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With recent advancements in imaging technology, it is now feasible to acquire X-ray imaging data with very high resolution. Desktop micro-CT (µCT) systems are the current standard for imaging cancellous bone due to their non-destructive nature1. When imaging microstructural features of cortical bone, however, µCT use has been more limited. Due to resolution constraints, desktop systems cannot attain the resolution required to image microstructural features smaller than cortical pores, such as osteocyte lacunae. For this application, SRµCT is ideal owing to the greater resolution of these systems1. For example, experiments at the Canadian Light Source (CLS) on the BioMedical Imaging and Therapy (BMIT) beamlines2 have produced images with voxels as small as 0.9 µm. Previous studies1,3,4,5 have used this resolution to acquire projections and subsequent three-dimensional (3D) renders from cortical bone specimens from human long bones (Figure 1) to quantify osteocyte lacunar density4,6,7,8,9 and variation in the lacunar shape and size3 across the human lifespan and between the sexes. Further studies have demonstrated the presence of osteon banding in humans10, a phenomenon previously recognized to be associated with only nonhuman mammals in the forensic anthropological literature.
In order to achieve exceptional resolution, the X-ray beam must be finely focused within the field of view (FOV), which often limits the maximum specimen size to a few millimeters in diameter. Currently, there have been no comprehensive, standardized procedures described in the literature outlining bone sample procurement that meet these restrictions. Centering specimens within the FOV is critical to ensure that 1) the sample remains centered as it rotates 180° during imaging, and 2) scan artifacts are limited since there is no image truncation. In other words, no portions of the sample outside of the FOV interfere with the beam entering its focal point inside the FOV. If this occurs, the reconstruction algorithm is deprived of some of the attenuation data needed for a fully correct reconstruction. It is further worth noting that 360° (full rotation) scans minimize the effects of beam hardening but increase artifacts caused by misalignment and sample movement during imaging. Thus, while a 360° scan will typically generate cleaner data, imaging time is doubled and so a compromise between experimental cost and data quality must be addressed.
An important and often overlooked aspect of bone imaging experiments is the accurate and replicable specimen preparation technique performed prior to scanning. Studies that incorporate SRµCT methods into their experiments briefly mention their sampling protocol, but the authors provide little to no detail regarding the particular methodology used to gather their specimens. Many such studies mention cutting rectilinear bone blocks of arbitrary dimensions, but generally provide no further information about the tools or embedding materials used3,4,10,11,12,13,14. Some researchers commonly use handheld rotary tools (e.g., Dremel) to remove rectilinear blocks of bone from a region of interest (ROI)3,4,10,11,12,13,14. This method results in nonuniformly sized samples that may be larger than the FOV, increasing the likelihood of scan artifacts and image truncation. Such specimens often require further refining using a precision diamond-wafer saw (e.g., Buehler Isomet). Procuring samples with consistent dimensions (to the two-hundredths/mm) is critical to ensure that the acquired datasets are of the highest quality and the subsequent results are replicable.
The limited reporting of sample procurement methodology adds an extra layer of difficulty when attempting to employ and/or validate methods performed in a previous study. Currently, researchers must contact authors directly for further details on their sampling procedures. The protocol detailed here provides biomedical researchers with a thoroughly documented, replicable, and cost-efficient sampling technique. The primary objective of this article is to provide a comprehensive tutorial regarding how to procure consistently sized cortical bone core samples using a mill-drill press and diamond coring bit for the accurate visualization and extraction of microarchitectural data. This method is modified from procedures used to routinely collect uniform, small-diameter (1-5 mm) cylinders from blocks of hard materials in high pressure rock mechanics15,16,17,18,19.