4-D Micro-CT of the Mouse Heart

Optimising 4D imaging of fast-oscillating structures using X-ray microtomography with retrospective gating

Imaging the internal architecture of fast-vibrating structures at micrometer scale and kilohertz frequencies poses great challenges for numerous applications, including the study of biological oscillators, mechanical testing of materials, and process engineering. Over the past decade, X-ray microtomography with retrospective gating has shown very promising advances in meeting these challenges. However, breakthroughs are still expected in acquisition and reconstruction procedures to keep improving the spatiotemporal resolution, and study the mechanics of fast-vibrating multiscale structures. Thereby, this works aims to improve this imaging technique by minimising streaking and motion blur artefacts through the optimisation of experimental parameters. For that purpose, we have coupled a numerical approach relying on tomography simulation with vibrating particles with known and ideal 3D geometry (micro-spheres or fibres) with experimental campaigns. These were carried out on soft composites, imaged in synchrotron X-ray beamlines while oscillating up to 400 Hz, thanks to a custom-developed vibromechanical device. This approach yields homogeneous angular sampling of projections and gives reliable predictions of image quality degradation due to motion blur. By overcoming several technical and scientific barriers limiting the feasibility and reproducibility of such investigations, we provide guidelines to enhance gated-CT 4D imaging for the analysis of heterogeneous, high-frequency oscillating materials.

Hybrid Molecular and Functional Micro-CT Imaging Reveals Increased Myocardial Apoptosis Preceding Cardiac Failure in Progeroid Ercc1 Mice

PURPOSE

In this study, we explored the role of apoptosis as a potential biomarker for cardiac failure using functional micro-CT and fluorescence molecular tomography (FMT) imaging techniques in Ercc1 mutant mice. Ercc1 is involved in multiple DNA repair pathways, and its mutations contribute to accelerated aging phenotypes in both humans and mice, due to the accumulation of DNA lesions that impair vital DNA functions. We previously found that systemic mutations and cardiomyocyte-restricted deletion of Ercc1 in mice results in left ventricular (LV) dysfunction at older age.

PROCEDURES AND RESULTS

Here we report that combined functional micro-CT and FMT imaging allowed us to detect apoptosis in systemic Ercc1 mutant mice prior to the development of overt LV dysfunction, suggesting its potential as an early indicator and contributing factor of cardiac impairment. The detection of apoptosis in vivo was feasible as early as 12 weeks of age, even when global LV function appeared normal, underscoring the potential of apoptosis as an early predictor of LV dysfunction, which subsequently manifested at 24 weeks.

CONCLUSIONS

This study highlights the utility of combined functional micro-CT and FMT imaging in assessing cardiac function and detecting apoptosis, providing valuable insights into the potential of apoptosis as an early biomarker for cardiac failure.

An optimized CT-dense agent perfusion and micro-CT imaging protocol for chick embryo developmental stages

Compared to classical techniques of morphological analysis, micro-CT (μ-CT) has become an effective approach allowing rapid screening of morphological changes. In the present work, we aimed to provide an optimized micro-CT dense agent perfusion protocol and μ-CT guidelines for different stages of chick embryo cardiogenesis. Our study was conducted over a period of 10 embryonic days (Hamburger-Hamilton HH36) in chick embryo hearts. During the perfusion of the micro-CT dense agent at different developmental stages (HH19, HH24, HH27, HH29, HH31, HH34, HH35, and HH36), we demonstrated that durations and volumes of the injected contrast agent gradually increased with the heart developmental stages contrary to the flow rate that was unchanged during the whole experiment. Analysis of the CT imaging confirmed the efficiency of the optimized parameters of the heart perfusion.

MCR toolkit: A GPU-based toolkit for multi-channel reconstruction of preclinical and clinical x-ray CT data

BACKGROUND

The advancement of x-ray CT into the domains of photon counting spectral imaging and dynamic cardiac and perfusion imaging has created many new challenges and opportunities for clinicians and researchers. To address challenges such as dose constraints and scanning times while capitalizing on opportunities such as multi-contrast imaging and low-dose coronary angiography, these multi-channel imaging applications require a new generation of CT reconstruction tools. These new tools should exploit the relationships between imaging channels during reconstruction to set new image quality standards while serving as a platform for direct translation between the preclinical and clinical domains.

PURPOSE

We outline and demonstrate a new Multi-Channel Reconstruction (MCR) Toolkit for GPU-based analytical and iterative reconstruction of preclinical and clinical multi-energy and dynamic x-ray CT data. To promote open science, open-source distribution of the Toolkit will coincide with the release of this publication (GPL v3; gitlab.oit.duke.edu/dpc18/mcr-toolkit-public).

METHODS

The MCR Toolkit source code is implemented in C/C++ and NVIDIA's CUDA GPU programming interface, with scripting support from MATLAB and Python. The Toolkit implements matched, separable footprint CT reconstruction operators for projection and backprojection in two geometries: planar, cone-beam CT (CBCT) and 3rd generation, cylindrical multi-detector row CT (MDCT). Analytical reconstruction is performed using filtered backprojection (FBP) for circular CBCT, weighted FBP (WFBP) for helical CBCT, and cone-parallel projection rebinning followed by WFBP for MDCT. Arbitrary combinations of energy and temporal channels are iteratively reconstructed under a generalized multi-channel signal model for joint reconstruction. We solve this generalized model algebraically using the split Bregman optimization method and the BiCGSTAB(l) linear solver interchangeably for both CBCT and MDCT data. Rank-sparse kernel regression (RSKR) and patch-based singular value thresholding (pSVT) are used to regularize the energy and time dimensions, respectively. Under a Gaussian noise model, regularization parameters are estimated automatically from the input data, dramatically reducing algorithm complexity for end users. Multi-GPU parallelization of the reconstruction operators is supported to manage reconstruction times.

RESULTS

Denoising with RSKR and pSVT and post-reconstruction material decomposition are illustrated with preclinical and clinical cardiac photon-counting (PC)CT data. A digital MOBY mouse phantom with cardiac motion is used to illustrate single energy (SE), multi-energy (ME), time resolved (TR), and combined multi-energy and time-resolved (METR) helical, CBCT reconstruction. A fixed set of projection data is used across all reconstruction cases to demonstrate the Toolkit's robustness to increasing data dimensionality. Identical reconstruction code is applied to in vivo cardiac PCCT data acquired in a mouse model of atherosclerosis (METR). Clinical cardiac CT reconstruction is illustrated using the XCAT phantom and the DukeSim CT simulator, while dual-source, dual-energy CT reconstruction is illustrated for data acquired with a Siemens Flash scanner. Benchmarking results with NVIDIA RTX 8000 GPU hardware demonstrate 61%-99% efficiency in scaling computation from one to four GPUs for these reconstruction problems.

CONCLUSIONS

The MCR Toolkit provides a robust solution for temporal and spectral x-ray CT reconstruction problems and was built from the ground up to facilitate translation of CT research and development between preclinical and clinical applications.

An optimized CT-dense agent perfusion and micro-CT imaging protocol for chick embryo developmental stages.. [DOI: 10.21203/rs.3.rs-2541863/v1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Compared to classical techniques of morphological analysis, micro-CT (µ-CT) has become an effective approach allowing rapid screening of morphological changes. In the present work, we aimed to provide an optimized µ-CT dense agent perfusion protocol and µ-CT guidelines for different stages of chick embryo cardiogenesis. Our study was conducted over a period of 10 embryonic days (Hamburger-Hamilton HH36) in chick embryo hearts. During the perfusion of the µ-CT dense agent at different developmental stages (HH19, HH24, HH27, HH29, HH31, HH34, HH35, and HH36), we demonstrated that durations and volumes of the injected contrast agent gradually increased with the heart developmental stages contrary to the flow rate that was unchanged during the whole experiment. Analysis of the CT imaging confirmed the efficiency of the optimized parameters of the heart perfusion.

