3D image registration is critical to ensure accurate detection of longitudinal changes in trabecular bone density, microstructure, and stiffness measurements in rat tibiae by in vivo microcomputed tomography (μCT)

Abstract

In the recent decade, in vivo μCT scanners have become available to monitor temporal changes in rodent bone in response to diseases and treatments. We investigated short-term and long-term precision of in vivo μCT measurements of trabecular bone density, microstructure and stiffness of rat tibiae and tested whether they can be improved by 3D image registration. Rats in the short-term precision group underwent baseline and follow-up scans within the same day (n=15) and those in the long-term precision group were scanned at day 0 and day 14 (n=16) at 10.5 μm voxel size. A 3D image-registration scheme was applied to register the trabecular bone compartments of baseline and follow-up scans. Prior to image registration, short-term precision ranged between 0.85% and 2.65% in bone volume fraction (BV/TV), trabecular number, thickness, and spacing (Tb.N*, Tb.Th*, Tb.Sp*), trabecular bone mineral density and tissue mineral density (Tb.BMD, and Tb.TMD), and was particularly high in structure model index (SMI), connectivity density (Conn.D), and stiffness (4.29%–8.83%). Image registration tended to improve the short-term precision, but the only statistically significant improvement was in Tb.N*, Tb.TMD, and stiffness. On the other hand, unregistered comparisons between day-0 and day-14 scans suggested significant increases in BV/TV, Tb.N*, Tb.Th*, Conn.D, and Tb.BMD and decrease in Tb.Sp* and SMI. However, the percent change in each parameter from registered comparisons was significantly different from unregistered comparisons. Registered results suggested a significant increase in BV/TV, Tb.BMD, and stiffness over 14 days, primarily caused by increased Tb.Th* and Tb.TMD. Due to the continuous growth of rodents, the direct comparisons between the unregistered baseline and follow-up scans were driven by changes due to global bone modeling instead of local remodeling. Our results suggested that 3D image registration is critical for detecting changes due to bone remodeling activities in rodent trabecular bone by in vivo μCT imaging.

μCT-based, in vivo dynamic bone histomorphometry allows 3D evaluation of the early responses of bone resorption and formation to PTH and alendronate combination therapy

“…A recent study has demonstrated protective effects of PTH treatment against radiation damage [36], and our previous study showed high rates of bone formation in rats treated with PTH despite having received additional scans [10]; thus it is expected that scans on days 4 and 8 had no detrimental effects. As described in [10] and [34], rats were anesthetized (4.0/1.75% isoflurane), and the right leg of each rat was inserted into a custom holder to ensure minimal movement during the scan. A 4 mm region of the tibia distal to the proximal growth plate was scanned at 10.5 μm resulution, 55 keV energy, 145 μA intensity, 200 ms integration time, and 1000 projections, using a 0.5mm Al filter and a standard, manufacturer-provided beam-hardening correction algorithm, resulting in a total scan time of about 20 minutes and approximate radiation dose of 0.307 Gy per scan.…”

Mechanical Stimulation of Bone Marrow In Situ Induces Bone Formation in Trabecular Explants

“…This was confirmed by greater increases in BV/TV, Tb.Th and decreases in BS/TV and slenderness in the explanted trabecular bone cores when exposed to LMHF loading compared to static and media flow only conditions. Image registration techniques were used to accurately determine changes in trabecular bone over the course of the experiment 37,52 . This finding agrees with the numerous other studies which display the positive effect of LMHF on trabecular bone 24,31,51,60 .…”

Enhanced Individual Trabecular Repair and Its Mechanical Implications in Parathyroid Hormone and Alendronate Treated Rat Tibial Bone

“…The scanner energy was 55 keV energy, intensity ¼ 142 lA, and integration time ¼ 200 ms. Rats were anesthetized and immobilized during the scans (4.0/1.75% isoflurane) using a customized foot holder to ensure minimal motion over the 20 min scan [28]. A two-dimensional (2D) scout view was used to select the scan region, and a total of 417 slices (corresponding to a 4.4 mm region) distal to the proximal tibial growth plate were acquired.…”

“…An open source software (National Library of Medicine Insight Segmentation and Registration Toolkit, USA) [32] was used to derive the transformation matrix between the image coordinates of baseline (F 1 ) and follow-up (F 2 ) lCT scans by landmarkinitialized, mutual-information-based optimization [33,34]. This optimization scheme has been described in detail in a previous publication [28]. In the traditional registration scheme, once a transformation matrix T between image coordinates of baseline (F 1 ) and follow-up (F 2 ) scans is determined, it will be applied to move the follow-up scan into (F 1 ).…”

“…Most techniques currently available for microscale imaging of bone structure are limited to taking a "snapshot" of a complicated process. Recently, the development of in vivo lCT allows longitudinal imaging of trabecular bone microstructure at a resolution <15 lm, which enables the tracking of structural alterations at an individual trabecula level in rodent bone [25][26][27][28][29][30][31]. However, the challenges of precise registration of longitudinal scans as well as automatic detection of connectivity changes in individual trabecular structures remain.…”