Micro-CT imaging: Developing criteria for examining fetal skeletons in regulatory developmental toxicology studies - A workshop report

Concordance between alizarin red stained skeleton and micro-CT skeleton evaluation methods: A case study in New Zealand White rabbits

OBJECTIVES

The objective of this study was to examine the fetal skeletons using both alizarin red stain and micro-computed tomography (CT) images; investigate differences, and to determine if the conclusions of the study were the same regardless of the examination method.

METHODS

A candidate drug was given orally by gavage to pregnant New Zealand White rabbits on gestation day (GD) 7 to GD 19 (mating = GD 0) at doses of 0 (control), 0.02, 0.5, 5, and 15 mg/kg/day. Maternal toxicity was evident at ≥0.02 mg/kg/day. The 199 fetal skeletons (totaling 50,546 skeletal elements) obtained at cesarean delivery on GD29 were first stained with Alizarin Red S, then imaged by a Siemens Inveon micro-CT scanner. All fetal skeletons were examined by both methods, without knowledge of dose group, and the results were compared.

RESULTS

In total, 33 types of skeletal abnormalities were identified. There was 99.8% concordance of results comparing stain to micro-CT. Ossification of the middle phalanx of the forepaw digit 5 showed the greatest difference between the two methods.

CONCLUSION

Overall, micro-CT imaging is a realistic, and robust alternative to skeletal staining to examine fetal rabbit skeletons in developmental toxicity studies.

Advances in micro-CT imaging of small animals

PURPOSE

Micron-scale computed tomography (micro-CT) imaging is a ubiquitous, cost-effective, and non-invasive three-dimensional imaging modality. We review recent developments and applications of micro-CT for preclinical research.

METHODS

Based on a comprehensive review of recent micro-CT literature, we summarize features of state-of-the-art hardware and ongoing challenges and promising research directions in the field.

RESULTS

Representative features of commercially available micro-CT scanners and some new applications for both in vivo and ex vivo imaging are described. New advancements include spectral scanning using dual-energy micro-CT based on energy-integrating detectors or a new generation of photon-counting x-ray detectors (PCDs). Beyond two-material discrimination, PCDs enable quantitative differentiation of intrinsic tissues from one or more extrinsic contrast agents. When these extrinsic contrast agents are incorporated into a nanoparticle platform (e.g. liposomes), novel micro-CT imaging applications are possible such as combined therapy and diagnostic imaging in the field of cancer theranostics. Another major area of research in micro-CT is in x-ray phase contrast (XPC) imaging. XPC imaging opens CT to many new imaging applications because phase changes are more sensitive to density variations in soft tissues than standard absorption imaging. We further review the impact of deep learning on micro-CT. We feature several recent works which have successfully applied deep learning to micro-CT data, and we outline several challenges specific to micro-CT.

CONCLUSIONS

All of these advancements establish micro-CT imaging at the forefront of preclinical research, able to provide anatomical, functional, and even molecular information while serving as a testbench for translational research.

In Vivo Imaging With Confirmation by Histopathology for Increased Rigor and Reproducibility in Translational Research: A Review of Examples, Options, and Resources

Preclinical noninvasive imaging can be an indispensable tool for studying animal models of disease. In vivo imaging to assess anatomical, functional, and molecular features requires verification by a comparison to the macroscopic and microscopic morphological features, since all noninvasive in vivo imaging methods have much lower resolution than standard histopathology. Comprehensive pathological evaluation of the animal model is underutilized; yet, many institutions have veterinary or human pathologists with necessary comparative pathology expertise. By performing a rigorous comparison to gross or histopathology for image interpretation, these trained individuals can assist scientists with the development of the animal model, experimental design, and evaluation of the in vivo imaging data. These imaging and pathology corroboration studies undoubtedly increase scientific rigor and reproducibility in descriptive and hypothesis-driven research. A review of case examples including ultrasound, nuclear, optical, and MRI is provided to illustrate how a wide range of imaging modalities data can be confirmed by gross or microscopic pathology. This image confirmation and authentication will improve characterization of the model and may contribute to decreasing costs and number of animals used and to more rapid translation from preclinical animal model to the clinic.