Detector-trigger-based cardiac multiphase micro-CT imaging for small animals

BACKGROUND

Micro-computed tomography is important in cardiac imaging for preclinical small animal models, but motion artifacts may appear due to the rapid heart rates. To avoid influence of motion artifacts, the prospective ECG gating schemes based on an X-ray source trigger have been investigated. However, due to the lack of pulsed X-ray exposure modes, high-resolution micro-focus X-ray sources do not support source triggering in most cases.

OBJECTIVE

To develop a fast-cardiac multiphase acquisition strategy using prospective ECG gating for micro-focus X-ray tubes with a continuous emission mode.

METHODS

The proposed detector-trigger-based prospective ECG gating acquisition scheme (DTB-PG) triggers the X-ray detector at the R peak of ECG, and then collects multiple phase projections of the heart in one ECG cycle by sequence acquisition. Cardiac multiphase images are reconstructed after performing the same acquisition in all views. The feasibility of this strategy was verified in multiphase imaging experiments of a phantom with 150 ms motion period and a mouse heart on a micro-focus micro-CT system with continuous emission mode.

RESULTS

Using a high frame-rate CMOS detector, DTB-PG discriminates the positions of the motion phantom well in 10 different phases and enables to distinguish the changes in the cardiac volume of the mouse in different phases. The acquisition rate of DTB-PG is much faster than other prospective gating schemes as demonstrated by theoretical analysis.

CONCLUSIONS

DTB-PG combines the advantages of prospective ECG gating strategies and X-ray detector-trigger mode to suppress motion artifacts, achieve ultra-fast acquisition rates, and relax hardware limitations.

An in-silico method to predict and quantify the effect of gold nanoparticles in X-ray imaging

PURPOSE

Over the last few years studies are conducted, highlighting the feasibility of Gold Nanoparticles (GNPs) to be used in clinical CT imaging and as an efficient contrast agent for cancer research. After ensuring that GNPs formulations are appropriate for in vivo or clinical use, the next step is to determine the parameters for an X-ray system's optimal contrast for applications and to extract quantitative information. There is currently a gap and need to exploit new X-ray imaging protocols and processing algorithms, through specific models avoiding trial-and-error procedures and provide an imaging prognosis tool. Such a model can be used to confirm the accumulation of GNPs in target organs before radiotherapy treatments with a system easily available in hospitals, as low energy X-rays.

METHODS

In this study a complete, easy-to-use, simulation platform is designed and built, where simple parameters, as the X-ray's specifications and experimentally defined biodistributions of specific GNPs are imported. The induced contrast and images can be exported, and accurate quantification can be performed. This platform is based on the GATE Monte Carlo simulation toolkit, based on the GEANT4 toolkit and the MOBY phantom, a realistic 4D digital mouse.

RESULTS

We have validated this simulation platform to predict the contrast induction and minimum detectable concentration of GNPs on any given X-ray system. The study was applied to preclinical studies but is also expandable to clinical studies.

CONCLUSIONS

According to our knowledge, no other such validated simulation model currently exists, and this model could help radiology imaging with GNPs to be truly deployed.

Refined Locally Linear Transform-Based Spectral-Domain Gradient Sparsity and Its Applications in Spectral CT Reconstruction

Spectral computed tomography (CT) is extension of the conventional single spectral CT (SSCT) along the energy dimension, which achieves superior energy resolution and material distinguishability. However, for the state-of-the-art photon counting detector (PCD) based spectral CT, because the emitted photons with a fixed total number for each X-ray beam are divided into several energy bins, the noise level is increased in each reconstructed channel image, and it further leads to an inaccurate material decomposition. To improve the reconstructed image quality and decomposition accuracy, in this work, we first employ a refined locally linear transform to convert the structural similarity among two-dimensional (2D) spectral CT images to a spectral-dimension gradient sparsity. By combining the gradient sparsity in the spatial domain, a global three-dimensional (3D) gradient sparsity is constructed, then measured with L 1-, L 0- and trace-norm, respectively. For each sparsity measurement, we propose the corresponding optimization model, develop the iterative algorithm, and verify the effectiveness and superiority with real datasets.

Coronary micro-computed tomography angiography in mice

Coronary computed tomography angiography is an established technique in clinical practice and a valuable tool in the diagnosis of coronary artery disease in humans. Imaging of coronaries in preclinical research, i.e. in small animals, is very difficult due to the high demands on spatial and temporal resolution. Mice exhibit heart rates of up to 600 beats per minute motivating the need for highest detector framerates while the coronaries show diameters below 100 μm indicating the requirement for highest spatial resolution. We herein use a custom built micro-CT equipped with dedicated reconstruction algorithms to illustrate that coronary imaging in mice is possible. The scanner provides a spatial and temporal resolution sufficient for imaging of smallest, moving anatomical structures and the dedicated reconstruction algorithms reduced radiation dose to less than 1 Gy but do not yet allow for longitudinal studies. Imaging studies were performed in ten mice administered with a blood-pool contrast agent. Results show that the course of the left coronary artery can be visualized in all mice and all major branches can be identified for the first time using micro-CT. This reduces the gap in cardiac imaging between clinical practice and preclinical research.

Locally linear transform based three-dimensional gradient L 0 -norm minimization for spectral CT reconstruction

PURPOSE

Spectral computed tomography (CT) is proposed by extending the conventional CT along the energy dimension. One newly implementation is to employ an energy-discriminating photon counting detector (PCD), which can distinguish photon energy and divide a whole x-ray spectrum into several energy bins with appropriate post-processing steps. The state-of-the-art PCD-based spectral CT has superior energy resolution and material distinguishability, and it further has a great potential in both medical and industrial applications. To improve the reconstruction quality and decomposition accuracy, in this work, we propose an optimization-based spectral CT reconstruction method with an innovational sparsity constraint.

METHODS

We first employ a locally linear transform to the reconstructed channel images, and the structural similarity along the spectral dimension is effectively converted to a one-dimensional (1D) gradient sparsity. Then, combining the prior knowledge of piecewise constant in the spatial domain (e.g., a two-dimensional (2D) gradient sparsity feature), we unify both spectral and spatial dimensions and establish a joint three-dimensional (3D) gradient sparsity. In addition, we use the L 0 -norm to measure the proposed sparsity and incorporate it as a smoothness constraint to concretize a general optimization framework. Furthermore, we develop the corresponding iterative algorithm to solve the optimization problem.

RESULTS

Both visual results and quantitative indexes of numerical simulations and phantom experiments demonstrate the proposed method outperform the conventional filtered backprojection (FBP), total variation (TV), 2D L₀ -norm (L₀ ), and TV with low rank (TVLR)-based methods. From the image and ROI comparisons, we find the proposed method performs well in noise suppression, detail maintenance, and decomposition accuracy. However, the FBP suffers severe noise, the TV and L₀ are difficult to work consistently among different energy bins, and the TVLR fails to avoid gray value shift. The image quality assessments, such as peak signal-to-noise ratio (PSNR), normal mean absolute deviation (NMAD). and structural similarity (SSIM), also consistently indicate the proposed method can effectively removing noise and keeping fine structures in both channel-wise reconstructions and material decompositions.

CONCLUSIONS

By employing a locally linear transform, the structural similarity among spectral channel images is converted to a 1D gradient sparsity and the gray value shift is effectively avoided when the difference measurement is minimized. The 3D L₀ -norm jointly and uniformly measures the gradient sparsity in both spectral and spatial dimensions. The cooperation of locally linear transform and 3D L₀ -norm well reinforces the global sparse features and keeps the correlation along spectral dimension without bringing gray-value distortions. The corresponding constraint optimization model is fast and stably solved by using an alternative direction technique. Both numerical simulations and phantom experiments confirm the superior performance of the proposed method in noise suppression, structure maintenance, and accurate decomposition.

