Assessment of morphometry of pulmonary acini in mouse lungs by nondestructive imaging using multiscale microcomputed tomography

Elossaily G, Ansari MN, Ali N. An Insight on Microfluidic Organ-on-a-Chip Models for PM2.5-Induced Pulmonary Complications

Pulmonary diseases like asthma, chronic obstructive pulmonary disorder, lung fibrosis, and lung cancer pose a significant burden to global human health. Many of these complications arise as a result of exposure to particulate matter (PM), which has been examined in several preclinical and clinical trials for its effect on several respiratory diseases. Particulate matter of size less than 2.5 μm (PM2.5) has been known to inflict unforeseen repercussions, although data from epidemiological studies to back this are pending. Conventionally utilized two-dimensional (2D) cell culture and preclinical animal models have provided insufficient benefits in emulating the in vivo physiological and pathological pulmonary conditions. Three-dimensional (3D) structural models, including organ-on-a-chip models, have experienced a developmental upsurge in recent times. Lung-on-a-chip models have the potential to simulate the specific features of the lungs. With the advancement of technology, an emerging and advanced technique termed microfluidic organ-on-a-chip has been developed with the aim of identifying the complexity of the respiratory cellular microenvironment of the body. In the present Review, the role of lung-on-a-chip modeling in reproducing pulmonary complications has been explored, with a specific emphasis on PM2.5-induced pulmonary complications.

The pathogenesis of influenza in intact alveoli: virion endocytosis and its effects on the lung's air-blood barrier

Lung infection by influenza A virus (IAV) is a major cause of global mortality from lung injury, a disease defined by widespread dysfunction of the lung's air-blood barrier. Endocytosis of IAV virions by the alveolar epithelium - the cells that determine barrier function - is central to barrier loss mechanisms. Here, we address the current understanding of the mechanistic steps that lead to endocytosis in the alveolar epithelium, with an eye to how the unique structure of lung alveoli shapes endocytic mechanisms. We highlight where future studies of alveolar interactions with IAV virions may lead to new therapeutic approaches for IAV-induced lung injury.

Development of a unilateral porcine emphysema model induced by porcine pancreatic elastase

Emphysema is one of the pathological hallmarks of chronic obstructive pulmonary disease. We have recently reported that radiofrequency therapy improves lung function in rodent models of emphysema. However, preclinical data using large animals is necessary for clinical translation. Here, we describe the work performed to establish a unilateral porcine emphysema model. Different doses of porcine pancreatic elastase (PPE) were instilled into the left lung of 10 Yucatan pigs. Three additional pigs were used as controls. Six weeks after instillation, lungs were harvested. Lung compliance was measured by a water displacement method and plethysmography. Systematic uniform random sampling of the left and right lungs was performed independently to measure alveolar surface area using micro-computed tomography (micro-CT) and histology. In pigs instilled with 725-750 U/kg of PPE (PPE group, n = 6), the compliance of the left lung was significantly higher by 37.6% than that of the right lung (P = 0.03) using the water displacement method. With plethysmography, the volume of the left lung was significantly larger than that of the right lung at 3, 5, and 10 cmH2O. Measurements from either micro-CT or histology images showed a significant decrease in alveolar surface area by 14.2% or 14.5% (P = 0.031) in the left lung compared with the right lung of the PPE group. A unilateral model for mild emphysema in Yucatan pigs has been established, which can now be used for evaluating novel therapeutics and interventional strategies.NEW & NOTEWORTHY For clinical translation, preclinical data using large animal models is necessary. However, papers describing an emphysema model in pigs, which are anatomically and physiologically similar to humans, are lacking. Here, we report success in creating a unilateral mild-emphysema model in pigs with only one single dose of porcine pancreatic elastase. This model will be useful in bringing novel technologies and therapies from small animals to humans with emphysema.

Microvascular maturation of the septal capillary layers takes place in parallel to alveolarization in human lungs

Primary and secondary septa formed during lung development contain a double-layered capillary network. To improve gas exchange, the capillary network is remodeled into a single-layered one, a process that is called microvascular maturation (MVM). It takes place during classical and continued alveolarization. Classical alveolarization is defined as a formation of new septa from immature septa and continued alveolarization as a formation from mature septa. Until now, MVM was never quantitatively evaluated in human lungs. To correlate alveolarization and MVM, and to determine the transition point from classical to continued alveolarization, the degree of MVM was stereologically estimated. In 12 human lungs (0.1-15 yr), the alveolar surface area of immature and mature septa was estimated stereologically by intersection counting. An MVM-quotient (RMVM) was defined as the mature alveolar surface area over total alveolar surface area. The MVM-quotient increased logarithmically over age and showed a biphasic increase similar to alveolarization. It did not reach 100% maturity in these samples. A linear correlation between the MVM-quotient and the logarithm of the number of alveoli was observed. We conclude that MVM increased logarithmically and biphasically in parallel to alveolarization until alveolarization ceased. However, at 2-3 yr of age three-quarters of the alveolar microvasculature are mature. This result may explain a previous postulate that MVM is finished at this age. We hypothesize that as long as alveolarization takes place, MVM will take place in parallel. We propose that the transition from classical to continued alveolarization takes place between the ages of 1-3 yr in humans.NEW & NOTEWORTHY Newly formed alveolar septa contain a double-layered capillary network. To optimize gas exchange, the two layers fuse to a single-layered capillary network during microvascular maturation. Because its timing is unknow in humans, microvascular maturation was stereologically estimated throughout postnatal human lung development. It is shown that maturation of the microvascular and alveolar septa takes place in parallel to alveolarization. At an age of 2-3 yr three-quarters of the septa are mature.

Airspace Diameter Map-A Quantitative Measurement of All Pulmonary Airspaces to Characterize Structural Lung Diseases

(1) Background: Stereological estimations significantly contributed to our understanding of lung anatomy and physiology. Taking stereology fully 3-dimensional facilitates the estimation of novel parameters. (2) Methods: We developed a protocol for the analysis of all airspaces of an entire lung. It includes (i) high-resolution synchrotron radiation-based X-ray tomographic microscopy, (ii) image segmentation using the free machine-learning tool Ilastik and ImageJ, and (iii) calculation of the airspace diameter distribution using a diameter map function. To evaluate the new pipeline, lungs from adult mice with cystic fibrosis (CF)-like lung disease (βENaC-transgenic mice) or mice with elastase-induced emphysema were compared to healthy controls. (3) Results: We were able to show the distribution of airspace diameters throughout the entire lung, as well as separately for the conducting airways and the gas exchange area. In the pathobiological context, we observed an irregular widening of parenchymal airspaces in mice with CF-like lung disease and elastase-induced emphysema. Comparable results were obtained when analyzing lungs imaged with μCT, sugges-ting that our pipeline is applicable to different kinds of imaging modalities. (4) Conclusions: We conclude that the airspace diameter map is well suited for a detailed analysis of unevenly distri-buted structural alterations in chronic muco-obstructive lung diseases such as cystic fibrosis and COPD.

Acinar micromechanics in health and lung injury: what we have learned from quantitative morphology

Within the pulmonary acini ventilation and blood perfusion are brought together on a huge surface area separated by a very thin blood-gas barrier of tissue components to allow efficient gas exchange. During ventilation pulmonary acini are cyclically subjected to deformations which become manifest in changes of the dimensions of both alveolar and ductal airspaces as well as the interalveolar septa, composed of a dense capillary network and the delicate tissue layer forming the blood-gas barrier. These ventilation-related changes are referred to as micromechanics. In lung diseases, abnormalities in acinar micromechanics can be linked with injurious stresses and strains acting on the blood-gas barrier. The mechanisms by which interalveolar septa and the blood-gas barrier adapt to an increase in alveolar volume have been suggested to include unfolding, stretching, or changes in shape other than stretching and unfolding. Folding results in the formation of pleats in which alveolar epithelium is not exposed to air and parts of the blood-gas barrier are folded on each other. The opening of a collapsed alveolus (recruitment) can be considered as an extreme variant of septal wall unfolding. Alveolar recruitment can be detected with imaging techniques which achieve light microscopic resolution. Unfolding of pleats and stretching of the blood-gas barrier, however, require electron microscopic resolution to identify the basement membrane. While stretching results in an increase of the area of the basement membrane, unfolding of pleats and shape changes do not. Real time visualization of these processes, however, is currently not possible. In this review we provide an overview of septal wall micromechanics with focus on unfolding/folding as well as stretching. At the same time we provide a state-of-the-art design-based stereology methodology to quantify microarchitecture of alveoli and interalveolar septa based on different imaging techniques and design-based stereology.

