Metal artifacts can be broadly classified into In Plane and Through plane artifacts based on the plane of distribution of the artifact.(Fig 2)
Fig. 2: Metal Artifacts classified based on plane of distribution.
IN PLANE ARTIFACTS: These artifacts appear along the direction of slice selection gradient.
Signal loss & Pile up
Near metal objects, the magnetic field variations can be very rapid, such that the magnetization within a voxel precesses at varying rates. This leads to intra voxel dephasing because of large susceptibility difference between the tissue and metal. This results in loss of coherence and signal.(Fig 3)
Fig. 3: Example of metal induced artifact due to the presence of stainless screws in a healthy 37-year-old man . The solid arrow here is showing the signal loss . The dotted arrow is representing the geometric distortion, whereas the dashed arrow is showing the signal pile up.
References: Hargreaves BA, Worters PW, Pauly KB, Pauly JM, Koch KM, Gold GE. Metal-induced artifacts in MRI. AJR Am J Roentgenol 2011;197(3):547–555
This can be reduced by using spin-echo sequences. Spin-echo–based sequences use multiple radiofrequency pulses to refocus and reduce the degree of T2* decay, thus reducing signal loss. Gradient echo–based sequences should be avoided, as such parameters amplify the degree of T2* decay, leading to substantial signal loss.
An additional problem-solving technique includes swapping the phase- and frequency encoding directions. Metal-related artifact tends to be least prominent when the lengthiest portion of the prosthesis is aligned parallel to the B0 field. Swapping the phase- and frequency-encoding directions may change the orientation of in-plane artifacts or reveal key anatomic structures near curved or curvilinear portions of a prosthesis. This strategy rarely eliminates artifacts but may shift signal loss or pileup artifacts to less anatomically important locations.
Poor or no Fat Suppression
Spectral-based fat suppression is not beneficial around metallic prostheses. For successful spectral fat saturation, the local B0 must be as homogeneous as possible to take advantage of the relatively small differences in the chemical shifts of fat and water. The ferromagnetic properties of the implant cause marked variability in the local field, and this perturbation of local field homogeneity leads to incomplete or absent fat suppression.
A more tenable alternative method is the STIR technique which relies on the difference in T1 relaxation times of fat and water unlike CHESS which is dependent on their chemical shift. STIR uses an inversion recovery approach to null fat on the basis of its short T1 relaxation time, which provides much more homogeneous fat suppression near metal.(Fig 4)
Fig. 4: Sagittal view of MRI Knee with titanium screw. A showing Conventional fat-saturated image. B shows STIR image of the same with more uniform fat suppression.
References: Hargreaves BA, Worters PW, Pauly KB, Pauly JM, Koch KM, Gold GE. Metal-induced artifacts in MRI. AJR Am J Roentgenol 2011;197(3):547–555
Likewise, Dixon techniques can track gradual magnetic field variations and perform well some distance from metal where fat saturation may fail. However, closer to the metal, even Dixon techniques fail, and the best choice is to use STIR imaging because it is completely independent of resonance frequency.
Geometric Distortion
Metals have high intrinsic magnetic susceptibilities that produce significant local field disturbances. By altering resonance frequencies metals shift image pixels away from their true positions leading to significant geometric distortions. This cohabitation of metal or metal alloy within the same voxel as the tissue of interest leads to the spreading out of the pixels over a larger range, creating a distorted image.
The spatial distortion in the slice direction is the ratio of frequency offset to slice bandwidth, multiplied by slice width. Therefore, using thin slices will reduce the amount of this distortion but it increases scan time and reduces SNR.( Fig 5)
A direct way to reduce distortion effects is to maximize the bandwidth used both during slice selection and during readout. On both slice selection and readout, the spatial distortion is inversely proportional to the gradient strength, which scales with the bandwidth. Increased radiofrequency bandwidth,comes at a cost of increased power deposition SAR, which may either force longer TRs or fewer interleaved slices per repetition.
Fig. 5: Effect of Slice-selection bandwidth on slice.
A. Drawing shows use of low slice-selection bandwidth results in substantial slice distortion in presence of frequency variations
B. Drawing shows use of higher excitation pulse bandwidth decreases distortion but at a cost of higher radio frequency power
C.Drawing shows use of unmatched bandwidths on excitation and refocusing pulses so that only overlapped region is imaged . This may reduce some artifact but can result in signal loss.
References: Hargreaves BA, Worters PW, Pauly KB, Pauly JM, Koch KM, Gold GE. Metal-induced artifacts in MRI. AJR Am J Roentgenol 2011;197(3):547–555
THROUGH PLANE ARTIFACTS : Abnormalities that violate boundaries between sections can be particularly troublesome and thereby lead to Through plane artifacts . Metal Suppression packages and sequences have been developed by vendors to correct a much more arbitrary range of resonance frequency offsets near metal. Some of them have been described here .
