Measurement of three-dimensional lung tree structures by using computed tomography

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.

In Silico Ventilation Within the Dose-Volume is Predictive of Lung Function Post-radiation Therapy in Patients with Lung Cancer

Lung cancer is a leading cause of death worldwide. Radiation therapy (RT) is one method to treat this disease. A common side effect of RT for lung cancer is radiation-induced lung damage (RILD) which leads to loss of lung function. RILD often compounds pre-existing smoking-related regional lung function impairment. It is difficult to predict patient outcomes due to large variability in individual response to RT. In this study, the capability of image-based modelling of regional ventilation in lung cancer patients to predict lung function post-RT was investigated. Twenty-five patient-based models were created using CT images to define the airway geometry, size and location of tumour, and distribution of emphysema. Simulated ventilation within the 20 Gy isodose volume showed a statistically significant negative correlation with the change in forced expiratory volume in 1 s 12-months post-RT (p = 0.001, R = - 0.61). Patients with higher simulated ventilation within the 20 Gy isodose volume had a greater loss in lung function post-RT and vice versa. This relationship was only evident with the combined impact of tumour and emphysema, with the location of the emphysema relative to the dose-volume being important. Our results suggest that model-based ventilation measures can be used in the prediction of patient lung function post-RT.

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.

Small airway segmentation in thoracic computed tomography scans: a machine learning approach

Small airway obstruction is a main cause for chronic obstructive pulmonary disease (COPD). We propose a novel method based on machine learning to extract the airway system from a thoracic computed tomography (CT) scan. The emphasis of the proposed method is on including the smallest airways that are still visible on CT. We used an optimized sampling procedure to extract airway and non-airway voxel samples from a large set of scans for which a semi-automatically constructed reference standard was available. We created a set of features which represent tubular and texture properties that are characteristic for small airway voxels. A random forest classifier was used to determine for each voxel if it belongs to the airway class. Our method was validated on a set of 20 clinical thoracic CT scans from the COPDGene study. Experiments show that our method is effective in extracting the full airway system and in detecting a large number of small airways that were missed by the semi-automatically constructed reference standard.

Multi-scale model of food drying: Current status and challenges

For a long time, food engineers have been trying to describe the physical phenomena that occur during food processing especially drying. Physics-based theoretical modeling is an important tool for the food engineers to reduce the hurdles of experimentation. Drying of food is a multi-physics phenomenon such as coupled heat and mass transfer. Moreover, food structure is multi-scale in nature, and the microstructural features play a great role in the food processing specially in drying. Previously simple macroscopic model was used to describe the drying phenomena which can give a little description about the smaller scale. The multiscale modeling technique can handle all the phenomena that occur during drying. In this special kind of modeling approach, the single scale models from bigger to smaller scales are interconnected. With the help of multiscale modeling framework, the transport process associated with drying can be studied on a smaller scale and the resulting information can be transferred to the bigger scale. This article is devoted to discussing the state of the art multi-scale modeling, its prospect and challenges in the field of drying technology. This article has also given some directions to how to overcome the challenges for successful implementation of multi-scale modeling.

A skeleton-tree-based approach to acinar morphometric analysis using microcomputed tomography with comparison of acini in young and old C57BL/6 mice

We seek to establish a method using interior tomographic techniques (Xradia MicroXCT-400) for acinar morphometric analysis using the pathway center lines from micro X-ray computed tomographic (Micro-CT) images as the road map. Through the application of these techniques, we present a method to extend the atlas of murine lungs to acinar levels and present a comparison between two age groups of the C57BL/6 strain. Lungs fixed via vascular perfusion were scanned using high-resolution Micro-CT protocols. Individual acini were segmented, and skeletonized paths to alveolar sacs from the entrance to the acinus were formed. Morphometric parameters, including branch lengths, diameters, and branching angles, were generated. Six mice each, at two age groups (∼20 and ∼90 wk of age), were studied. Additive Gaussian noise (0 mean and SD 1, 2, 5, and 10) was used to test the robustness of the analytical method. Noise-based variations were within ±6 μm for branch lengths and ±5 μm for diameters. At a noise level of 10, errors increased. Branch diameters were less susceptible to noise than lengths. There was >95% center line overlap across all noise levels. The measurements obtained using the center lines as a road map were not affected by added noise. Acini from younger mice had smaller branch diameters and lengths at all generations without significant differences in branching angles. The relative distribution of volume in the alveolar ducts was similar across both age groups. The method has been demonstrated to be repeatable and robust to image noise and provides a new, nondestructive technique to assess and compare acinar morphometry quantitatively.

The Prognostic Value of Residual Volume/Total Lung Capacity in Patients with Chronic Obstructive Pulmonary Disease

The prognostic role of resting pulmonary hyperinflation as measured by residual volume (RV)/total lung capacity (TLC) in chronic obstructive pulmonary disease (COPD) remains poorly understood. Therefore, this study aimed to identify the factors related to resting pulmonary hyperinflation in COPD and to determine whether resting pulmonary hyperinflation is a prognostic factor in COPD. In total, 353 patients with COPD in the Korean Obstructive Lung Disease cohort recruited from 16 hospitals were enrolled. Resting pulmonary hyperinflation was defined as RV/TLC ≥ 40%. Multivariate logistic regression analysis demonstrated that older age (P = 0.001), lower forced expiratory volume in 1 second (FEV1) (P < 0.001), higher St. George Respiratory Questionnaire (SGRQ) score (P = 0.019), and higher emphysema index (P = 0.010) were associated independently with resting hyperinflation. Multivariate Cox regression model that included age, gender, dyspnea scale, SGRQ, RV/TLC, and 6-min walking distance revealed that an older age (HR = 1.07, P = 0.027), a higher RV/TLC (HR = 1.04, P = 0.025), and a shorter 6-min walking distance (HR = 0.99, P < 0.001) were independent predictors of all-cause mortality. Our data showed that older age, higher emphysema index, higher SGRQ score, and lower FEV1 were associated independently with resting pulmonary hyperinflation in COPD. RV/TLC is an independent risk factor for all-cause mortality in COPD.