Analysis of the alveolar shape in 3-D

Mechanical forces affect the alveolar shape, depending on location and tissue composition, and vary during the respiratory cycle. This study performs alveolar morphomics in different lobes of human lungs using models generated from three-dimensional (3-D) micro-computed tomography (microCT) images. Cylindrical tissue samples (1.6 cm × 2 cm) were extracted from two nontransplantable donor lungs (one ex-smoker and one smoker, 3 samples per subject) that were air-inflated and frozen solid in liquid nitrogen vapor. Samples were scanned with microCT (11 µm/voxel). Within representative cubic regions of interest (5.5 mm edge length), alveoli were segmented to produce corresponding 3-D models from which quantitative data were obtained. The surface of segmented alveoli (n_alv_total = 23,587) was divided into individual planar surfaces (facets) and angles between facet normals were calculated. Moreover, the number of neighboring alveoli was estimated for every alveolus. In this study, we examined intraindividual differences in alveolar morphology, which were reproducible in the lungs of two subjects. The main aspects are higher mean alveolar volumes (v_alv: 6.64 × 10⁶ and 6.63 × 10⁶ µm³ vs. 5.78 × 10⁶ and 6.29 × 10⁶ µm³) and surface sizes (s_alv: 0.19 and 0.18 mm² vs. 0.17 mm² in both lower lobes) in both upper lung lobes compared with the lower lobes. An increasing number of facets (f_alv) from top to bottom (12 and 14 in the upper lobes; 14 and 15 in the lower lobes), as well as a decreasing number of alveolar neighbors (nei_alv: 9 and 8 in the upper lobes; 8 and 7 in the lower lobes) from the upper lobes to the lower lobes were observed. We could observe an increasing ratio of alveolar entrance size to the surface size of the alveoli from top to bottom (S_ratio_alv: 0.71 and 0.64 in the upper lobes, 0.73 and 0.70 in the lower lobes). The angles between facet normals (ang_alv) were larger in the upper lobes (67.72° and 62.44°) of both lungs than in the lower lobes (66.19° and 61.30°). By using this new approach of analyzing alveolar 3-D data, which enables the estimation of facet, neighbor, and shape characteristics, we aimed to establish the baseline measures for in-depth studies of mechanical conditions and morphology.

Application of Machine Learning for Segmentation of the Pulmonary Acinus Imaged by Synchrotron X-Ray Tomography

Background: To assess the effectiveness of inhalation therapy, it is important to evaluate the lungs' structure; thus, visualization of the entire lungs at the level of the alveoli is necessary. To achieve this goal, the applied visualization technique must satisfy the following two conditions simultaneously: (1) it has to obtain images of the entire lungs, since one part of the lungs is influenced by the other parts, and (2) the images have to capture the detailed structure of the alveolus/acinus in which gas exchange occurs. However, current visualization techniques do not fulfill these two conditions simultaneously. Segmentation is a process in which each pixel of the obtained high-resolution images is simplified (i.e., the representation of an image is changed by categorizing and modifying each pixel) so that we can perform three-dimensional volume rendering. One of the bottlenecks of current approaches is that the accuracy of the segmentation of each image has to be evaluated on the outcome of the process (mainly by an expert). It is a formidable task to evaluate the astronomically large numbers of images that would be required to resolve the entire lungs in high resolution. Methods: To overcome this challenge, we propose a new approach based on machine learning (ML) techniques for the validation step. Results: We demonstrate the accuracy of the segmentation process itself by comparison with previously validated images. In this ML approach, to achieve a reasonable accuracy, millions/billions of parameters used for segmentation have to be optimized. This computationally demanding new approach is achievable only due to recent dramatic increases in computation power. Conclusion: The objective of this article is to explain the advantages of ML over the classical approach for acinar imaging.

Quantitative Imaging Metrics for the Assessment of Pulmonary Pathophysiology: An Official American Thoracic Society and Fleischner Society Joint Workshop Report

Multiple thoracic imaging modalities have been developed to link structure to function in the diagnosis and monitoring of lung disease. Volumetric computed tomography (CT) renders three-dimensional maps of lung structures and may be combined with positron emission tomography (PET) to obtain dynamic physiological data. Magnetic resonance imaging (MRI) using ultrashort-echo time (UTE) sequences has improved signal detection from lung parenchyma; contrast agents are used to deduce airway function, ventilation-perfusion-diffusion, and mechanics. Proton MRI can measure regional ventilation-perfusion ratio. Quantitative imaging (QI)-derived endpoints have been developed to identify structure-function phenotypes, including air-blood-tissue volume partition, bronchovascular remodeling, emphysema, fibrosis, and textural patterns indicating architectural alteration. Coregistered landmarks on paired images obtained at different lung volumes are used to infer airway caliber, air trapping, gas and blood transport, compliance, and deformation. This document summarizes fundamental "good practice" stereological principles in QI study design and analysis; evaluates technical capabilities and limitations of common imaging modalities; and assesses major QI endpoints regarding underlying assumptions and limitations, ability to detect and stratify heterogeneous, overlapping pathophysiology, and monitor disease progression and therapeutic response, correlated with and complementary to, functional indices. The goal is to promote unbiased quantification and interpretation of in vivo imaging data, compare metrics obtained using different QI modalities to ensure accurate and reproducible metric derivation, and avoid misrepresentation of inferred physiological processes. The role of imaging-based computational modeling in advancing these goals is emphasized. Fundamental principles outlined herein are critical for all forms of QI irrespective of acquisition modality or disease entity.

Mathematical modeling of pulmonary acinus structure: Verification of acinar shape effects on pathway structure using rat lungs

The pulmonary acinus is the gas exchange unit in the lung and has a very complex microstructure. The structure model is essential to understand the relationship between structural heterogeneity and mechanical phenomena at the acinus level with computational approaches. We propose an acinus structure model represented by a cluster of truncated octahedra in conical, double-conical, inverted conical, or chestnut-like conical confinement to accommodate recent experimental information of rodent acinar shapes. The basis of the model is the combined use of Voronoi and Delaunay tessellations and the optimization of the ductal tree assuming the number of alveoli and the mean path length as quantities related to gas exchange. Before applying the Voronoi tessellation, controlling the seed coordinates enables us to model acinus with arbitrary shapes. Depending on the acinar shape, the distribution of path length varies. The lengths are more widely spread for the cone acinus, with a bias toward higher values, while most of the lengths for the inverted cone acinus primarily take a similar value. Longer pathways have smaller tortuosity and more generations, and duct length per generation is almost constant irrespective of generation, which agrees well with available experimental data. The pathway structure of cone and chestnut-like cone acini is similar to the surface acini's features reported in experiments. According to space-filling requirements in the lung, other conical acini may also be acceptable. The mathematical acinus structure model with various conical shapes can be a platform for computational studies on regional differences in lung functions along the lung surface, underlying respiratory physiology and pathophysiology.

Origins of and lessons from quantitative functional X-ray computed tomography of the lung

Functional CT of the lung has emerged from quantitative CT (qCT). Structural details extracted at multiple lung volumes offer indices of function. Additionally, single volumetric images, if acquired at standardized lung volumes and body posture, can be used to model function by employing such engineering techniques as computational fluid dynamics. With the emergence of multispectral CT imaging including dual energy from energy integrating CT scanners and multienergy binning using the newly released photon counting CT technology, function is tagged via use of contrast agents. Lung disease phenotypes have previously been lumped together by the limitations of spirometry and plethysmography. QCT and its functional embodiment have been imbedded into studies seeking to characterize chronic obstructive pulmonary disease, severe asthma, interstitial lung disease and more. Reductions in radiation dose by an order of magnitude or more have been achieved. At the same time, we have seen significant increases in spatial and density resolution along with methodologic validations of extracted metrics. Together, these have allowed attention to turn towards more mild forms of disease and younger populations. In early applications, clinical CT offered anatomic details of the lung. Functional CT offers regional measures of lung mechanics, the assessment of functional small airways disease, as well as regional ventilation-perfusion matching (V/Q) and more. This paper will focus on the use of quantitative/functional CT for the non-invasive exploration of dynamic three-dimensional functioning of the breathing lung and beating heart within the unique negative pressure intrathoracic environment of the closed chest.

