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Background
The purpose of this exhibit is to review fundamentals of how susceptibility effects from metal can interfere magnetic resonance imaging (MRI). We divide the effects into groups based on alteration of the resonance frequency and indication of gradients in the magnetic field. Based on the understanding of the underlying mechanisms and their effects on different image sequence, we will illustrate strategies and techniques to reduce the effects of several metal-induced artifacts.

Educational Goals / Teaching Points
Review underlying physics concepts and imaging findings of different types of metal artifacts in MRI, including magnetic susceptibility, frequency change, induction of field gradients, interference with tissue suppression techniques and others. We will show how the proper identification of the mechanisms underlying the artifacts will guide strategies to reduce metal artifacts in MRI. Discuss the pros and cons of changing various MRI scan parameters. Understand what shimming is, and how its use can be affected by metal.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Metal within the MR scanner can affect image generation in several ways, depending on the sequence. Metal results in changes to the Larmor frequency and induces gradients in the magnetic field. Depending on the sequence, one effect may dominate. For example, in gradient echo sequences, the dominant effect is often dephasing due to induction of gradient, not distortion. Measures that address distortion will have no visible positive effect, but can have negative effects. Instead, it is best practice to reduce the TE or consider a spin echo sequence. Strategies to reduce spin echo sequences distortion include making the phase encoding direction perpendicular to the hardware and increasing transmit and receive bandwidth, although the latter also decreases signal to noise ratio. In chemical saturation sequences, such as sequences with fat saturation, the saturation pulse can miss fat and mistakenly saturate water, causing extensive signal loss. It is commonly misunderstood that many FLAIR and DWI sequences are baseline fat saturated, and so failure of fat saturation and accidental water saturation can affect these sequences. Additional strategies to reduce metal artifact include Dixon sequences and STIR. Dixon is less sensitive to main magnetic field inhomogeneities but can result in swap artifact. A few new advanced metal artifact correction sequences such as view angle tilting and Slice Encoding for Metal Artifact Correction SEMAC/MAVRIC) are becoming more available. Lastly, we will show how metal artifact can interfere with interpretation of images and can obscure pathology, but also give the appearance of pathology (water saturation giving appearance of lack of enhancement or lack of CSF suppression on FLAIR giving the appearance of abnormal CSF).

Conclusion
Metal artifacts in MRI include geometric distortion, dephasing, frequency shift, failure of chemical saturation, and signal loss. Understanding of the underlying mechanisms will allow thoughtful adjustments to imaging sequence selection and settings can diminish artifact and improve image quality and interpretation.