Time-course study of a gold nanoparticle contrast agent for cardiac-gated micro-CT imaging in mice

Although micro-computed tomography (micro-CT) images have high contrast for bone or air, between soft tissues the contrast is typically low. To overcome this inherent issue, attenuating exogenous contrast agents are used to provide contrast enhancement in the vasculature and abdominal organs. The aim of this study is to measure the contrast enhancement time course for a gold nanoparticle blood-pool contrast agent and use it to perform cardiac-gated 4D micro-CT scans of the heart. Six healthy female C57BL/6 mice were anesthetized and imaged after receiving an injected dose of MVivo gold nanoparticle blood-pool contrast agent. Following the injection, we performed micro-CT scans at 0, 0.25, 0.5, 0.75, 1, 2, 4, 8, 24, 48 and 72 h. The mean CT number was measured for 7 different organs. No contrast enhancement was noticed in the bladder, kidneys or muscle during the time-course study. However, it clearly appears that the contrast enhancement is high in both right ventricle and vena cava. To perform cardiac-gated imaging, either the gold nanoparticle agent (n = 3) or an iodine-based (n = 3) contrast agent was introduced and images representing 9 phases of the cardiac cycle were obtained in 6 additional mice. A few typical cardiac parameters were measured or calculated, with similar accuracy between the gold and iodinated agents, but better visualization of structures with the gold agent. The MVivo Au contrast agent can be used for investigations of cardiac or vascular disease with a single bolus injection, with an optimal cardiac imaging window identified during the first hour after injection, demonstrating similar image quality to iodinated contrast agents and excellent measurement accuracy. Furthermore, the long-lasting contrast enhancement of up to 8 h can be very useful for scanning protocols that require longer acquisition times.

Vascular Endothelial Growth Factor Receptor 3 Regulates Endothelial Function Through β-Arrestin 1

BACKGROUND

Receptor signaling is central to vascular endothelial function and is dysregulated in vascular diseases such as atherosclerosis and pulmonary arterial hypertension (PAH). Signaling pathways involved in endothelial function include vascular endothelial growth factor receptors (VEGFRs) and G protein-coupled receptors, which classically activate distinct intracellular signaling pathways and responses. The mechanisms that regulate these signaling pathways have not been fully elucidated and it is unclear what nodes for cross talk exist between these diverse signaling pathways. For example, multifunctional β-arrestin (ARRB) adapter proteins are best known as regulators of G protein-coupled receptor signaling, but their role at other receptors and their physiological importance in the setting of vascular disease are unclear.

METHODS

We used a combination of human samples from PAH, human microvascular endothelial cells from lung, and Arrb knockout mice to determine the role of ARRB1 in endothelial VEGFR3 signaling. In addition, a number of biochemical analyses were performed to determine the interaction between ARRB1 and VEGFR3, signaling mediators downstream of VEGFR3, and the internalization of VEGFR3.

RESULTS

Expression of ARRB1 and VEGFR3 was reduced in human PAH, and the deletion of Arrb1 in mice exposed to hypoxia led to worse PAH with a loss of VEGFR3 signaling. Knockdown of ARRB1 inhibited VEGF-C-induced endothelial cell proliferation, migration, and tube formation, along with reduced VEGFR3, Akt, and endothelial nitric oxide synthase phosphorylation. This regulation was mediated by direct ARRB1 binding to the VEGFR3 kinase domain and resulted in decreased VEGFR3 internalization.

CONCLUSIONS

Our results demonstrate a novel role for ARRB1 in VEGFR regulation and suggest a mechanism for cross talk between G protein-coupled receptors and VEGFRs in PAH. These findings also suggest that strategies to promote ARRB1-mediated VEGFR3 signaling could be useful in the treatment of pulmonary hypertension and other vascular disease.

Uniform light delivery in volumetric optoacoustic tomography

Accurate image reconstruction in volumetric optoacoustic tomography implies the efficient generation and collection of ultrasound signals around the imaged object. Non-uniform delivery of the excitation light is a common problem in optoacoustic imaging often leading to a diminished field of view, limited dynamic range and penetration, as well as impaired quantification abilities. Presented here is an optimized illumination concept for volumetric tomography that utilizes additive manufacturing via 3D printing in combination with custom-made optical fiber illumination. The custom-designed sample chamber ensures convenient access to the imaged object along with accurate positioning of the sample and a matrix array ultrasound transducer used for collection of the volumetric image data. Ray tracing is employed to optimize the positioning of the individual fibers in the chamber. Homogeneity of the generated light excitation field was confirmed in tissue-mimicking agar spheres. Applicability of the system to image entire mouse organs ex vivo has been showcased. The new approach showed a clear advantage over conventional, single-sided illumination strategies by eliminating the need to correct for illumination variances and resulting in enhancement of the effective field of view, greater penetration depth and significant improvements in the overall image quality.

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.

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.

Low-dose 4D cardiac imaging in small animals using dual source micro-CT

Micro-CT is widely used in preclinical studies, generating substantial interest in extending its capabilities in functional imaging applications such as blood perfusion and cardiac function. However, imaging cardiac structure and function in mice is challenging due to their small size and rapid heart rate. To overcome these challenges, we propose and compare improvements on two strategies for cardiac gating in dual-source, preclinical micro-CT: fast prospective gating (PG) and uncorrelated retrospective gating (RG). These sampling strategies combined with a sophisticated iterative image reconstruction algorithm provide faster acquisitions and high image quality in low-dose 4D (i.e. 3D  +  Time) cardiac micro-CT. Fast PG is performed under continuous subject rotation which results in interleaved projection angles between cardiac phases. Thus, fast PG provides a well-sampled temporal average image for use as a prior in iterative reconstruction. Uncorrelated RG incorporates random delays during sampling to prevent correlations between heart rate and sampling rate. We have performed both simulations and animal studies to validate these new sampling protocols. Sampling times for 1000 projections using fast PG and RG were 2 and 3 min, respectively, and the total dose was 170 mGy each. Reconstructions were performed using a 4D iterative reconstruction technique based on the split Bregman method. To examine undersampling robustness, subsets of 500 and 250 projections were also used for reconstruction. Both sampling strategies in conjunction with our iterative reconstruction method are capable of resolving cardiac phases and provide high image quality. In general, for equal numbers of projections, fast PG shows fewer errors than RG and is more robust to undersampling. Our results indicate that only 1000-projection based reconstruction with fast PG satisfies a 5% error criterion in left ventricular volume estimation. These methods promise low-dose imaging with a wide range of preclinical applications in cardiac imaging.

An update on carbon nanotube-enabled X-ray sources for biomedical imaging

A new imaging technology has emerged that uses carbon nanotubes (CNT) as the electron emitter (cathode) for the X-ray tube. Since the performance of the CNT cathode is controlled by simple voltage manipulation, CNT-enabled X-ray sources are ideal for the repetitive imaging steps needed to capture three-dimensional information. As such, they have allowed the development of a gated micro-computed tomography (CT) scanner for small animal research as well as stationary tomosynthesis, an experimental technology for large field-of-view human imaging. The small animal CT can acquire images at specific points in the respiratory and cardiac cycles. Longitudinal imaging therefore becomes possible and has been applied to many research questions, ranging from tumor response to the noninvasive assessment of cardiac output. Digital tomosynthesis (DT) is a low-dose and low-cost human imaging tool that captures some depth information. Known as three-dimensional mammography, DT is now used clinically for breast imaging. However, the resolution of currently-approved DT is limited by the need to swing the X-ray source through space to collect a series of projection views. An array of fixed and distributed CNT-enabled sources provides the solution and has been used to construct stationary DT devices for breast, lung, and dental imaging. To date, over 100 patients have been imaged on Institutional Review Board-approved study protocols. Early experience is promising, showing an excellent conspicuity of soft-tissue features, while also highlighting technical and post-acquisition processing limitations that are guiding continued research and development. Additionally, CNT-enabled sources are being tested in miniature X-ray tubes that are capable of generating adequate photon energies and tube currents for clinical imaging. Although there are many potential applications for these small field-of-view devices, initial experience has been with an X-ray source that can be inserted into the mouth for dental imaging. Conceived less than 20 years ago, CNT-enabled X-ray sources are now being manufactured on a commercial scale and are powering both research tools and experimental human imaging devices. WIREs Nanomed Nanobiotechnol 2018, 10:e1475. doi: 10.1002/wnan.1475 This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