Three-Dimensional Whole-Organ Characterization of the Regional Alveolar Morphology in Normal Murine Lungs

Alveolar architecture plays a fundamental role in the processes of ventilation and perfusion in the lung. Alterations in the alveolar surface area and alveolar cavity volume constitute the pathophysiological basis of chronic respiratory diseases such as pulmonary emphysema. Previous studies based on micro-computed tomography (micro-CT) of lung samples have allowed the geometrical study of acinar units. However, our current knowledge is based on the study of a few tissue samples in random locations of the lung that do not give an account of the spatial distributions of the alveolar architecture in the whole lung. In this work, we combine micro-CT imaging and computational geometry algorithms to study the regional distribution of key morphological parameters throughout the whole lung. To this end, 3D whole-lung images of Sprague–Dawley rats are acquired using high-resolution micro-CT imaging and analyzed to estimate porosity, alveolar surface density, and surface-to-volume ratio. We assess the effect of current gold-standard dehydration methods in the preparation of lung samples and propose a fixation protocol that includes the application of a methanol-PBS solution before dehydration. Our results show that regional porosity, alveolar surface density, and surface-to-volume ratio have a uniform distribution in normal lungs, which do not seem to be affected by gravitational effects. We further show that sample fixation based on ethanol baths for dehydration introduces shrinking and affects the acinar architecture in the subpleural regions. In contrast, preparations based on the proposed dehydration protocol effectively preserve the alveolar morphology.

Pulmonary acini exhibit complex changes during postnatal rat lung development

Pulmonary acini represent the functional gas-exchanging units of the lung. Due to technical limitations, individual acini cannot be identified on microscopic lung sections. To overcome these limitations, we imaged the right lower lobes of instillation-fixed rat lungs from postnatal days P4, P10, P21, and P60 at the TOMCAT beamline of the Swiss Light Source synchrotron facility at a voxel size of 1.48 μm. Individual acini were segmented from the three-dimensional data by closing the airways at the transition from conducting to gas exchanging airways. For a subset of acini (N = 268), we followed the acinar development by stereologically assessing their volume and their number of alveoli. We found that the mean volume of the acini increases 23 times during the observed time-frame. The coefficients of variation dropped from 1.26 to 0.49 and the difference between the mean volumes of the fraction of the 20% smallest to the 20% largest acini decreased from a factor of 27.26 (day 4) to a factor of 4.07 (day 60), i.e. shows a smaller dispersion at later time points. The acinar volumes show a large variation early in lung development and homogenize during maturation of the lung by reducing their size distribution by a factor of 7 until adulthood. The homogenization of the acinar sizes hints at an optimization of the gas-exchange region in the lungs of adult animals and that acini of different size are not evenly distributed in the lungs. This likely leads to more homogeneous ventilation at later stages in lung development.

Sirtuin 3 Ameliorates Lung Senescence and Improves Type II Alveolar Epithelial Cell Function by Enhancing the FoxO3a-Dependent Antioxidant Defense Mechanism

Lung aging alters the intrinsic structure of the lung and pulmonary surfactant system and increases the mortality and morbidity due to respiratory diseases in elderly individuals. We hypothesized that lung aging results from an insufficiency of type II alveolar epithelial cells (AECIIs) in the lung tissue. Sirtuin 3 (SIRT3) is a member of the sirtuin family of proteins that promote longevity in many organisms. Increased SIRT3 expression has been linked to an extended life span in humans. Hence, we speculated that the overexpression of SIRT3 may help to ameliorate lung senescence and improve AECII function. AECIIs were isolated from young and old patients with pneumothorax caused by pulmonary bullae. The expression of SIRT3, manganese superoxide dismutase, and catalase, as well as cell function and senescence indicators of young and old AECIIs, was measured before and after SIRT3 overexpression. After SIRT3 overexpression, the aged state of old AECIIs improved, and antiapoptotic activity, proliferation, and secretion were dramatically enhanced. Surfactant protein C (SPC), which is secreted by AECIIs, reduces alveolar surface tension, repairs the alveolar structure, and regulates inflammation. SPC deficiency in patients is associated with increased inflammation and delayed repair. SIRT3 deacetylated forkhead box O3a, thereby protecting mitochondria from oxidative stress and improving cell function and the senescent state of old AECIIs. These findings provide a possible direction for aging-delaying therapies and interventions for diseases of the respiratory system.

Validating in vivo hyperpolarized 129 Xe diffusion MRI and diffusion morphometry in the mouse lung

PURPOSE

Diffusion and lung morphometry imaging using hyperpolarized gases are promising tools to quantify pulmonary microstructure noninvasively in humans and in animal models. These techniques assume the motion encoded is exclusively diffusive gas displacement, but the impact of cardiac motion on measurements has never been explored. Furthermore, although diffusion morphometry has been validated against histology in humans and mice using 3 He, it has never been validated in mice for 129 Xe. Here, we examine the effect of cardiac motion on diffusion imaging and validate 129 Xe diffusion morphometry in mice.

THEORY AND METHODS

Mice were imaged using gradient-echo-based diffusion imaging, and apparent diffusion-coefficient (ADC) maps were generated with and without cardiac gating. Diffusion-weighted images were fit to a previously developed theoretical model using Bayesian probability theory, producing morphometric parameters that were compared with conventional histology.

RESULTS

Cardiac gating had no significant impact on ADC measurements (dual-gating: ADC = 0.020 cm2 /s, single-gating: ADC = 0.020 cm2 /s; P = .38). Diffusion-morphometry-generated maps of ADC (mean, 0.0165 ± 0.0001 cm2 /s) and acinar dimensions (alveolar sleeve depth [h] = 44 µm, acinar duct radii [R] = 99 µm, mean linear intercept [Lm ] = 74 µm) that agreed well with conventional histology (h = 45 µm, R = 108 µm, Lm = 63 µm).

CONCLUSION

Cardiac motion has negligible impact on 129 Xe ADC measurements in mice, arguing its impact will be similarly minimal in humans, where relative cardiac motion is reduced. Hyperpolarized 129 Xe diffusion morphometry accurately and noninvasively maps the dimensions of lung microstructure, suggesting it can quantify the pulmonary microstructure in mouse models of lung disease.

Combination of µCT and light microscopy for generation-specific stereological analysis of pulmonary arterial branches: a proof-of-concept study

Various lung diseases, including pulmonary hypertension, chronic obstructive pulmonary disease or bronchopulmonary dysplasia, are associated with structural and architectural alterations of the pulmonary vasculature. The light microscopic (LM) analysis of the blood vessels is limited by the fact that it is impossible to identify which generation of the arterial tree an arterial profile within a LM microscopic section belongs to. Therefore, we established a workflow that allows for the generation-specific quantitative (stereological) analysis of pulmonary blood vessels. A whole left rabbit lung was fixed by vascular perfusion, embedded in glycol methacrylate and imaged by micro-computed tomography (µCT). The lung was then exhaustively sectioned and 20 consecutive sections were collected every 100 µm to obtain a systematic uniform random sample of the whole lung. The digital processing involved segmentation of the arterial tree, generation analysis, registration of LM sections with the µCT data as well as registration of the segmentation and the LM images. The present study demonstrates that it is feasible to identify arterial profiles according to their generation based on a generation-specific color code. Stereological analysis for the first three arterial generations of the monopodial branching of the vasculature included volume fraction, total volume, lumen-to-wall ratio and wall thickness for each arterial generation. In conclusion, the correlative image analysis of µCT and LM-based datasets is an innovative method to assess the pulmonary vasculature quantitatively.