Bone development in laboratory mammals used in developmental toxicity studies

Evaluation of the skeleton in laboratory animals is a standard component of developmental toxicology testing. Standard methods of performing the evaluation have been established, and modification of the evaluation using imaging technologies is under development. The embryology of the rodent, rabbit, and primate skeleton has been characterized in detail and summarized herein. The rich literature on variations and malformations in skeletal development that can occur in the offspring of normal animals and animals exposed to test articles in toxicology studies is reviewed. These perturbations of skeletal development include ossification delays, alterations in number, shape, and size of ossification centers, and alterations in numbers of ribs and vertebrae. Because the skeleton is undergoing developmental changes at the time fetuses are evaluated in most study designs, transient delays in development can produce apparent findings of abnormal skeletal structure. The determination of whether a finding represents a permanent change in embryo development with adverse consequences for the organism is important in study interpretation. Knowledge of embryological processes and schedules can assist in interpretation of skeletal findings.

Regulatory Forum Opinion Piece*: Imaging Applications in Toxicologic Pathology-Recommendations for Use in Regulated Nonclinical Toxicity Studies

Available imaging systems for use in preclinical toxicology studies increasingly show utility as important tools in the toxicologic pathologist's armamentarium, permit longitudinal evaluation of functional and morphological changes in tissues, and provide important information such as organ and lesion volume not obtained by conventional toxicology study parameters. Representative examples of practical imaging applications in toxicology research and preclinical studies are presented for ultrasound, positron emission tomography/single-photon emission computed tomography, optical, magnetic resonance imaging, and matrix-assisted laser desorption ionization-imaging mass spectrometry imaging. Some of the challenges for making imaging systems good laboratory practice-compliant for regulatory submission are presented. Use of imaging data on a case-by-case basis as part of safety evaluation in regulatory submissions is encouraged.

Implementation of Good Laboratory Practices (GLP) in basic scientific research: Translating the concept beyond regulatory compliance

The principles of Good Laboratory Practices (GLPs) are mainly intended for the laboratories performing studies for regulatory compliances. However, today GLP can be applied to broad disciplines of science to cater to the needs of the experimental objectives, generation of quality data and assay reproducibility. Considering its significance, it can now be applied in academics; industries as well as government set ups throughout the world. GLP is the best way to promote the reliability, reproducibility of the test data and hence facilitates the international acceptability. Now it is high time to translate and implement the concept of GLP beyond regulatory studies. Thus, it can pave the way for better understanding of scientific problems and help to maintain a good human and environmental health. Through this review, we have made an attempt to explore the uses of GLP principles in different fields of science and its acceptability as well as looking for its future perspectives.

Phenotyping cardiac and structural birth defects in fetal and newborn mice

Mouse models are invaluable for investigating the developmental etiology and molecular pathogenesis of structural birth defects. While this has been deployed for studying a wide spectrum of birth defects, mice are particularly valuable for modeling congenital heart disease, given they have the same four-chamber cardiac anatomy as in humans. We have developed the use of noninvasive fetal ultrasound together with micro-computed tomography (micro-CT) imaging for high throughput phenotyping of mice for congenital heart defects (CHD) and other developmental anomalies. We showed the efficacy of fetal ultrasound and micro-CT imaging for diagnosis of a wide spectrum of CHD. With fetal ultrasound, longitudinal scans can be conducted to track the developmental profile of disease pathogenesis, providing both structural information with two-dimensional (2D) imaging, as well as functional data from hemodynamic assessments with color flow and spectral Doppler imaging. In contrast, with micro-CT imaging, virtual necropsies can be conducted rapidly postmortem for diagnosis of not only CHD, but also other structural birth defects. To validate the CHD diagnosis, we further showed the use of a novel histological technique with episcopic confocal microscopy to obtain rapid 3D reconstructions for accurate diagnosis of even the most complex anatomical defect. The latter histological imaging technique when combined with the use of ultrasound and micro-CT imaging provides a powerful combination of imaging modalities that will be invaluable in meeting the accelerating demands for high throughput mouse phenotyping of genetically engineered mice in the coming age of functional genomics. Birth Defects Research 109:778-790, 2017. © 2017 Wiley Periodicals, Inc.