Comparison of a New Integral-Based Half-Band Method for CT Measurement of Peripheral Airways in COPD With a Conventional Full-Width Half-Maximum Method Using Both Phantom and Clinical CT Images

OBJECTIVES

To compare a new integral-based half-band method (IBHB) and a conventional full-width half-maximum (FWHM) method in measuring peripheral airway dimensions at airway phantoms and thin-section computed tomography of chronic obstructive pulmonary disease (COPD).

METHODS

The IBHB was validated and compared using airway phantoms and 50 patients with COPD. Airway parameters (wall area percentage [WA%], mean lumen radius, and mean wall thickness) were measured at fourth to sixth generations of the right apical bronchus. Matched results from 2 methods were compared and correlated with forced expiratory volume (FEV) in 1 second (FEV1), FEV1 / forced vital capacity (FVC), and global initiative for chronic obstructive lung disease (GOLD) stage. Linear regression analysis was performed using airway dimensions and emphysema index.

RESULTS

The IBHB generated more accurate measurements at phantom study. Measured airway parameters by both methods at thin-section computed tomography study were significantly different (all P < 0.05, paired t test). The IBHB method-measured WA% and wall thickness were significantly smaller. Mean WA% with IBHB also showed better correlation than that with FWHM (FEV1, r = -0.52 vs -0.28; FEV1 / FVC, r = -0.41 vs r = -0.20; GOLD, 0.52 vs 0.33, respectively). Linear regression analysis revealed fifth-generation WA% measured by IBHB was an independent variable, and addition to emphysema index increased predictability (FEV1, r = 0.63; FEV1 / FVC, r = 0.61; GOLD, r = 0.70).

CONCLUSIONS

The new IBHB measured peripheral airway dimensions differently than FWHM and showed better correlations with functional parameters in COPD.

Volumetric quantification of airway wall in CT via collision-free active surface model: application to asthma assessment

Emerging idea in asthma phenotyping, incorporating local morphometric information on the airway wall thickness would be able to better account for the process of airway remodeling as indicator of pathology or therapeutic impact. It is thus important that such information be provided uniformly along the airway tree, not on a sparse (cross-section) sampling basis. The volumetric segmentation of the airway wall from CT data is the issue addressed in this paper by exploiting a patient-specific surface active model. An original aspect taken into account in the proposed deformable model is the management of auto-collisions for this complex morphology. The analysis of several solutions ended up with the design of a motion vector field specific to the patient geometry to guide the deformation. The segmentation result, presented as two embedded inner/outer surfaces of the wall, allows the quantification of the tissue thickness based on a locally-defined measure sensitive to even small surface irregularities. The method is validated with respect to several ground truth simulations of pulmonary CT data with different airway geometries and acquisition protocols showing accuracy within the CT resolution range. Results from an ongoing clinical study on moderate and severe asthma are presented and discussed.

Chronic obstructive pulmonary disease: CT quantification of airways disease

Chronic obstructive pulmonary disease (COPD) is an increasing cause of morbidity and mortality worldwide and results in substantial social and economic burdens. COPD is a heterogeneous disease with both extrapulmonary and pulmonary components. The pulmonary component is characterized by an airflow limitation that is not fully reversible. In the authors' opinion, none of the currently available classifications combining airflow limitation measurements with clinical parameters is sufficient to determine the prognosis and treatment of a particular patient with COPD. With regard to the causes of airflow limitation, CT can be used to quantify the two main contributions to COPD: emphysema, and small airways disease (a narrowing of the airways). CT quantification--with subsequent COPD phenotyping--can contribute to improved patient care, assessment of COPD progression, and identification of severe COPD with increasing risk of mortality. Small airways disease can be quantified through measurements reflecting morphology, quantification of obstruction, and changes in airways walls. This article details these three approaches and concludes with perspectives and directions for further research.

CT based computerized identification and analysis of human airways: a review

As one of the most prevalent chronic disorders, airway disease is a major cause of morbidity and mortality worldwide. In order to understand its underlying mechanisms and to enable assessment of therapeutic efficacy of a variety of possible interventions, noninvasive investigation of the airways in a large number of subjects is of great research interest. Due to its high resolution in temporal and spatial domains, computed tomography (CT) has been widely used in clinical practices for studying the normal and abnormal manifestations of lung diseases, albeit there is a need to clearly demonstrate the benefits in light of the cost and radiation dose associated with CT examinations performed for the purpose of airway analysis. Whereas a single CT examination consists of a large number of images, manually identifying airway morphological characteristics and computing features to enable thorough investigations of airway and other lung diseases is very time-consuming and susceptible to errors. Hence, automated and semiautomated computerized analysis of human airways is becoming an important research area in medical imaging. A number of computerized techniques have been developed to date for the analysis of lung airways. In this review, we present a summary of the primary methods developed for computerized analysis of human airways, including airway segmentation, airway labeling, and airway morphometry, as well as a number of computer-aided clinical applications, such as virtual bronchoscopy. Both successes and underlying limitations of these approaches are discussed, while highlighting areas that may require additional work.