Stereology as the 3D tool to quantitate lung architecture

Stereology is the method of choice for the quantitative assessment of biological objects in microscopy. It takes into account the fact that, in traditional microscopy such as conventional light and transmission electron microscopy, although one has to rely on measurements on nearly two-dimensional sections from fixed and embedded tissue samples, the quantitative data obtained by these measurements should characterize the real three-dimensional properties of the biological objects and not just their “flatland” appearance on the sections. Thus, three-dimensionality is a built-in property of stereological sampling and measurement tools. Stereology is, therefore, perfectly suited to be combined with 3D imaging techniques which cover a wide range of complementary sample sizes and resolutions, e.g. micro-computed tomography, confocal microscopy and volume electron microscopy. Here, we review those stereological principles that are of particular relevance for 3D imaging and provide an overview of applications of 3D imaging-based stereology to the lung in health and disease. The symbiosis of stereology and 3D imaging thus provides the unique opportunity for unbiased and comprehensive quantitative characterization of the three-dimensional architecture of the lung from macro to nano scale.

A semi-automated method for unbiased alveolar morphometry: Validation in a bronchopulmonary dysplasia model

Reproducible and unbiased methods to quantify alveolar structure are important for research on many lung diseases. However, manually estimating alveolar structure through stereology is time consuming and inter-observer variability is high. The objective of this work was to develop and validate a fast, reproducible and accurate (semi-)automatic alternative. A FIJI-macro was designed that automatically segments lung images to binary masks, and counts the number of test points falling on tissue and the number of intersections of the air-tissue interface with a set of test lines. Manual selection remains necessary for the recognition of non-parenchymal tissue and alveolar exudates. Volume density of alveolar septa (VVsep) and mean linear intercept of the airspaces (Lm) as measured by the macro were compared to theoretical values for 11 artificial test images and to manually counted values for 17 lungs slides using linear regression and Bland-Altman plots. Inter-observer agreement between 3 observers, measuring 8 lungs both manually and automatically, was assessed using intraclass correlation coefficients (ICC). VVsep and Lm measured by the macro closely approached theoretical values for artificial test images (R² of 0.9750 and 0.9573 and bias of 0.34% and 8.7%). The macro data in lungs were slightly higher for VVsep and slightly lower for Lm in comparison to manually counted values (R² of 0.8262 and 0.8288 and bias of -6.0% and 12.1%). Visually, semi-automatic segmentation was accurate. Most importantly, manually counted VVsep and Lm had only moderate to good inter-observer agreement (ICC 0.859 and 0.643), but agreements were excellent for semi-automatically counted values (ICC 0.956 and 0.900). This semi-automatic method provides accurate and highly reproducible alveolar morphometry results. Future efforts should focus on refining methods for automatic detection of non-parenchymal tissue or exudates, and for assessment of lung structure on 3D reconstructions of lungs scanned with microCT.

Image-based scaling laws for somatic growth and pulmonary artery morphometry from infancy to adulthood

Pulmonary artery (PA) morphometry has been extensively explored in adults, with particular focus on intra-acinar arteries. However, scaling law relationships for length and diameter of extensive preacinar PAs by age have not been previously reported for in vivo human data. To understand preacinar PA growth spanning children to adults, we performed morphometric analyses of all PAs visible in the computed tomography (CT) and magnetic resonance (MR) images from a healthy subject cohort [n = 16; age: 1-51 yr; body surface area (BSA): 0.49-2.01 m²]. Subject-specific anatomic PA models were constructed from CT and MR images, and morphometric information-diameter, length, tortuosity, bifurcation angle, and connectivity-was extracted and sorted into diameter-defined Strahler orders. Validation of Murray's law, describing optimal scaling exponents of radii for branching vessels, was performed to determine how closely PAs conform to this classical relationship. Using regression analyses of vessel diameters and lengths against orders and patient metrics (BSA, age, height), we found that diameters increased exponentially with order and allometrically with patient metrics. Length increased allometrically with patient metrics, albeit weakly. The average tortuosity index of all vessels was 0.026 ± 0.024, average bifurcation angle was 28.2 ± 15.1°, and average Murray's law exponent was 2.92 ± 1.07. We report a set of scaling laws for vessel diameter and length, along with other morphometric information. These provide an initial understanding of healthy structural preacinar PA development with age, which can be used for computational modeling studies and comparison with diseased PA anatomy.NEW & NOTEWORTHY Pulmonary artery (PA) morphometry studies to date have focused primarily on large arteries and intra-acinar arteries in either adults or children, neglecting preacinar arteries in both populations. Our study is the first to quantify in vivo preacinar PA morphometry changes spanning infants to adults. For preacinar arteries > 1 mm in diameter, we identify scaling laws for vessel diameters and lengths with patient metrics of growth and establish a healthy PA morphometry baseline for most preacinar PAs.

Alveolar size effects on nanoparticle deposition in rhythmically expanding-contracting terminal alveolar models

Significant differences in alveolar size exist in humans of different ages, gender, health, and among different species. The effects of alveolar sizes, as well as the accompanying breathing frequencies, on regional and local dosimetry of inhaled nanoparticles have not been sufficiently studied. Despite a well-accepted qualitative understanding of the advection-diffusion-sedimentation mechanism in the acinar region, a quantitative picture of the interactions among these factors remains inchoate. The objective of this study is to quantify the effects of alveolar size on the regional and local deposition of inhaled nanoparticles in alveolar models of varying complexities and to understand the dynamic interactions among different deposition mechanisms. Three different models were considered that retained 1, 4, and 45 alveoli, respectively. For each model, the baseline geometry was scaled by ¼, ½, 2, 4, and 8 times by volume. Temporal evolution and spatial distribution of particle deposition were tracked using a discrete-phase Lagrangian model. Lower retentions of inhaled nanoparticles were observed in the larger alveoli under the same respiration frequency, while similar retentions were found among different geometrical scales if breathing frequencies allometrically matched the alveolar size. Dimensional analysis reveals a manifold deposition mechanism with tantamount contributions from advection, diffusion, and gravitational sedimentation, each of which can become dominant depending on the location in the alveoli. Results of this study indicate that empirical correlations obtained from one sub-population cannot be directly applied to others, nor can they be simply scaled as a function of the alveolar size or respiration frequency due to the regime-transiting deposition mechanism that is both localized and dynamic.

Comprehensive stereological assessment of the human lung using multiresolution computed tomography

The application of stereology to lung casts and two-dimensional microscopy images is the gold standard for quantification of the human lung anatomy. However, these techniques are labor intensive, involving fixation, embedding, and histological sectioning of samples and thus have prevented comprehensive studies. Our objective was to demonstrate the application of stereology to volumetric multiresolution computed tomography (CT) to efficiently and extensively quantify the human lung anatomy. Nontransplantable donor lungs from individuals with no evidence of respiratory disease (n = 13) were air inflated, frozen at 10 cmH₂O, and scanned using CT. Systematic uniform random samples were taken, scanned using micro-CT, and assessed using stereology. The application of stereology to volumetric CT imaging enabled comprehensive quantification of total lung volume, volume fractions of alveolar, alveolar duct, and tissue, mean linear intercept, alveolar surface area, alveolar surface area density, septal wall thickness, alveolar number, number-weighted mean alveolar volume, and the number and morphometry of terminal and transitional bronchioles. With the use of this data set, we found that women and men have the same number of terminal bronchioles (last generation of conducting airways), but men have longer terminal bronchioles, a smaller wall area percentage, and larger lungs due to a greater number of alveoli per acinus. The application of stereology to multiresolution CT imaging enables comprehensive analysis of the human lung parenchyma that identifies differences between men and women. The reported data set of normal donor lungs aged 25-77 yr provides reference data for future studies of chronic lung disease to determine exact changes in tissue pathology.NEW & NOTEWORTHY Stereology has been the gold standard to quantify the three-dimensional lung anatomy using two-dimensional microscopy images. However, such techniques are labor intensive. This study provides a method that applies stereology to volumetric computed tomography images of frozen whole human lungs and systematic uniform random samples. The method yielded a comprehensive data set on the small airways and parenchymal lung structures, highlighting morphometric sex differences and providing a reference data set for future pathological studies.

