Reproducibility of Densitometric and Biomechanical Assessment of the Mouse Tibia From In Vivo Micro-CT Images

Abstract

Interventions for bone diseases (e.g. osteoporosis) require testing in animal models before clinical translation and the mouse tibia is among the most common tested anatomical sites. In vivo micro-Computed Tomography (microCT) based measurements of the geometrical and densitometric properties are non-invasive and therefore constitute an important tool in preclinical studies. Moreover, validated micro-Finite Element (microFE) models can be used for predicting the bone mechanical properties non-invasively. However, considering that the image processing pipeline requires operator-dependant steps, the reproducibility of these measurements has to be assessed. The aim of this study was to evaluate the intra- and inter-operator reproducibility of several bone parameters measured from microCT images. Ten in vivo microCT images of the right tibia of five mice (at 18 and 22 weeks of age) were processed. One experienced operator (intra-operator analysis) and three different operators (inter-operator) aligned each image to a reference through a rigid registration and selected a volume of interest below the growth plate. From each image the following parameters were measured: total bone mineral content (BMC) and density (BMD), BMC in 40 subregions (ten longitudinal sections, four quadrants), microFE-based stiffness and failure load. Intra-operator reproducibility was acceptable for all parameters (precision error, PE < 3.71%), with lowest reproducibility for stiffness (3.06% at week 18, 3.71% at week 22). The inter-operator reproducibility was slightly lower (PE < 4.25%), although still acceptable for assessing the properties of most interventions. The lowest reproducibility was found for BMC in the lateral sector at the midshaft (PE = 4.25%). Densitometric parameters were more reproducible than most standard morphometric parameters calculated in the proximal trabecular bone. In conclusion, microCT and microFE models provide reproducible measurements for non-invasive assessment of the mouse tibia properties.

Keywords: bone mineral; finite element; microCT; morphometric; mouse tibia; reproducibility.

Figure 1

Overview of the study. The right tibia of five mice was microCT scanned longitudinally at week 18 and week 22 of age. Each image acquired at week 18 of age was aligned to a reference and a volume of interest (VOI, 80% Length) was selected. Each image acquired at week 22 of age was aligned to the corresponding baseline image and a VOI was selected. These operations were repeated three times by operator 1 and once by operators 2 and 3.

Figure 2

Overview of the workflow. A user uploads microCT data to the webservice (A). An operator downloads the image and performs the alignment and VOI selection (B). Pre-processed images are uploaded back to the webservice. Bone mineral content (BMC) and microFE analyses (C) are run automatically in the HPC ShARC (Sheffield Advanced Research Computer). The results are sent back to the service and to the user.

Figure 3

Frequency plots for the distribution of the third principal strain obtained from intra-operator and inter-operator microFE analyses of one mouse tibia at week 18 (top) or 22 (bottom) of age.