Abstract
DVC is a novel full-field and contactless measurement technique for calculating displacements and strains inside bones (Grassi and Isaksson 2015) through the comparison of 3D reconstructions (CT, micro-CT, MRI, etc.) from unloaded and loaded samples. Recent in zero-strain tests to estimate the measurement precision by applying a known state of strain (Palanca, Tozzi et al. 2015) suggested that DVC is suitable to identify regions where bone tissue is yielded (i.e. subjected to high strains). Conversely to reliably measure strain in the physiological range a severe compromise with spatial resolution is necessary (Dall'Ara, Barber et al. 2014, Palanca, Tozzi et al. 2015). In order to use DVC to explore the relationship between the local physiological strain and bone microarchitecture, an error lower than 200 microstrain (an order of magnitude lower than the mean strain) and a spatial resolution of the strain measurement lower than 100 μm is required. The aim of this work is to define if, and to what extend, high-quality images obtained by synchrotron radiation micro computed tomography (SR-μCT) improve the precision of a global DVC approach.
Cylindrical specimens of cortical and trabecular bone were extracted from a fresh bovine femur and embedded in acrylic resin. Both samples were scanned twice without any repositioning (‘repeated scantest’) at beamline l13–2 of Diamond Light Source (Oxford, UK). 4000 projections of 53 ms exposure were collected via fly-scanning with a CdWO4scintillator-coupled pco.edge 5.5 detector with 4× magnification and an effective pixel size of 1.6μm. Strains were evaluated using a global DVC approach (ShIRT-FE) in two cubic volumes of interest (VOI) of 1,000 voxels in side length, for each specimen, exploring a DVC spatial resolution from 16 to 498 μm. The precision of measurements was evaluated extracting a similar indicator to (Liu and Morgan 2007).
Precision improved with decreasing spatial resolution, confirming a trend similar to that obtained with ‘laboratory source’ μCT on similar specimens (Palanca, Tozzi et al. 2015). To obtain a precision of better than 200 microstrains the cortical and trabecular samples required spatial resolutions of 41 and 80 μm respectively. Comparing these results to those of previous studies, where similar specimens were scanned with ‘laboratory source’ μCT (effective voxel size of the order of ten μm) the errors were vastly reduced (approximately one order of magnitude). In fact, in order to obtain a precision of better than 200 microstrain, spatial resolutions of 550 (cortical) and 480 (trabecular) μm were needed (Dall'Ara, Barber et al. 2014).
This work showed that using high-quality tomograms obtained by synchrotron radiation μCT decreases the measurement uncertainties of a global DVC approach with respect to those obtained with laboratory source μCT. DVC could therefore be used with μCT data to evaluate displacement and strain in the physiological range with remarkable spatial resolution.