CT Super-Resolution GAN Constrained by the Identical, Residual, and Cycle Learning Ensemble (GAN-CIRCLE)

In this paper, we present a semi-supervised deep learning approach to accurately recover high-resolution (HR) CT images from low-resolution (LR) counterparts. Specifically, with the generative adversarial network (GAN) as the building block, we enforce the cycle-consistency in terms of the Wasserstein distance to establish a nonlinear end-to-end mapping from noisy LR input images to denoised and deblurred HR outputs. We also include the joint constraints in the loss function to facilitate structural preservation. In this process, we incorporate deep convolutional neural network (CNN), residual learning, and network in network techniques for feature extraction and restoration. In contrast to the current trend of increasing network depth and complexity to boost the imaging performance, we apply a parallel 1×1 CNN to compress the output of the hidden layer and optimize the number of layers and the number of filters for each convolutional layer. The quantitative and qualitative evaluative results demonstrate that our proposed model is accurate, efficient and robust for super-resolution (SR) image restoration from noisy LR input images. In particular, we validate our composite SR networks on three large-scale CT datasets, and obtain promising results as compared to the other state-of-the-art methods.

Age-Related Structural and Functional Changes in the Mouse Lung

Lung function declines with advancing age. To improve our understanding of the structure-function relationships leading to this decline, we investigated structural alterations in the lung and their impact on micromechanics and lung function in the aging mouse. Lung function analysis was performed in 3, 6, 12, 18, and 24 months old C57BL/6 mice (n = 7-8/age), followed by lung fixation and stereological sample preparation. Lung parenchymal volume, total, ductal and alveolar airspace volume, alveolar volume and number, septal volume, septal surface area and thickness were quantified by stereology as well as surfactant producing alveolar epithelial type II (ATII) cell volume and number. Parenchymal volume, total and ductal airspace volume increased in old (18 and 24 months) compared with middle-aged (6 and 12 months) and young (3 months) mice. While the alveolar number decreased from young (7.5 × 10⁶) to middle-aged (6 × 10⁶) and increased again in old (9 × 10⁶) mice, the mean alveolar volume and mean septal surface area per alveolus conversely first increased in middle-aged and then declined in old mice. The ATII cell number increased from middle-aged (8.8 × 10⁶) to old (11.8 × 10⁶) mice, along with the alveolar number, resulting in a constant ratio of ATII cells per alveolus in all age groups (1.4 ATII cells per alveolus). Lung compliance and inspiratory capacity increased, whereas tissue elastance and tissue resistance decreased with age, showing greatest changes between young and middle-aged mice. In conclusion, alveolar size declined significantly in old mice concomitant with a widening of alveolar ducts and late alveolarization. These changes may partly explain the functional alterations during aging. Interestingly, despite age-related lung remodeling, the number of ATII cells per alveolus showed a tightly controlled relation in all age groups.

Assessment of the Alveolar Capillary Network in the Postnatal Mouse Lung in 3D Using Serial Block-Face Scanning Electron Microscopy

The alveolar capillary network (ACN) has a large surface area that provides the basis for an optimized gas exchange in the lung. It needs to adapt to morphological changes during early lung development and alveolarization. Structural alterations of the pulmonary vasculature can lead to pathological functional conditions such as in bronchopulmonary dysplasia and various other lung diseases. To understand the development of the ACN and its impact on the pathogenesis of lung diseases, methods are needed that enable comparative analyses of the complex three-dimensional structure of the ACN at different developmental stages and under pathological conditions. In this study a newborn mouse lung was imaged with serial block-face scanning electron microscopy (SBF-SEM) to investigate the ACN and its surrounding structures before the alveolarization process begins. Most parts but not all of the examined ACN contain two layers of capillaries, which were repeatedly connected with each other. A path from an arteriole to a venule was extracted and straightened to allow cross-sectional visualization of the data along the path within a plane. This allows a qualitative characterization of the structures that erythrocytes pass on their way through the ACN. One way to define regions of the ACN supplied by specific arterioles is presented and used for analyses. Pillars, possibly intussusceptive, were found in the vasculature but no specific pattern was observed in regard to parts of the saccular septa. This study provides 3D information with a resolution of about 150 nm on the microscopic structure of a newborn mouse lung and outlines some of the potentials and challenges of SBF-SEM for 3D analyses of the ACN.

The role of three-dimensionality and alveolar pressure in the distribution and amplification of alveolar stresses

Alveolar stresses are fundamental to enable the respiration process in mammalians and have recently gained increasing attention due to their mechanobiological role in the pathogenesis and development of respiratory diseases. Despite the fundamental physiological role of stresses in the alveolar wall, the determination of alveolar stresses remains challenging, and our current knowledge is largely drawn from 2D studies that idealize the alveolar septal wall as a spring or a planar continuum. Here we study the 3D stress distribution in alveolar walls of normal lungs by combining ex-vivo micro-computed tomography and 3D finite-element analysis. Our results show that alveolar walls are subject to a fully 3D state of stresses rather than to a pure axial stress state. To understand the contributions of the different components and deformation modes, we decompose the stress tensor field into hydrostatic and deviatoric components, which are associated with isotropic and distortional stresses, respectively. Stress concentrations arise in localized regions of the alveolar microstructure, with magnitudes that can be up to 27 times the applied alveolar pressure. Interestingly, we show that the stress amplification factor strongly depends on the level of alveolar pressure, i.e, stresses do not scale proportional to the applied alveolar pressure. In addition, we show that 2D techniques to assess alveolar stresses consistently overestimate the stress magnitude in alveolar walls, particularly for lungs under high transpulmonary pressure. These findings take particular relevance in the study of stress-induced remodeling of the emphysematous lung and in ventilator-induced lung injury, where the relation between transpulmonary pressure and alveolar wall stress is key to understand mechanotransduction processes in pneumocytes.

Heterogeneous structure and surface tension effects on mechanical response in pulmonary acinus: A finite element analysis

BACKGROUND

The pulmonary acinus is a dead-end microstructure that consists of ducts and alveoli. High-resolution micro-CT imaging has recently provided detailed anatomical information of a complete in vivo acinus, but relating its mechanical response with its detailed acinar structure remains challenging. This study aimed to investigate the mechanical response of acinar tissue in a whole acinus for static inflation using computational approaches.

METHODS

We performed finite element analysis of a whole acinus for static inflation. The acinar structure model was generated based on micro-CT images of an intact acinus. A continuum mechanics model of the lung parenchyma was used for acinar tissue material model, and surface tension effects were explicitly included. An anisotropic mechanical field analysis based on a stretch tensor was combined with a curvature-based local structure analysis.

FINDINGS

The airspace of the acinus exhibited nonspherical deformation as a result of the anisotropic deformation of acinar tissue. A strain hotspot occurred at the ridge-shaped region caused by a rod-like deformation of acinar tissue on the ridge. The local structure becomes bowl-shaped for inflation and, without surface tension effects, the surface of the bowl-shaped region primarily experiences isotropic deformation. Surface tension effects suppressed the increase in airspace volume and inner surface area, while facilitating anisotropic deformation on the alveolar surface.

INTERPRETATION

In the lungs, the heterogeneous acinar structure and surface tension induce anisotropic deformation at the acinar and alveolar scales. Further research is needed on structural variation of acini, inter-acini connectivity, or dynamic behavior to understand multiscale lung mechanics.

MRI and CT lung biomarkers: Towards an in vivo understanding of lung biomechanics

BACKGROUND

The biomechanical properties of the lung are necessarily dependent on its structure and function, both of which are complex and change over time and space. This makes in vivo evaluation of lung biomechanics and a deep understanding of lung biomarkers, very challenging. In patients and animal models of lung disease, in vivo evaluations of lung structure and function are typically made at the mouth and include spirometry, multiple-breath gas washout tests and the forced oscillation technique. These techniques, and the biomarkers they provide, incorporate the properties of the whole organ system including the parenchyma, large and small airways, mouth, diaphragm and intercostal muscles. Unfortunately, these well-established measurements mask regional differences, limiting their ability to probe the lung's gross and micro-biomechanical properties which vary widely throughout the organ and its subcompartments. Pulmonary imaging has the advantage in providing regional, non-invasive measurements of healthy and diseased lung, in vivo. Here we summarize well-established and emerging lung imaging tools and biomarkers and how they may be used to generate lung biomechanical measurements.

METHODS

We review well-established and emerging lung anatomical, microstructural and functional imaging biomarkers generated using synchrotron x-ray tomographic-microscopy (SRXTM), micro-x-ray computed-tomography (micro-CT), clinical CT as well as magnetic resonance imaging (MRI).