Comparison of clinico-physiologic and CT imaging risk factors for COPD exacerbation

To date, clinico-physiologic indices have not been compared with quantitative CT imaging indices in determining the risk of chronic obstructive pulmonary disease (COPD) exacerbation. We therefore compared clinico-physiologic and CT imaging indices as risk factors for COPD exacerbation in patients with COPD. We retrospectively analyzed 260 COPD patients from pulmonary clinics at 11 hospitals in Korea from June 2005 to November 2009 and followed-up for at least one year. At the time of enrollment, none of these patients had COPD exacerbations for at least 2 months. All underwent clinico-physiologic and radiological evaluation for risk factors of COPD exacerbation. After 1 yr, 106 of the 260 patients had at least one exacerbation of COPD. Multiple logistic regression analysis showed that old age, high Charlson Index, and low FEV(1) were significant in a clinico-physiologic model, with C-statistics of 0.69, and that increased age and emphysema index were significant in a radiologic model, with C-statistics of 0.64. The difference between the two models was statistically significant (P = 0.04 by bootstrap analysis). Combinations of clinico-physiologic risk factors may be better than those of imaging risk factors in predicting COPD exacerbation.

Response patterns to bronchodilator and quantitative computed tomography in chronic obstructive pulmonary disease

BACKGROUND

Patients with chronic obstructive pulmonary disease (COPD) show different spirometric response patterns to bronchodilator, such that some patients show improvement principally in expiratory flow (forced expiratory volume in 1 s; FEV(1)), whereas others respond by improvement of lung volume (forced vital capacity; FVC). The mechanisms of these different response patterns to bronchodilator remain unclear. We investigated the associations between bronchodilator responsiveness and quantitative computed tomography (CT) indices in patients with COPD.

METHODS

Data on a total of 101 patients with stable COPD were retrospectively analysed. Volume and flow responses to bronchodilator were assessed by FVC and FEV(1) changes before and after inhalation of salbutamol (400 μg). Volumetric CT was performed to quantify emphysema, air trapping and large airway thickness. Emphysema was assessed by the volume fraction of the lung under -950 Hounsfield units (HU; V(950)) at full inspiration and air trapping by the ratio of mean lung density (MLD) at full expiration and inspiration. Airway wall thickness and wall area percentage (WA%; defined as wall area/[wall area + lumen area] × 100), were measured near the origin of right apical and left apico-posterior bronchus.

RESULTS

Among quantitative CT indices, the CT emphysema index (V(950 insp)) showed a significant negative correlation with postbronchodilator FEV(1) change (R = -0·213, P = 0·004), and the CT air-trapping index correlated positively with postbronchodilator FVC change(R = 0·286, P≤0·001). Multiple linear regression analysis showed that CT emphysema index had independent association with postbronchodilator FEV(1) change and CT air-trapping index with postbronchodilator FVC change.

CONCLUSION

The degrees of emphysema and air trapping may contribute to the different response patterns to bronchodilator in patients with COPD.

Lung imaging in asthmatic patients: the picture is clearer

Imaging of the lungs in patients with asthma has evolved dramatically over the last decade with sophisticated techniques, such as computed tomography, magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography. New insights into current and future modalities for imaging in asthmatic patients and their application are discussed to potentially shed a clearer picture of the underlying pathophysiology of asthma, especially severe asthma, and the proposed clinical utility of imaging in patients with this common disease.

Predictors of pulmonary function response to treatment with salmeterol/fluticasone in patients with chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a heterogeneous disease and responses to therapies are highly variable. The aim of this study was to identify the predictors of pulmonary function response to 3 months of treatment with salmeterol/fluticasone in patients with COPD. A total of 127 patients with stable COPD from the Korean Obstructive Lung Disease (KOLD) Cohort, which were prospectively recruited from June 2005 to September 2009, were analyzed retrospectively. The prediction models for the FEV(1), FVC and IC/TLC changes after 3 months of treatment with salmeterol/fluticasone were constructed by using multiple, stepwise, linear regression analysis. The prediction model for the FEV(1) change after 3 months of treatment included wheezing history, pre-bronchodilator FEV(1), post-bronchodilator FEV(1) change and emphysema extent on CT (R = 0.578). The prediction models for the FVC change after 3 months of treatment included pre-bronchodilator FVC, post-bronchodilator FVC change (R = 0.533), and those of IC/ TLC change after 3 months of treatment did pre-bronchodilator IC/TLC and post-bronchodilator FEV(1) change (R = 0.401). Wheezing history, pre-bronchodilator pulmonary function, bronchodilator responsiveness, and emphysema extent may be used for predicting the pulmonary function response to 3 months of treatment with salmeterol/fluticasone in patients with COPD.

Individual canine airway response variability to a deep inspiration

In healthy individuals, a DI can reverse (bronchodilation) or prevent (bronchoprotection) induced airway constriction. For individuals with asthma or COPD, these effects may be attenuated or absent. Previous work showed that the size and duration of a DI affected the subsequent response of the airways. Also, increased airway tone lead to increased airway size variability. The present study examined how a DI affected the temporal variability in individual airway baseline size and after methacholine challenge in dogs using High-Resolution Computed Tomography. Dogs were anesthetized and ventilated, and on 4 separate days, HRCT scans were acquired before and after a DI at baseline and during a continuous intravenous infusion of methacholine (Mch) at 3 dose rates (17, 67, and 200 μg/min). The Coefficient of Variation was used as an index of temporal variability in airway size.We found that at baseline and the lowest dose of Mch, variability decreased immediately and 5 minutes after the DI (P < 0.0001). In contrast, with higher doses of Mch, the DI caused a variable response. At a rate of 67 μg/min of Mch, the temporal variability increased after 5 minutes, while at a rate of 200 μg/min of Mch, the temporal variability increased immediately after the DI. Increased airway temporal variability has been shown to be associated with asthma. Although the mechanisms underlying this temporal variability are poorly understood, the beneficial effects of a DI to decrease airway temporal variability was eliminated when airway tone was increased. If this effect is absent in asthmatics, this may suggest a possible mechanism for the loss of bronchoprotective and bronchodilatory effects after a DI in asthma.

