Decreased computed tomographic lung density during exacerbation of asthma

Small airways in asthma: From inflammation and pathophysiology to treatment response

Small airways are characterized as those with an inner diameter less than 2 mm and constitute a major site of pathology and inflammation in asthma disease. It is estimated that small airways dysfunction may occur before the emergence of noticeable symptoms, spirometric abnormalities and imaging findings, thus characterizing them as "the quiet or silent zone" of the lungs. Despite their importance, measuring and quantifying small airways dysfunction presents a considerable challenge due to their inaccessibility in usual functional measurements, primarily due to their size and peripheral localization. Several pulmonary function tests have been proposed for the assessment of the small airways, including impulse oscillometry, nitrogen washout, body plethysmography, as well as imaging methods. Nevertheless, none of these methods has been established as the definitive "gold standard," thus, a combination of them should be used for an effective assessment of the small airways. Widely used asthma treatments seem to also affect several parameters of the small airways. Emerging biologic treatments show promising results in reducing small airways inflammation and remodelling, providing evidence for potential alterations in the disease's progression and outcomes. These novel therapies have implications not only in the clinical aspects of asthma but also in its inflammatory and functional aspects.

The effect of exacerbations on lung density in α1-antitrypsin deficiency

Background

Acute exacerbations of COPD (AECOPD) have unclear impacts on emphysema measurement using computed tomography (CT)-derived 15th percentile lung density (PD15). The aim of this study was to assess the influence of AECOPD on PD15 lung density in α₁-antitrypsin deficiency.

Methods

In a post hoc analysis of the RAPID (Randomised Trial of Augmentation Therapy in α₁-Proteinase Inhibitor Deficiency) trial, raw marginal residuals of PD15 (measured - predicted) were determined by fitting a regression line to individual patient CT data. These deviations from the expected slope were compared by age, sex, baseline forced expiratory volume in 1 s, diffusing capacity of the lungs for carbon monoxide % predicted and PD15, inhaled corticosteroid use and treatment group.

Results

Positive and negative residuals (reflecting higher or lower lung density than predicted from regression) were observed, which declined in magnitude over time following AECOPD events. Logistic regression confirmed a limited effect of patient characteristics on the absolute size of residuals, whereas AECOPD within 6 weeks of CT had a notable effect versus no AECOPD within 6 weeks (OR 5.707, 95% CI 3.375-9.652; p<0.0001).

Conclusion

AECOPD result in higher or lower CT lung density estimates; the effect is greatest in the 2 weeks immediately after an AECOPD and persists for <6 weeks. Patient characteristics were less relevant than AECOPD within 6 weeks, supporting the reliability of PD15 as a measure of lung density. An exacerbation-free period prior to CT scan is advisable to reduce signal-to-noise ratio in future clinical trials.

Quantification of lung function on CT images based on pulmonary radiomic filtering

PURPOSE

To develop a radiomics filtering technique for characterizing spatial-encoded regional pulmonary ventilation information on lung computed tomography (CT).

METHODS

The lung volume was segmented on 46 CT images, and a 3D sliding window kernel was implemented across the lung volume to capture the spatial-encoded image information. Fifty-three radiomic features were extracted within the kernel, resulting in a fourth-order tensor object. As such, each voxel coordinate of the original lung was represented as a 53-dimensional feature vector, such that radiomic features could be viewed as feature maps within the lungs. To test the technique as a potential pulmonary ventilation biomarker, the radiomic feature maps were compared to paired functional images (Galligas PET or DTPA-SPECT) based on the Spearman correlation (ρ) analysis.

RESULTS

The radiomic feature maps GLRLM-based Run-Length Non-Uniformity and GLCOM-based Sum Average are found to be highly correlated with the functional imaging. The achieved ρ (median [range]) for the two features are 0.46 [0.05, 0.67] and 0.45 [0.21, 0.65] across 46 patients and 2 functional imaging modalities, respectively.

CONCLUSIONS

The results provide evidence that local regions of sparsely encoded heterogeneous lung parenchyma on CT are associated with diminished radiotracer uptake and measured lung ventilation defects on PET/SPECT imaging. These findings demonstrate the potential of radiomics to serve as a complementary tool to the current lung quantification techniques and provide hypothesis-generating data for future studies.

Menstrual cycle impacts lung structure measures derived from quantitative computed tomography

OBJECTIVE

Quantitative computed tomography (qCT) is being increasingly incorporated in research studies and clinical trials aimed at understanding lung disease risk, progression, exacerbations, and intervention response. Menstrual cycle-based changes in lung function are recognized; however, the impact on qCT measures is currently unknown. We hypothesize that the menstrual cycle impacts qCT-derived measures of lung structure in healthy women and that the degree of measurement change may be mitigated in subjects on cyclic hormonal birth control.

