Dosimetric Evaluation in Micro-CT Studies Used in Preclinical Molecular Imaging

In microCT imaging, there is a close relationship between the dose of radiation absorbed by animals and the image quality, or spatial resolution. Although the radiation levels used in these systems are generally non-lethal, they can induce cellular or molecular alterations that affect the experimental results. Here, we describe a dosimetric characterization of the different image acquisition modalities used by the microCT unit of the Albira microPET/SPECT/CT scanner, which is a widely used multimodal imaging system in preclinical research. The imparted dose at the animal surface (IDS) was estimated based on Boone’s polynomial interpolation method and experimental measurements using an ionization chamber and thermoluminescent dosimeters. The results indicated that the imparted dose at surface level delivered to the mice was in the 30 to 300 mGy range. For any combination of current (0.2 or 0.4 mA) and voltage (35 or 45 kV), in the Standard, Good, and Best image acquisition modalities, the dose imparted at surface level in rodents was below its threshold of deterministic effects (250 mGy); however, the High Res modality was above that threshold.

Exposure dating of detrital magnetite using ³He enabled by microCT and calibration of the cosmogenic ³He production rate in magnetite

We test whether X-ray micro-computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted detrital magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of the expected spallation production. We present Li concentrations, major and trace element analyses, and estimated magnetite (U–Th) / He cooling ages of samples in order to model the contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to 4 times that of the spallation component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ± 2.2 ka, we calibrate a production rate for magnetite 3He at sea level and high latitude (SLHL) of 116 ± 13 at g−1 a−1. We suggest that this microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.

The 3D construction of leaves is coordinated with water use efficiency in conifers

Conifers dominate the canopies of many terrestrial biomes, holding a great ecological and economic importance globally. Current increases in temperature and aridity are resulting in conifer mortality and imposing high transpirational demands to global vegetation. Therefore, identifying leaf structural determinants of carbon acquisition and water use efficiency is essential in predicting physiological impacts due to environmental variation. Using synchrotron-generated microCT imaging, we extracted leaf volumetric anatomy and stomatal traits in 34 species across the conifers with a special focus on Pinus, the richest conifer genus. We show that intrinsic water use efficiency (WUEi) is driven by leaf vein volumetrics, with both traits scaling positively. The ratios of stomatal pore number per unit mesophyll or intercellular airspace volume emerged as powerful explanatory variables, accurately predicting both stomatal conductance and WUEi. Our results clarify how the three-dimensional organization of tissues within the leaf has a direct impact on plant water use and carbon uptake.

The presence of Wormian bones increases the fracture resistance of equine cranial bone

Wormian (intrasutural) bones are small, irregular bones, that are found in the cranial sutures of the skull. The occurrence of Wormian bones in human skulls has been well documented but few studies have detected the presence of such bones in domestic animals. Although some research has linked the presence of Wormian bones to bone pathology, its anatomical significance in healthy individuals is not known. To the best of our knowledge, no previous study has examined the biomechanical features of Wormian bone. This study uses microCT imaging of the parietal bone region to determine the frequency of occurrence of Wormian bones in horse skulls and, through 3-point bending tests, to calculate the mechanical differences that result from the presence of such bones. In addition, bone properties such as bone mineral density (BMD) and stiffness were measured and analysed to determine the influence of Wormian bone. Our findings on 54 specimens taken from 10 horses (ages ranging from 4 to 29 years) showed that Wormian bone was present in 70% of subjects and that its occurrence was unrelated to age or sex. 3-point bend tests revealed that the stiffness normalised by cross section area (P = 0.038) was lower in samples where Wormian bone was present. An idealised Finite Element simulation confirmed that the presence of Wormian bone reduced the maximum stress and strain, as well as their distribution throughout the sample. We consequently conclude that the presence of Wormian bones, which are confined to the calvaria, increase the compliance of the bone and reduce the likelihood of skull fracture. As all skull samples were collected from a local abattoir, ethical approval was not required for this work.

Exposure dating of detrital magnetite using 3He enabled by microCT and calibration of the cosmogenic 3He production rate in magnetite

We test whether X-ray micro computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic 3He exposure dating. We extracted magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning Strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of expected cosmogenic production. We present Li concentrations, major and trace element analysis, and magnetite (U-Th)/He cooling ages of samples in order to model the contribution from radiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to four times that of the cosmogenic 3He component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite 3He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have 3He concentrations far in excess of the values expected from existing 10Be and 26Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured 3He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ka, we calibrate a magnetite 3He SLHL production rate of 116 ± 13 at g−1 a−1. We suggest that the microCT screening approach can be used to improve the quality of cosmogenic 3He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.