The lung physiome: merging imaging-based measures with predictive computational models

Global measurements of the lung provided by standard pulmonary function tests do not give insight into the regional basis of lung function and lung disease. Advances in imaging methodologies, computer technologies, and subject-specific simulations are creating new opportunities to study structure-function relationships in the lung through multidisciplinary research. The digital Human Lung Atlas is an image-based resource compiled from male and female subjects spanning several decades of age. The Atlas comprises both structural and functional measures, and includes computational models derived to match individual subjects for personalized prediction of function. The computational models in the Atlas form part of the Lung Physiome project, which is an international effort to develop integrative models of lung function at all levels of biological organization. The computational models provide mechanistic interpretation of imaging measures; the Atlas provides structural data on which to base model geometry, and functional data against which to test hypotheses. The example of simulating airflow on a subject-specific basis is considered. Methods for deriving multiscale models of the airway geometry for individual subjects in the Atlas are outlined, and methods for modeling turbulent flows in the airway are reviewed.

The bronchodilator response of in vivo specific airway compliance in adults with asthma

A new technique has been developed to determine in vivo airway compliance in humans that is specific to airway size and transpulmonary pressure, and can be represented as a three-dimensional surface. As yet, the ability of this technique to detect changes in specific airway compliance with disease status has not been demonstrated. The aim of this study was to assess whether this technique could determine changes in airway compliance which are thought to occur with altered smooth muscle tone in adults with asthma. Airway compliance was measured and displayed as a surface in adults with asthma before and after a reduction in smooth muscle tone by bronchodilator administration. Compliance, with respect to airway size, was calculated at three specific lung volumes; functional residual capacity (FRC), total lung capacity (TLC), and midway between FRC and TLC (MID). After bronchodilator, airway compliance increased at FRC and MID in the smaller airways (<3 mm). Furthermore, airway compliance under both conditions was greater in the smaller airways compared to the larger airways. In conclusion, our method may have future utility in assessing changes in airway compliance in respiratory diseases such as asthma.

Measuring small airways in transverse CT images correction for partial volume averaging and airway tilt

RATIONALE AND OBJECTIVES

Airway wall dimensions can be determined in vivo using transverse computed tomographic (CT) images, but the measurement of airway phantoms shows that the wall thickness is consistently overestimated for small airways. This phantom study was performed to derive and test corrections to the measurements on the basis of consideration of partial volume averaging and tilt effects.

MATERIALS AND METHODS

A lung phantom with six polycarbonate tubes embedded in foam was scanned, and the cross-sectional dimensions of the tubes were determined using the full width at half maximum, zero crossing, and phase congruency edge detection methods. Equations were derived using the reported wall intensity to correct for partial volume averaging. Corrections for the overestimation of the wall thickness due to the tilt of the tube with respect to the CT z-axis were also derived.

RESULTS

All three methods (full width at half maximum, zero crossing, and phase congruency) overestimated the wall thickness of the small polycarbonate tubes. It was verified that two sources of error were partial volume averaging and tilt that was introduced when the phantom was positioned with tube axes at an angle to the CT z-axis. The corrections were applied to the measured tube wall dimensions and substantially reduced the deviation of the CT measurements from the true values.

CONCLUSIONS

Correcting for partial volume effects and airway tilt greatly increases the accuracy of simulated airway wall measurements in transverse CT images.

Effect of parenchymal stiffness on canine airway size with lung inflation

Although airway patency is partially maintained by parenchymal tethering, this structural support is often ignored in many discussions of asthma. However, agonists that induce smooth muscle contraction also stiffen the parenchyma, so such parenchymal stiffening may serve as a defense mechanism to prevent airway narrowing or closure. To quantify this effect, specifically how changes in parenchymal stiffness alter airway size at different levels of lung inflation, in the present study, we devised a method to separate the effect of parenchymal stiffening from that of direct airway narrowing. Six anesthetized dogs were studied under four conditions: baseline, after whole lung aerosol histamine challenge, after local airway histamine challenge, and after complete relaxation of the airways. In each of these conditions, we used High resolution Computed Tomography to measure airway size and lung volume at five different airway pressures (0, 12, 25, 32, and 45 cm H(2)O). Parenchymal stiffening had a protective effect on airway narrowing, a fact that may be important in the airway response to deep inspiration in asthma. When the parenchyma was stiffened by whole lung aerosol histamine challenge, at every lung volume above FRC, the airways were larger than when they were directly challenged with histamine to the same initial constriction. These results show for the first time that a stiff parenchyma per se minimizes the airway narrowing that occurs with histamine challenge at any lung volume. Thus in clinical asthma, it is not simply increased airway smooth muscle contraction, but perhaps a lack of homogeneous parenchymal stiffening that contributes to the symptomatic airway hyperresponsiveness.

A method to determine in vivo, specific airway compliance, in humans

In order to understand the pathophysiology of diseases such as asthma and chronic obstructive pulmonary disease, it is essential to measure the mechanical properties of the airways. Currently, there are no methods to measure and quantify in vivo airway compliance in humans. In order to develop a method, we generated a curve-fitting algorithm that combines airway diameter measurements by high resolution computed tomography with pressure-volume curves obtained by the esophageal balloon technique. Our method allows the description of diameter-pressure curves for airways of varying size, presented as a 3D surface, from which specific airway compliance can be determined at any transpulmonary pressure. Applying this method to data from two healthy subjects, we found that small airways are more compliant than large airways and specific airway compliance was greatest at low transpulmonary pressures. In conclusion, our 3D surface is a useful tool to measure and quantify in vivo specific airway compliance in humans.

Influence of rapid fluid loading on airway structure and function in healthy humans

BACKGROUND

The present study examined the influence of rapid intravenous fluid loading (RFL) on airway structure and pulmonary vascular volumes using computed tomography imaging and the subsequent impact on pulmonary function in healthy adults (n = 16).