FINDINGS

Pulmonary imaging provides measurements of lung structure, function and biomechanics with high spatial and temporal resolution. Imaging biomarkers that reflect the biomechanical properties of the lung are now being validated to provide a deeper understanding of the lung that cannot be achieved using measurements made at the mouth.

A co-culture model of the bovine alveolus

The epithelial lining of the lung is often the first point of interaction between the host and inhaled pathogens, allergens and medications. Epithelial cells are therefore the main focus of studies which aim to shed light on host-pathogen interactions, to dissect the mechanisms of local host immunity and study toxicology. If these studies are not to be conducted exclusively in vivo, it is imperative that in vitro models are developed with a high in vitro- in vivo correlation. We describe here a co-culture model of the bovine alveolus, designed to overcome some of the limitations encountered with mono-culture and live animal models. Our system includes bovine pulmonary arterial endothelial cells (BPAECs) seeded onto a permeable membrane in 24 well Transwell format. The BPAECs are overlaid with immortalised bovine alveolar type II epithelial cells and cultured at air-liquid interface for 14 days before use; in our case to study host-mycobacterial interactions.

Characterisation of novel cell lines and the co-culture model have provided compelling evidence that immortalised bovine alveolar type II cells are an authentic substitute for primary alveolar type II cells and their co-culture with BPAECs provides a physiologically relevant in vitro model of the bovine alveolus.  

The co-culture model may be used to study dynamic intracellular and extracellular host-pathogen interactions, using proteomics, genomics, live cell imaging, in-cell ELISA and confocal microscopy. The model presented in this article enables other researchers to establish an in vitro model of the bovine alveolus that is easy to set up, malleable and serves as a comparable alternative to in vivo models, whilst allowing study of early host-pathogen interactions, currently not feasible in vivo. The model therefore achieves one of the 3Rs objectives in that it replaces the use of animals in research of bovine respiratory diseases.

Three-Dimensional Quantitative Co-Mapping of Pulmonary Morphology and Nanoparticle Distribution with Cellular Resolution in Nondissected Murine Lungs

Deciphering biodistribution, biokinetics, and biological effects of nanoparticles (NPs) in entire organs with cellular resolution remains largely elusive due to the lack of effective imaging tools. Here, light sheet fluorescence microscopy in combination with optical tissue clearing was validated for concomitant three-dimensional mapping of lung morphology and NP biodistribution with cellular resolution in nondissected ex vivo murine lungs. Tissue autofluorescence allowed for label-free, quantitative morphometry of the entire bronchial tree, acinar structure, and blood vessels. Co-registration of fluorescent NPs with lung morphology revealed significant differences in pulmonary NP distribution depending on the means of application (intratracheal instillation and ventilator-assisted aerosol inhalation under anesthetized conditions). Inhalation exhibited a more homogeneous NP distribution in conducting airways and acini indicated by a central-to-peripheral (C/P) NP deposition ratio of unity (0.98 ± 0.13) as compared to a 2-fold enhanced central deposition (C/P = 1.98 ± 0.37) for instillation. After inhalation most NPs were observed in the proximal part of the acini as predicted by computational fluid dynamics simulations. At cellular resolution patchy NP deposition was visualized in bronchioles and acini, but more pronounced for instillation. Excellent linearity of the fluorescence intensity-dose response curve allowed for accurate NP dosimetry and revealed ca. 5% of the inhaled aerosol was deposited in the lungs. This single-modality imaging technique allows for quantitative co-registration of tissue architecture and NP biodistribution, which could accelerate elucidation of NP biokinetics and bioactivity within intact tissues, facilitating both nanotoxicology studies and the development of nanomedicines.

Mice-to-men comparison of inhaled drug-aerosol deposition and clearance

Part of the effective prediction of the pharmacokinetics of drugs (or toxic particles) requires extrapolation of experimental data sets from animal studies to humans. As the respiratory tracts of rodents and humans are anatomically very different, there is a need to study airflow and drug-aerosol deposition patterns in lung airways of these laboratory animals and compare them to those of human lungs. As a first step, interspecies computational comparison modeling of inhaled nano-to-micron size drugs (50 nm < d<15μm) was performed using mouse and human upper airway models under realistic breathing conditions. Critical species-specific differences in lung physiology of the upper airways and subsequently in local drug deposition were simulated and analyzed. In addition, a hybrid modeling methodology, combining Computational Fluid-Particle Dynamics (CF-PD) simulations with deterministic lung deposition models, was developed and predicted total and regional drug-aerosol depositions in lung airways of both mouse and man were compared, accounting for the geometric, kinematic and dynamic differences. Interestingly, our results indicate that the total particle deposition fractions, especially for submicron particles, are comparable in rodent and human respiratory models for corresponding breathing conditions. However, care must be taken when extrapolating a given dosage as considerable differences were noted in the regional particle deposition pattern. Combined with the deposition model, the particle retention and clearance kinetics of deposited nanoparticles indicates that the clearance rate from the mouse lung is higher than that in the human lung. In summary, the presented computer simulation models provide detailed fluid-particle dynamics results for upper lung airways of representative human and mouse models with a comparative analysis of particle lung deposition data, including a novel mice-to-men correlation as well as a particle-clearance analysis both useful for pharmacokinetic and toxicokinetic studies.

How high resolution 3-dimensional imaging changes our understanding of postnatal lung development

During the last 10 + years biologically and clinically significant questions about postnatal lung development could be answered due to the application of modern cutting-edge microscopic and quantitative histological techniques. These are in particular synchrotron radiation based X-ray tomographic microscopy (SRXTM), but also ³Helium Magnetic Resonance Imaging, as well as the stereological estimation of the number of alveoli and the length of the free septal edge. First, the most important new finding may be the following: alveolarization of the lung does not cease after the maturation of the alveolar microvasculature but continues until young adulthood and, even more important, maybe reactivated lifelong if needed to rescue structural damages of the lungs. Second, the pulmonary acinus represents the functional unit of the lung. Because the borders of the acini could not be detected in classical histological sections, any investigation of the acini requires 3-dimensional (imaging) methods. Based on SRXTM it was shown that in rat lungs the number of acini stays constant, meaning that their volume increases by a factor of ~ 11 after birth. The latter is very important for acinar ventilation and particle deposition.

Recent developments in 3-D reconstruction and stereology to study the pulmonary vasculature

Alterations of the pulmonary vasculature are an important feature of human lung diseases such as chronic obstructive pulmonary disease, pulmonary hypertension, and bronchopulmonary dysplasia. Experimental studies to investigate the pathogenesis or a therapeutic intervention in animal models of these diseases often require robust, meaningful, and efficient morphometric data that allow for appropriate statistical testing. The gold standard for obtaining such data is design-based stereology. However, certain morphological characteristics of the pulmonary vasculature make the implementation of stereological methods challenging. For example, the alveolar capillary network functions according to the sheet flow principle, thus making unbiased length estimations impossible and requiring other strategies to obtain mechanistic morphometric data. Another example is the location of pathological changes along the branches of the vascular tree. For developmental defects like in bronchopulmonary dysplasia or for pulmonary hypertension, it is important to know whether certain segments of the vascular tree are preferentially altered. This cannot be overcome by traditional stereological methods but requires the combination of a three-dimensional data set and stereology. The present review aims at highlighting the great potential while discussing the major challenges (such as time consumption and data volume) of this combined approach. We hope to raise interest in the potential of this approach and thus stimulate solutions to overcome the existing challenges.

Biomimetics of the pulmonary environment in vitro: A microfluidics perspective

The entire luminal surface of the lungs is populated with a complex yet confluent, uninterrupted airway epithelium in conjunction with an extracellular liquid lining layer that creates the air-liquid interface (ALI), a critical feature of healthy lungs. Motivated by lung disease modelling, cytotoxicity studies, and drug delivery assessments amongst other, in vitro setups have been traditionally conducted using macroscopic cultures of isolated airway cells under submerged conditions or instead using transwell inserts with permeable membranes to model the ALI architecture. Yet, such strategies continue to fall short of delivering a sufficiently realistic physiological in vitro airway environment that cohesively integrates at true-scale three essential pillars: morphological constraints (i.e., airway anatomy), physiological conditions (e.g., respiratory airflows), and biological functionality (e.g., cellular makeup). With the advent of microfluidic lung-on-chips, there have been tremendous efforts towards designing biomimetic airway models of the epithelial barrier, including the ALI, and leveraging such in vitro scaffolds as a gateway for pulmonary disease modelling and drug screening assays. Here, we review in vitro platforms mimicking the pulmonary environment and identify ongoing challenges in reconstituting accurate biological airway barriers that still widely prevent microfluidic systems from delivering mainstream assays for the end-user, as compared to macroscale in vitro cell cultures. We further discuss existing hurdles in scaling up current lung-on-chip designs, from single airway models to more physiologically realistic airway environments that are anticipated to deliver increasingly meaningful whole-organ functions, with an outlook on translational and precision medicine.

