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

The Problem 14cm2

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

Hip Stem Materials Titanium Alloys Cobalt Alloys Ti-6Al-7Nb alloy ProtasulTM-100 ISO 5832-11 mass density: 4.52 g/cm3 Zimmer CLS hip stem Cobalt Alloys CoNi35Cr20Mo10 ProtasulTM-10 ISO 5832-6 mass density: 8.43 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.

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

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

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

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

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

> 3071 HU

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

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

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

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: 4100 HU for Ti and 6600 HU for Co Co segmented with Ti thresholds: 15% volume error, 4% error in diameter

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): 2012. Philips Healthcare: Metal Artifact Reduction for Orthopedic Implants (O-MAR). White Paper, Philips: 2011. Thomas Buchsbaum: Orthopedic Hip Implants, Extended Hounsfield Units, and Artifact Reduction: Evaluation of a Commercial CT System. Master’s Thesis, TU Kaiserslautern, 2013.

Do not hesitate to contact me buchsbaum@ieee.org

Backup

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

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

3D CT-Value Distribution

90%-Quantile vs. Cross-Section Area

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

Images, Co, 3.8 cm2

Images, Co, 14.7 cm2