Primer on Commonly Occurring MRI Artifacts and How to Overcome Them

Abstract

Various artifacts occur at MRI, and MRI technologists and radiologists must know how to identify and avoid these artifacts so they can consistently produce high-quality images.

The full digital presentation is available online.

TEACHING POINTS

• ■ By learning the most common causes of each type of artifact, the impact on image quality can be mitigated.

• ■ System conditions, patient physiology, and tissue characteristics contribute to the appearance of artifacts at MRI.

• ■ Each artifact reduction strategy has pitfalls and trade-offs.

Various artifacts can occur at MRI. To avoid confusion when dictating reports, radiologists must be aware of these artifacts. In addition, MRI technologists must be able to identify artifacts and understand their causes and solutions to produce consistently high-quality images. This online presentation reviews the artifacts that are commonly encountered at MRI. For each artifact, there is a description of the phenomenon and its cause, as well as suggestions of how to mitigate or avoid similar artifacts. The presentation is targeted toward MRI technologists and trainees, but it can also serve as a useful reference for physicians who review MR images. However, it is assumed that the reader already has a basic knowledge of MR physics.

MRI artifacts can be caused by software, hardware, pulse sequences, or factors related to the patient (eg, tissue heterogeneity or movement) (Fig 1). Multiple factors may contribute to a single artifact. Many artifacts can be minimized or completely eliminated by changing the imaging parameters (Fig 2).

Figure 1.

Overview of commonly occurring artifacts at routine MRI. PI = parallel imaging, RF = radiofrequency.

Figure 2.

Minimizing artifact by changing imaging parameters. (A) Axial T2-weighted MR image shows chemical shift artifact as high signal intensity (arrowhead) in the orbit. The principal imaging parameters were a matrix of 256 × 256 and a bandwidth of 140 Hz/pixel. The phase-encoding direction was right to left. The fat-saturation pulse was turned off. (B–D) Axial T2-weighted MR images that were obtained after increasing the bandwidth from 140 Hz/pixel to 488 Hz/pixel (B), after increasing the frequency matrix from 256 to 512 (C), and after adding the fat-saturation pulse (D) demonstrate how chemical shift artifact can be minimized.

Zipper noise and spike noise are caused by hardware. These artifacts are often eradicated by eliminating the noise source (eg, the charger for a contrast agent injector), but in severe cases, service personnel may need to inspect the system.

Aliasing, also called fold-over, wraps signals from outside the field of view into the image. At two-dimensional imaging, this artifact can be avoided by carefully planning both the field of view and the phase-encoding direction. At three-dimensional imaging, wrapping also occurs in the section direction. Therefore, it is necessary to increase the slab thickness or oversampling to the section direction in consideration of wrapping.

Streak artifacts are unique to sequences that rely on radial sampling. The cause of these artifacts is the radial dispersion of motion or aliasing, the intensity of which depends on the amount of data sampling. It is necessary to optimize these scan parameters carefully.

Susceptibility artifacts due to metal are primarily displayed as localized signal loss, although high signal intensity (often called pile-up) may be seen along the periphery of the signal void. Removing the metallic material eliminates the artifact, but for items that cannot be removed (eg, cerebral artery clip, embolic coil, or pacemaker), some level of artifact typically remains evident, even if the parameters are properly modified.

Motion artifacts can be caused by periodic motion (eg, heartbeat or breathing) and random motion (eg, peristalsis or eye movement) during the scan. For periodic movements, artifacts can be reduced by using gated scan techniques (eg, cardiac gating or respiratory gating). There is no complete solution for artifacts that occur because of random motion. However, radial sampling and motion-corrected techniques may reduce the impact of patient motion on the image. When these are not available, it may be easier to read images by switching the phase-encoding direction. If the phase-encoding matrix can be lowered, it is also possible to reduce the severity of motion artifacts by acquiring the image in a shorter time. Finally, to minimize artifacts caused by patient movement, proper immobilization and good communication between the technologist and the patient are critical.