MARS
MARS is referred to as the Metal Artifact Reduction Sequence (MARS). In its original formulation, this sequence uses an increased radiofrequency bandwidth, thin section selection, an increased echo train length, decreased echo spacing, and an increased image matrix.
WARP with VAT ( View Angle Tilting )
WARP is used to optimize the standard principles of MARS and to include multidirectional VAT to further reduce distortions. VAT is a concept which uses an additional gradient to reduce artefact.
VAT uses an extra gradient in the slice select direction during the read-out gradient, and the slice is effectively viewed from an angle.( Fig 7)
Fig. 7: VAT minimizes in-plane distortions by the addition of an altered readout gradient, given as the angle theta (right graph). This will primarily reduce signal loss and pileup artifacts, with the trade-off of a small degree of blurring. Note the apparent gap between the bone (speckled) and the metal (gray) in the graph on the left. After application of VAT, the apparent gap is reduced. VAT is a key component of WARP and some proprietary MARS sequences. GX = reading gradient, GZ = concentration gradient, tan = tangent.
References: Brett S. Talbot et al. (2016) MRI Imaging with metal suppression .RSNA 36:209–225
The sum of artefactual frequency shifts in the slice select and the read-out (frequency encoding) direction results in a frequency shift with oblique direction. By viewing from this oblique angle during read-out, the received signals can be projected into the correct pixel of the image matrix. Although distortion is considerably reduced, blur is introduced into the MR image.(Fig 8)
Fig. 8: 37-year-old man with stainless steel screws in tibia. A and B, Spin-echo (A) and view-angle tilting (B) images obtained using identical parameters show geometric distortion is completely corrected in viewangle tilting (arrows) but through-slice signal loss and pile-up artifacts remain (bright areas in tibia).
References: Hargreaves BA, Worters PW, Pauly KB, Pauly JM, Koch KM, Gold GE. Metal-induced artifacts in MRI. AJR Am J Roentgenol 2011;197(3):547–555
MAVRIC
Multiacquisition variable-resonance image Combination (MAVRIC) is a spin-echo–based sequence that uses a series of frequency-selective excitations, multidirectional VAT, computational postprocessing, and a standard three-dimensional readout .
MAVRIC uses a frequency-selective excitation (rather than exciting a slice or slab) to limit the range of frequency offsets imaged at one time.
This is followed by a standard 3D imaging readout, typically using a spin-echo train. Here , when the range of frequencies is limited, the in-plane displacement is also limited, to within about a pixel in this case.(Fig 9)
Fig. 9: MAVRIC process (a) The sequence begins with frequency-selective excitation and refocusing. A representative bone (speckled) with a central metal rod (gray) is shown. A region of interest is excited across the frequency range (blue rectangle in a and b). (b)Three-dimensional fast spin-echo imaging is used to obtain the range of excited frequencies over the given region. Many section-encoding steps may be required to cover the entire field of view (green rhombi in b).
References: Brett S. Talbot et al. (2016) MRI Imaging with metal suppression .RSNA 36:209–225
SEMAC
Slice-encoding for metal artifact correction (SEMAC) corrects both in-plane and through-slice distortions near metal.At its core, SEMAC is a two-dimensional fast spin-echo or turbo spin-echo sequence in which each section is phase encoded in the third dimension as follows,
1. Excitation: Equal section-selection gradients are applied to the region of interest. The excited area is depicted as bone (speckled) with a metal prosthesis (gray) .
2. Section distortion: Ferromagnetic effects lead to a series of distorted sections, one of which is shown here. With large frequency offsets, the distorted area may extend outside the encoded field of view (red areas).
3. Profile resolution: Individual section profiles are then resolved with additional Z-phase encoding (green rhombi). A specific section is sampled and is reconstructed in a pixel-by-pixel fashion.
4. Reconstruction: Regions of high distortion are suppressed and essentially “taper.” After multiple distorted sections are mapped, the signals from different excited sections are combined. and through-plane distortions are corrected.(Fig 10)
Fig. 10: SEMAC process. (a) Excitation: Equal section-selection gradients are applied to the region of interest. The excited area is depicted as bone (speckled) with a metal prosthesis (gray) in a–d. (b) Section distortion: Ferromagnetic effects lead to a series of distorted sections, one of which is shown here. With large frequency offsets, the distorted area may extend outside the encoded field of view (red areas). (c) Profile resolution: Individual section profiles are then resolved with additional Z-phase encoding (green rhombi). A specific section is sampled and is reconstructed in a pixel-by-pixel fashion. (d) Reconstruction: Regions of high distortion are suppressed and essentially “taper.” After multiple distorted sections are mapped, the signals from different excited sections are combined. and through-plane distortions are corrected.
References: Brett S. Talbot et al. (2016) MRI Imaging with metal suppression .RSNA 36:209–225