Imaging the lungs in asthmatic patients by using hyperpolarized helium-3 magnetic resonance: assessment of response to methacholine and exercise challenge

Hyperpolarized 129Xe MRI, 99mTc scintigraphy, and SPECT in lung ventilation imaging: a quantitative comparison

RATIONALE AND OBJECTIVES

The current clinical standard for functional imaging of patients with lung ailments is nuclear medicine scintigraphy and Single Photon Emission Computed Tomography (SPECT) which detect the gamma decay of inhaled radioactive tracers. Hyperpolarized (HP) Xenon-129 MRI (XeMRI) of the lungs has recently been FDA approved and provides similar functional images of the lungs with higher spatial resolution than scintigraphy and SPECT. Here we compare Technetium-99m (99mTc) diethylene-triamine-pentaacetate scintigraphy and SPECT with HP XeMRI in healthy controls, asthma, and chronic obstructive pulmonary disorder (COPD) patients.

MATERIALS AND METHODS

59 subjects, healthy, with asthma, and with COPD, underwent 99mTc scintigraphy/SPECT, standard spirometry, and HP XeMRI. XeMRI and SPECT images were registered for direct voxel-wise signal comparisons. Images were also compared using ventilation defect percentage (VDP), and a standard 6-compartment method. VDP calculated from XeMRI and SPECT images was compared to spirometry.

RESULTS

Median Pearson correlation coefficient for voxel-wise signal comparison was 0.698 (0.613-0.782) between scintigraphy and XeMRI and 0.398 (0.286-0.502) between SPECT and XeMRI. Correlation between VDP measures was r = 0.853, p < 0.05. VDP separated asthma and COPD from the control group and was significantly correlated with FEV1, FEV1/FVC, and FEF 25-75.

CONCLUSION

HP XeMRI provides equivalent information to 99mTc SPECT and standard spirometry measures. Additionally, XeMRI is non-invasive, hence it could be used for longitudinal studies for evaluating emerging treatment for lung ailments.

Pulmonary MRI with hyperpolarized xenon-129 demonstrates novel alterations in gas transfer across the air-blood barrier in asthma

BACKGROUND

Individuals with asthma can vary widely in clinical presentation, severity, and pathobiology. Hyperpolarized xenon-129 (Xe129) MRI is a novel imaging method to provide 3-D mapping of both ventilation and gas exchange in the human lung.

PURPOSE

To evaluate the functional changes in adults with asthma as compared to healthy controls using Xe129 MRI.

METHODS

All subjects (20 controls and 20 asthmatics) underwent lung function measurements and Xe129 MRI on the same day. Outcome measures included the pulmonary ventilation defect and transfer of inspired Xe129 into two soluble compartments: tissue and blood. Ten asthmatics underwent Xe129 MRI before and after bronchodilator to test whether gas transfer measures change with bronchodilator effects.

RESULTS

Initial analysis of the results revealed striking differences in gas transfer measures based on age, hence we compared outcomes in younger (n = 24, ≤ 35 years) versus older (n = 16, > 45 years) asthmatics and controls. The younger asthmatics exhibited significantly lower Xe129 gas uptake by lung tissue (Asthmatic: 0.98% ± 0.24%, Control: 1.17% ± 0.12%, P = 0.035), and higher Xe129 gas transfer from tissue to the blood (Asthmatic: 0.40 ± 0.10, Control: 0.31% ± 0.03%, P = 0.035) than the younger controls. No significant difference in Xe129 gas transfer was observed in the older group between asthmatics and controls (P > 0.05). No significant change in Xe129 transfer was observed before and after bronchodilator treatment.

CONCLUSIONS

By using Xe129 MRI, we discovered heterogeneous alterations of gas transfer that have associations with age. This finding suggests a heretofore unrecognized physiological derangement in the gas/tissue/blood interface in young adults with asthma that deserves further study.

