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

Schmutz, Beat, Wullschleger, Martin E., & Schuetz, Michael A. (2007) The effect of CT slice spacing on the geometry of 3D models. In 6th Australasian Biomechanics Conference, 15-17 Feburary, The University of Auckland, New Zealand.


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

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ID Code: 6302
Item Type: Conference Paper
Refereed: Yes
Additional URLs:
Keywords: Femur, 3D model, CT scans, CT slice spacing
Subjects: Australian and New Zealand Standard Research Classification > ENGINEERING (090000) > BIOMEDICAL ENGINEERING (090300) > Biomedical Engineering not elsewhere classified (090399)
Australian and New Zealand Standard Research Classification > ENGINEERING (090000) > BIOMEDICAL ENGINEERING (090300)
Divisions: Past > QUT Faculties & Divisions > Faculty of Built Environment and Engineering
Copyright Owner: Copyright 2007 (The authors)
Deposited On: 22 Feb 2007 00:00
Last Modified: 09 Jun 2010 12:37

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