METHODS AND RESULTS

Total lung capacity (DeltaTLC = -6%), forced vital capacity (DeltaFVC = -14%), and peak expiratory flow (DeltaPEF = -19%) decreased, and residual volume (DeltaRV = +38%) increased post-RFL (P < .05). Airway luminal cross-sectional area (CSA) decreased at the trachea, and at airway generation 3 (P < .05), wall thickness changed minimally with a tendency for increasing in generation five (P = .13). Baseline pulmonary function was positively associated with airway luminal CSA; however, this relationship deteriorated after RFL. Lung tissue volume and pulmonary vascular volumes increased 28% (P < .001) post-RFL, but did not fully account for the decline in TLC.

CONCLUSIONS

These data suggest that RFL results in obstructive/restrictive PF changes that are most likely related to structural changes in smaller airways or changes in extrapulmonary vascular beds.

Automatic lung segmentation in CT images with accurate handling of the hilar region

A fully automated and three-dimensional (3D) segmentation method for the identification of the pulmonary parenchyma in thorax X-ray computed tomography (CT) datasets is proposed. It is meant to be used as pre-processing step in the computer-assisted detection (CAD) system for malignant lung nodule detection that is being developed by the Medical Applications in a Grid Infrastructure Connection (MAGIC-5) Project. In this new approach the segmentation of the external airways (trachea and bronchi), is obtained by 3D region growing with wavefront simulation and suitable stop conditions, thus allowing an accurate handling of the hilar region, notoriously difficult to be segmented. Particular attention was also devoted to checking and solving the problem of the apparent 'fusion' between the lungs, caused by partial-volume effects, while 3D morphology operations ensure the accurate inclusion of all the nodules (internal, pleural, and vascular) in the segmented volume. The new algorithm was initially developed and tested on a dataset of 130 CT scans from the Italung-CT trial, and was then applied to the ANODE09-competition images (55 scans) and to the LIDC database (84 scans), giving very satisfactory results. In particular, the lung contour was adequately located in 96% of the CT scans, with incorrect segmentation of the external airways in the remaining cases. Segmentation metrics were calculated that quantitatively express the consistency between automatic and manual segmentations: the mean overlap degree of the segmentation masks is 0.96 ± 0.02, and the mean and the maximum distance between the mask borders (averaged on the whole dataset) are 0.74 ± 0.05 and 4.5 ± 1.5, respectively, which confirms that the automatic segmentations quite correctly reproduce the borders traced by the radiologist. Moreover, no tissue containing internal and pleural nodules was removed in the segmentation process, so that this method proved to be fit for the use in the framework of a CAD system. Finally, in the comparison with a two-dimensional segmentation procedure, inter-slice smoothness was calculated, showing that the masks created by the 3D algorithm are significantly smoother than those calculated by the 2D-only procedure.

Prevalence of tracheal collapse in an emphysema cohort as measured with end-expiration CT

RATIONALE AND OBJECTIVES

To retrospectively investigate the prevalence of tracheal collapse in an emphysema cohort. The occurrence of a large degree of tracheal collapse may have important implications for the clinical management of respiratory symptoms and air trapping in patients with emphysema.

MATERIALS AND METHODS

Paired full-inspiratory and end-expiratory thin-section volumetric computed tomographic scans were available for 1071 long-term smokers with clinically and physiologically confirmed emphysema. The percentage reduction in the cross-sectional tracheal luminal area from full-inspiration to end-expiration was automatically computed at 2.5-mm intervals along the centerline of the trachea using customized software.

RESULTS

Maximal tracheal collapse did not follow a normal distribution in the emphysema cohort (P < .0001, skewness/kurtosis tests for normality); the median collapse was 18% (intraquartile range, 11%-30%). Statistically significant differences were found in the distribution of maximal collapse by gender (P < .005, Wilcoxon rank sum test). Overall, 10.5% of men and 17.1% of women showed evidence of tracheomalacia on the basis of the criterion of a reduction of 50% or greater in cross-sectional tracheal luminal area at end-expiration.

CONCLUSION

This study offers insights into the prevalence of tracheal collapse in a cohort of patients with emphysema; future work is needed to determine the possible relationship between tracheal collapse and air trapping in subjects with emphysema.

Finite element 3D reconstruction of the pulmonary acinus imaged by synchrotron X-ray tomography

The alveolated structure of the pulmonary acinus plays a vital role in gas exchange function. Three-dimensional (3D) analysis of the parenchymal region is fundamental to understanding this structure-function relationship, but only a limited number of attempts have been conducted in the past because of technical limitations. In this study, we developed a new image processing methodology based on finite element (FE) analysis for accurate 3D structural reconstruction of the gas exchange regions of the lung. Stereologically well characterized rat lung samples (Pediatr Res 53: 72-80, 2003) were imaged using high-resolution synchrotron radiation-based X-ray tomographic microscopy. A stack of 1,024 images (each slice: 1024 x 1024 pixels) with resolution of 1.4 mum(3) per voxel were generated. For the development of FE algorithm, regions of interest (ROI), containing approximately 7.5 million voxels, were further extracted as a working subunit. 3D FEs were created overlaying the voxel map using a grid-based hexahedral algorithm. A proper threshold value for appropriate segmentation was iteratively determined to match the calculated volume density of tissue to the stereologically determined value (Pediatr Res 53: 72-80, 2003). The resulting 3D FEs are ready to be used for 3D structural analysis as well as for subsequent FE computational analyses like fluid dynamics and skeletonization.

Investigation of hyperpolarized 3He magnetic resonance imaging utility in examining human airway diameter behavior in asthma through comparison with high-resolution computed tomography

RATIONALE AND OBJECTIVES

Application of a previously developed model-based algorithm on hyperpolarized (HP) (3)He magnetic resonance (MR) dynamic projection images of phantoms was extended to investigate the utility of HP (3)He MR imaging (MRI) in quantifying airway caliber changes associated with asthma.