Deep Learning Approaches for Quantifying Ventilation Defects in Hyperpolarized Gas Magnetic Resonance Imaging of the Lung: A Review

This paper provides an in-depth overview of Deep Neural Networks and their application in the segmentation and analysis of lung Magnetic Resonance Imaging (MRI) scans, specifically focusing on hyperpolarized gas MRI and the quantification of lung ventilation defects. An in-depth understanding of Deep Neural Networks is presented, laying the groundwork for the exploration of their use in hyperpolarized gas MRI and the quantification of lung ventilation defects. Five distinct studies are examined, each leveraging unique deep learning architectures and data augmentation techniques to optimize model performance. These studies encompass a range of approaches, including the use of 3D Convolutional Neural Networks, cascaded U-Net models, Generative Adversarial Networks, and nnU-net for hyperpolarized gas MRI segmentation. The findings highlight the potential of deep learning methods in the segmentation and analysis of lung MRI scans, emphasizing the need for consensus on lung ventilation segmentation methods.

Feasibility of free-breathing 19 F MRI image acquisition to characterize ventilation defects in CF and healthy volunteers at wash-in

PURPOSE

To explore the feasibility of measuring ventilation defect percentage (VDP) using 19 F MRI during free-breathing wash-in of fluorinated gas mixture with postacquisition denoising and to compare these results with those obtained through traditional Cartesian breath-hold acquisitions.

METHODS

Eight adults with cystic fibrosis and 5 healthy volunteers completed a single MR session on a Siemens 3T Prisma. 1 H Ultrashort-TE MRI sequences were used for registration and masking, and ventilation images with 19 F MRI were obtained while the subjects breathed a normoxic mixture of 79% perfluoropropane and 21% oxygen (O2 ). 19 F MRI was performed during breath holds and while free breathing with one overlapping spiral scan at breath hold for VDP value comparison. The 19 F spiral data were denoised using a low-rank matrix recovery approach.

RESULTS

VDP measured using 19 F VIBE and 19 F spiral images were highly correlated (r = 0.84) at 10 wash-in breaths. Second-breath VDPs were also highly correlated (r = 0.88). Denoising greatly increased SNR (pre-denoising spiral SNR, 2.46 ± 0.21; post-denoising spiral SNR, 33.91 ± 6.12; and breath-hold SNR, 17.52 ± 2.08).

CONCLUSION

Free-breathing 19 F lung MRI VDP analysis was feasible and highly correlated with breath-hold measurements. Free-breathing methods are expected to increase patient comfort and extend ventilation MRI use to patients who are unable to perform breath holds, including younger subjects and those with more severe lung disease.

Functional lung imaging using novel and emerging MRI techniques

Respiratory diseases are leading causes of death and disability in the world. While early diagnosis is key, this has proven difficult due to the lack of sensitive and non-invasive tools. Computed tomography is regarded as the gold standard for structural lung imaging but lacks functional information and involves significant radiation exposure. Lung magnetic resonance imaging (MRI) has historically been challenging due to its short T2 and low proton density. Hyperpolarised gas MRI is an emerging technique that is able to overcome these difficulties, permitting the functional and microstructural evaluation of the lung. Other novel imaging techniques such as fluorinated gas MRI, oxygen-enhanced MRI, Fourier decomposition MRI and phase-resolved functional lung imaging can also be used to interrogate lung function though they are currently at varying stages of development. This article provides a clinically focused review of these contrast and non-contrast MR imaging techniques and their current applications in lung disease.

Heterogeneity of Airway Smooth Muscle Remodeling in Asthma

Rationale: Ventilatory defects in asthma are heterogeneous and may represent the distribution of airway smooth muscle (ASM) remodeling. Objectives: To determine the distribution of ASM remodeling in mild-severe asthma. Methods: The ASM area was measured in nine airway levels in three bronchial pathways in cases of nonfatal (n = 30) and fatal asthma (n = 20) and compared with control cases without asthma (n = 30). Correlations of ASM area within and between bronchial pathways were calculated. Asthma cases with 12 large and 12 small airways available (n = 42) were classified on the basis of the presence or absence of ASM remodeling (more than two SD of mean ASM area of control cases, n = 86) in the large or small airway or both. Measurements and Main Results: ASM remodeling varied widely within and between cases of nonfatal asthma and was more widespread and confluent and more marked in fatal cases. There were weak correlations of ASM between levels within the same or separate bronchial pathways; however, predictable patterns of remodeling were not observed. Using mean data, 44% of all asthma cases were classified as having no ASM remodeling in either the large or small airway despite a three- to 10-fold increase in the number of airways with ASM remodeling and 81% of asthma cases having ASM remodeling in at least one large and small airway. Conclusions: ASM remodeling is related to asthma severity but is heterogeneous within and between individuals and may contribute to the heterogeneous functional defects observed in asthma. These findings support the need for patient-specific targeting of ASM remodeling.