3He diffusion MRI in human lungs

Hyperpolarized 3He gas allows the air spaces of the lungs to be imaged via MRI. Imaging of restricted diffusion is addressed here, which allows the microstructure of the lung to be characterized through the physical restrictions to gas diffusion presented by airway and alveolar walls in the lung. Measurements of the apparent diffusion coefficient (ADC) of 3He at time scales of milliseconds and seconds are compared; measurement of acinar airway sizes by determination of the microscopic anisotropy of diffusion is discussed. This is where Dr. JJH Ackerman's influence was greatest in aiding the formation of the Washington University 3He group, involving early a combination of physicists, radiologists, and surgeons, as the first applications of 3He ADC were to COPD and its destruction/modification of lung microstructure via emphysema. The sensitivity of the method to early COPD is demonstrated, as is its validation by direct comparison to histology. More recently the method has been used broadly in adult and pediatric obstructive lung diseases, from severe asthma to cystic fibrosis to bronchopulmonary dysplasia, a result of premature birth. These applications of the technique are discussed briefly.

Multi-scale X-ray computed tomography to detect and localize metal-based nanomaterials in lung tissues of in vivo exposed mice

In this methodological study, we demonstrated the relevance of 3D imaging performed at various scales for the ex vivo detection and location of cerium oxide nanomaterials (CeO₂-NMs) in mouse lung. X-ray micro-computed tomography (micro-CT) with a voxel size from 14 µm to 1 µm (micro-CT) was combined with X-ray nano-computed tomography with a voxel size of 63 nm (nano-CT). An optimized protocol was proposed to facilitate the sample preparation, to minimize the experimental artifacts and to optimize the contrast of soft tissues exposed to metal-based nanomaterials (NMs). 3D imaging of the NMs biodistribution in lung tissues was consolidated by combining a vast variety of techniques in a correlative approach: histological observations, 2D chemical mapping and speciation analysis were performed for an unambiguous detection of NMs. This original methodological approach was developed following a worst-case scenario of exposure, i.e. high dose of exposure with administration via intra-tracheal instillation. Results highlighted both (i) the non-uniform distribution of CeO₂-NMs within the entire lung lobe (using large field-of-view micro-CT) and (ii) the detection of CeO₂-NMs down to the individual cell scale, e.g. macrophage scale (using nano-CT with a voxel size of 63 nm).

A Gaussian extension for Diffraction Enhanced Imaging

Unlike conventional x-ray attenuation one of the advantages of phase contrast x-ray imaging is its capability of extracting useful physical properties of the sample. In particular the possibility to obtain information from small angle scattering about unresolvable structures with sub-pixel resolution sensitivity has drawn attention for both medical and material science applications. We report on a novel algorithm for the analyzer based x-ray phase contrast imaging modality, which allows the robust separation of absorption, refraction and scattering effects from three measured x-ray images. This analytical approach is based on a simple Gaussian description of the analyzer transmission function and this method is capable of retrieving refraction and small angle scattering angles in the full angular range typical of biological samples. After a validation of the algorithm with a simulation code, which demonstrated the potential of this highly sensitive method, we have applied this theoretical framework to experimental data on a phantom and biological tissues obtained with synchrotron radiation. Owing to its extended angular acceptance range the algorithm allows precise assessment of local scattering distributions at biocompatible radiation doses, which in turn might yield a quantitative characterization tool with sufficient structural sensitivity on a submicron length scale.

One (sub-)acinus for all: Fate of inhaled aerosols in heterogeneous pulmonary acinar structures

Computational Fluid Dynamics (CFD) have offered an attractive gateway to investigate in silico respiratory flows and aerosol transport in the depths of the lungs. Yet, not only do existing models lack sufficient anatomical realism in capturing the heterogeneity and morphometry of the acinar environment, numerical simulations have been widely restricted to domains capturing a mere few percent of a single acinus. Here, we present to the best of our knowledge the most detailed and comprehensive in silico simulations to date on the fate of aerosols in the acinar depths. Our heterogeneous acinar domains represent complete sub-acinar models (i.e. 1/8th of a full acinus) based on the recent algorithm of Koshiyama & Wada (2015), capturing statistics of human acinar morphometry (Ochs et al. 2004). Our simulations deliver high-resolution, 3D spatial-temporal data on aerosol transport and deposition, emphasizing how variances in acinar heterogeneity only play a minor role in determining general deposition outcomes. With such tools at hand, we revisit whole-lung deposition predictions (i.e. ICRP) based on past 1D lung models. While our findings under quiet breathing substantiate general deposition trends obtained with past predictions in the alveolar regions, we underscore how deposition fractions are anticipated to increase, in particular during deep inhalation. For such inhalation maneuver, our simulations support the notion of significantly augmented deposition for all aerosol sizes (0.005-5.0μm). Overall, our efforts not only help consolidate our mechanistic understanding of inhaled aerosol transport in the acinar depths but also continue to bridge the gap between "bottom-up" in silico models and regional deposition predictions from whole-lung models. Such quantifications provide what is deemed more accurate deposition predictions in morphometrically-faithful models and are particularly useful in assessing inhalation strategies for deep airway deposition (e.g. systemic delivery).

Automated computer-assisted quantitative analysis of intact murine lungs at the alveolar scale

Using state-of-the-art X-ray tomographic microscopy we can image lung tissue in three dimensions in intact animals down to a micrometer precision. The structural complexity and hierarchical branching scheme of the lung at this level of details, however, renders the extraction of biologically relevant quantities particularly challenging. We have developed a methodology for a detailed description of lung inflation patterns by measuring the size and the local curvature of the parenchymal airspaces. These quantitative tools for morphological and topological analyses were applied to high-resolution murine 3D lung image data, inflated at different pressure levels under immediate post mortem conditions. We show for the first time direct indications of heterogeneous intra-lobar and inter-lobar distension patterns at the alveolar level. Furthermore, we did not find any indication that a cyclic opening-and-collapse (recruitment) of a large number of alveoli takes place.

Parenchymal Airspace Profiling: Sensitive Quantification and Characterization of Lung Structure Evaluating Parenchymal Destruction

Lung morphometry was introduced over 50 years ago to provide quantitative evaluation of the lung structure. The existing parameters, such as mean linear intercept and destructive index, suffer from simplistic data interpretation and a subjective data acquisition process. To overcome these existing shortcomings, parenchymal airspace profiling (PAP) was developed to provide a more detailed and unbiased quantitative method. Following the standard protocols of fixation, embedding, and sectioning, lung micrographs were: (1) marked with nonparenchymal area, preprocessed, and binarized under the researcher's supervision; (2) analyzed with a statistical learning method, Gaussian mixture model, to provide an unbiased categorization of parenchymal airspace compartments, corresponding to a single alveolus, alveolar sac, and ductal/destructive airspace; and (3) further quantified into morphometric parameters, including reference volume, alveolar count, and ductal/destructive fraction (DF) based on stereological principles. PAP was performed on hematoxylin and eosin-stained lung sections from mice and rabbits. Unbiased categorization revealed differences in alveolar size among several mouse strains (NZW/LacJ<AKR/J<A/J<C57BL/6J) and across species (mouse<rabbit). Further quantification indicates that parenchymal destruction, modeled in mouse lungs with 1-month smoke exposure, resulted in decreased alveolar count, increased DF, but no significant differences in mean linear intercept. DF also provides a robust measurement that is not biased by processing artifacts, magnification, or reference volume, which are common limitations in human lung biopsies or data obtained from different laboratories. PAP is a novel approach to lung morphometry that offers more detailed characterization of the lung structure, sensitivity, and robustness than presently used methods for evaluating parenchymal destruction.

