1. Reduction of Metal Artefacts in MRI of Hip Arthroplasty
The following presentation was given at the Radiological Society of North America (RSNA)
Reduction of Metal Artefacts in MRI of Hip Arthroplasty
D. McRobbie, A. Papadaki, K. Satchithananda,
A Mitchell, A Hart, J Henckel and C Todd

2. Reduction of Metal Artefacts in MRI of Hip Arthroplasty
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Reduction of Metal Artefacts in MRI of Hip Arthroplasty
D. McRobbie, A. Papadaki, K. Satchithananda,
A Mitchell, A Hart, J Henckel and C Todd

3. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Introduction
Implant-derived metal debris from metal-on-metal hip replacements is thought to be responsible for pseudotumours, a fluid or solid mass of tissue visible on cross-sectional imaging. These pseudotumours have been linked to pain and tissue destruction but it is not known how common they are in well functioning hips imaged by MRI. This is key to understanding their clinical significance.
Metal implants cause susceptibility artefacts in MRI. We will describe ways to reduce these artefacts to visualization of structures adjacent to implants.

4. Parameters and techniques that can affect
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Parameters and techniques that can affect
the appearance of metal implants in MRI:
TITLE
Implant
Type
Size/geometry
Direction in relation to Bo
Bo effect
Magnetic field strength
Sequence and sequence parameters
Type of sequence TE
Slice Thickness
Read out bandwidth
Matrix
FOV
Phase/Frequency encode direction
Sequence Design
View angle tilting (VAT) and Multiple VAT
Slice encoding for Metal Artifact Correction in MRI (SEMAC)
Multi-acquisition variable-resonance image combination (MAVRIC)

5. Methods TITLE Co-Cr Titanium
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Methods
To investigate the effect of some of the sequence parameters, a Co-Cr hip implant (Figure 1) was inserted in water and held in place by strings. A Titanium implant was also imaged for comparison.
Images of the phantom were acquired mostly on a 1.5T MRI system. A 3T system was also used for comparison.
Images of patients were obtained on a 1.5 T MRI system
Co-Cr
Titanium

6. Orientation relative to Bo
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Orientation relative to Bo
To reduce artefact, optimal direction of the long axis of the implant is parallel to Bo Bo

7. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Type of implant
Titanium cause less artefacts than cobalt-chromium and stainless steel.
Co-Cr
Titanium
Theory: The different magnetic susceptibility  of the implant from tissue results in unwanted field gradients that dephase the MR signal and lead to image distortions.
CoCr has  ~ 900 ppm, about 5 times  for titanium [1].

8. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
High field MRI systems
Susceptibility artefacts are worse on 3T scanners compared to 1.5T
1.5 T 3T
Theory : Field inhomogeneities ΔΒ scale with B0. The distortion is due to the size of ΔΒ relative to the frequency encode gradient GFE.. T2* is also reduced at higher B0.

9. Sequence Parameters to change
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Sequence Parameters to change
TITLE
INTRODUCTION
These are sequence parameters that can be changed easily on any commercially available clinical MRI system [2]
Type of sequence TE
Slice Thickness
Read out bandwidth
Matrix
FOV
Phase/Frequency encode direction
Parallel imaging
SEQUENCE PARAMETERS
VAT
METHODS fMRI
RESULTS EEG
RESULTS fMRI
OTHER
CONCLUSION
SUMMARY

10. Type of sequence Choose SE or TSE/FSE over gradient echo TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Type of sequence
Choose SE or TSE/FSE over gradient echo
Gradient Echo
SE or TSE
Theory: Spin echo and turbo spin echo sequences are preferred because they employ 180o RF refocusing pulses that can compensate for field inhomogeneities caused by metal implants.

11. Type of sequence Choose TSE/FSE over SE TITLE TE 6.5 TE 97 TE 6.5
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Type of sequence
Choose TSE/FSE over SE
TE 6.5
TE 97
TE 6.5
TE 97
Turbo/fast spin echo
Spin echo
Theory: Longer TE in spin echo leads to more signal dephasing and uneven distortion between the echoes. Consistent image quality is maintained between echoes for TSE/FSE.

12. For spin echo In SE reduce TE DRAWBACK
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
For spin echo
TITLE
In SE reduce TE
DRAWBACK
Does not work well for T2-weighting
INTRODUCTION
TE 50 ms
TE 6.5 ms
TE 25 ms
TE 100 ms
SEQUENCE PARAMETERS
VAT
METHODS fMRI
RESULTS EEG SE
OTHER
RESULTS fMRI
Theory: Decreasing TE will allow less time for signal dephasing and reduce the artefact. Increasing bandwidth will allow shorter TE. For T2 –weighted images TSE should be used. The shortest inter-echo spacing should be used. Increasing the bandwidth will allow shorter echo spacing.
SUMMARY
CONCLUSION

13. For Fast/turbo spin echo
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
For Fast/turbo spin echo
IES 5.06 ms
IES 7.36 ms
IES 9.42 ms
reduce inter-echo spacing
DRAWBACK
Increase SAR
Theory: The dephasing caused by the metal is substantially refocused by the train of 180 degree pulses. Distortions will still occur due to frequency misregistration but T2* signal voids are reduced.