A Novel Mouse Segmentation Method Based on Dynamic Contrast Enhanced Micro-CT Images

With the development of hybrid imaging scanners, micro-CT is widely used in locating abnormalities, studying drug metabolism, and providing structural priors to aid image reconstruction in functional imaging. Due to the low contrast of soft tissues, segmentation of soft tissue organs from mouse micro-CT images is a challenging problem. In this paper, we propose a mouse segmentation scheme based on dynamic contrast enhanced micro-CT images. With a homemade fast scanning micro-CT scanner, dynamic contrast enhanced images were acquired before and after injection of non-ionic iodinated contrast agents (iohexol). Then the feature vector of each voxel was extracted from the signal intensities at different time points. Based on these features, the heart, liver, spleen, lung, and kidney could be classified into different categories and extracted from separate categories by morphological processing. The bone structure was segmented using a thresholding method. Our method was validated on seven BALB/c mice using two different classifiers: a support vector machine classifier with a radial basis function kernel and a random forest classifier. The results were compared to manual segmentation, and the performance was assessed using the Dice similarity coefficient, false positive ratio, and false negative ratio. The results showed high accuracy with the Dice similarity coefficient ranging from 0.709 ± 0.078 for the spleen to 0.929 ± 0.006 for the kidney.

Development of computational small animal models and their applications in preclinical imaging and therapy research

The development of multimodality preclinical imaging techniques and the rapid growth of realistic computer simulation tools have promoted the construction and application of computational laboratory animal models in preclinical research. Since the early 1990s, over 120 realistic computational animal models have been reported in the literature and used as surrogates to characterize the anatomy of actual animals for the simulation of preclinical studies involving the use of bioluminescence tomography, fluorescence molecular tomography, positron emission tomography, single-photon emission computed tomography, microcomputed tomography, magnetic resonance imaging, and optical imaging. Other applications include electromagnetic field simulation, ionizing and nonionizing radiation dosimetry, and the development and evaluation of new methodologies for multimodality image coregistration, segmentation, and reconstruction of small animal images. This paper provides a comprehensive review of the history and fundamental technologies used for the development of computational small animal models with a particular focus on their application in preclinical imaging as well as nonionizing and ionizing radiation dosimetry calculations. An overview of the overall process involved in the design of these models, including the fundamental elements used for the construction of different types of computational models, the identification of original anatomical data, the simulation tools used for solving various computational problems, and the applications of computational animal models in preclinical research. The authors also analyze the characteristics of categories of computational models (stylized, voxel-based, and boundary representation) and discuss the technical challenges faced at the present time as well as research needs in the future.

4D x-ray phase contrast tomography for repeatable motion of biological samples

X-ray phase contrast tomography based on a grating interferometer was applied to fast and dynamic measurements of biological samples. To achieve this, the scanning procedure in the tomographic scan was improved. A triangle-shaped voltage signal from a waveform generator to a Piezo stage was used for the fast phase stepping in the grating interferometer. In addition, an optical fiber coupled x-ray scientific CMOS camera was used to achieve fast and highly efficient image acquisitions. These optimizations made it possible to perform an x-ray phase contrast tomographic measurement within an 8 min scan with density resolution of 2.4 mg/cm³. A maximum volume size of 13 × 13 × 6 mm³ was obtained with a single tomographic measurement with a voxel size of 6.5 μm. The scanning procedure using the triangle wave was applied to four-dimensional measurements in which highly sensitive three-dimensional x-ray imaging and a time-resolved dynamic measurement of biological samples were combined. A fresh tendon in the tail of a rat was measured under a uniaxial stretching and releasing condition. To maintain the freshness of the sample during four-dimensional phase contrast tomography, the temperature of the bathing liquid of the sample was kept below 10° using a simple cooling system. The time-resolved deformation of the tendon and each fascicle was measured with a temporal resolution of 5.7 Hz. Evaluations of cross-sectional area size, length of the axis, and mass density in the fascicle during a stretching process provided a basis for quantitative analysis of the deformation of tendon fascicle.

Application of advanced X-ray methods in life sciences

BACKGROUND

Synchrotron radiation (SR) sources provide diverse X-ray methods for the investigation of structure-function relationships in biological macromolecules.

SCOPE OF REVIEW

Recent developments in SR sources and in the X-ray tools they offer for life sciences are reviewed. Specifically, advances in macromolecular crystallography, small angle X-ray solution scattering, X-ray absorption and fluorescence spectroscopy, and imaging are discussed with examples.

MAJOR CONCLUSIONS

SR sources offer a range of X-ray techniques that can be used in a complementary fashion in studies of biological systems at a wide range of resolutions from atomic to cellular scale. Emerging applications of X-ray techniques include the characterization of disordered proteins, noncrystalline and nonequilibrium systems, elemental imaging of tissues, cells and organs, and detection of time-resolved changes in molecular structures.

GENERAL SIGNIFICANCE

X-ray techniques are in the center of hybrid approaches that are used to gain insight into complex problems relating to biomolecular mechanisms, disease and possible therapeutic solutions. This article is part of a Special Issue entitled "Science for Life". Guest Editors: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Left ventricular function measurements in a mouse myocardial infarction model. Comparison between 3D-echocardiography and ECG-gated SPECT

AIM

To assess the accuracy of ECG-gated micro (µ)-SPECT in a mouse myocardial infarction (MI) model in comparison to 3D-echocardiography.

ANIMALS, METHODS

In a mouse (Swiss mice) MI model we compared the accuracy of technetium-99m sestamibi (99mTc-sestamibi) myocardial perfusion, electrocardiogram (ECG) gated µSPECT to 3D-echocardiography in determining left ventricular function. 3D-echocardiography and myocardial perfusion ECG-gated µSPECT data were acquired in the same animal at baseline (n = 11) and 7 (n = 8) and 35 (n = 9) days post ligation of the left anterior descending coronary artery (LAD). Sham operated mice were used as a control (8, 6 and 7 mice respectively). Additionally, after day 35 µSPECT scans, hearts were harvested and 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) staining and autoradiography was performed to determine infarct size.

RESULTS

In both infarcted and sham-operated mice we consistently found comparable values for the end-diastolic volume (EDV), end-systolic volume (ESV) and ejection fraction (EF) obtained by 3D-echocardiography and ECG-gated µSPECT. Excellent correlations between measurements from 3D-echocardiography and ECG-gated µSPECT were found for EDV, ESV and EF (r = 0.9532, r = 0.9693 respectively and r = 0.9581) in infarcted mice. Furthermore, comparable infarct size values were found at day 35 post MI by TTC staining and autoradiography (27.71 ± 1.80% and 29.20 ± 1.18% with p = 0.43).

CONCLUSION

We have demonstrated that ECG-gated µSPECT imaging provides reliable left ventricular function measurements in a mouse MI model. Obtained results were comparable to the highly accurate 3D-echocardiography. This, in addition to the opportunity to simultaneously image multiple biological processes during a single acquisition makes µSPECT imaging a serious option for studying cardiovascular disease in small animals.

Comparison of small animal CT contrast agents

Non-invasive in vivo small animal computed tomography (CT) imaging provides high resolution bone scans but cannot differentiate between soft tissues. For most applications injections of contrast agents (CAs) are necessary. Aim of this study was to uncover the advantages and disadvantages of commercially available CT CAs (ExiTron nano 12 000 and 6000, eXIA 160 and 160XL, Fenestra VC and LC) regarding their pharmacokinetics, toxicological side-effects and the influence of anesthesia on the biodistribution, based on an injection volume of 100 μL/25 g body weight. The pharmacokinetics of the CAs were determined for up to five days. The CA-induced toxicological/physiological side-effects were evaluated by determining blood counts, liver enzymes, thyroxine and total protein values, pro-inflammatory mediators (messenger ribonucleic acid (mRNA)), histology and immunohistochemistry. ExiTron nano 12 000 and 6000 yielded a long-term contrast enhancement (CE) in the liver and spleen for up to five days. Some of the evaluated CAs did not show any CE at all. Anesthesia did not impair the CAs' biodistribution. The CAs differentially affected the body weight, blood counts, liver enzymes, thyroxine and total protein values. ExiTron nano 12 000 and 6000 induced histiocytes in the liver and spleen. Moreover, ExiTron nano 12 000 and eXIA 160 enhanced tumor necrosis factor (TNF) mRNA expression levels in the kidneys. Thus, we recommend ExiTron nano 12 000 and 6000 when multiple injections should be avoided. We recommend careful selection of the employed CA in order to achieve an acceptable CE in the organs of interest and to avoid influences on the animal physiology. Copyright © 2016 John Wiley & Sons, Ltd.

