The effect of CT slice spacing on the geometry of 3D models

摘要: THE EFFECT OF CT SLICE SPACING ON GEOMETRY 3D MODELS Beat Schmutz*, Martin E. Wullschleger*, Michael A. Schuetz*^ *Queensland University of Technology, Brisbane, Australia ^Princess Alexandra Hospital, Australia INTRODUCTION CT imaging is the current gold standard for acquisition morphological data for subsequent 3D reconstruction of virtual bone models. In addition to clinical diagnostics, image-based 3D reconstructions are frequently being used in a broad range other fields such as kinematic studies (Dennis et al., 2005), finite element modelling (FEM) (Penna et 2006; Taddei 2006) and implant design (Robertson 2000). Accurate bone models from requires acquisition of thin and overlapping slices (Hopper et al., 1996; Shin 2002). Such high resolution image data can be obtained from cadaver specimens without any problems. However, in an effort to reduce radiation exposure patient sections are not always clinically justified. Depending on application, may not always have sufficient resolution to useful for research purposes. The study quantitatively investigates effect various image slice spacing’s on geometry 3D models long bones. METHOD The femur fresh intact ovine cadaver hind limb was scanned at 120KVP and 0.5, 1, 2, 3 4 mm for the axial slices. The pixel size 0.4 x 0.4 mm. was reconstructed using middle-hard bone kernel 50% slice overlap. 3D models outer contour the femur were generated image processing registration software Amira 4.1 (Mercury Computer Systems, France). segmented using semi-automatic threshold based method. The differences surface geometry between model interest the reference (0.5 spaced CT data) quantified with reverse engineering software RapidForm2006 (INUS Technology). Values were generated recorded mean distance, maximum distance standard deviation. percentage fit between a model reference model was evaluated within tolerance ± 0.5 mm. In quantifying differences between entire surfaces two models their deviations five anatomical regions (Figure 1) bone were also quantified. Figure 1. anatomical sections: Distal epiphysis, distal metaphysis, shaft, trochanter region head. RESULTS The effects slice spacing’s subsequent 3D summarized Table 1. The has been validated against obtained 3D laser scan (Roland, LPX-250) the dissected bone. CONCLUSIONS For all geometric accuracy decreased increase slice spacing. As expected, shaft region was least affected by variations in slice spacing’s. epiphyseal regions were most due larger changes relative to axis. For 0.5 mm, the results indicate that spacing 1 mm or less required accurately reconstruct articulating a long bone. A 2 will enable reconstruction metaphyseal areas with reasonable accuracy. Whereas a spacing – a reconstruction region. This demonstrates usefulness of clinically acquired depend on accuracy requirements the application which are intended for. The findings this be useful development optimal scanning protocols of long bones (Zannoni al. 1998). To extend validation applicability of results pilot study, further tests involve larger samples and different required. REFERENCES Dennis J. Biomech. 38:241-253, 2005. Pena 39(9), 1686-1701, 2006. Taddei 39(13), 2457-2467, 2006. Robertson Bone Joint Surg. 82-A(11), 1594-1602, 2000. Hopper Comput. Assist. Tomo. 20(5), 841-847, 1996. Shin Korean Radiol. 3(1), 49-56, 2002. Zannoni IEEE Trans Med Imag 17(4), 663- 666, 1998. Table Differences surface geometries relative (0.5 mm data) slice spacing’s. Model Slice spacing (mm) Ave distance (mm) Max distance (mm) SD Within ± 0.5mm (%) Entire 1 0.15 0.97 0.12 98 Entire 0.28 3.08 0.38 86 Entire 0.40 4.88 0.50 75 Entire 0.77 6.83 0.83 52 Dist epiphysis 0.22 0.91 96 Dist 0.44 0.52 74 Dist 0.55 4.72 61 Dist 0.95 6.29 0.94 42 Dist meta 0.79 0.07 100 Dist 0.21 1.86 0.26 93 Dist 2.46 0.24 87 Dist 0.65 3.55 0.60 54 Shaft 0.10 0.32 0.05 100 Shaft 0.09 0.49 0.88 0.11 99 Shaft 1.23 0.18 92 Trochanter 0.14 0.85 99 Trochanter 1.67 93 Trochanter 0.43 2.18 0.67 77 Trochanter 2.92 49 Femur head 0.20 0.16 96 Femur 2.36 0.48 63 Femur 0.58 51 Femur 1.31 1.04 27 Dr Beat Schmutz Institute Health & Biomedical Innovation Queensland Technology 60 Musk Avenue Kelvin Grove QLD 4059, Australia Fax: +61 7 3138 6030 Email: b.schmutz@qut.edu.au