9
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MRI Artifacts: Mechanism and Control

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