Cardiorespiratory motion-compensated micro-CT image reconstruction using an artifact model-based motion estimation

PURPOSE

Cardiac in vivo micro-CT imaging of small animals typically requires double gating due to long scan times and high respiratory rates. The simultaneous respiratory and cardiac gating can either be done prospectively or retrospectively. In any case, for true 5D imaging, i.e., three spatial dimensions plus one respiratory-temporal dimension plus one cardiac temporal dimension, the amount of information corresponding to a given respiratory and cardiac phase is orders of magnitude lower than the total amount of information acquired. Achieving similar image quality for 5D than for usual 3D investigations would require increasing the amount of data and thus the applied dose to the animal. Therefore, the goal is phase-correlated imaging with high image quality but without increasing the dose level.

METHODS

To achieve this, the authors propose a new image reconstruction algorithm that makes use of all available projection data, also of that corresponding to other motion windows. In particular, the authors apply a motion-compensated image reconstruction approach that sequentially compensates for respiratory and cardiac motion to decrease the impact of sparsification. In that process, all projection data are used no matter which motion phase they were acquired in. Respiratory and cardiac motion are compensated for by using motion vector fields. These motion vector fields are estimated from initial phase-correlated reconstructions based on a deformable registration approach. To decrease the sensitivity of the registration to sparse-view artifacts, an artifact model-based approach is used including a cyclic consistent nonrigid registration algorithm.

RESULTS

The preliminary results indicate that the authors' approach removes the sparse-view artifacts of conventional phase-correlated reconstructions while maintaining temporal resolution. In addition, it achieves noise levels and spatial resolution comparable to that of nongated reconstructions due to the improved dose usage. By using the proposed motion estimation, no sensitivity to streaking artifacts has been observed.

CONCLUSIONS

Using sequential double gating combined with artifact model-based motion estimation allows to accurately estimate respiratory and cardiac motion from highly undersampled data. No sensitivity to streaking artifacts introduced by sparse angular sampling has been observed for the investigated dose levels. The motion-compensated image reconstruction was able to correct for both, respiratory and cardiac motion, by applying the estimated motion vector fields. The administered dose per animal can thus be reduced for 5D imaging allowing for longitudinal studies at the highest image quality.

In vivo small animal micro-CT using nanoparticle contrast agents

Computed tomography (CT) is one of the most valuable modalities for in vivo imaging because it is fast, high-resolution, cost-effective, and non-invasive. Moreover, CT is heavily used not only in the clinic (for both diagnostics and treatment planning) but also in preclinical research as micro-CT. Although CT is inherently effective for lung and bone imaging, soft tissue imaging requires the use of contrast agents. For small animal micro-CT, nanoparticle contrast agents are used in order to avoid rapid renal clearance. A variety of nanoparticles have been used for micro-CT imaging, but the majority of research has focused on the use of iodine-containing nanoparticles and gold nanoparticles. Both nanoparticle types can act as highly effective blood pool contrast agents or can be targeted using a wide variety of targeting mechanisms. CT imaging can be further enhanced by adding spectral capabilities to separate multiple co-injected nanoparticles in vivo. Spectral CT, using both energy-integrating and energy-resolving detectors, has been used with multiple contrast agents to enable functional and molecular imaging. This review focuses on new developments for in vivo small animal micro-CT using novel nanoparticle probes applied in preclinical research.

Assessment of dedicated low-dose cardiac micro-CT reconstruction algorithms using the left ventricular volume of small rodents as a performance measure

PURPOSE

Phase-correlated microcomputed tomography (micro-CT) imaging plays an important role in the assessment of mouse models of cardiovascular diseases and the determination of functional parameters as the left ventricular volume. As the current gold standard, the phase-correlated Feldkamp reconstruction (PCF), shows poor performance in case of low dose scans, more sophisticated reconstruction algorithms have been proposed to enable low-dose imaging. In this study, the authors focus on the McKinnon-Bates (MKB) algorithm, the low dose phase-correlated (LDPC) reconstruction, and the high-dimensional total variation minimization reconstruction (HDTV) and investigate their potential to accurately determine the left ventricular volume at different dose levels from 50 to 500 mGy. The results were verified in phantom studies of a five-dimensional (5D) mathematical mouse phantom.

METHODS

Micro-CT data of eight mice, each administered with an x-ray dose of 500 mGy, were acquired, retrospectively gated for cardiac and respiratory motion and reconstructed using PCF, MKB, LDPC, and HDTV. Dose levels down to 50 mGy were simulated by using only a fraction of the projections. Contrast-to-noise ratio (CNR) was evaluated as a measure of image quality. Left ventricular volume was determined using different segmentation algorithms (Otsu, level sets, region growing). Forward projections of the 5D mouse phantom were performed to simulate a micro-CT scan. The simulated data were processed the same way as the real mouse data sets.

RESULTS

Compared to the conventional PCF reconstruction, the MKB, LDPC, and HDTV algorithm yield images of increased quality in terms of CNR. While the MKB reconstruction only provides small improvements, a significant increase of the CNR is observed in LDPC and HDTV reconstructions. The phantom studies demonstrate that left ventricular volumes can be determined accurately at 500 mGy. For lower dose levels which were simulated for real mouse data sets, the HDTV algorithm shows the best performance. At 50 mGy, the deviation from the reference obtained at 500 mGy were less than 4%. Also the LDPC algorithm provides reasonable results with deviation less than 10% at 50 mGy while PCF and MKB reconstruction show larger deviations even at higher dose levels.

CONCLUSIONS

LDPC and HDTV increase CNR and allow for quantitative evaluations even at dose levels as low as 50 mGy. The left ventricular volumes exemplarily illustrate that cardiac parameters can be accurately estimated at lowest dose levels if sophisticated algorithms are used. This allows to reduce dose by a factor of 10 compared to today's gold standard and opens new options for longitudinal studies of the heart.

Comparison of 4D-microSPECT and microCT for murine cardiac function

PURPOSE

The objective of this study was to compare a new generation of four-dimensional micro-single photon emission computed tomography (microSPECT) with microCT for the quantitative in vivo assessment of murine cardiac function.

PROCEDURES

Four-dimensional isotropic cardiac images were acquired from anesthetized normal C57BL/6 mice with either microSPECT (n = 6) or microCT (n = 6). One additional mouse with myocardial infarction (MI) was scanned with both modalities. Prior to imaging, mice were injected with either technetium tetrofosmin for microSPECT or a liposomal blood pool contrast agent for microCT. Segmentation of the left ventricle (LV) was performed using Vitrea (Vital Images) software, to derive global and regional function.

RESULTS

Measures of global LV function between microSPECT and microCT groups were comparable (e.g., ejection fraction = 71 ± 6 % microSPECT and 68 ± 4 % microCT). Regional functional indices (wall motion, wall thickening, regional ejection fraction) were also similar for the two modalities. In the mouse with MI, microSPECT identified a large perfusion defect that was not evident with microCT.

CONCLUSIONS

Despite lower spatial resolution, microSPECT was comparable to microCT in the quantitative evaluation of cardiac function. MicroSPECT offers an advantage over microCT in the ability to evaluate simultaneously myocardial radiotracer distribution and function, simultaneously. MicroSPECT should be considered as an alternative to microCT and magnetic resonance for preclinical cardiac imaging in the mouse.