MATERIALS AND METHODS

Airways of seven volunteers were imaged and measured using HP (3)He MRI and multidetector-row computed tomography (MDCT) before and after a methacholine (MCh) challenge. MDCT data were obtained at functional residual capacity and 1 L above functional residual capacity.

RESULTS

Comparison of the resultant data showed that HP (3)He MRI did not match MDCT in measuring the ratios of airway calibers before and after the MCh challenge in 37% to 43% of the airways from the first six generations at the two lung volumes tested. However, MDCT did yield the observation that 49% to 69% of these airways displayed bronchodilation following MCh challenge.

CONCLUSION

The current implementation of HP (3)He MRI did not match the MCh-induced postchallenge-to-prechallenge airway caliber ratios as measured with MDCT. Elevated parenchymal tethering due to bronchoconstriction-induced hyperinflation was proposed as a possible explanation for this airway dilation.

Semi-automatic measurement of the airway dimension by computed tomography using the full-width-half-maximum method: a study on the measurement accuracy according to the CT parameters and size of the airway

OBJECTIVE

To assess the influence of variable factors such as the size of the airway and the CT imaging parameters such as the reconstruction kernel, field-of-view (FOV), and slice thickness on the automatic measurement of airway dimension.

MATERIALS AND METHODS

An airway phantom was fabricated that contained eleven poly-acryl tubes of various lumen diameters and wall thicknesses. The measured density of the poly-acryl wall was 150 HU, and the measured density of the airspace filled with polyurethane foam was -900 HU. CT images were obtained using a 16-MDCT (multidetector CT) scanner and were reconstructed with various reconstruction kernels, thicknesses and FOV. The luminal radius and wall thickness were measured using in-house software based on the full-width-half-maximum method. The measured values as determined by CT and the actual dimensions of the tubes were compared.

RESULTS

Measurements were most accurate on images reconstructed with use of a standard kernel (mean error: -0.03 +/- 0.21 mm for wall thickness and -0.12 +/- 0.11 mm for the luminal radius). There was no significant difference in accuracy among images with the use of variable slice thicknesses or a variable FOV. Below a 1-mm threshold, the measurement failed to represent the change of the real dimensions.

CONCLUSION

Measurement accuracy was strongly influenced by the specific reconstruction kernel utilized. For accurate measurement, standardization of the imaging protocol and selection of the appropriate anatomic level are essential.

Quantitative assessment of emphysema, air trapping, and airway thickening on computed tomography

The severity of chronic obstructive pulmonary disease (COPD) is evaluated not only by airflow limitation but also by factors such as exercise capacity and body mass index. Recent advances in CT technology suggest that it might be a useful tool for evaluating the severity of the disease components of COPD. The aim of this study is to evaluate the correlation between the parameters measured on volumetric CT, including the extent of emphysema, air trapping, and airway thickening, and clinical parameters. CT scans were performed in 34 patients with COPD at full inspiration and expiration. We used in-house software to measure CT parameters, including volume fraction of emphysema (V(950)), mean lung density (MLD), CT air trapping index (CT ATI), segmental bronchial wall area (WA), lumen area (LA), and wall area percent (WA%). We found that the CT parameters were correlated with the pulmonary function test (PFT) results, body mass index (BMI), the modified Medical Research Council Dyspnea scale (MMRC scale), the six-minute-walk distance (6MWD), and the BODE index. V(950 insp) correlated to the BMI, FEV(1), 6MWD, and the BODE index. The CT ATI correlated with the physiologic ATI (VC-FVC) (R=0.345, p=0.045) and the MMRC scale (R=0.532, p=0.001). There was a positive correlation between the WA% and the BMI (R=0.563, p<0.001). MLD(exp) showed the strongest correlation with the BODE index (R= -0.756, p<0.001). We conclude that the severity of emphysema and air trapping measured on CT correlated with the PFT parameters 6MWD and BMI.

Functional imaging: CT and MRI

Numerous imaging techniques permit evaluation of regional pulmonary function. Contrast-enhanced CT methods now allow assessment of vasculature and lung perfusion. Techniques using spirometric controlled multi-detector row CT allow for quantification of presence and distribution of parenchymal and airway pathology; xenon gas can be employed to assess regional ventilation of the lungs, and rapid bolus injections of iodinated contrast agent can provide a quantitative measure of regional parenchymal perfusion. Advances in MRI of the lung include gadolinium-enhanced perfusion imaging and hyperpolarized gas imaging, which allow functional assessment, including ventilation/perfusion, microscopic air space measurements, and gas flow and transport dynamics.

Temporal variability in the responses of individual canine airways to methacholine

Previous work showed that individual airway size, before any spasmogen, varied widely in the same animals on different days. The effect of this variable baseline size on the airway response to a subsequent challenge is unknown. The present study examined how the variability in individual airway baseline size in dogs was related to that after methacholine challenge on 4 different days using high-resolution computed tomography scans. Dogs were anesthetized and ventilated, and on 4 separate days randomly varying between 1 and 8 wk apart, baseline scans were acquired, followed by a continuous intravenous infusion of methacholine at three rates in increasing order (17, 67, and 200 microg/min). As the measure of variability, we used the coefficient of variation (CV) of the four airway luminal measurements of each airway at baseline and at each dose of methacholine. For most airways, there was wide variability both between and within dogs in the response to a given dose of methacholine (CV = 33-38%). Airways with any level of methacholine stimulation had greater variability than those at baseline. The airway variability was greatest at the lowest dose of methacholine administered but was elevated at all the doses. In conclusion, there was substantial day-to-day variability in baseline airway size. Most importantly, the same dose of methacholine to the same individual airway showed even greater variability than that at baseline. If we consider that increased heterogeneity may potentiate clinical symptoms, then airway response variability may play an important role in the manifestation of airway disease.

