Magnetic resonance imaging relies on a homogenous magnetic field. When we introduce magnetic field variations across the patient with magnetic field gradients, the magnetic field strength relates to position, and is used to encode the MR signal.
But not in the presence of metal.
Magnetic susceptibility can be thought of as the “magnetisability” of a substance. Tissues with differing magnetic susceptibility will have a different static magnetic field (B0) strength within them. Thus, adjacent tissues of differing magnetic susceptibility have microscopic magnetic field gradients between them. As you know from the Larmor equation, this means that the precessional frequency of net magnetisation vectors in those tissues will be different. These tiny gradients cause dephasing within a voxel, which leads to reduced signal. However, the magnetic susceptibility of metal is much higher than that of tissue, such that around metal very large variations in Larmor frequency occur. This not only causes signal reduction, but signal loss. (But lost to where? …we’ll get back to that later.) So called signal pile-ups can also occur due to non-linear frequency-position mapping.
As a result, around metal implants, anatomy can be obscured. How can these unwanted effects be mitigated in MRI?
(Note: All acronyms and abbreviations in this post are explained at revisemri.com/abbrev)
Use Turbo Spin Echo
In gradient-echo based sequences, the transverse magnetisation decays according to T2*, which includes effects from magnetic field variations. Image artefacts due to metal are from the inhomogeneity they cause in the static magnetic field, and so gradient echo based sequences are more prone to metal artefacts (especially EPI). However spin echo based methods use RF refocusing pulses to return the T2* decay of transverse magnetisation to (longer) T2 decay, mitigating signal loss.
Many pulse sequences are based on gradient echo because of its speed, including the first sequence in an MR examination: the survey. It is sensible to use a turbo spin echo based sequence for the survey, to allow better planning of subsequent scans. If possible, turbo spin echo should be the sequence of choice for all the subsequent scans too. However, if gradient-echo based sequences must be used, the tips listed below which are not related to TSE still apply.
Set a shorter TSE echo spacing. This will allow collection of more echoes in an echo train before the signal has decayed away (according to T2 in a TSE sequence). On some scanners, the TE and echo spacing parameters may be decoupled (set independently) by selection of an asymmetric k-space profile order.
Use an intermediate/high number of TSE echoes in each echo train (shot duration about 4*TE).
If, in any T2w TSE scan, you usually employ the Fast Recovery method of driven equilibrium (FR aka DRIVE, RESTORE), it would be wise to turn this off, since the assumption that the turbo spin echo refocusing pulses properly refocus transverse magnetisation (so that the flip-back 90° pulse can return it to the longitudinal axis) is violated around the metal.
No Parallel Imaging
Parallel imaging methods necessarily cause a loss of SNR, which is dependent on the g-factor of the coil in use and the acceleration attempted. Since many of the metal artefact reduction techniques listed here will also cause SNR reduction, further SNR loss is to be avoided.
Parallel imaging techniques which rely on a low resolution coil sensitivity information (SENSE, ASSET, mSENSE) will have erroneous or missing sensitivity information around metal implants. Artefacts which arise from these data (or rather lack of data) can be unpredictable. The artefact power can be high in the centre of the field-of-view (FOV). Whilst the artefact power of k-space based methods (GRAPPA, GEM, ARC) is more likely to be smeared across the field-of-view and therefore may not obscure anatomy as much, they have less SNR than SENSE based methods, and SNR loss is to be avoided.
No Sensitivity-Based Homogeneity Correction
As a corollary to “no parallel imaging”, sensitivity based homogeneity correction is also best avoided (CLEAR, PURE, Prescan Normalize). Artefacts due to erroneous or missing coil sensitivity information from the presence of metal is likely to propagate into the “corrected” image; the process will cause further signal loss where tissue is located.
Increase Receiver Bandwidth
Just as chemical shift artefact in the frequency encoding direction occurs in voxels containing water and fat, due to the different resonant frequencies of the two, geometric distortion arises from “incorrect” Larmor frequencies produced around metal implants. The effect of chemical shift artefact is reduced by increasing the receiver bandwith (rBW). On some scanners, this is achieved by reducing the water-fat shift parameter. This causes the range of resonant frequencies over which the distortion is spread to cover a smaller pixel range, and the in-plane geometric distortion is contained within a smaller area within the FOV.