Effect of exercise-induced bronchoconstriction on the configuration of the maximal expiratory flow-volume curve in adults with asthma

We determined the effect of exercise-induced bronchoconstriction (EIB) on the shape of the maximal expiratory flow-volume (MEFV) curve in asthmatic adults. The slope-ratio index (SR) was used to quantitate the shape of the MEFV curve. We hypothesized that EIB would be accompanied by increases in SR and thus increased curvilinearity of the MEFV curve. Adult asthmatic ( n  = 10) and non-asthmatic control subjects ( n  = 9) cycled for 6-8 min at 85% of peak power. Following exercise, subjects remained on the ergometer and performed a maximal forced exhalation every 2 min for a total 20 min. In each MEFV curve, the slope-ratio index (SR) was calculated in 1% volume increments beginning at peak expiratory flow (PEF) and ending at 20% of forced vital capacity (FVC). Baseline spirometry was lower in asthmatics compared to control subjects (FEV₁ % predicted, 89.1 ± 14.3 vs. 96.5 ± 12.2% [SD] in asthma vs. control; p  < 0.05). In asthmatic subjects, post-exercise FEV₁ decreased by 29.9 ± 13.2% from baseline (3.48 ± 0.74 and 2.24 ± 0.59 [SD] L for baseline and post-exercise nadir; p  < 0.001). At baseline and at all timepoints after exercise, average SR between 80 and 20% of FVC was larger in asthmatic than control subjects (1.48 ± 0.02 vs. 1.23 ± 0.02 [SD] for asthma vs. control; p < 0.005). This averaged SR did not change after exercise in either subject group. In contrast, post-exercise SR between PEF and 75% of FVC was increased from baseline in subjects with asthma, suggesting that airway caliber heterogeneity increases with EIB. These findings suggest that the SR-index might provide useful information on the physiology of acute airway narrowing that complements traditional spirometric measures.

Imaging Regional Airway Involvement of Asthma: Heterogeneity in Ventilation, Mucus Plugs and Remodeling

The imaging of asthma using chest computed tomography (CT) is well-established (Jarjour et al., Am J Respir Crit Care Med 185(4):356-62, 2012; Castro et al., J Allergy Clin Immunol 128:467-78, 2011). Moreover, recent advances in functional imaging of the lungs with advanced computer analysis of both CT and magnetic resonance images (MRI) of the lungs have begun to play a role in quantifying regional obstruction. Specifically, quantitative measurements of the airways for bronchial wall thickening, luminal narrowing and distortion, the amount of mucus plugging, parenchymal density, and ventilation defects that could contribute to the patient's disease course are instructive for the entire care team. In this chapter, we will review common imaging methods and findings that relate to the heterogeneity of asthma. This information can help to guide treatment decisions. We will discuss mucous plugging, quantitative assessment of bronchial wall thickening, delta lumen phenomenon, parenchymal low-density lung on CT, and ventilation defect percentage on MRI as metrics for assessing regional ventilatory dysfunction.

Preclinical Four-Dimensional Functional Lung Imaging and Quantification of Regional Airflow: A New Standard in Lung Function Evaluation in Murine Models

The current standard for lung function evaluation in murine models is based on forced oscillation technology, which provides a measure of the total airway function but cannot provide information on regional heterogeneity in function. Limited detection of regional airflow may contribute to a discontinuity between airway inflammation and airflow obstruction in models of asthma. Here, we describe quantification of regional airway function using novel dynamic quantitative imaging and analysis to quantify and visualize lung motion and regional pulmonary airflow in four dimensions (4D). Furthermore, temporo-spatial specific ventilation (ml/ml) is used to determine ventilation heterogeneity indices for lobar and sublobar regions, which are directly compared to ex vivo biological analyses in the same sublobar regions. In contrast, oscillation-based technology in murine genetic models of asthma have failed to demonstrate lung function change despite altered inflammation, whereas 4D functional lung imaging demonstrated diminished regional lung function in genetic models relative to wild-type mice. Quantitative functional lung imaging assists in localizing the regional effects of airflow. Our approach reveals repeatable and consistent differences in regional airflow between lung lobes in all models of asthma, suggesting that asthma is characterized by regional airway dysfunctions that are often not detectable in composite measures of lung function. 4D functional lung imaging technology has the potential to transform discovery and development in murine models by mapping out regional areas heterogeneously affected by the disease, thus deciphering pathobiology with greater precision.

Multi-nuclear magnetic resonance spectroscopy: state of the art and future directions

With the development of heteronuclear fluorine, sodium, phosphorus, and other probes and imaging technologies as well as the optimization of magnetic resonance imaging (MRI) equipment and sequences, multi-nuclear magnetic resonance (multi-NMR) has enabled localize molecular activities in vivo that are central to a variety of diseases, including cardiovascular disease, neurodegenerative pathologies, metabolic diseases, kidney, and tumor, to shift from the traditional morphological imaging to the molecular imaging, precision diagnosis, and treatment mode. However, due to the low natural abundance and low gyromagnetic ratios, the clinical application of multi-NMR has been hampered. Several techniques have been developed to amplify the NMR sensitivity such as the dynamic nuclear polarization, spin-exchange optical pumping, and brute-force polarization. Meanwhile, a wide range of nuclei can be hyperpolarized, such as 2H, 3He, 13C, 15 N, 31P, and 129Xe. The signal can be increased and allows real-time observation of biological perfusion, metabolite transport, and metabolic reactions in vivo, overcoming the disadvantages of conventional magnetic resonance of low sensitivity. HP-NMR imaging of different nuclear substrates provides a unique opportunity and invention to map the metabolic changes in various organs without invasive procedures. This review aims to focus on the recent applications of multi-NMR technology not only in a range of preliminary animal experiments but also in various disease spectrum in human. Furthermore, we will discuss the future challenges and opportunities of this multi-NMR from a clinical perspective, in the hope of truly bridging the gap between cutting-edge molecular biology and clinical applications.

Lung MRI with hyperpolarised gases: current & future clinical perspectives

The use of pulmonary MRI in a clinical setting has historically been limited. Whilst CT remains the gold-standard for structural lung imaging in many clinical indications, technical developments in ultrashort and zero echo time MRI techniques are beginning to help realise non-ionising structural imaging in certain lung disorders. In this invited review, we discuss a complementary technique - hyperpolarised (HP) gas MRI with inhaled ³He and ¹²⁹Xe - a method for functional and microstructural imaging of the lung that has great potential as a clinical tool for early detection and improved understanding of pathophysiology in many lung diseases. HP gas MRI now has the potential to make an impact on clinical management by enabling safe, sensitive monitoring of disease progression and response to therapy. With reference to the significant evidence base gathered over the last two decades, we review HP gas MRI studies in patients with a range of pulmonary disorders, including COPD/emphysema, asthma, cystic fibrosis, and interstitial lung disease. We provide several examples of our experience in Sheffield of using these techniques in a diagnostic clinical setting in challenging adult and paediatric lung diseases.

Imaging in Asthma Management

Asthma is a heterogeneous disease characterized by chronic airway inflammation that affects more than 300 million people worldwide. Clinically, asthma has a widely variable presentation and is defined based on a history of respiratory symptoms alongside airflow limitation. Imaging is not needed to confirm a diagnosis of asthma, and thus the use of imaging in asthma has historically been limited to excluding alternative diagnoses. However, significant advances continue to be made in novel imaging methodologies, which have been increasingly used to better understand respiratory impairment in asthma. As a disease primarily impacting the airways, asthma is best understood by imaging methods with the ability to elucidate airway impairment. Techniques such as computed tomography, magnetic resonance imaging with gaseous contrast agents, and positron emission tomography enable assessment of the small airways. Others, such as optical coherence tomography and endobronchial ultrasound enable high-resolution imaging of the large airways accessible to bronchoscopy. These imaging techniques are providing new insights in the pathophysiology and treatments of asthma and are poised to impact the clinical management of asthma.

Pulmonary functional imaging (PFI): A historical review and perspective

PFI Pulmonary Functional Imaging (PFI) refers to visualization and measurement of ventilation, perfusion, gas flow and exchange as well as biomechanics. In this review, we will highlight the historical development of PFI, describing recent advances and listing the various techniques for PFI offered per modality. Challenges PFI is facing and requirements for PFI from a clinical point of view will be pointed out. Hereby the review is meant as an introduction to PFI.

Hyperpolarized 129Xenon MRI Ventilation Defect Quantification via Thresholding and Linear Binning in Multiple Pulmonary Diseases

RATIONALE

There is no agreed upon method for quantifying ventilation defect percentage (VDP) with high sensitivity and specificity from hyperpolarized (HP) gas ventilation MR images in multiple pulmonary diseases for both pediatrics and adults, yet identifying such methods will be necessary for future multi-site trials. Most HP gas MRI ventilation research focuses on a specific pulmonary disease and utilizes one quantification scheme for determining VDP. Here we sought to determine the potential of different methods for quantifying VDP from HP 129Xe images in multiple pulmonary diseases through comparison of the most utilized quantification schemes: linear binning and thresholding.

MATERIALS AND METHODS

HP 129Xe MRI was performed in a total of 176 subjects (125 pediatrics and 51 adults, age 20.98±16.48 years) who were either healthy controls (n = 23) or clinically diagnosed with cystic fibrosis (CF) (n = 37), lymphangioleiomyomatosis (LAM) (n = 29), asthma (n = 22), systemic juvenile idiopathic arthritis (sJIA) (n = 11), interstitial lung disease (ILD) (n = 7), or were bone marrow transplant (BMT) recipients (n = 47). HP 129Xe ventilation images were acquired during a ≤16 second breath-hold using a 2D multi-slice gradient echo sequence on a 3T Philips scanner (TR/TE 8.0/4.0ms, FA 10-12°, FOV 300 × 300mm, voxel size≈3 × 3 × 15mm). Images were analyzed using 5 different methods to quantify VDPs: linear binning (histogram normalization with binning into 6 clusters) following either linear or a variant of a nonparametric nonuniform intensity normalization algorithm (N4ITK) bias-field correction, thresholding ≤60% of the mean signal intensity with linear bias-field correction, and thresholding ≤60% and ≤75% of the mean signal intensity following N4ITK bias-field correction. Spirometry was successfully obtained in 84% of subjects.

RESULTS

All quantification schemes were able to label visually identifiable ventilation defects in similar regions within all subjects. The VDPs of control subjects were significantly lower (p<0.05) compared to BMT, CF, LAM, and ILD subjects for most of the quantification methods. No one quantification scheme was better able to differentiate individual disease groups from the control group. Advanced statistical modeling of the VDP quantification schemes revealed that in comparing controls to the combined disease group, N4ITK bias-field corrected 60% thresholding had the highest predictive efficacy, sensitivity, and specificity at the VDP cut-point of 2.3%. However, compared to the thresholding quantification schemes, linear binning was able to capture and label subtle low-ventilation regions in subjects with milder obstruction, such as subjects with asthma.

CONCLUSION

The difference in VDP between healthy controls and patients varied between the different disease states for all quantification methods. Although N4ITK bias-field corrected 60% thresholding was superior in separating the combined diseased group from controls, linear binning is able to better label low-ventilation regions unlike the current, 60% thresholding scheme. For future clinical trials, a consensus will need to be reached on which VDP scheme to utilize, as there are subtle advantages for each for specific disease.

Comparison of Hyperpolarized 3He and 129Xe MR Imaging in Cystic Fibrosis Patients

PURPOSE

In this study, we compared hyperpolarized ³He and ¹²⁹Xe images from patients with cystic fibrosis using two commonly applied magnetic resonance sequences, standard gradient echo (GRE) and balanced steady-state free precession (TrueFISP) to quantify regional similarities and differences in signal distribution and defect analysis.

MATERIALS AND METHODS

Ten patients (7M/3F) with cystic fibrosis underwent hyperpolarized gas MR imaging with both ³He and ¹²⁹Xe. Six had MRI with both GRE, and TrueFISP sequences and four patients had only GRE sequence but not TrueFISP. Ventilation defect percentages (VDPs) were calculated as lung voxels with <60% of the whole-lung hyperpolarized gas signal mean and was measured in all datasets. The voxel signal distributions of both ¹²⁹Xe and ³He gases were visualized and compared using violin plots. VDPs of hyperpolarized 3 He and 129 Xe were compared in Bland-Altman plots; Pearson correlation coefficients were used to evaluate the relationships between inter-gas and inter-scan to assess the reproducibility.

RESULTS

A significant correlation was demonstrated between ¹²⁹Xe VDP and ³He VDP for both GRE and TrueFISP sequences (ρ = 0.78, p<0.0004). The correlation between the GRE and TrueFISP VDP for ³He was ρ = 0.98 and was ρ = 0.91 for ¹²⁹Xe. Overall, ¹²⁹Xe (27.2±9.4) VDP was higher than ³He (24.3±6.9) VDP on average on cystic fibrosis patients.

