Implant Imaging with PMRI

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Implant Imaging with PMRI
Ross Venook, Meena Ramachandran, Sharon Ungersma,
Nathaniel Matter, Nicholas Giori1, Garry Gold,
Albert Macovski, Greig Scott Steven Conolly2
1Orthopedics, Palo Alto VA 2Bioengineering, U.C.
Berkeley
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Outline

• Motivation
• Why should we image implants?
• Background on Implants
• Susceptibility
• Imaging Experiments
• Conclusion

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Implantsso hot right now

• 300,000 total knee replacements per year
• 40-50 of orthopedic surgeries result in a
  patient with some metal inside
• All trauma, joint replacement, or spine
• Half of hand or foot

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Why image implants? (short term)

• Post-operative evaluation is limited to
  traditional radiographs
• No soft-tissue imaging modality to track progress
  or identify complications

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Why image implants? (long term)

• Loosening is a longer-term complication
• Septic loosening gt Removal
• Immediate surgery, serious risks
• Loss of function
• Aseptic loosening gt Revision
• Lower risks
• Restores function
• Average implant age increases as people live
  longer and as younger people get more implants

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Outline

• Motivation
• Background on Implants
• Show and Tell
• Orthopedic methods, materials, manufacturers
• Problems with imaging implants
• Susceptibility
• Imaging Experiments
• Conclusion

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Show and Tell
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Orthopedic Methods

• Once involved mostly screws and plates
• Still used for traumatic cases, vertebrae
• Now working with bone cements, and special
  surface geometries
• Certain surface features promote bone adhesion
• Previously very few sizes/shapes of implants
• Now implants are modular for optimal size and
  shape to match anatomy

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Manufacturers and Materials

• Zimmer
• Alphatech
• Synthes
• Smith Nephew
• DePuy (J J)
• Howmedica (?)
• Others

• Stainless Steel
• Cobalt-chrome
• Titanium
• Titanium alloys
• Tivanium
• Zirconium
• Zirconium alloys
• Oximium
• Zimalloy

Optimized for safety and efficacy
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Problem with Imaging Metal Implants is
they are made of metal.

• Radiography works fine
• Soft tissue somewhat lacking

Cyteval, et al., Rad 2002
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Why not use CT?

• People do

Cyteval, et al., Rad 2002
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Why not use CT?

• but there are problems
• Beam hardening
• Streaking artifacts
• Unable to differentiate aseptic loosening

Cyteval, et al., Rad 2002
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Why not use MR?

• Short answer MR is just so darn sensitive
• Jonghos talk
• Lung air susceptibility
• B0 changes 1Hz

• Air has 9 ppm shift
• More than 1 radius from lungs
• Titanium has 180 ppm shift
• Image right on top of it

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Outline

• Motivation
• Background on Implants
• Susceptibility
• Basics
• Why PMRI
• Imaging Experiments
• Conclusion

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Susceptibility Basics

• All materials have mr
• Magnetic permeability
• Magnetic analog of electric polarizability
• Susceptibility defined
• c mr 1
• How susceptible to applied magnetic field

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Susceptibility Wide Range
Schenck, JF, Med Phys 1996
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Susceptibility in an MR Magnet

• Off-resonance artifacts depend on
• Orientation of object with respect to B0
• Magnitude of B0 (ppm)
• Susceptibility difference
• Dcci-ce

Ludeke, et al., MRI 1985 Butts, et al., JMRI 1999
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Susceptibility in an MR Magnet

• Creates an object-dependent, orientation-dependent
  , serious off-resonance artifact
• DwDcgB0/2 (for right cylinder)
• a Dcw0

Materials Dc (ppm)
HbO2, dHb Air, Water Water, Titanium 0.3 9 180
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Susceptibility Wrap-up

• As complicated as you want it to be
• Trajectory
• Readout Gradient Strength
• Slice Selection (RF and Gradient)
• Problems a DcgB0
• Material properties Dc, g
• Scanner property B0 (if only we had a
  low-field)

Woohoo!
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Outline

• Motivation
• Background on Implants
• Susceptibility
• Imaging Experiments
• PMRI (27mT) vs. 1.5T Spin Echo
• Conclusion

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Goals

• Compare standard spin-echo images
• 1.5T Signa scanner (64MHz)
• TE 10ms, 31.25 kHz BW, 256x128, 24cm FOV, 3mm
  slice
• 27mT PMRI scanner (1.1MHz)
• TE 6ms, 16 kHz BW, 128x128, 12cm FOV, 1cm
  slice
• Simple experiment with actual implant
• Titanium tibial knee joint replacement

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Images
1.5T, Signa
27mT, PMRI
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Images
1.5T, Signa
27mT, PMRI
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Images
1.5T, Signa
27mT, PMRI
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Outline

• Motivation
• Background on Implants
• Automatic Tuning
• Imaging Experiments
• Conclusion
• Wait a minute
• Future work

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Techniques for 1.5T

• View Angle Tilting (VAT)
• Re-registers water-fat and other inhomogeneities
• Presumes good slice
• Some blurring
• MARS
• VAT with bigger gradients
• VAT deblurring
• Kim, John, Garry
• Quadratic-phase RF

Standard SE
with MARS
Olsen, et al., Radiographics 2000
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Future Work

• Image every implant in our collection
• Catalog artifacts at low-field
• Do susceptibility artifacts scale with field?
• Compare with 0.5T, 1.5T
• Compare with different PMRI fields (1MHz-2MHz)
• Other artifacts
• RF eddy currents
• Gradient switching
• Optimal field?

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Acknowledgements

• GE Medical Systems
• NIH
• Nick Giori (implants)