Wormhole Growth in Dissolving Limestones: Insights from 4D Tomography

<p>Dissolution of porous media introduces a positive feedback between fluid transport and chemical reactions at mineral surfaces leading to the formation of pronounced wormhole-like channels. While the impact of flow rate and reaction rate on the shapes of the wormholes is now well understood, much less is known about the dynamics of their propagation. In this study we capture the evolution of wormholes and their effects on flow patterns by in-situ X-ray microCT imaging of dissolving limestone cores. 4D tomography allows us in particular to correlate the permeability changes in a dissolving core with the advancement of the tip position of the wormhole. Surprisingly, we find that the relation between the two is highly nonlinear, with extensive periods of relatively fast growth of the wormhole which is nevertheless not reflected in any significant change in the overall permeability.  We hypothesize that this is caused by the presence of highly cemented regions in the core which act as permeability barriers for the flow. The presence of such regions is confirmed by a detailed analysis of the pore geometry based on the tomographic data. The results demonstrate that the analysis of the wormhole dynamics in 4d tomography can be used to probe the internal structure of the rock. </p>

Stomach serosal arteries distinguish gastric regions of the rat

Because the stomach in situ has few distinctive surface features and changes shape dramatically with food intake, we have used microCT imaging to (1) characterize the pattern of arteries, potential landmarks, on the stomach wall and (2) evaluate how meal-related shape changes affect the size of the different regions. The stomach receives its blood supply primarily from two pairs of vessels, the gastric and gastroepiploic arteries. Each of the three regions of the stomach is delineated by a distinctive combination of arterial fields: The corpus, consistent with its dynamic secretory activity and extensive mucosa, is supplied by extensive arterial trees formed by the left and right gastric arteries. These major arteries course circularly from the lesser towards the greater curvature, distally along both left and right walls of the corpus, and branch rostrally to supply the region. The muscular antrum is characterized by smaller arterial branches arising primarily from the right gastroepiploic artery that follows the distal greater curvature and secondarily from small, distally directed arteries supplied by the large vessels of the left and right gastric arteries. The forestomach, essentially devoid of mucosal tissue and separated from the corpus by the limiting ridge, is vascularized predominantly by a network of small arteries issued from the left gastroepiploic artery coursing around the proximal greater curvature, as well as from higher order and smaller branches issued by the gastric and celiac arteries. The regions of the stomach empty at different rates, thus changing the dimensions of the organ regions non-linearly.

Longitudinal time-lapse in vivo micro-CT reveals differential patterns of peri-implant bone changes after subclinical bacterial infection in a rat model

Subclinical infection associated with orthopedic devices can be challenging to diagnose. The goal of this study was to evaluate longitudinal, microcomputed tomography (microCT) imaging in a rat model of subclinical orthopedic device-related infection caused by Staphylococcus epidermidis and four different Cutibacterium (previously Propionibacterium) acnes strains, and compare outcomes with non-inoculated and historical S. aureus-inoculated controls. Sterile screws or screws colonized with bacteria were placed in the tibia of 38 adult Wistar rats [n = 6 sterile screws; n = 6 S. epidermidis-colonized screws; n = 26 C. acnes-colonized screws (covering all three main subspecies)]. Regular microCT scans were taken over 28 days and processed for quantitative time-lapse imaging with dynamic histomorphometry. At euthanasia, tissues were processed for semiquantitative histopathology or quantitative bacteriology. All rats receiving sterile screws were culture-negative at euthanasia and displayed progressive bony encapsulation of the screw. All rats inoculated with S. epidermidis-colonized screws were culture-positive and displayed minor changes in peri-implant bone, characteristic of subclinical infection. Five of the 17 rats in the C. acnes inoculated group were culture positive at euthanasia and displayed bone changes at the interface of the screw and bone, but not deeper in the peri-implant bone. Dynamic histomorphometry revealed significant differences in osseointegration, bone remodeling and periosteal reactions between groups that were not measurable by visual observation of still microCT images. Our study illustrates the added value of merging 3D microCT data from subsequent timepoints and producing inherently richer 4D data for the detection and characterization of subclinical orthopedic infections, whilst also reducing animal use.

Innovative high-resolution microCT imaging of animal brain vasculature

Analysis of the angioarchitecture and quantification of the conduit vessels and microvasculature is of paramount importance for understanding the physiological and pathological processes within the central nervous system (CNS). Most of the available in vivo imaging methods lack penetration depth and/or resolution. Some ex vivo methods may provide better resolution, but are mainly destructive, as they are designed for imaging the CNS tissues after their removal from the skull or vertebral column. The removal procedure inevitably alters the in situ relations of the investigated structures and damages the dura mater and leptomeninges. µAngiofil, a polymer-based contrast agent, permits a qualitatively novel postmortem microangio-computed tomography (microangioCT) approach with excellent resolution and, therefore, visualization of the smallest brain capillaries. The datasets obtained empower a rather straightforward quantitative analysis of the vascular tree, including the microvasculature. The µAngiofil has an excellent filling capacity as well as a radio-opacity higher than the one of bone tissue, which allows imaging the cerebral microvasculature even within the intact skull or vertebral column. This permits in situ visualization and thus investigation of the dura mater and leptomeningeal layers as well as their blood supply in their original geometry. Moreover, the methodology introduced here permits correlative approaches, i.e., microangioCT followed by classical histology, immunohistochemistry and even electron microscopy. The experimental approach presented here makes use of common desktop microCT scanners, rendering it a promising everyday tool for the evaluation of the (micro)vasculature of the central nervous system in preclinical and basic research.