Anatomical and functional imaging of myocardial infarction in mice using micro-CT and eXIA 160 contrast agent

Noninvasive small animal imaging techniques are essential for evaluation of cardiac disease and potential therapeutics. A novel preclinical iodinated contrast agent called eXIA 160 has recently been developed, which has been evaluated for micro-CT cardiac imaging. eXIA 160 creates strong contrast between blood and tissue immediately after its injection and is subsequently taken up by the myocardium and other metabolically active tissues over time. We focus on these properties of eXIA and show its use in imaging myocardial infarction in mice. Five C57BL/6 mice were imaged ~2 weeks after left anterior descending coronary artery ligation. Six C57BL/6 mice were used as controls. Immediately after injection of eXIA 160, an enhancement difference between blood and myocardium of ~340 HU enabled cardiac function estimation via 4D micro-CT scanning with retrospective gating. Four hours post-injection, the healthy perfused myocardium had a contrast difference of ~140 HU relative to blood while the infarcted myocardium showed no enhancement. These differences allowed quantification of infarct size via dual-energy micro-CT. In vivo micro-SPECT imaging and ex vivo triphenyl tetrazolium chloride (TTC) staining provided validation for the micro-CT findings. Root mean squared error of infarct measurements was 2.7% between micro-CT and SPECT, and 4.7% between micro-CT and TTC. Thus, micro-CT with eXIA 160 can be used to provide both morphological and functional data for preclinical studies evaluating myocardial infarction and potential therapies. Further studies are warranted to study the potential use of eXIA 160 as a CT molecular imaging tool for other metabolically active tissues in the mouse.

Micro-CT of rodents: state-of-the-art and future perspectives

Micron-scale computed tomography (micro-CT) is an essential tool for phenotyping and for elucidating diseases and their therapies. This work is focused on preclinical micro-CT imaging, reviewing relevant principles, technologies, and applications. Commonly, micro-CT provides high-resolution anatomic information, either on its own or in conjunction with lower-resolution functional imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT). More recently, however, advanced applications of micro-CT produce functional information by translating clinical applications to model systems (e.g., measuring cardiac functional metrics) and by pioneering new ones (e.g. measuring tumor vascular permeability with nanoparticle contrast agents). The primary limitations of micro-CT imaging are the associated radiation dose and relatively poor soft tissue contrast. We review several image reconstruction strategies based on iterative, statistical, and gradient sparsity regularization, demonstrating that high image quality is achievable with low radiation dose given ever more powerful computational resources. We also review two contrast mechanisms under intense development. The first is spectral contrast for quantitative material discrimination in combination with passive or actively targeted nanoparticle contrast agents. The second is phase contrast which measures refraction in biological tissues for improved contrast and potentially reduced radiation dose relative to standard absorption imaging. These technological advancements promise to develop micro-CT into a commonplace, functional and even molecular imaging modality.

Three-dimensional microvasculature in rat and human hearts using a non-injection Ca2+-ATPase method on thick and ultra-thick sections

Currently there are no methods available for staining rat and human myocardial microvasculature on thick sections that would allow for specific staining and differentiation of arterioles, venules, and capillaries. A non-injection technique is described that allows for labeling of the microvascular bed (MVB) in formalin-fixed pieces of the myocardium from humans and the white rat Rattus norvegicus, as well as human full-mount pericardium. Vessel staining is based on the activity of phosphatases (ATPases) and the precipitation of the released phosphate with calcium ions at high pH (pH 10.5-11.5). The resulting precipitate subsequently is converted to black or brown lead sulfide. The specificity of this reaction to vessels of the MVB allows arterioles, venules, capillaries, and pre- and postcapillaries to be clearly visualized in thick (60-100 µm) and ultra-thick (300-500 µm) sections against an unstained background of muscle and connective tissue. In addition, smooth muscle cells of arterioles are also stained allowing for differentiation between arteriolar and venular beds. These observations have not been reported in rat or human myocardium using other methods. This procedure should benefit studies of coronary microcirculation in experimental and pathological conditions, as well as in pharmacological investigations.

In vivo time-resolved microtomography reveals the mechanics of the blowfly flight motor

Time-resolved X-ray microtomography permits a real-time view of the blowfly in flight at a previously unprecedented level of detail, revealing how the tiny steering muscles work.

Dipteran flies are amongst the smallest and most agile of flying animals. Their wings are driven indirectly by large power muscles, which cause cyclical deformations of the thorax that are amplified through the intricate wing hinge. Asymmetric flight manoeuvres are controlled by 13 pairs of steering muscles acting directly on the wing articulations. Collectively the steering muscles account for <3% of total flight muscle mass, raising the question of how they can modulate the vastly greater output of the power muscles during manoeuvres. Here we present the results of a synchrotron-based study performing micrometre-resolution, time-resolved microtomography on the 145 Hz wingbeat of blowflies. These data represent the first four-dimensional visualizations of an organism's internal movements on sub-millisecond and micrometre scales. This technique allows us to visualize and measure the three-dimensional movements of five of the largest steering muscles, and to place these in the context of the deforming thoracic mechanism that the muscles actuate. Our visualizations show that the steering muscles operate through a diverse range of nonlinear mechanisms, revealing several unexpected features that could not have been identified using any other technique. The tendons of some steering muscles buckle on every wingbeat to accommodate high amplitude movements of the wing hinge. Other steering muscles absorb kinetic energy from an oscillating control linkage, which rotates at low wingbeat amplitude but translates at high wingbeat amplitude. Kinetic energy is distributed differently in these two modes of oscillation, which may play a role in asymmetric power management during flight control. Structural flexibility is known to be important to the aerodynamic efficiency of insect wings, and to the function of their indirect power muscles. We show that it is integral also to the operation of the steering muscles, and so to the functional flexibility of the insect flight motor.

A blowfly's wingbeat is 50 times shorter than a blink of a human eye, and is controlled by numerous tiny steering muscles—some of which are as thin as a human hair. To visualize the movements of these muscles and the deformations of the surrounding exoskeleton, we developed a technique to allow us to look inside the insects during tethered flight. We used a particle accelerator to record high-speed X-ray images of the flying blowflies, which we used to reconstruct three-dimensional tomograms of their flight motor at ten different stages of the wingbeat. We measured the asymmetric movements of the steering muscles associated with turning flight, together with the accompanying movements of the wing hinge—arguably the most complex joint in nature. The steering muscles represent <3% of total flight muscle mass, so a key question has been how they can modulate the output of the much larger power muscles. We show that by shifting the flight motor between different modes of oscillation, the fly is able to divert mechanical energy into a steering muscle that is specialized to absorb mechanical energy. In general, we find that deformations of the muscles and thorax are key to understanding this remarkable mechanism.

A comparative analysis of water-soluble and blood-pool contrast agents for in vivo vascular imaging with micro-CT

RATIONALE AND OBJECTIVES

In recent years, micro-computed tomography (micro-CT) has emerged as a high-resolution modality for vascular exploration in vivo. Several x-ray contrast agents for in vivo imaging are on the market and are based on different formulations. The objective of this study was to compare contrast-related and pharmacokinetic properties of a water-soluble compound containing iomeprol (Iomeron 400) and blood-pool agents (eXIA160XL, AuroVist 15 nm, and ExiTron nano 12000) for the identification of suitable in vivo vascular imaging applications.

MATERIALS AND METHODS

Forty-four healthy C57BL/6J mice were used in this study. Iomeprol was administered with a continuous infusion protocol; the other agents as a bolus. Anatomical micro-CT was applied at the head, neck, and lower hind limb before (baseline) and immediately after contrast injection, and used to quantify contrast-related properties of the agents. Dynamic micro-CT was applied at the same regions to characterize the agents pharmacokinetics.

RESULTS

All contrast media revealed safe, except for eXIA160XL, which caused death in four of eight tested animals and was therefore excluded early from the study. AuroVist 15 nm provided the highest attenuation (2.33/mm) as compared to iomeprol (1.97/mm) and ExiTron nano 12000 (1.58/mm) and a maximum temporal variation of contrast of 20% after 30 minutes, but the appearance of a dark skin staining did not allow multiple injections of the agent. Iomeprol passively diffused across capillary membranes, and after 30 minutes doubled the tissue contrast with respect to its initial levels. ExiTron nano 12000 revealed temporal variations of contrast below 10% and significantly reduced clearance rates after the third consecutive injection.