14. For Fast/turbo spin echo
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
For Fast/turbo spin echo
TE 22 ms
reduce inter-echo spacing
TE has little effect
DRAWBACK
Increase SAR
TE 94 ms
Theory: The dephasing caused by the metal is substantially refocused by the train of 180 degree pulses. Distortions will still occur due to frequency misregistration but T2* signal voids are reduced.

15. For Fast/turbo spin echo
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
For Fast/turbo spin echo
reduce inter-echo spacing
TE has little effect
Echo train length or turbo factor has no effect.
DRAWBACK
Increase SAR
TF 15
TF 7
TF 3
Theory: The dephasing caused by the metal is substantially refocused by the train of 180 degree pulses. Distortions will still occur due to frequency misregistration but T2* signal voids are reduced.

16. Decrease slice thickness
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Decrease slice thickness
TITLE
DRAWBACKS
Signal-to-noise ratio (SNR) is decreased
More slices required for coverage
INTRODUCTION
SEQUENCE PARAMETERS
VAT
METHODS fMRI
METHODS fMRI
METHODS fMRI
RESULTS EEG
RESULTS EEG
RESULTS EEG
OTHER
RESULTS fMRI
RESULTS fMRI
RESULTS fMRI
Theory: Decreasing the slice thickness increases the slice select gradient so more frequencies are included within the same voxel and artefact spreads over less voxels. Below a certain limit, the slice thickness may be controlled by the RF transmission bandwidth and no further improvement occurs.
SUMMARY
CONCLUSION
CONCLUSION
CONCLUSION

17. Decrease slice thickness
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Decrease slice thickness
TITLE
DRAWBACKS
Signal-to-noise ratio (SNR) is decreased
More slices required for coverage
INTRODUCTION
7mm
5mm
3mm
SEQUENCE PARAMETERS
VAT
METHODS fMRI
METHODS fMRI
METHODS fMRI
RESULTS EEG
RESULTS EEG
RESULTS EEG
RESULTS fMRI
OTHER
RESULTS fMRI
RESULTS fMRI
Theory: Decreasing the slice thickness increases the slice select gradient so more frequencies are included within the same voxel and artefact spreads over less voxels. Below a certain limit, the slice thickness may be controlled by the RF transmission bandwidth and no further improvement occurs.
CONCLUSION
SUMMARY
CONCLUSION
CONCLUSION

18. Increase bandwidth DRAWBACK Signal-to-noise ratio (SNR) is decreased
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Increase bandwidth
TITLE
INTRODUCTION
DRAWBACK
Signal-to-noise ratio (SNR) is decreased
SEQUENCE PARAMETERS
SEQUENCE PARAMETERS
VAT
METHODS fMRI
METHODS fMRI
METHODS fMRI
RESULTS EEG
RESULTS EEG
RESULTS EEG
RESULTS fMRI
OTHER
RESULTS fMRI
RESULTS fMRI
CONCLUSION
SUMMARY
CONCLUSION
Theory: Increasing the pixel bandwidth increases the frequency encode gradient so more frequencies are included within the same voxel and artefact spreads over less voxels.
CONCLUSION

19. Increase bandwidth DRAWBACK Signal-to-noise ratio (SNR) is decreased
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Increase bandwidth
DRAWBACK
Signal-to-noise ratio (SNR) is decreased
781 Hx/px
400 Hz/px
600 Hz/px
199 Hz/px
Theory: Increasing the pixel bandwidth increases the frequency encode gradient so more frequencies are included within the same voxel and artefact spreads over less voxels.

20. Increase matrix or reduce field-of-view
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Increase matrix or reduce field-of-view
DRAWBACK
Increased matrix will decrease SNR and increase scan time
Theory: Increasing the matrix for constant FOV increases the frequency encode gradient so more frequencies are included within the same voxel and artefact spreads over less voxels. Decreasing FOV has the same effect although is not often changed as anatomy of interest should be covered.

