J Korean Soc Radiol.  2020 Jan;81(1):41-57. 10.3348/jksr.2020.81.1.41.

Metallic Artifacts on MR Imaging and Methods for Their Reduction

• Choo HJ
• Lee SJ
• Lee YH

• KMID: 2469181
• DOI: http://doi.org/10.3348/jksr.2020.81.1.41

Abstract

Metallic artifacts on MR imaging are typically induced by differences in magnetic susceptibility between the metallic implant and surrounding tissue. Conventional techniques for metal artifact reduction require MR machines with low field strength, shift in the frequency-encoding and phase-encoding directions according to the axis of metallic implant, increased receiver bandwidth and matrix, decreased slice thickness, and utilization of the short tau inversion recovery or Dixon method for fat-suppression. Slice-encoding for metal artifact correction and multi-acquisition variable-resonance image combination can dramatically reduce the number of metallic artifacts. However, these sequences have a considerably long acquisition time. Furthermore, the recently developed acceleration techniques including compressed sensing can solve this problem.

MeSH Terms

Acceleration
Artifacts*
Magnetic Resonance Imaging*
Metals
Methods*
Prostheses and Implants
Metals

Figure

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  Fig. 1. 74-year-old man with posterior fusion of L2-3-4. A, B. Sagittal T2-weighted image shows the signal void and pile-up next to the screws (A). The out-pouching of the intervertebral discs (arrows) can be identified on this image (A). However, it may be a pseudo-lesion associated with a geographic distortion considering the fact that the out-pouching of the intervertebral disc (arrows) is less prominent on the slice encoding for metal artifact correction combined with T2-weighted image (B).

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  Fig. 2. Types of metallic materials. A, B. T1-weighted images with spinal fusion using titanium in 59-year-old man (A) and using stainless steel in 63-year-old man (B). There are more metal artifacts in (B) than in (A).

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  Fig. 3. 63-year-old man with postoperative spinal infection. A, B. Axial T2-weighted image (TR/TE = 2000/100, receiver bandwidth = 230 Hz/pixel) obtained by a 3 T MR machine shows the prevertebral abscesses (arrowheads) (A). The followup MRI outcome after a 3-month antibiotic therapy has been imaged by a 1.5 T machine (B). The image (TR/TE = 3550/123, receiver bandwidth = 163 Hz/pixel) shows resolution of prevertebral abscesses. The metal artifact of the screws (B) is smaller than that in the initial MRI with 3 T (A). TE = echo time, TR = repetition time

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  Fig. 4. Effect of receiver bandwidth. Each voxel is encoded by a wider frequency range in a high receiver bandwidth than in a low bandwidth. Therefore, the local frequency errors generated by metal implant do not significantly influence the spatial encoding in a high receiver bandwidth. BW = bandwidth, Gr = readout gradient axis, Gs = slice selection gradient axis Modified from Khodarahmi et al. Semin Musculoskelet Radiol 2019; 23:e68-e81, with permission of Thieme (10)

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  Fig. 5. 66-year-old woman with posterior fusion of L3-4-5. A, B. The receiver bandwidths for the two axial T2-weighted images are 150 Hz/pixel (A) and 550 Hz/pixel (B), and the other parameters for (A) and (B) are the same. The metal artifact size for the bilateral screws is considerably larger in (A) than in (B).

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  Fig. 6. 59-year-old man with septic arthritis at left hip joint. A. Simple radiography shows the presence of antibiotic beads, spacer, and metallic device at the left hip joint and femur. Right hip joint and right femur are deformed. B. The frequency-selective fat suppressed T2 weighted image applied with SEMAC shows incomplete fat suppression around the metallic implant. C. The water-only image from Dixon technique shows more homogenous fat suppression than that in (B). However, it shows zebra artifact or Moire fringes (arrowheads) around the metallic instrument. D. Although the short tau inversion recovery with SEMAC shows more homogenous fat suppression than that in (B) and (C), its signal-to-noise ratio is relatively poor. E, F. Compared with T1 weighted image with SEMAC after contrast administration (E), subtraction image (F) between pre-contrast and post-contrast T1 weighted images with SEMAC shows obvious contrast-enhancement around the left hip joint (arrowheads) and right acetabulum (arrow). SEMAC = slice-encoding for metal artifact correction

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  Fig. 7. Concepts of VAT. A, B. Three materials with different magnetic susceptibility are assumed to be aligned in a row (A). The materials are displaced under the magnetic field and demonstrate signal overlap and signal void (A and B). C. VAT by applying the gradient in slice selection direction during the readout results in tilting the angle of the readout direction and resolves the in-plane artifacts. However, the image blurring can be seen at the margins of the materials. VAT = view angle tilting Modified from Khodarahmi et al. Semin Musculoskelet Radiol 2019;23:e68-e81, with permission of Thieme (10)

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  Fig. 8. SEMAC with SEMAC factor of 19. Individual encoding step images are obtained by additional phase-encoding steps in a slice-selection direc-tion. The combination of these partitions makes a final SEMAC image. SEMAC = slice encoding for metal artifact correction

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  Fig. 9. 39-year-old man with ACL, PLC, and medial collateral ligament repair. A. Simple radiography shows the several staples and anchor for ACL and PLC reconstruction. B. Conventional T2-weighted image shows the signal void and signal pile-up around the metallic devices inserted at the lateral femoral condyle. C. SEMAC T2-weighted image shows a metal artifact, which is smaller than that in (A), at the lateral femoral condyle. The typical ripple artifact around the metallic implants is present on the SEMAC image (arrowheads). ACL = anterior cruciate reconstruction, PLC = posterolateral corner reconstruction, SEMAC = slice-encoding for metal artifact correction

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  Fig. 10. 75-year-old woman with posterior fusion of L3-4-5 and interbody fusion of L3-4, L4-5, and L5-S1. A, B. Sagittal T2-weighted image with high receiver bandwidth (545 Hz/pixel) shows fluid collection (arrows) at the posterior epidural space of L4-5 (A). However, this fluid collection is obscured by the signal void (arrows) at the spinal central canal on slice-encoding for metal artifact correction T2-weighted image (B).

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  Fig. 11. 55-year-old man with acetabular fixation. A, B. Compared with conventional coronal proton density-weighted image (A), the artifact size of the metallic instruments is small in MAVRIC-SL (B). However, MAVRIC-SL demonstrates image blurring. MAVRIC-SL = multi-acquisition variable-resonance image combination-selective

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  Fig. 12. 75-year-old woman with posterior fusion of L4-5. A, B. T2-weighted images with SEMAC (A) and T2-weighted images with compressed-sensing SEMAC (B) show similar image quality. The acquisition times for compressed-sensing SEMAC (B) is particularly shorter than that for SEMAC (A) and are quite different: 6 minutes 35 seconds for T2-weighted images with SEMAC and 3 minutes 10 seconds for compressed-sensing SEMAC. SEMAC = slice-encoding for metal artifact correction

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