METHODS

Thirty-one non-smoking, healthy women with regular menstrual cycles (16 of which were on cyclic hormonal birth control) underwent pulmonary function testing and qCT imaging at both menses and early luteal phase time points. Data were evaluated to identify lung measurements which changed significantly across the two key time points and to compare degree of change across metrics for the sub-cohort with versus without birth control.

RESULTS

The segmental airway measurements were larger and mean lung density was higher at menses compared to the early luteal phase. The sub-cohort with cyclic hormonal birth control did not have less evidence of measurement difference over the menstrual cycle compared to the sub-cohort without hormonal birth control.

CONCLUSIONS

This study provides evidence that qCT-derived measures from the lung are impacted by the female menstrual cycle. This indicates studies seeking to use qCT as a more sensitive measure of cross-sectional differences or longitudinal changes in these derived lung measurements should consider acquiring data at a consistent time in the menstrual cycle for pre-menopausal women and warrants further exploration.

KEY POINTS

• Lung measurements from chest computed tomography are used in multicenter studies exploring lung disease progression and treatment response. • The menstrual cycle impacts lung structure measurements. • Cyclic variability should be considered when evaluating longitudinal change with CT in menstruating women.

Quantitative evaluation of computed tomography findings in patients with bronchial asthma: prediction of therapeutic response

INTRODUCTION

To compare pretreatment quantitative CT parameters between patients with well-controlled and those with poorly controlled bronchial asthma after treatment.

METHODS

We retrospectively reviewed 785 patients with clinical diagnosis of bronchial asthma from January 2009 to April 2015. Of these, 43 patients underwent high-resolution CT and pulmonary function tests at initial diagnosis. According to the Global Initiative for Asthma (GINA) 2015 guidelines, the patients were classified into two groups (well-controlled (GINA1), n = 18; poorly controlled (GINA2 and GINA3), n = 25). Quantitative measurements for airways (total cross-sectional area (TA), lumen area (LA), wall area (WA) and wall area percentage (WA%)), air trapping and emphysema were performed on initial pretreatment CT scans. We compared CT measurements for airways between well-controlled and poorly controlled groups and also compared those between ever-smokers and never-smokers. The significant quantitative CT parameters were evaluated with multiple regression analysis.

RESULTS

The TA and the WA demonstrated significantly higher values in the poorly controlled than in the well-controlled patient group (TA in RB1 and LB1, each P < 0.05; WA in RB1 and RB8, each P < 0.05). Quantitative parameters for air trapping and emphysema did not show significant differences between the two patient groups. On multiple regression analysis, the TA and the WA were statistically significant in comparison of two patient groups with an adjustment for age, sex, body mass index and smoking history (each P < 0.05). Significantly higher values of the WA and the WA% were revealed in the asthmatics with smoking history comparing to non-smokers (P < 0.05).

CONCLUSION

Most of the quantitative CT measurements did not correlate significantly with clinical outcomes in patients with bronchial asthma. And, further research that supplements the limitations of this study is needed to support and validate the association between CT parameters and therapeutic response. This can be justified by the fact that a trend of higher values in airway measurements on initial pretreatment HRCT scan in the poorly controlled than in the well-controlled patient group.

An Exploratory Radiomics Approach to Quantifying Pulmonary Function in CT Images

Contemporary medical imaging is becoming increasingly more quantitative. The emerging field of radiomics is a leading example. By translating unstructured data (i.e., images) into structured data (i.e., imaging features), radiomics can potentially characterize clinically useful imaging phenotypes. In this paper, an exploratory radiomics approach is used to investigate the potential association between quantitative imaging features and pulmonary function in CT images. Thirty-nine radiomic features were extracted from the lungs of 64 patients as potential imaging biomarkers for pulmonary function. Collectively, these features capture the morphology of the lungs, as well as intensity variations, fine-texture, and coarse-texture of the pulmonary tissue. The extracted lung radiomics data was compared to conventional pulmonary function tests. In general, patients with larger lungs of homogeneous, low attenuating pulmonary tissue (as measured via radiomics) were found to be associated with poor spirometry performance and a lower diffusing capacity for carbon monoxide. Unsupervised dynamic data clustering revealed subsets of patients with similar lung radiomic patterns that were found to be associated with similar forced expiratory volume in one second (FEV₁) measurements. This implies that patients with similar radiomic feature vectors also presented with comparable spirometry performance, and were separable by varying degrees of pulmonary function as measured by imaging.

Recent Advances in Inflammation and Treatment of Small Airways in Asthma

Small airways were historically considered to be almost irrelevant in the development and control of pulmonary chronic diseases but, as a matter of fact, in the past few years we have learned that they are not so "silent". Asthma is still a worldwide health issue due to the great share of patients being far from optimal management. Several studies have shown that the deeper lung inflammation plays a critical role in asthma pathogenesis, mostly in these not well-controlled subjects. Therefore, assessing the degree of small airways inflammation and impairment appears to be a pivotal step in the asthmatic patient's management. It is now possible to evaluate them through direct and indirect measurements, even if some obstacles still affect their clinical application. The success of any treatment obviously depends on several factors but reaching the deeper lung has become a priority and, for inhaled drugs, this is strictly connected to the molecule's size. The aim of the present review is to summarize the recent evidence concerning the small airway involvement in asthma, its physiopathological characteristics and how it can be evaluated in order to undertake a personalized pharmacological treatment and achieve a better disease control.

Novel imaging approaches in adult asthma and their clinical potential

Currently, imaging in asthma is confined to chest radiography and CT. The emergence of new imaging techniques and tremendous improvement of existing imaging methods, primarily due to technological advancement, has completely changed its research and clinical prospects. In research, imaging in asthma is now being employed to provide quantitative assessment of morphology, function and pathogenic processes at the molecular level. The unique ability of imaging for non-invasive, repeated, quantitative, and in vivo assessment of structure and function in asthma could lead to identification of 'imaging biomarkers' with potential as outcome measures in future clinical trials. Emerging imaging techniques and their utility in the research and clinical setting is discussed in this review.

Techniques of assessing small airways dysfunction

The small airways are defined as those less than 2 mm in diameter. They are a major site of pathology in many lung diseases, not least chronic obstructive pulmonary disease (COPD) and asthma. The small airways are frequently involved early in the course of these diseases, with significant pathology demonstrable often before the onset of symptoms or changes in spirometry and imaging. Despite their importance, they have proven relatively difficult to study. This is in part due to their relative inaccessibility to biopsy and their small size which makes their imaging difficult. Traditional lung function tests may only become abnormal once there is a significant burden of disease within them. This has led to the term 'the quiet zone' of the lung. In recent years, more specialised tests have been developed which may detect these changes earlier, perhaps offering the possibility of earlier diagnosis and intervention. These tests are now moving from the realms of clinical research laboratories into routine clinical practice and are increasingly useful in the diagnosis and monitoring of respiratory diseases. This article gives an overview of small airways physiology and some of the routine and more advanced tests of airway function.

Markers of vascular perturbation correlate with airway structural change in asthma

RATIONALE

Air trapping and ventilation defects on imaging are characteristics of asthma. Airway wall thickening occurs in asthma and is associated with increased bronchial vascularity and vascular permeability. Vascular endothelial cell products have not been explored as a surrogate to mark structural airway changes in asthma.

OBJECTIVES

Determine whether reporters of vascular endothelial cell perturbation correlate with airway imaging metrics in patients with asthma of varying severity.

METHODS

Plasma from Severe Asthma Research Program subjects was analyzed by ELISAs for soluble von Willebrand factor mature protein (VWF:Ag) and propeptide (VWFpp), P-selectin, and platelet factor 4. Additional subjects were analyzed over 48 hours after whole-lung antigen challenge. We calculated ventilation defect volume by hyperpolarized helium-3 magnetic resonance imaging and areas of low signal density by multidetector computed tomography (less than -856 Hounsfield units [HU] at functional residual capacity and -950 HU at total lung capacity [TLC]).

MEASUREMENTS AND MAIN RESULTS

VWFpp and VWFpp/Ag ratio correlated with and predicted greater percentage defect volume on hyperpolarized helium-3 magnetic resonance imaging. P-selectin correlated with and predicted greater area of low density on chest multidetector computed tomography less than -950 HU at TLC. Platelet factor 4 did not correlate. Following whole-lung antigen challenge, variation in VWFpp, VWFpp/Ag, and P-selectin among time-points was less than that among subjects, indicating stability and repeatability of the measurements.

CONCLUSIONS

Plasma VWFpp and P-selectin may be useful as surrogates of functional and structural defects that are evident on imaging. The results raise important questions about why VWFpp and P-selectin are associated specifically with different imaging abnormalities.

Techniques for assessing small airways function: Possible applications in asthma and COPD

In recent years special interest has been expressed for the contribution of small airways in the pathophysiology, clinical manifestations and treatment of asthma and COPD. Small airways contribute little to the total respiratory resistance so that extensive damage of small airways may occur before the appearance of any symptoms, and this is the reason why they are characterized as the "silent zone" of airways. Furthermore, the peripheral localization of the small airways and their small diameter constitutes difficult their direct assessment. Thus, they are usually studied indirectly, taking advantage of the effects of their obstruction, such as premature closure, air trapping, heterogeneity of ventilation, and lung volume dependence of airflow limitation. Today, several heterogeneous methods for the assessment of small airways are available. These can be either functional (spirometry, plethysmography, resistance measurements, nitrogen washout, alveolar nitric oxide, frequency dependence of compliance, flow-volume curves breathing mixture of helium-oxygen) or imaging (mainly through high resolution computed tomography). The above-mentioned methods are summarized in Table 1. However, no method is currently considered as the "gold standard" and it seems that combinations of tests are needed. Furthermore, it is not clear whether the small airways are affected in all patients with asthma or COPD and their clinical significance remains under investigation. Well-designed future studies with large numbers of patients are expected to reveal which of the methods for assessing the small airways is the most accurate, reliable and reproducible, for which patients, and which can be used for the evaluation of the effects of treatment.

Extrafine inhaled corticosteroid therapy in the control of asthma

Small airways disease plays an important role in the pathogenesis of asthma, but assessment of small airways impairment is not easy in everyday clinical practice. The small airways can be examined by several invasive and noninvasive methods, most of which can at present be used only in the experimental setting. Inhalers providing extrafine inhaled corticosteroid particle sizes may achieve sufficient deposition in the peripheral airways. Many studies have reported the beneficial effects of extrafine inhaled corticosteroids on inflammation, ie, on dysfunction in both the central and distal airways in asthmatics, and there are some data on asthma phenotypes in which the small airways seem to be affected more than in other phenotypes, including nocturnal asthma, severe steroid-dependent or difficult-to-treat asthma, asthma complicated by smoking, elderly asthmatic patients and/or patients with fixed airflow obstruction, and asthmatic children. The relevant randomized controlled clinical trials indicate that the efficacy of extrafine and nonextrafine inhaled corticosteroid formulations is similar in terms of primary endpoints, but there are certain clinically important endpoints for which the extrafine formulations show additional benefits.

Biomarkers in asthma: a real hope to better manage asthma

Diagnosis and treatment of asthma are currently based on assessment of patient symptoms and physiologic tests of airway reactivity. Research over the past decade has identified an array of biochemical and cellular biomarkers, which reflect the heterogeneous and multiple mechanistic pathways that may lead to asthma. These mechanistic biomarkers offer hope for optimal design of therapies targeting the specific pathways that lead to inflammation. This article provides an overview of blood, urine, and airway biomarkers; summarizes the pathologic pathways that they signify; and begins to describe the utility of biomarkers in the future care of patients with asthma.

Assessing and treating small airways disease in asthma and chronic obstructive pulmonary disease

Asthma and chronic obstructive pulmonary disease (COPD) are chronic inflammatory disorders of the respiratory tract that are characterized by airflow limitation. They are distinct conditions with different causes, structural changes, and immunopathology. The pathophysiology in asthma and COPD involves not only the proximal large airways, but also the distal small airways, and thus the small airways are an important therapeutic target in the treatment of both diseases. The assessment of diseased distal small airways is challenging. Extensive disease can be present in the small airways with little abnormality in conventional pulmonary function tests. Recent advances in imaging technologies have led to better spatial resolution to assess small airways morphology non-invasively. New physiological tests have been developed to detect disease and response to therapy in regional airways. Improving the efficiency of existing aerosolized therapy to direct drug to the appropriate lung regions may improve clinical efficacy. Approaches to target distal lung regions include developing new drug formulations with smaller aerosol particle size or using inhaler devices that emit aerosolized drug at slow inhalation flows. Large studies are needed to determine whether better distal lung deposition leads to improvements in small airways function that are translated into clinically significant patient outcomes.

Computed tomographic imaging of the airways in COPD and asthma

Computed tomography (CT) is the modality of choice for imaging the airways. Volumetric data sets with isotropic spatial resolution based on multidetector thin-section CT with overlapping reconstruction should be used. Chronic obstructive pulmonary disease and asthma are the 2 most common disease entities that are defined by airflow obstruction. The morphologic correlates of airway changes are dilation of the lumen, thickening of the wall, visibility of small airways due to mucus or edema, air trapping, hypoxic vasoconstriction, and collapsibility. To assess air trapping, additional expiratory low-dose scans are recommended. In clinical routine, these findings are visually assessed and should be routinely reported. However, the interobserver variability is high, and there is a clear need for objective software-based measurements. The development of such tools is challenging, and they are just becoming available on a broader scale. Novel techniques based on dual-energy CT aim to measure iodine distribution maps to assess pulmonary perfusion as well as the distribution of inhaled xenon gas to assess the distribution and time course of pulmonary ventilation. However, these techniques are still being investigated in clinical studies. This review will provide an overview of CT for the diagnosis of chronic obstructive pulmonary disease and asthma, its role in phenotyping these diseases, and the measurement of disease severity and functional compromise.

Assessment of imaging techniques for evaluating small-airway disease in asthma

The imaging techniques used to investigate patients with asthma and to assess the effects of asthma treatments include computed tomography (CT), helium magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron-emission tomography (PET). Only MRI does not involve radiation exposure. Technical improvements in CT, together with the imaging advantages inherent in the presence of air in the lung, have diminished the radiation exposure required for lung CT. High-resolution low-dose lung CT protocols deliver a dose roughly equal to 1 year of natural radiation exposure and can be used even in paediatric patients. To date, CT is the most extensively studied lung imaging method, the simplest to perform, and the least expensive. In patients with asthma, CT may show several structural changes related to small-airway disease including cylindrical bronchiolectasis, bronchial wall thickening, and air trapping; an indirect marker for bronchiolar obstruction. A robust body of evidence indicates that valid CT markers for small-airway disease can be derived from quantitative lung density measurements and that these markers correlate with clinical severity and lung function test results. In addition, these CT markers are sufficiently sensitive to demonstrate therapeutic effects.

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.

Demonstration of the heterogeneous distribution of asthma in the lungs using CT and hyperpolarized helium-3 MRI

Asthma is a chronic inflammatory disease that affects both the large and small airways and results in bronchoconstriction, mucous hypersecretion, smooth muscle hypertrophy, and subepithelial fibrosis. To gain insight into the pathophysiology of asthma, chest computed tomography (CT) has been investigated as a noninvasive method to evaluate airway wall thickness of medium and large airways. Hyperpolarized gas MRI can assess the functional alterations of airflow within the lung resulting from the structural changes in the airways. In this article, we review the application of CT-based techniques and hyperpolarized gas MRI to study structural and functional changes in asthma. From the result of studies with CT and hyperpolarized gas MRI, it is becoming apparent that asthma has a regional distribution within the lung, that is, some areas of the lung are more affected than others. Furthermore, there appears to be some persistence to this distribution which may explain the observed patterns of airway remodeling and provide targets for localized therapies such as local application of anti-inflammatory agents or bronchial thermoplasty. Thus, cross sectional imaging in asthma is providing new insights into the pathophysiology of the disease and has the potential to become essential in the guidance of localized treatments.

Airway Measurement for Airway Remodeling Defined by Post-Bronchodilator FEV1/FVC in Asthma: Investigation Using Inspiration-Expiration Computed Tomography

PURPOSE

Airway remodeling may be responsible for irreversible airway obstruction in asthma, and a low post-bronchodilator FEV1/FVC ratio can be used as a noninvasive marker of airway remodeling. We investigated correlations between airway wall indices on computed tomography (CT) and various clinical indices, including post-bronchodilator FEV1/FVC ratio, in patients with asthma.

METHODS

Volumetric CT was performed on 22 stable asthma patients who were taking inhaled corticosteroids. Airway dimensions were measured at four segmental bronchi using in-house software based on the full-width/half-maximum method. Parameters included luminal area, wall thickness (WT), wall thickness percentage (WT%), wall area percentage (WA%), bronchial-to-arterial diameter (BA) ratio on inspiration CT, airway collapsibility (AC), and air trapping index (ATI). Correlations were analyzed between CT parameters and clinical indices, including %FEV1, FEV1/FVC, FEF(25-75%), and post-bronchodilator FEV1/FVC ratio.

RESULTS

Post-bronchodilator FEV1/FVC showed significant correlations with WT%, WT, BA ratio, AC, and ATI (r=-0.503, -0.576, 0.454, 0.475, and -0.610, respectively). WT showed negative correlations with FEV1/FVC and FEF(25-75%) (r=-0.431 and -0.581), and WT% was negatively correlated with %FEV1, FEV1/FVC, and FEF(25-75%) (r=-0.434, -0.431, and -0.540, respectively). WA% showed correlations with FEF(25-75%) and body mass index (r=-0.459 and 0.453). The BA ratio was positively correlated with %FEV1 (r=0.459) and FEF(25-75%) (r=0.479). AC showed strong positive correlation with FEV1/FVC (r=0.592), and ATI showed negative correlations with FEV1/FVC (r=-0.534) and FEF(25-75%) (r=-0.591).

CONCLUSIONS

WT%, WT, BA ratio, and AC on inspiration and expiration CT are good indices for measuring airway remodeling defined by post-bronchodilator FEV1/FVC in stable asthma patients treated with inhaled corticosteroids.

Multidetector row computed tomography to assess changes in airways linked to asthma control

BACKGROUND

In asthma, multidetector row computed tomography (MDCT) detects abnormalities that are related to disease severity, including increased bronchial wall thickness. However, whether these abnormalities could be related to asthma control has not been investigated yet.

OBJECTIVE

Our goal was to determine which changes in airways could be linked to disease control.

METHODS

Twelve patients with poor asthma control were included and received a salmeterol/fluticasone propionate combination daily for 12 weeks. Patients underwent clinical, functional, and MDCT examinations before and after the treatment period. MDCT examinations were performed using a low-dose protocol at a controlled lung volume (65% TLC). Bronchial lumen (LA) and wall areas (WA) were evaluated at a segmental and subsegmental level using BronCare software. Lung density was measured at the base of the lung. Baseline and end-of-treatment data were compared using the Wilcoxon signed-rank test.

RESULTS

After the 12-week treatment period, asthma control was achieved. Airflow obstruction and air trapping decreased as assessed by the changes in FEV(1) (p < 0.01) and expiratory reserve volume (p < 0.01). Conversely, LA and WA did not vary significantly. However, a median decrease in LA of >10% was observed in half of the patients with a wide intra- and intersubject response heterogeneity. This was concomitant with a decrease in lung density (p < 0.02 in the anteroinferior areas).

CONCLUSIONS

MDCT is insensitive for demonstrating any decrease in bronchial wall thickness. This is mainly due to changes in bronchial caliber which may be linked to modifications of the elastic properties of the bronchopulmonary system under treatment.

Radiological approach to asthma and COPD--the role of computed tomography

Asthma and chronic obstructive pulmonary disease (COPD) are among the most prevalent lung diseases. In both asthma and COPD, airway inflammation leads to airway remodeling. Parenchyma of the lung is also influenced by disease conditions. Airway wall thickening/lumen narrowing and parenchymal destruction occur in COPD. In asthma, airway remodeling contributes to the lung parenchyma. Computed tomography (CT) has been widely used as an imaging tool for lung diseases. With the technical advancement of CT, together with the development of analysis software, it is now possible to analyze the lung parenchymal change and airway remodeling quantitatively using CT. This article reviews the role of CT in assessing the lung structure and functions of patients with asthma and COPD.

[Assessment of imaging techniques in the evaluation of small airways disease in asthma]

The imaging techniques that have been used in the exploration and evaluation of treatment in asthma include computed tomography (CT), helium-magnetic resonance imaging (MRI), single photon emission computerized tomoscintigraphy (SPECT) and positron emission tomography (PET). Only MRI does not involve radiation. However, in the case of CT, technical improvements and the advantages of the air-filled lung have resulted in a decreased radiation burden. High resolution examinations, using a low dose of about one-year of natural background irradiation, are possible even in infancy. CT is the best evaluated so far, the simplest to perform and the least expensive. In asthma several morphological changes related to small airway disease can be visualised on CT images: cylindrical bronchiolectasis, thickening of the bronchial walls and air trapping, an indirect marker of bronchiolar obstruction. Today there is a robust body of evidence that valid indices of small airways disease can be deduced from quantitative analysis of lung density, indices that correlate well with clinical severity and functional measurements. In addition, the sensitivity of the method is sufficient to demonstrate therapeutic effects.

Oxygen-enhanced MRI vs. quantitatively assessed thin-section CT: pulmonary functional loss assessment and clinical stage classification of asthmatics

PURPOSE

The purpose of this study was to prospectively compare the efficacy of oxygen-enhanced MR imaging (O(2)-enhanced MRI) and CT for pulmonary functional loss assessment and clinical stage classification of asthmatics.

MATERIALS AND METHODS

O(2)-enhanced MRI, CT and %FEV(1) measurement were used 34 consecutive asthmatics classified into four stages ('Mild Intermittent [n=7]', 'Mild Persistent [n=8], 'Moderate Persistent [n=14]' and 'Severe Persistent [n=5]'). Relative enhancement ratio maps for every subject were generated, and determine mean relative enhancement ratios (MRERs). Mean lung density (MLD) and the airway wall area (WA) corrected by body surface area (WA/BSA) were also measured on CT. To compare the efficacy of the two methods for pulmonary functional loss assessment, all indexes were correlated with %FEV(1). To determine the efficacy of the two methods for clinical stage classification, all parameters for the four clinical stages were statistically compared.

RESULTS

%FEV(1) showed fair or moderate correlation with all parameters (0.15≤r(2)≤0.30, p<0.05). WA, WA/BSA and MRER of the 'Severe Persistent' group were significantly larger than those of 'Mild Intermittent' and 'Mild Persistent' groups (p<0.05), and MRER of the 'Moderate Persistent' group significantly lower than that of the 'Mild Intermittent' group (p<0.05).

CONCLUSION

O(2)-enhanced MRI is as effective as CT for pulmonary functional loss assessment and clinical stage classification of asthmatics.

Changes in regional airflow obstruction over time in the lungs of patients with asthma: evaluation with 3He MR imaging

PURPOSE

To determine changes in regional airflow obstruction over time in the lungs of patients with asthma, as demonstrated with hyperpolarized helium 3 ((3)He) magnetic resonance (MR) imaging, and to assess correlations with disease severity and use of asthma medications.

MATERIALS AND METHODS

Institutional review board approval and written informed consent were obtained for this HIPAA-compliant study. Use of (3)He was approved by the U.S. Food and Drug Administration. Forty-three patients underwent 103 MR imaging studies in total; 26 were imaged twice within 42-82 minutes (same day), and 17 were imaged on 3 days between 1 and 476 days (multiday). Each day, spirometry was performed, disease severity was determined, and the use of asthma medications was recorded. Images were reviewed in a pairwise fashion to determine total ventilation defect number, defects in same location between imaging studies, and size. Parametric and nonparametric statistical methods were used.

RESULTS

For the same-day examinations, the mean number of defects per image section was similar at baseline and repeat imaging (1.8 +/- 1.9 [standard deviation] vs 1.6 +/- 1.9, respectively; P = .15), with 75% of defects remaining in the same location and 71% of these not changing size. For the multiday examinations, the mean number of defects per section was higher for study 2 (2.4 +/- 1.5) than study 1 (1.7 +/- 0.9, P = .02), was lower for study 3 (1.5 +/- 1.1) than for study 2 (P < .01), and was similar for studies 1 and 3 (P = .56). Time between examinations was not associated with change in mean number of defects per section (median intrasubject correlation [r(m)] = 0.01, P = .64) or change in spirometric values (range of r(m) values: -0.56 to -0.31; range of P values: .09-.71). Defects in the same location decreased with time (r(m) = -0.83, P < .01), with 67% persisting between studies 1 and 2 (median interval, 31 days), 43% persisting between studies 2 and 3 (median interval, 41 days), and 38% persisting between studies 1 and 3 (median interval, 85 days); 46%-58% of defects remained unchanged in size. These trends were the same regardless of disease severity or medication use.

CONCLUSION

In asthma, focal airflow impediment within the lungs can be markedly persistent over time, regardless of disease severity or treatment.

MR for the evaluation of obstructive pulmonary disease

Obstructive lung diseases include emphysema, chronic bronchitis, chronic obstructive pulmonary disease, asthma, and cystic fibrosis. These diseases are a heterogeneous group of pulmonary disorders that share in common obstruction of air flow and deranged gas exchange. Traditionally these diseases are evaluated with clinical testing, such as pulmonary function tests, but such tests provide only global measures of respiratory function. MR techniques designed for obstructive lung disease have the capability of directly imaging the anatomic and pathophysiologic derangements and may prove useful for monitoring response to therapy.

Heterogeneity of bronchoconstriction does not distinguish mild asthmatic subjects from healthy controls when supine

Heterogeneity is a fundamental property of airway constriction; however, whether it is a distinguishing feature of mild asthma is not clear. We used computerized tomography and the forced oscillation technique to compare lung heterogeneity between 18 mildly asthmatic and 19 healthy control subjects at similar levels of bronchoconstriction while subjects were supine. We also assessed the effects of deep inhalation and albuterol on supine lung mechanics. Measures of heterogeneity included lung attenuation, from which we derived a novel index of air-space size, and the frequency dependence of respiratory system resistance between 1 and 20 Hz. We found that asthmatic subjects had airways hyperresponsiveness to methacholine in the sitting position compared with controls, but both groups had similar falls in forced expiratory volume in 1 s after inhaling methacholine while supine. There were no baseline differences between the groups in the frequency dependence of resistance, or lung attenuation, before methacholine, and both groups responded similarly with an increase in air-space size (+9.2% vs. +3.4%), air-space size heterogeneity (+9.8% vs. +4.2%), and frequency dependence of resistance (+76% vs. +86%) after methacholine. Deep inhalation did not affect resistance in either group, but albuterol significantly reduced resistance in both groups. We conclude that both computerized tomography and the forced oscillation technique demonstrate increased heterogeneity of airway narrowing during induced bronchoconstriction while supine and that this heterogeneity is equivalent between subjects with mild asthma and healthy controls when bronchoconstricted to the same degree. Thus heterogeneity appears to be a fundamental feature of bronchoconstriction and is not unique to mild asthma.

Evaluation of Asthma With Hyperpolarized Helium-3 MRI

BACKGROUND

Accurate characterization of asthma severity is difficult due to the variability of symptoms. Hyperpolarized helium-3 MRI (H(3)HeMR) is a new technique in which the airspaces are visualized, depicting regions with airflow obstruction as "ventilation defects." The objective of this study was to compare the extent of H(3)HeMR ventilation defects with measures of asthma severity and spirometry.

METHODS

Patients with a physician diagnosis of asthma and normal control subjects underwent H(3)HeMR. For each person, the number and size of ventilation defects were scored and the average number of ventilation defects per slice (VDS) was calculated. The correlations of the imaging findings with measures of asthma severity and spirometry were determined.