21. Increase matrix or reduce field-of-view
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Increase matrix or reduce field-of-view
TITLE
The reduction of artefact is not significant if both the matrix and bandwidth are changed [3]
Hint: Select resolution required for clinical scan and optimise bandwidth.
DRAWBACK
Increased matrix will decrease SNR and increase scan time
128 x 76
512 x 306
320 x 192
256 x 153
INTRODUCTION
SEQUENCE PARAMETERS
VAT
METHODS fMRI
METHODS fMRI
METHODS fMRI
RESULTS EEG
RESULTS EEG
RESULTS EEG
OTHER
RESULTS fMRI
RESULTS fMRI
RESULTS fMRI
SUMMARY
CONCLUSION
Theory: Increasing the matrix for constant FOV increases the frequency encode gradient so more frequencies are included within the same voxel and artefact spreads over less voxels. Decreasing FOV has the same effect although is not often changed as anatomy of interest should be covered.
CONCLUSION
CONCLUSION

22. Frequency encode direction
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Frequency encode direction
The metal artefact is worse in the frequency encode direction, generally select frequency encode parallel to the long axis of the implant
Phase encode LR (coronal plane)
Phase encode HF (coronal plane)

23. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Fat suppression
Use STIR instead of spectral fat saturation or water excitation
T2 tse fat sat
STIR
Theory: STIR relies upon nulling the signal from fat due to its T1relaxation time following an inversion pulse. This is unaffected by inhomogeneities caused by metal. Fat-sat, water-excitation and Dixon depend upon the resonant frequencies of water and fat. These are altered by the presence of the metal.

24. Parallel Imaging Parallel imaging has little effect on the artefact
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
Parallel Imaging
TITLE
INTRODUCTION
Parallel imaging has little effect on the artefact
Use it if necessary to reduce scan time
VAT
METHODS fMRI
SEQUENCE DEVELOPMENT
RESULTS EEG
R=1
R=2
R=3
R=4
OTHER
RESULTS fMRI
Theory: Parallel imaging reduces the number of phase encode lines acquired using array coil element sensitivities either in image space or k-space. Distortion is still present due to frequency encoding.
CONCLUSION
SUMMARY

25. Standard MARS Protocol Optimised MARS protocol
TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETERS
REFERENCES
NEW SEQUENCES
OPTIMISED SEQUENCE T1w coronal TSE
Standard MARS Protocol
TE/TR = 7/679 ms
Matrix 320 x 192
Bandwidth 744 Hz/px
Slice thickness = 5mm
PE direction HF
Optimised MARS protocol
TE/TR = 7.7/694 ms
Matrix 512 x 384
Bandwidth 751 Hz/px
Slice thickness = 3mm
PE direction RL

26. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETRS
REFERENCES
NEW SEQUENCES
Sequence development
New sequences under development offer the prospect of further reducing the metal artefact.
VAT – view angle tilting [4]
MAVRIC – Multi-acquisition variable-resonance image combination [1]
SEMAC - Slice encoding for metal artefact correction in MRI [5]
Theory: View Angle Tilting partially corrects the in-plane distortion, but at the cost of introducing blurring. To unwrap the distortion extreme tilt angles are necessary. Butts et al [4] have proposed refinements to the VAT method to reduce the effect of the blurring[. Several non-standard methods are in development but are as yet unavailable on commercial scanners: MAVRIC [1] and SEMAC [5] utilise modified 3D methods taking advantage of the insensitivity of the phase encoding process to magnetic field distortions. Single point scanning [6] involves the use of 3 phase encodes but results in impractically long scan times.

27. TITLE
METHODS EEG
METHODS fMRI
RESULTS EEG
CONCLUSION
INTRODUCTION
METHODS
PHYSICAL PARAMETERS
SEQUENCE PARAMETRS
REFERENCES
NEW SEQUENCES
REFERENCES:
Koch KM, Lorbiecki JE, Hinks RS,King KF, A multispectral three-dimensional acquisition technique for imaging near metal implants, Magn Reson Med, 2009, 61:
Vandevenne JE, Vanhoenacker FM, Parizel PM, Pauly KB, Lang PK, Reduction of metal artefacts in musculoskeletal MR Imaging, JBR-BTR,2007, 90:
Toms AP, Smith-Bateman C, Malcolm PN, Cahir J, Graves M Optimization of metal artefact reduction (MAR) sequences for MRI of total hip prostheses, Clin Radiol, 2010, 65:
Butts K, Pauly JM, Gold GE, Reduction of blurring in View Angle Tilting MRI, Magn Reson Med, 2005, 53:
Lu W, Pauly KB, Gold GE, Pauly JM, Hargreaves BA, SEMAC: Slice Encoding for Metal Artifact Correction in MRI, MRM, 2009, 62: 66-76
Ramos-Cabrer P, Duynhoven JPM, van der Toorn A, Nicolay K (2004) MRI of hip prostheses using single-point methods: in vitro studies towards the artefact free imaging of individuals with metal implants, Magn. Reson. Imag. 22: