1. 2013 Annual Scientific Meeting of the SSRMP
Thomas Buchsbaum Lukas Hirschi, Federico Hasenbalg, Peter Pemler
Orthopedic Hip Implants Extended Hounsfield Unit Scale Metal Artifact Reduction Evaluation of the Philips Brilliance Big Bore
2013 Annual Scientific Meeting of the SSRMP

2. The Problem
14cm2

3. A Solution by Philips: O-MAR?
Artifact Strength CT Value Accuracy ?
CT Value Properties
Segmentation Thresholds

4. Hip Stem Materials Titanium Alloys Cobalt Alloys Ti-6Al-7Nb alloy
ProtasulTM-100
ISO
mass density: g/cm3
Zimmer CLS hip stem
Cobalt Alloys
CoNi35Cr20Mo10
ProtasulTM-10
ISO
mass density: g/cm3
Sulzer
Acknowledgment:
Thanks to Dr. Jörg Huber, leading orthopedist at Triemli hospital Zürich, for making available the hip implant materials for this study; and to Norman Stark, manager of Materials and Retrievals Research at Zimmer GmbH, for providing specialized information about hip stem materials.

5. Analyses in a Water Phantom
Example: Co 400 ext.-HU voxels (3.8 cm2)
O-MAR
Std.
36.5 cm
18 cm
Artifact Strength
S.D. of CT values within ROI CT Value Accuracy
Mean of CT values within ROI
ROI
ROI
For illustration purposes, let‘s consider the a center ROI…, about 5 cm x 4 cm, 400(O-MAR)/420(Std) extended HU voxels

6. Artifact Strength: Kept Low by O-MAR
S.D. in HU
3.8 cm2
3.8 cm2
14.7 cm2

7. Influence on dose? CT Value Accuracy Mean CT value in HU 3.8 cm2

8. Plan Setup (AAA, 15X, AP-PA, 10 x 6)
100% dose in ISO of non-metal calculation; same MUs for every plan

9. O-MAR: Smallest Differences to Non-Metal Dose
Standard Reconstruction
O-MAR %
14 cm

10. > 3071 HU

11. Material Properties Apparent in Extended HU Range
1.95 cm Ti Co
O-MAR Profiles
Dependence on cross-section (and material)  nevertheless, threshold such that volume equals physical volume

12. Segmentation of Prosthesis Material
CT Value Threshold
225 HU
1000 HU
3071 HU
4100 HU
6600 HU
8000 HU Ti Co
225 HU
1000 HU
3071 HU
4100 HU
6600 HU
8000 HU

13. Conclusions (1) O-MAR Improves Image Quality
Simplifies Contouring/Segmentation of Organs
Improves AAA Dose Calculation; differences are small Co
6.7 cm2

14. Conclusions (2) Metals in extended Hounsfield unit range
Co and Ti can be classified, e.g. by 90%-quantiles
Strong cupping effect
CT value distribution depends on metal cross-section area
Water phantom and patient data agree with each other
Segmentation thresholds: HU for Ti and 6600 HU for Co
Co segmented with Ti thresholds: 15% volume error, 4% error in diameter

15. References
Li, Hua et al.: Clinical evaluation of a commercial orthopedic metal artifact reduction tool for CT simulations in radiation therapy. Med. Phys. 39(12): Philips Healthcare: Metal Artifact Reduction for Orthopedic Implants (O-MAR). White Paper, Philips: Thomas Buchsbaum: Orthopedic Hip Implants, Extended Hounsfield Units, and Artifact Reduction: Evaluation of a Commercial CT System. Master’s Thesis, TU Kaiserslautern, 2013.

16. Do not hesitate to contact me

17. Backup

18. Philips O-MAR – Contraindications
External metals
Bismuth-Abschirmung
Source: Philips White Paper: Metal Artifact Reduction for Orthopedic Implants (O-MAR)

19. Better Accuracy with Std. Recon.; 2-sided Co

20. 3D CT-Value Distribution

21. 90%-Quantile vs. Cross-Section Area

22. Threshold-based Segmentation
Co segmented with Ti thresholds: 15% volume error, 4% error in diameter

23. Images, Co, 3.8 cm2

24. Images, Co, 14.7 cm2