Development of the lung

To fulfill the task of gas exchange, the lung possesses a huge inner surface and a tree-like system of conducting airways ventilating the gas exchange area. During lung development, the conducting airways are formed first, followed by the formation and enlargement of the gas exchange area. The latter (alveolarization) continues until young adulthood. During organogenesis, the left and right lungs have their own anlage, an outpouching of the foregut. Each lung bud starts a repetitive process of outgrowth and branching (branching morphogenesis) that forms all of the future airways mainly during the pseudoglandular stage. During the canalicular stage, the differentiation of the epithelia becomes visible and the bronchioalveolar duct junction is formed. The location of this junction stays constant throughout life. Towards the end of the canalicular stage, the first gas exchange may take place and survival of prematurely born babies becomes possible. Ninety percent of the gas exchange surface area will be formed by alveolarization, a process where existing airspaces are subdivided by the formation of new walls (septa). This process requires a double-layered capillary network at the basis of the newly forming septum. However, in parallel to alveolarization, the double-layered capillary network of the immature septa fuses to a single-layered network resulting in an optimized setup for gas exchange. Alveolarization still continues, because, at sites where new septa are lifting off preexisting mature septa, the required second capillary layer will be formed instantly by angiogenesis. The latter confirms a lifelong ability of alveolarization, which is important for any kind of lung regeneration.

Nondestructive cryomicro-CT imaging enables structural and molecular analysis of human lung tissue

Micro-computed tomography (CT) enables three-dimensional (3D) imaging of complex soft tissue structures, but current protocols used to achieve this goal preclude cellular and molecular phenotyping of the tissue. Here we describe a radiolucent cryostage that permits micro-CT imaging of unfixed frozen human lung samples at an isotropic voxel size of (11 µm)³ under conditions where the sample is maintained frozen at -30°C during imaging. The cryostage was tested for thermal stability to maintain samples frozen up to 8 h. This report describes the methods used to choose the materials required for cryostage construction and demonstrates that whole genome mRNA integrity and expression are not compromised by exposure to micro-CT radiation and that the tissue can be used for immunohistochemistry. The new cryostage provides a novel method enabling integration of 3D tissue structure with cellular and molecular analysis to facilitate the identification of molecular determinants of disease.

NEW & NOTEWORTHY

The described micro-CT cryostage provides a novel way to study the three-dimensional lung structure preserved without the effects of fixatives while enabling subsequent studies of the cellular matrix composition and gene expression. This approach will, for the first time, enable researchers to study structural changes of lung tissues that occur with disease and correlate them with changes in gene or protein signatures.

The total number of acini remains constant throughout postnatal rat lung development

The pulmonary airways are subdivided into conducting and gas-exchanging airways. The small tree of gas-exchanging airways which is fed by the most distal conducting airway represents an acinus. Very little is known about the development of the number of acini. The goal of this study was to estimate their number throughout rat postnatal development. Right middle rat lung lobes were obtained at postnatal day 4-60, stained with heavy metals, paraffin embedded, and scanned by synchrotron radiation-based X-ray tomographic microscopy or imaged with micro computed tomography after critical point drying. The acini were counted by detection of the transitional bronchioles [bronchioalveolar duct junction (BADJ)] by using morphological criteria (thickness of the walls of airways and appearance of alveoli) during examination of the resulting three-dimensional (3D) image stacks. Between postnatal days 4-60, the number of acini per lung remained constant (5,840 ± 547 acini), but their volume increased significantly. We concluded that the acini are formed before the end of the saccular stage (before postnatal day 4) and that the developmental increase of the lung volume is achieved by an increase of the acinar volume and not by an increase of their number. Furthermore, our results propose that the bronchioalveolar stem cells, which are residing in the BADJ, are as constant in their location as the BADJ itself.

Multiscale Opening of Conjoined Fuzzy Objects: Theory and Applications

Theoretical properties of a multi-scale opening (MSO) algorithm for two conjoined fuzzy objects are established, and its extension to separating two conjoined fuzzy objects with different intensity properties is introduced. Also, its applications to artery/vein (A/V) separation in pulmonary CT imaging and carotid vessel segmentation in CT angiograms (CTAs) of patients with intracranial aneurysms are presented. The new algorithm accounts for distinct intensity properties of individual conjoined objects by combining fuzzy distance transform (FDT), a morphologic feature, with fuzzy connectivity, a topologic feature. The algorithm iteratively opens the two conjoined objects starting at large scales and progressing toward finer scales. Results of application of the method in separating arteries and veins in a physical cast phantom of a pig lung are presented. Accuracy of the algorithm is quantitatively evaluated in terms of sensitivity and specificity on patients' CTA data sets and its performance is compared with existing methods. Reproducibility of the algorithm is examined in terms of volumetric agreement between two users' carotid vessel segmentation results. Experimental results using this algorithm on patients' CTA data demonstrate a high average accuracy of 96.3% with 95.1% sensitivity and 97.5% specificity and a high reproducibility of 94.2% average agreement between segmentation results from two mutually independent users. Approximately, twenty-five to thirty-five user-specified seeds/separators are needed for each CTA data through a custom designed graphical interface requiring an average of thirty minutes to complete carotid vascular segmentation in a patient's CTA data set.

Aging-related changes in respiratory system mechanics and morphometry in mice

Previous work investigating respiratory system mechanics in mice has reported an aging-related increase in compliance and mean linear intercept (Lm). However, these changes were assessed using only a young (2-mo-old) and old (20- and 26-mo-old) group yet were interpreted to reflect a linear evolution across the life span. Therefore, to investigate respiratory system mechanics and lung morphometry across a more complete spectrum of ages, we utilized 2 (100% survival, n = 6)-, 6 (100% survival, n = 12)-, 18 (90% survival, n = 12)-, 24 (75% survival, n = 12)-, and 30 (25% survival, n = 12)-mo-old C57BL/6 mice. We found a nonlinear aging-related decrease in respiratory system resistance and increase in dynamic compliance and hysteresis between 2- and 24-mo-old mice. However, in 30-mo-old mice, respiratory system resistance increased, and dynamic compliance and hysteresis decreased relative to 24-mo-old mice. Respiratory system impedance spectra were measured between 1-20.5 Hz at positive end-expiratory pressures (PEEP) of 1, 3, 5, and 7 cmH2O. Respiratory system resistance and reactance at each level of PEEP were increased and decreased, respectively, only in 2-mo-old animals. No differences in the respiratory system impedance spectra were observed in 6-, 18-, 24-, and 30-mo-old mice. Additionally, lungs were fixed following tracheal instillation of 4% paraformaldehyde at 25 cmH2O and processed for Lm and airway collagen deposition. There was an aging-related increase in Lm consistent with emphysematous-like changes and no evidence of increased airway collagen deposition. Accordingly, we demonstrate nonlinear aging-related changes in lung mechanics and morphometry in C57BL/6 mice.

Morphometric differences between central vs. surface acini in A/J mice using high-resolution micro-computed tomography

Through interior tomography, high-resolution microcomputed tomography (μCT) systems provide the ability to nondestructively assess the pulmonary acinus at micron and submicron resolutions. With the application of systematic uniform random sampling (SURS) principles applied to in situ fixed, intact, ex vivo lungs, we have sought to characterize morphometric differences in central vs. surface acini to better understand how well surface acini reflect global acinar geometry. Lungs from six mice (A/J strain, 15-20 wk of age) were perfusion fixed in situ and imaged using a multiresolution μCT system (Micro XCT 400, Zeiss). With the use of lower-resolution whole lung images, SURS methods were used for identification of central and surface foci for high-resolution imaging. Acinar morphometric metrics included diameters, lengths, and branching angles for each alveolar duct and total path lengths from entrance of the acinus to the terminal alveolar sacs. In addition, acinar volume, alveolar surface area, and surface area/volume ratios were assessed. A generation-based analysis demonstrated that central acini have significantly smaller branch diameters at each generation with no significant increase in branch lengths. In addition to larger-diameter alveolar ducts, surface acini had significantly increased numbers of branches and terminal alveolar sacs. The total path lengths from the acinar entrance to the terminal nodes were found to be higher in the case of surface acini. Volumes and surface areas of surface acini are greater than central acini, but there were no differences in surface/volume ratios. In conclusion, there are significant structural differences between surface and central acini in the A/J mouse.