RESULTS

There were 58 patients with asthma (mild-intermittent, n = 13; mild-persistent, n = 13; moderate-persistent, n = 20; and severe-persistent, n = 12) and 18 control subjects. Mean +/- SE VDS for asthmatics was significantly greater than for control subjects (0.99 +/- 0.15 vs 0.26 +/- 0.22, p = 0.004). Among asthmatics, VDS was significantly higher for the group with moderate-persistent and severe-persistent disease than for the group with mild-intermittent and mild-persistent disease (1.37 +/- 0.24 vs 0.53 +/- 0.12, p < 0.001). VDS correlated significantly with FEV(1)/FVC (r = - 0.51, p = 0.002), forced expiratory flow between 25% and 75% from the beginning of FVC (FEF(25-75%)) percentage of predicted for height, sex, and race (%predicted) [r = - 0.50, p = 0.001], and FEV(1) %predicted (r = - 0.40, p = 0.002), but not with FVC %predicted (r = - 0.26, p = 0.057) and peak expiratory flow %predicted (r = - 0.16, p = 0.231). Many asthmatics had an elevated VDS, but their spirometric indexes, except FEF(25%-75%), were normal. Most ventilation defects were < 3 cm in size for all asthmatics. In the group of patients with moderate-to-severe persistent asthma, there were more defects > or =3 cm than in the group with mild-intermittent and mild-persistent disease (p = 0.021).

CONCLUSIONS

Regional changes of airflow obstruction in asthmatics depicted by H(3)HeMR correlate with measures of asthma severity and spirometry.

The use of computed tomography to assess asthma severity

PURPOSE OF REVIEW

Chronic inflammation in asthma can also lead to airway remodeling, which contributes to airway narrowing. It may be possible to assess and quantify the extent of airway remodeling in vivo using computed tomography. This review examines recent developments in the evaluation of asthma severity using computed tomography, and the effect of treatment assessed by computed tomography.

RECENT FINDINGS

Asthma patients have thicker airways on computed tomography scans than do healthy control individuals, and the degree of thickening is related to the severity of disease, airflow obstruction, and airway reactivity. Recent studies have indicated that patients with severe asthma and irreversible airflow obstruction had longer disease duration, a greater inflammatory process and more airway abnormalities, assessed by high-resolution computed tomography, suggestive of airway remodeling. Other studies have shown that high-resolution computed tomography lung density correlates with airflow limitation and lung volume (but not with lung transfer factor), and also correlates with patient age and severity of asthma. More recently, two publications demonstrated the effect of treatment on airway wall thickness and lung density assessed by computed tomography in patients with asthma.

SUMMARY

High-resolution computed tomography is one of the most useful tools for imaging airways and parenchyma. Computed tomography scanning may be useful in determining which patients might benefit from more or less treatment. With additional advances in technology, it is likely that quantitative assessment by computed tomography will ultimately be a valuable tool for the study and treatment of chronic airway diseases.

Clinical assessment of airway remodeling in asthma: utility of computed tomography

Airway remodeling is an established feature of asthma. Histologic examination is essential in the assessment of remodeling that is a pathologic concept. Examinations of autopsied or resected lung have enabled detailed morphologic and morphometric studies and have provided fundamental knowledge of airway remodeling in asthma. However, such materials are only accidentally available, and clinical information may often be insufficient in autopsied cases. Bronchoscopic mucosal biopsy has been widely used since the 1980s, and has contributed substantially to basic investigations of inflammation and remodeling. However such specimens are limited in size and depth, limited to central airways, and the procedure might be too invasive to be repeated. Remodeling can also be assessed indirectly. Pulmonary function tests to evaluate chronic airflow obstruction are available in clinical settings and suitable for screening or mass studies, but they may be affected by concomitant diseases or short-term asthma control. Computed tomography (CT) has recently been utilized to assess remodeling. It cannot discern pathologic details but provides a broader range of airway/lung morphology and may be less invasive compared to biopsy. In addition to classic subjective evaluations,quantitative assessment has been reported for central airway dimensions, such as airway wall area, luminal area and wall thickness, and for peripheral airway abnormality or air trapping as measured by decreased lung attenuation or increased mosaic perfusion. This article summarizes the merits and limitations of various methods to assess airway remodeling, and describes the details of methodologies, interpretations, pathophysiologic relevance, and future directions of asthmatic airway remodeling assessed by CT.

Optimization and Standardization of Lung Densitometry in the Assessment of Pulmonary Emphysema

Currently, lung densitometry for the assessment of pulmonary emphysema has been fully validated against pathology, pulmonary function, and health status, and it is therefore being applied in pharmacotherapeutic trials. Nevertheless, its application for the early detection of emphysema has not yet been introduced in daily clinical practice. The main reason for this is the fact that it is not yet regarded a fully optimized and standardized technique. In this work, an overview is given on the current status of different standardization aspects that play an important role in this, ie, image acquisition, choice of densitometric parameter and image processing. To address these issues, solutions have been sought from the literature and from original data from previous studies. Standardization and optimization of lung densitometry has reached a more advanced stage than has been reported so far. If normal values will become available, this technique will be feasible for clinical practice. As a result, standardization for the detection and assessment of other density-related lung diseases can be achieved in a shorter period of time.