Effect of Bronchial Thermoplasty on Airway Closure

BACKGROUND: Bronchial Thermoplasty, a procedure that applies thermal energy to the airway wall has been shown to impair the ability of airway to contract in response to methacholine chloride (Mch). The technique has been advocated as an alternative treatment for asthma that may permanently limit airway narrowing. In previous experimental studies in dogs and humans, it was shown that those airways treated with bronchial thermoplasty had significant impairment of Mch responsiveness. METHODS: In the present study, we investigated the ability of canine airways to close completely with very high concentrations of Mch after bronchial thermoplasty. Bronchial thermoplasty was performed on dogs using the Alair System, comprising a low power RF controller and a basket catheter with four electrodes. A local atomization of Mch agonist was delivered directly to the epithelium of the same airway locations with repeated challenges. Airway size was measured with computed tomography, and closure was considered to occur in any airway where the lumen fell below the resolution of the scanner (< 1 mm). RESULTS: Our results show that, while treated airways still have the capacity to close at very high doses of Mch, this ability is seriously impaired after treatment, requiring much higher doses. CONCLUSIONS: Bronchial thermoplasty as currently applied seems to simply shift the entire dose response curve toward increasing airway size. Thus, this procedure simply serves to minimize the ability of airways to narrow under any level of stimulation.

Automatic segmentation of 3D micro-CT coronary vascular images

Although there are many algorithms available in the literature aimed at segmentation and model reconstruction of 3D angiographic images, many are focused on characterizing only a part of the vascular network. This study is motivated by the recent emerging prospects of whole-organ simulations in coronary hemodynamics, autoregulation and tissue oxygen delivery for which anatomically accurate vascular meshes of extended scale are highly desirable. The key requirements of a reconstruction technique for this purpose are automation of processing and sub-voxel accuracy. We have designed a vascular reconstruction algorithm which satisfies these two criteria. It combines automatic seeding and tracking of vessels with radius detection based on active contours. The method was first examined through a series of tests on synthetic data, for accuracy in reproduced topology and morphology of the network and was shown to exhibit errors of less than 0.5 voxel for centerline and radius detections, and 3 degrees for initial seed directions. The algorithm was then applied on real-world data of full rat coronary structure acquired using a micro-CT scanner at 20 microm voxel size. For this, a further validation of radius quantification was carried out against a partially rescanned portion of the network at 8 microm voxel size, which estimated less than 10% radius error in vessels larger than 2 voxels in radius.

The effects of systemic lidocaine on airway tone and pulmonary function in asthmatic subjects

BACKGROUND

To prevent reflex-induced bronchoconstriction in patients with asthma, local anesthetics are commonly administered by aerosol or IV as adjunct medication. Lidocaine attenuates responsiveness to a neurally active stimulus that increases tone, but there is scant information about the effect of lidocaine on baseline airway tone. Therefore we examined the effects of IV lidocaine on baseline airway tone in asthmatic subjects.

METHODS

Small, medium, and large airways (2-5, 5-8, >8 mm diameter) were analyzed by computed tomography in 15 asthmatic volunteers under baseline conditions and during infusion of lidocaine. Changes in luminal airway diameter and wall thickness from baseline to during lidocaine infusion, and the change in pulmonary function induced by lidocaine, were analyzed.

RESULTS

Lidocaine caused a significant decrease in the forced expiratory volume in 1 s pulmonary function measure (7 +/- 2%, P = 0.006). There was also a small but significant decrease in the airway luminal diameter at total lung capacity during lidocaine infusion compared to baseline (-3 +/- 0.5%, P < 0.001). Moreover, there was a significant correlation between the change in forced expiratory volume in 1 s and the change in airway luminal diameter at total lung capacity (r2 = 0.47, P = 0.01).

CONCLUSION

Lidocaine, which reduces airway responsiveness to drugs that cause bronchospasm through sensory nerve activation, did not reduce baseline airway tone. Instead, even when administered IV, lidocaine significantly increased airway tone and caused airway narrowing. Therefore, while the administration of lidocaine can prevent intubation-induced bronchospasm, the airways should be constantly monitored by auscultation even during IV lidocaine administration.

The Use of MDCT-Based Computer-Aided Pathway Finding for Mediastinal and Perihilar Lymph Node Biopsy: A Randomized Controlled Prospective Trial

BACKGROUND

Mediastinal and perihilar lymph node samples can be acquired safely through the transbronchial approach during a bronchoscopic examination that is usually required as part of the evaluation of suspected lung cancer. Typically, needle aspiration samples are performed and needle cores can be sampled if the operator is confident that the needle is within the lymph node target, partly because of the risk of bleeding if a large blood vessel is sampled during core biopsy, especially in the perihilar region. Many bronchoscopists have difficulty assessing the three-dimensional (3D) positioning for needle sampling during these procedures, especially when relying on multidetector-row computerized tomography (MDCT) images displayed two-dimensionally seen prior to and usually during the procedure.

OBJECTIVE

We have developed and evaluated a process model and associated software for providing interactive 3D displays of the MDCT data for procedure planning and real-time virtual bronchoscopic pathfinding for these procedures.

METHODS

We undertook a prospective randomized clinical study for evaluating the computer-aided pathfinding assistance in mediastinal lymph node biopsies in 87 consenting subjects.

RESULTS

We demonstrate that the addition of this computer-aided pathfinding improved operator performance in perihilar and paratracheal lymph node sampling (100 vs. 69%) but not in subcarinal sampling (82 vs. 85%). Overall success with lymph node sampling is 92% using the computer-aided method and 77% using standard clinical practice.

CONCLUSIONS

The type of computer-aided pathway assistance described here, using 3D MDCT scanning information obtained before the procedure, but interacting with real-time bronchoscopic images during the bronchoscopic procedure, should improve the confidence of most bronchoscopists in performing these procedures, with improved clinical outcomes, and will add to the personalization of medicine through imaging.