CONCLUSION

AuroVist 15 nm is best suited for anatomical investigation of the vascular network, while the high extravasation levels of iomeprol can be exploited for perfusion analysis. ExiTron nano 12000 is indicated for use in longitudinal monitoring with repeated injections.

Quantitative analysis of microscopic X-ray computed tomography imaging: Japanese quail embryonic soft tissues with iodine staining

Rapid three-dimensional imaging of embryos to better understand the complex process of morphogenesis has been challenging. Recently introduced iodine staining protocols (I2 KI and alcoholic iodine stains) combined with microscopic X-ray computed tomography allows visualization of soft tissues in diverse small organisms and tissue specimens. I2 KI protocols have been developed specifically for small animals, with a limited number of quantitative studies of soft tissue contrasts. To take full advantage of the low X-ray attenuation of ethanol and retain bound iodine while dehydrating the specimen in ethanol, we developed an ethanol I2 KI protocol. We present comparative microscopic X-ray computed tomography analyses of ethanol I2 KI and I2 KI staining protocols to assess the performance of this new protocol to visualize soft tissue anatomy in late stage Japanese quail embryos using quantitative measurements of soft tissue contrasts and sample shrinkage. Both protocols had only 5% shrinkage compared with the original harvested specimen, supporting the use of whole mounts to minimize tissue shrinkage effects. Discrimination within and among the selected organs with each staining protocol and microscopic X-ray computed tomography imaging were comparable to those of a gray scale histological section. Tissue discrimination was assessed using calibrated computed tomography values and a new discrimination index to quantify the degree of computed tomography value overlaps between selected soft tissue regions. Tissue contrasts were dependent on the depth of the tissue within the embryos before the embryos were saturated with each stain solution, and optimal stain saturations for the entire embryo were achieved at 14 and 28 days staining for I2 KI and ethanol I2 KI, respectively. Ethanol I2 KI provided superior soft tissue contrasts by reducing overstaining of fluid-filled spaces and differentially modulating staining of some tissues, such as bronchial and esophageal walls and spinal cord. Delineating the selected soft tissues using optimal threshold ranges derived from the quantitative analyses of the contrast enhancement in optimally stained embryos is possible. The protocols presented here are expected to be applicable to other organisms with modifications to staining time and contribute toward rapid and more efficient segmentation of soft tissues for three-dimensional visualization.

Least-error projection sorting to optimize retrospectively gated cardiac micro-CT of free-breathing mice

PURPOSE

To develop and characterize a technique for optimizing image quality by eliminating streaking artifacts in retrospectively gated microcomputed tomography (micro-CT) images of mice caused by insufficient and irregular angular sampling.

METHODS

A least-error sorting technique was developed to minimize streak artifacts in retrospectively gated cardiac micro-CT images. To ensure complete filling of projection space, for each angular position, the projection acquired closest to the desired cardiac phase is used to reconstruct a volumetric image. An acrylic slanted-edge phantom undergoing cyclic motion was used to characterize the system's resolution. The phantom was scanned using a volumetric micro-CT scanner equipped with a flat-panel detector mounted on a slip-ring gantry. Projection images of the moving phantom were collected over a period of 60 s using a variety of acquisition protocols with the rotation period of the gantry ranging from 1 to 5 s. The modulation transfer function (MTF) of the reconstructed images was measured for many combinations of acquisition and reconstruction parameters. The use of the least-error technique was also demonstrated in vivo.

RESULTS

The motion blurring introduced into the images at physiologically significant velocities of 6 cm∕s agreed well with predicted values; limiting resolution (frequency at 10% MTF) degraded from 2.5 to 1.0 mm(-1) for a velocity of 6 cm∕s and 5 s∕rotation gantry speed. Faster gantry rotation speeds led to improved temporal resolution but the scanner's data storage and transfer rates and field of view limitations made scanning at gantry speeds faster than 2 s∕rotation impractical.

CONCLUSIONS

The least-error technique effectively eliminates streaking artifact caused by missing views and allows for optimization of image quality in retrospectively gated micro-CT.

Noninvasive technique for monitoring drug transport through the murine cochlea using micro-computed tomography

Local delivery of drugs to the inner ear has the potential to treat inner ear disorders including permanent hearing loss or deafness. Current mathematical models describing the pharmacokinetics of drug delivery to the inner ear have been based on large rodent studies with invasive measurements of concentration at few locations within the cochlea. Hence, estimates of clearance and diffusion parameters are based on fitting measured data with limited spatial resolution to a model. To overcome these limitations, we developed a noninvasive imaging technique to monitor and characterize drug delivery inside the mouse cochlea using micro-computed tomography (μCT). To increase the measurement accuracy, we performed a subject-atlas image registration to exploit the information readily available in the atlas image of the mouse cochlea and pass segmentation or labeling information from the atlas to our μCT scans. The approach presented here has the potential to quantify concentrations at any point along fluid-filled scalae of the inner ear. This may permit determination of spatially dependent diffusion and clearance parameters for enhanced models.

X-ray dose delivered during a longitudinal micro-CT study has no adverse effect on cardiac and pulmonary tissue in C57BL/6 mice

BACKGROUND

Micro-computed tomography (micro-CT) offers numerous advantages for small animal imaging, including the ability to monitor the same animals throughout a longitudinal study. However, concerns are often raised regarding the effects of X-ray dose accumulated over the course of the experiment.

PURPOSE

To scan C57BL/6 mice multiple times per week for 6 weeks, in order to determine the effect of the cumulative dose on pulmonary and cardiac tissue at the end of the study.

MATERIAL AND METHODS

C57BL/6 male mice were split into two groups (irradiated group = 10, control group = 10). The irradiated group was scanned (80 kVp/50 mA) three times weekly for 6 weeks, resulting in a weekly dose of 0.84 Gy, and a total study dose of 5.04 Gy. The control group was scanned on the final week. Scans from week 6 were reconstructed and the lungs and heart were analyzed.

RESULTS

Overall, there was no significant difference in lung volume or lung density or in left ventricular volume or ejection fraction between the control group and the irradiated group. Histological samples taken from excised lung and myocardial tissue also showed no evidence of inflammation or fibrosis in the irradiated group.

CONCLUSION

This study demonstrated that a 5 Gy X-ray dose accumulated over 6 weeks during a longitudinal micro-CT study had no significant effects on the pulmonary and myocardial tissue of C57BL/6 mice. As a result, the many advantages of micro-CT imaging, including rapid acquisition of high-resolution, isotropic images in free-breathing mice, can be taken advantage of in longitudinal studies without concern for negative dose-related effects.

The utility of micro-CT and MRI in the assessment of longitudinal growth of liver metastases in a preclinical model of colon carcinoma

RATIONALE AND OBJECTIVES

Liver is a common site for distal metastases in colon and rectal cancer. Numerous clinical studies have analyzed the relative merits of different imaging modalities for detection of liver metastases. Several exciting new therapies are being investigated in preclinical models. But, technical challenges in preclinical imaging make it difficult to translate conclusions from clinical studies to the preclinical environment. This study addresses the technical challenges of preclinical magnetic resonance imaging (MRI) and micro-computed tomography (CT) to enable comparison of state-of-the-art methods for following metastatic liver disease.

MATERIALS AND METHODS

We optimized two promising preclinical protocols to enable a parallel longitudinal study tracking metastatic human colon carcinoma growth in a mouse model: T2-weighted MRI using two-shot PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) and contrast-enhanced micro-CT using a liposomal contrast agent. Both methods were tailored for high throughput with attention to animal support and anesthesia to limit biological stress.

RESULTS AND CONCLUSIONS

Each modality has its strengths. Micro-CT permitted more rapid acquisition (<10 minutes) with the highest spatial resolution (88-micron isotropic resolution). But detection of metastatic lesions requires the use of a blood pool contrast agent, which could introduce a confound in the evaluation of new therapies. MRI was slower (30 minutes) and had lower anisotropic spatial resolution. But MRI eliminates the need for a contrast agent and the contrast-to-noise between tumor and normal parenchyma was higher, making earlier detection of small lesions possible. Both methods supported a relatively high-throughput, longitudinal study of the development of metastatic lesions.