If your scanner has an option to use a higher gradient performance level, select it. This is because higher receiver bandwidth is achieved (all other things being equal) by employing a higher frequency encoding gradient amplitude. Thus, freedom to reduce geometric distortion will be extended.
An increased receiver bandwidth (rBW) will also allow a shorter echo-spacing in the TSE echo train, and a shorter minimum TE (required for T1w and PDw images).
Note that higher receiver bandwidth causes an SNR loss (SNR ∝ 1/√rBW), because the noise power is increased relative to lower rBW. Therefore, don’t necessarily set the a receiver bandwidth to maximum; use a selection equivalent to a water-fat shift of about 0.5 pixels.
Use Higher Resolution
With the receiver bandwidth set—fixing the water-fat shift in pixels to a specified value—the anatomy over which those pixels extend can be reduced by increasing in-plane resolution. There will be a number of consequences of this. SNR will decrease (fewer protons per voxel), scan time will increase (more phase encoding steps), truncation artefacts will decrease. However, note that since MRI magnets are clinical tools, the prescribed resolution parameters will be preserved if the requested receiver bandwidth is not compatible; check that rBW or WFS haven’t changed when you specify your voxel resolution.
Acquire Thinner Slices
Susceptibility effects not only cause signal loss and distortion in-plane, they also cause slice profiles which deviate from the expected planar sheet. As a result, thicker slices can result in partial volume effects through-plane, which can cause SNR loss. So, contrary to expectation, thinner slices might actually increase SNR around a metal implant. However, SNR will decrease in the rest of the image as normal.
Increase Signal Averages
Since most of these measures cost SNR, an increase in signal averages (NSA, NEX, ACQ) is necessary. This will prolong scan time, but a scan time of 5 minutes ought to be achievable, by trading off with in-plane resolution if necessary.
Fat Sat: Use STIR, not a Spectral Method
Spectral selection (CHESS) based fat saturation methods (including SPIR, SPAIR, “Fat Sat” and SPECIAL) are dependent on good main magnetic field (B0) homogeneity. This is why over large FOVs, even in the absence of metal implants, STIR is sometimes preferred. In the presence of metal implants, the B0 homogeneity is significantly compromised, and CHESS-based methods are compromised. STIR is based on the difference in T1 relaxation times between water and lipid, not their chemical shift. Thus, the false apparent “chemical” shift around the metal does not affect STIR.
More about fat saturation methods may be found in a recent fat suppression methods post.
An in-plane resolution increase has the smallest effect in reducing metal artefact. So if, after following the other tips (including signal averaging and allowing sufficient scan time), you need to sacrifice something, start with in-plane resolution.
In addition to these general metal artefact reduction principles, there may be one or two more esoteric tweaks specific to your manufacturer’s pulse sequence implementations. Your magnet manufacturer’s Applications expert will advise.
Even when the metal artefact reduction strategies listed here are applied, SNR of the resultant image will be lower than a conventional protocol. The radiologist assessing the image must be aware of this unavoidable effect.
We’ve talked about signal loss around a metal implant. However, that signal isn’t completely “lost”; it does go somewhere. The signal from tissue around a metal implant has a different resonant frequency than required, which can correspond to a different location in space. A number of techniques have been reported in the research literature to return some of that signal back into the image. For example, View Angle Tilting (VAT) achieves some in-plane correction, but can result in blurring. Slice Encoding for Metal Artifact Correction (SEMAC) extends VAT with z-encoding to resolve distorted excitation profiles that cause through-plane distortions. Some resolution limitation occurs to keep scan times reasonable, but this technique is promising. A longer method is Multi-Acquisition with Variable Resonances Image Combination (MAVRIC), in which multiple 3D acquisitions are acquired with different frequency offsets, and the resultant range of off-resonance images are summed at each slice location. All of the these methods, and others, are still under active research and development in the research community.
Thanks to Marius van Meel for his excellent MARS talk which was basis of this post.