|ECR 2019 / C-2064|
|How to take-on Metal-on Metal in MRI: A Step-By-Step Guide to read Musculoskeletal MRI with Metallic implants.|
Metal associated artifacts arise due to the difference in magnetic properties of the tissues in the body (muscle, ligaments, fat, bone- that are diamagnetic and contain variable water protons) and metal (that are markedly paramagnetic or ferromagnetic and typically do not contain water protons).
Types of metal associated artifacts
The effect of the metal prosthesis in the magnetic field leads to geometric distortion, that in the frequency encoding direction leads to in-plane distortion, (curved appearance of straight lines) while that in the slice-selective direction leads to through-plane distortion, (shifting of information from the excited slice - the so-called "potato-chip" effect, or unintended thinner and thicker slices)
Signal Pile-up or Signal-voids
Due to metal, when signal is erroneously shifted to an adjacent voxel, or out of plane to an adjacent slice, it appears as a signal void, (a more common occurrence) while when signal is erroneously attributed to a voxel, it results in a pile-up of signal or very bright areas. The larger the voxel size, the more partial volume averaging leads to darker areas and the larger the artifact.
Failure of fat suppression
Frequency selective fat suppression / Chemical fat suppression utilizes a narrow bandwidth suppression pulse (on a setting of background magnetic field homogeneity) to flip fatty signal out of the transverse magnetization plane. In the presence of metal, this suppression pulse can lead to marked signal drop-out from all tissues, or absence of fat suppression or even artefactually appearing bright areas, and hence should be avoided.
Typical strategies to reduced metal artifact
- The implant should be in the frequency encoding direction or the longer dimension of the rectangular FOV - as this is usually in the long axis of B0 (main magnetic field). Some implants may, however, not be amenable to such orientation.
- The smallest size possible voxel should be used to minimize partial volume averaging artifacts. Increasing image noise is a disadvantage with this strategy.
- Use of MARS (Metal Artefact Reduction Sequence) higher receiver bandwidth, shorter TE (less than 40ms), typically giving Proton Density or T1 weighted images, that are optimal for musculoskeletal imaging. The disadvantage of the MARS images is usually increased image noise due to increased bandwidth.
- Use of non-frequency selective fat suppression, e.g. STIR (Short Tau Inversion Recovery) is satisfactory to evaluate bone marrow edema/ collections/ infection but not as good for articular pathology. 2-Point, or 3-Point DIXON techniques can be used to substitute for PD images, to evaluate ligaments, tendons, and cartilage in the presence of metal, but typically take longer times to acquire, and have inherent blurring.
- Specialized MR sequences have been developed that help in reducing the artifacts due to metal.
- Slice-Encoding for Metal Artifact Correction (SEMAC)- a 2D Spin echo technique that uses phase encoding in the third dimension (slice selection direction), and adjustments (View Angle Tilt) for in- and through- plane signal pile-up, or loss. The disadvantage is a longer scan time to preserve SNR and slightly more distortion.
- The other sequence is called Multi-Acquisition Variable-Resonance Image Combination (MAVRIC) that uses a spatially non-selective 3-D acquisition, with different central frequencies during transmission and acquisition. Limitation includes lack of phase unwrapping (larger Field of View leads to reduced resolution), blurring and longer scan times. Originally only PD or T2 images were available with MAVRIC.
- MAVRIC-SL (or MAVRIC- Slice selective) is a hybrid technique that uses the z-phase encoding of SEMAC and the spectral selectivity of MAVRIC technique. STIR, T1, T2 and PD sequences are available with MAVRIC SL and scan times are typically shorter.
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