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Implant Imaging with PMRI. Ross Venook, Meena Ramachandran, Sharon Ungersma, Nathaniel Matter, Nicholas Giori 1 , Garry Gold, Albert Macovski, Greig Scott & Steven Conolly 2. 1 Orthopedics, Palo Alto VA 2 Bioengineering, U.C. Berkeley. Outline. Motivation Why should we image implants?

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Implant Imaging with PMRI


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    1. 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

    2. Outline • Motivation • Why should we image implants? • Background on Implants • Susceptibility • Imaging Experiments • Conclusion PMRIL

    3. Implants—so 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 PMRIL

    4. Why image implants? (short term) • Post-operative evaluation is limited to traditional radiographs • No soft-tissue imaging modality to track progress or identify complications PMRIL

    5. Why image implants? (long term) “Loosening” is a longer-term complication: • Septic loosening => Removal • Immediate surgery, serious risks • Loss of function • Aseptic loosening => “Revision” • Lower risks • Restores function • Average implant age increases as people live longer and as younger people get more implants PMRIL

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

    7. Acetabular Cup Femoral Joint Tibial Joint Intermedullary Nail Screw Hips Show and Tell PMRIL

    8. 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 PMRIL

    9. Zimmer Alphatech Synthes Smith & Nephew DePuy (J & J) Howmedica (?) Others… Stainless Steel Cobalt-chrome Titanium Titanium alloys Tivanium™ Zirconium Zirconium alloys Oximium™ Zimalloy™ Manufacturers and Materials Optimized for safety and efficacy PMRIL

    10. Problem with Imaging Metal Implants is … they are made of metal. • Radiography works fine • Soft tissue somewhat lacking Cyteval, et al., Rad 2002 PMRIL

    11. Why not use CT? • People do… Cyteval, et al., Rad 2002 PMRIL

    12. Why not use CT? • …but there are problems • Beam hardening • ‘Streaking’ artifacts • Unable to differentiate aseptic loosening Cyteval, et al., Rad 2002 PMRIL

    13. Why not use MR? • Short answer: MR is just so darn sensitive • Jongho’s 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 PMRIL

    14. PMRIL

    15. Outline • Motivation • Background on Implants • Susceptibility • Basics • Why PMRI • Imaging Experiments • Conclusion PMRIL

    16. http://antigravitypower.tripod.com/BioGravity/clarklev.html Susceptibility: Basics • All materials have mr • Magnetic permeability • Magnetic analog of electric polarizability • Susceptibility defined: c = mr – 1 • How ‘susceptible’ to applied magnetic field PMRIL

    17. Susceptibility: Wide Range Schenck, JF, Med Phys 1996 PMRIL

    18. Susceptibility in an MR Magnet • Off-resonance artifacts depend on: • Orientation of object with respect to B0 • Magnitude of B0 (ppm) • Susceptibility difference Dc=ci-ce Ludeke, et al., MRI 1985 Butts, et al., JMRI 1999 PMRIL

    19. Susceptibility in an MR Magnet • Creates an object-dependent, orientation-dependent, serious off-resonance artifact Dw=DcgB0/2 (for right cylinder) a Dcw0 PMRIL

    20. 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! PMRIL

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

    22. 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 PMRIL

    23. Images 1.5T, Signa 27mT, PMRI PMRIL

    24. Images 1.5T, Signa 27mT, PMRI PMRIL

    25. Images 1.5T, Signa 27mT, PMRI PMRIL

    26. Outline • Motivation • Background on Implants • Automatic Tuning • Imaging Experiments • Conclusion • Wait a minute… • Future work PMRIL

    27. 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 PMRIL

    28. 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? PMRIL

    29. Acknowledgements • GE Medical Systems • NIH • Nick Giori (implants) PMRIL