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Technical Briefs

Whole Bone Strain Quantification by Image Registration: A Validation Study

[+] Author and Article Information
Michael R. Hardisty

Institute of Biomaterials and Biomedical Engineering, University of Toronto; Orthopaedic and Biomechanics Laboratory, Sunnybrook Health Sciences Centre, 2075 Bayview Avenue, Room UB-19, Toronto, ON, M4N 3M5, Canadam.hardisty@utoronto.ca

Cari M. Whyne

Institute of Biomaterials and Biomedical Engineering, University of Toronto; Orthopaedic and Biomechanics Laboratory, Sunnybrook Health Sciences Centre, 2075 Bayview Avenue, Room UB-19, Toronto, ON, M4N 3M5, Canadacari.whyne@sunnybrook.ca

J Biomech Eng 131(6), 064502 (May 08, 2009) (6 pages) doi:10.1115/1.3127249 History: Received October 19, 2007; Revised March 27, 2009; Published May 08, 2009

Quantification of bone strain can be used to better understand fracture risk, bone healing, and bone turnover. The objective of this work was to develop and validate an intensity matching image registration method to accurately measure and spatially resolve strain in vertebrae using μCT imaging. A strain quantification method was developed that used two sequential μCT scans, taken in loaded and unloaded configurations. The image correlation algorithm implemented was a multiresolution intensity matching deformable registration that found a series of affine mapping between the unloaded and loaded scans. Once the registration was completed, the displacement field and strain field were calculated from the mappings obtained. Validation was done in two distinct ways: the first was to look at how well the method could quantify zero strain; the second was to look at how the method was able to reproduce a known applied strain field. Analytically defined strain fields that linearly varied in space and strain fields resulting from finite element analysis were used to test the strain measurement algorithm. The deformable registration method showed very good agreement with all cases imposed, establishing a detection limit of 0.0004 strain and displaying agreement with the imposed strain cases (average R2=0.96). The deformable registration routine developed was able to accurately measure both strain and displacement fields in whole rat vertebrae. A rigorous validation of any strain measurement method is needed that reports on the ability of the routine to measure strain in a variety of strain fields with differing spatial extents, within the structure of interest.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Finite element model: A μCT scan of an rnu/rnu rat-tail vertebra was used to create a specimen specific FE model, consisting of ∼20,000 tetrahedral elements. (Table 2) The bone was modeled as isotropic with a modulus of elasticity of 250 MPa (24), loaded under 200 N of axial compression. (I-DEAS 10, Siemens, Plano, TX, USA)

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Figure 2

Sagital contour plots of z displacement fields (A and B) and axial strain fields (C and D): (A) Displacement fields found by registration, (B) the actual z displacements applied to the two scans registered, (C) axial strain contours found by registration, and (D) the actual axial strain contours of the two scans registered. The fields presented correspond to the validation tests described in the methods, specifically of (1) a rat-tail μCT scan deformed with a field having axial strains varying from 0 to 0.2, (2) half of a rat-tail μCT scan deformed with a field having axial strains varying from 0 to 0.1, (3) a rat-tail μCT scan deformed with a field having axial strains varying from 0 to 0.02, (4) a rat-tail μCT scan deformed with a displacement field generated from FEA.

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Figure 3

Scatter plot of calculated axial strains against actual imposed axial strains for the four test cases presented

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