Imaging of cardiac perfusion of free-breathing small animals using dynamic phase-correlated micro-CT

PURPOSE

Mouse models of cardiac diseases have proven to be a valuable tool in preclinical research. The high cardiac and respiratory rates of free breathing mice prohibit conventional in vivo cardiac perfusion studies using computed tomography even if gating methods are applied. This makes a sacrification of the animals unavoidable and only allows for the application of ex vivo methods.

METHODS

To overcome this issue the authors propose a low dose scan protocol and an associated reconstruction algorithm that allows for in vivo imaging of cardiac perfusion and associated processes that are retrospectively synchronized to the respiratory and cardiac motion of the animal. The scan protocol consists of repetitive injections of contrast media within several consecutive scans while the ECG, respiratory motion, and timestamp of contrast injection are recorded and synchronized to the acquired projections. The iterative reconstruction algorithm employs a six-dimensional edge-preserving filter to provide low-noise, motion artifact-free images of the animal examined using the authors' low dose scan protocol.

RESULTS

The reconstructions obtained show that the complete temporal bolus evolution can be visualized and quantified in any desired combination of cardiac and respiratory phase including reperfusion phases. The proposed reconstruction method thereby keeps the administered radiation dose at a minimum and thus reduces metabolic inference to the animal allowing for longitudinal studies.

CONCLUSIONS

The authors' low dose scan protocol and phase-correlated dynamic reconstruction algorithm allow for an easy and effective way to visualize phase-correlated perfusion processes in routine laboratory studies using free-breathing mice.

Registration-based segmentation of murine 4D cardiac micro-CT data using symmetric normalization

Micro-CT can play an important role in preclinical studies of cardiovascular disease because of its high spatial and temporal resolution. Quantitative analysis of 4D cardiac images requires segmentation of the cardiac chambers at each time point, an extremely time consuming process if done manually. To improve throughput this study proposes a pipeline for registration-based segmentation and functional analysis of 4D cardiac micro-CT data in the mouse. Following optimization and validation using simulations, the pipeline was applied to in vivo cardiac micro-CT data corresponding to ten cardiac phases acquired in C57BL/6 mice (n = 5). After edge-preserving smoothing with a novel adaptation of 4D bilateral filtration, one phase within each cardiac sequence was manually segmented. Deformable registration was used to propagate these labels to all other cardiac phases for segmentation. The volumes of each cardiac chamber were calculated and used to derive stroke volume, ejection fraction, cardiac output, and cardiac index. Dice coefficients and volume accuracies were used to compare manual segmentations of two additional phases with their corresponding propagated labels. Both measures were, on average, >0.90 for the left ventricle and >0.80 for the myocardium, the right ventricle, and the right atrium, consistent with trends in inter- and intra-segmenter variability. Segmentation of the left atrium was less reliable. On average, the functional metrics of interest were underestimated by 6.76% or more due to systematic label propagation errors around atrioventricular valves; however, execution of the pipeline was 80% faster than performing analogous manual segmentation of each phase.

Temporal and spectral imaging with micro-CT

PURPOSE

Micro-CT is widely used for small animal imaging in preclinical studies of cardiopulmonary disease, but further development is needed to improve spatial resolution, temporal resolution, and material contrast. We present a technique for visualizing the changing distribution of iodine in the cardiac cycle with dual source micro-CT.

METHODS

The approach entails a retrospectively gated dual energy scan with optimized filters and voltages, and a series of computational operations to reconstruct the data. Projection interpolation and five-dimensional bilateral filtration (three spatial dimensions + time + energy) are used to reduce noise and artifacts associated with retrospective gating. We reconstruct separate volumes corresponding to different cardiac phases and apply a linear transformation to decompose these volumes into components representing concentrations of water and iodine. Since the resulting material images are still compromised by noise, we improve their quality in an iterative process that minimizes the discrepancy between the original acquired projections and the projections predicted by the reconstructed volumes. The values in the voxels of each of the reconstructed volumes represent the coefficients of linear combinations of basis functions over time and energy. We have implemented the reconstruction algorithm on a graphics processing unit (GPU) with CUDA. We tested the utility of the technique in simulations and applied the technique in an in vivo scan of a C57BL∕6 mouse injected with blood pool contrast agent at a dose of 0.01 ml∕g body weight. Postreconstruction, at each cardiac phase in the iodine images, we segmented the left ventricle and computed its volume. Using the maximum and minimum volumes in the left ventricle, we calculated the stroke volume, the ejection fraction, and the cardiac output.

RESULTS

Our proposed method produces five-dimensional volumetric images that distinguish different materials at different points in time, and can be used to segment regions containing iodinated blood and compute measures of cardiac function.

CONCLUSIONS

We believe this combined spectral and temporal imaging technique will be useful for future studies of cardiopulmonary disease in small animals.

Evaluation of eXIA 160 cardiac-related enhancement in C57BL/6 and BALB/c mice using micro-CT

Evaluation of cardiovascular function in mice using micro-CT requires that a contrast agent be administered to differentiate the blood from the myocardium. eXIA 160, an aqueous colloidal poly-disperse contrast agent with a high iodine concentration (160 mg I ml(-1)), creates strong contrast between blood and tissue with a low injection volume. In this study, the blood-pool enhancement time-course of eXIA 160 is monitored over a 48 h period to determine its optimal use during cardiac function studies in C57BL/6 and BALB/c mice. Eight-second scans were performed (80 kV(p), 110 mA) using the GE Locus Ultra micro-CT scanner. Six C57BL/6 and six BALB/c male mice (22-24 g) were injected via tail vein with 5 µl g(-1) body weight eXIA 160. A precontrast scan was performed; following injection, mice were scanned at 5, 15, 30, 45 and 60 min, and 2, 4, 8, 12, 24 and 48 h. Images were reconstructed, and enhancement-time curves were generated for each of the following tissues: left ventricle (LV), myocardium, liver, spleen, renal cortex, bladder and brown adipose tissue. The highest contrast in the LV occurred at 5 min in both strains (~670 HU above precontrast value). Uptake of the contrast agent by the myocardium was also observed: myocardial tissue showed increasing enhancement over a 4 h period in both strains, remaining even once the contrast was eliminated from the vasculature. In both C57BL/6 and BALB/c strains, eXIA 160 provided high contrast between blood and myocardial tissue for a period of 30 min following injection. Notably, this contrast agent was also taken up by the myocardium and provided continued enhancement when it was eliminated from the blood, making LV wall motion studies possible. In conclusion, eXIA 160, with its high iodine concentration and targeted tissue uptake characteristics, is an ideal agent to use when evaluating cardiovascular function in mice.

X-ray phase contrast for CO2 microangiography

We demonstrate a laboratory method for imaging small blood vessels using x-ray propagation-based phase-contrast imaging and carbon dioxide (CO(2)) gas as a contrast agent. The limited radiation dose in combination with CO(2) being clinically acceptable makes the method promising for small-diameter vascular visualization. We investigate the possibilities and limitations of the method for small-animal angiography and compare it with conventional absorption-based x-ray angiography. Photon noise in absorption-contrast imaging prevents visualization of blood vessels narrower than 50 µm at the highest radiation doses compatible with living animals, whereas our simulations and experiments indicate the possibility of visualizing 20 µm vessels at radiation doses as low as 100 mGy. Experimental computed tomography of excised rat kidney shows blood vessels of diameters down to 60 µm with improved image quality compared to absorption-based methods. With our present prototype x-ray source, the acquisition time for a tomographic dataset is approximately 1 h, which is long compared to the 1-20 min common for absorption-contrast micro-CT systems. Further development of the liquid-metal-jet microfocus x-ray sources used here and high-resolution x-ray detectors shows promise to reduce exposure times and make this high-resolution method practical for imaging of living animals.