Assessment of Airways with Three-dimensional Quantitative Thin-Section CT: In Vitro and in Vivo Validation

PURPOSE

To prospectively validate the ability of customized three-dimensional (3D) software to enable bronchial tree skeletonization, orthogonal reconstruction of the main bronchial axis, and measurement of cross-sectional wall area (WA) and lumen area (LA) of any visible bronchus on thin-section computed tomographic (CT) images.

MATERIALS AND METHODS

Institutional review board approval and patient agreement and informed consent were obtained. Software was validated in a phantom that consisted of seven tubes and an excised human lung obtained and used according to institutional guidelines. In vivo validation was performed with multi-detector row CT in six healthy subjects (mean age, 47 years; range, 20-55 years). Intra- and interobserver agreement and reproducibility over time for bronchial tree skeletonization were evaluated with Bland-Altman analysis. Concordance in identifying bronchial generation was assessed with the kappa statistic. WA and LA obtained with the manual method were compared with WA and LA obtained with validated software by means of the Wilcoxon test and Bland-Altman analysis.

RESULTS

WA and LA measurements in the phantom were reproducible over multiple sessions (P > .90) and were not significantly different from WA and LA assessed with the manual method (P > .62). WA and LA measurements in the excised lung and the subjects were not different from measurements obtained with the manual method (intraclass correlation coefficient > 0.99). All lobar bronchi and 80.8% of third generation bronchi, 72.5% of fourth generation bronchi, and 37.7% of fifth generation bronchi were identified in vivo. Intra- and interobserver agreement and reproducibility over time for airway skeletonization and concordance in identifying bronchial generation were good to excellent (intraclass correlation coefficient > 0.98, kappa > 0.54, respectively).

CONCLUSION

This method enables accurate and reproducible measurement of WA and LA on reformatted CT sections perpendicular to the main axis of bronchi visible on thin-section CT scans.

Mechanisms of limited airway dimension with lung inflation

Airways distend with each inspiration, while a sigh or deep inspiration (DI) leads to a significant or a maximum distension of the airways. Distension of the airways is thought to play an important role in maintaining airway patency. Limited distension of the airways with lung inflation may be a major factor in certain lung diseases such as asthma and chronic obstructive pulmonary disease (COPD). High resolution computed tomography (HRCT) has gained wide acceptance as a diagnostic and investigational radiological tool for the evaluation of airway function. HRCT has been used to measure dynamic changes in airway caliber in vivo that are not detectable by conventional global lung measurements such as airway and lung resistance. HRCT is uniquely capable of imaging and quantifying airway size at different lung volumes. The current paper reviews the use of HRCT to examine the role of lung inflation on airway distension in animal models, and discusses potential mechanisms for limited distension of the airways with lung inflation in individuals with asthma and COPD.

Airflow limitation and airway dimensions in chronic obstructive pulmonary disease

RATIONALE

Chronic obstructive pulmonary disease (COPD) is characterized by airflow limitation caused by emphysema and/or airway narrowing. Computed tomography has been widely used to assess emphysema severity, but less attention has been paid to the assessment of airway disease using computed tomography.

OBJECTIVES

To obtain longitudinal images and accurately analyze short axis images of airways with an inner diameter>or=2 mm located anywhere in the lung with new software for measuring airway dimensions using curved multiplanar reconstruction.

METHODS

In 52 patients with clinically stable COPD (stage I, 14; stage II, 22; stage III, 14; stage IV, 2), we used the software to analyze the relationship of the airflow limitation index (FEV1, % predicted) with the airway dimensions from the third to the sixth generations of the apical bronchus (B1) of the right upper lobe and the anterior basal bronchus (B8) of the right lower lobe.

MEASUREMENTS AND MAIN RESULTS

Airway luminal area (Ai) and wall area percent (WA%) were significantly correlated with FEV1 (% predicted). More importantly, the correlation coefficients (r) improved as the airways became smaller in size from the third (segmental) to sixth generations in both bronchi (Ai: r=0.26, 0.37, 0.58, and 0.64 for B1; r=0.60, 0.65, 0.63, and 0.73 for B8).

CONCLUSIONS

We are the first to use three-dimensional computed tomography to demonstrate that airflow limitation in COPD is more closely related to the dimensions of the distal (small) airways than proximal (large) airways.

Matching and anatomical labeling of human airway tree

Matching of corresponding branchpoints between two human airway trees, as well as assigning anatomical names to the segments and branchpoints of the human airway tree, are of significant interest for clinical applications and physiological studies. In the past, these tasks were often performed manually due to the lack of automated algorithms that can tolerate false branches and anatomical variability typical for in vivo trees. In this paper, we present algorithms that perform both matching of branchpoints and anatomical labeling of in vivo trees without any human intervention and within a short computing time. No hand-pruning of false branches is required. The results from the automated methods show a high degree of accuracy when validated against reference data provided by human experts. 92.9% of the verifiable branchpoint matches found by the computer agree with experts' results. For anatomical labeling, 97.1% of the automatically assigned segment labels were found to be correct.

Quantitative analysis of pulmonary airway tree structures

A method for computationally efficient skeletonization of three-dimensional tubular structures is reported. The method is specifically targeting skeletonization of vascular and airway tree structures in medical images but it is general and applicable to many other skeletonization tasks. The developed approach builds on the following novel concepts and properties: fast curve-thinning algorithm to increase computational speed, endpoint re-checking to avoid generation of spurious side branches, depth-and-length sensitive pruning, and exact tree-branch partitioning allowing branch volume and surface measurements. The method was validated in computer and physical phantoms and in vivo CT scans of human lungs. The validation studies demonstrated sub-voxel accuracy of branch point positioning, insensitivity to changes of object orientation, and high reproducibility of derived quantitative indices of the tubular structures offering a significant improvement over previously reported methods (p<<0.001).