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 VA2Bioengineering, U.C. Berkeley

4 Feb, 2005PMRIL 2

Outline

• Motivation– Why should we image implants?

• Background on Implants• Susceptibility• Imaging Experiments• Conclusion

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 6

Outline

• Motivation• Background on Implants

– Show and Tell– Orthopedic methods, materials, manufacturers– Problems with imaging implants

• Susceptibility• Imaging Experiments• Conclusion

4 Feb, 2005PMRIL 7

Show and Tell

Hips

AcetabularCup Tibial Joint

Femoral Joint

Screw

Intermedullary Nail

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 9

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

4 Feb, 2005PMRIL 10

Problem with Imaging Metal Implants is …

• Radiography works fine

• Soft tissue somewhat lacking

they are made of metal.

Cyteval, et al., Rad 2002

4 Feb, 2005PMRIL 11

Why not use CT?

• People do…

Cyteval, et al., Rad 2002

4 Feb, 2005PMRIL 12

Why not use CT?• …but there are problems

– Beam hardening– ‘Streaking’ artifacts

• Unable to differentiate aseptic loosening Cyteval, et al., Rad 2002

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 14

4 Feb, 2005PMRIL 15

Outline

• Motivation• Background on Implants• Susceptibility

– Basics– Why PMRI

• Imaging Experiments• Conclusion

4 Feb, 2005PMRIL 16

Susceptibility: Basics

• All materials have r

– Magnetic permeability– Magnetic analog of

electric polarizability

• Susceptibility defined:

r – 1– How ‘susceptible’ to

applied magnetic field

http://antigravitypower.tripod.com/BioGravity/clarklev.html

4 Feb, 2005PMRIL 17

Susceptibility: Wide Range

Schenck, JF, Med Phys 1996

4 Feb, 2005PMRIL 18

Susceptibility in an MR Magnet

• Off-resonance artifacts depend on:– Orientation of object with

respect to B0

– Magnitude of B0 (ppm)

– Susceptibility difference

i-e

Ludeke, et al., MRI 1985Butts, et al., JMRI 1999

4 Feb, 2005PMRIL 19

Susceptibility in an MR Magnet

• Creates an object-dependent, orientation-dependent, serious off-resonance artifact

(for right cylinder)

Materials (ppm)

HbO2, dHb

Air, Water

Water, Titanium

0.3

9

180

4 Feb, 2005PMRIL 20

Susceptibility Wrap-up

• As complicated as you want it to be– Trajectory– Readout Gradient Strength– Slice Selection (RF and Gradient)

• Problems

– Material properties: – Scanner property: B0 (if only we had a low-field…)

Woohoo!

4 Feb, 2005PMRIL 21

Outline

• Motivation• Background on Implants• Susceptibility• Imaging Experiments

– PMRI (27mT) vs. 1.5T Spin Echo

• Conclusion

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 23

Images

1.5T, Signa 27mT, PMRI

4 Feb, 2005PMRIL 24

Images

1.5T, Signa 27mT, PMRI

4 Feb, 2005PMRIL 25

Images

1.5T, Signa 27mT, PMRI

4 Feb, 2005PMRIL 26

Outline

• Motivation• Background on Implants• Automatic Tuning• Imaging Experiments• Conclusion

– Wait a minute…– Future work

4 Feb, 2005PMRIL 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

4 Feb, 2005PMRIL 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?

4 Feb, 2005PMRIL 29

Acknowledgements

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