CONCLUSION

In patients with cystic fibrosis, the selection of hyperpolarized ¹²⁹Xe or ³He gas is most likely inconsequential when it comes to measure the overall lung function by VDP although ¹²⁹Xe may be more sensitive to starker lung defects, particularly when using a TrueFISP sequence.

Pulmonary functional MRI: Detecting the structure-function pathologies that drive asthma symptoms and quality of life

Pulmonary functional MRI (PfMRI) using inhaled hyperpolarized, radiation-free gases (such as ³ He and ¹²⁹ Xe) provides a way to directly visualize inhaled gas distribution and ventilation defects (or ventilation heterogeneity) in real time with high spatial (~mm³ ) resolution. Both gases enable quantitative measurement of terminal airway morphology, while ¹²⁹ Xe uniquely enables imaging the transfer of inhaled gas across the alveolar-capillary tissue barrier to the red blood cells. In patients with asthma, PfMRI abnormalities have been shown to reflect airway smooth muscle dysfunction, airway inflammation and remodelling, luminal occlusions and airway pruning. The method is rapid (8-15 s), cost-effective (~$300/scan) and very well tolerated in patients, even in those who are very young or very ill, because unlike computed tomography (CT), positron emission tomography and single-photon emission CT, there is no ionizing radiation and the examination takes only a few seconds. However, PfMRI is not without limitations, which include the requirement of complex image analysis, specialized equipment and additional training and quality control. We provide an overview of the three main applications of hyperpolarized noble gas MRI in asthma research including: (1) inhaled gas distribution or ventilation imaging, (2) alveolar microstructure and finally (3) gas transfer into the alveolar-capillary tissue space and from the tissue barrier into red blood cells in the pulmonary microvasculature. We highlight the evidence that supports a deeper understanding of the mechanisms of asthma worsening over time and the pathologies responsible for symptoms and disease control. We conclude with a summary of approaches that have the potential for integration into clinical workflows and that may be used to guide personalized treatment planning.

Image- versus histogram-based considerations in semantic segmentation of pulmonary hyperpolarized gas images

PURPOSE

To characterize the differences between histogram-based and image-based algorithms for segmentation of hyperpolarized gas lung images.

METHODS

Four previously published histogram-based segmentation algorithms (ie, linear binning, hierarchical k-means, fuzzy spatial c-means, and a Gaussian mixture model with a Markov random field prior) and an image-based convolutional neural network were used to segment 2 simulated data sets derived from a public (n = 29 subjects) and a retrospective collection (n = 51 subjects) of hyperpolarized 129Xe gas lung images transformed by common MRI artifacts (noise and nonlinear intensity distortion). The resulting ventilation-based segmentations were used to assess algorithmic performance and characterize optimization domain differences in terms of measurement bias and precision.

RESULTS

Although facilitating computational processing and providing discriminating clinically relevant measures of interest, histogram-based segmentation methods discard important contextual spatial information and are consequently less robust in terms of measurement precision in the presence of common MRI artifacts relative to the image-based convolutional neural network.

CONCLUSIONS

Direct optimization within the image domain using convolutional neural networks leverages spatial information, which mitigates problematic issues associated with histogram-based approaches and suggests a preferred future research direction. Further, the entire processing and evaluation framework, including the newly reported deep learning functionality, is available as open source through the well-known Advanced Normalization Tools ecosystem.

Xenon ventilation MRI in difficult asthma: initial experience in a clinical setting

Background

Hyperpolarised gas magnetic resonance imaging (MRI) can be used to assess ventilation patterns. Previous studies have shown the image-derived metric of ventilation defect per cent (VDP) to correlate with forced expiratory volume in 1 s (FEV₁)/forced vital capacity (FVC) and FEV₁ in asthma.

Objectives

The aim of this study was to explore the utility of hyperpolarised xenon-129 (¹²⁹Xe) ventilation MRI in clinical care and examine its relationship with spirometry and other clinical metrics in people seen in a severe asthma service.

Methods

26 people referred from a severe asthma clinic for MRI scanning were assessed by contemporaneous ¹²⁹Xe MRI and spirometry. A subgroup of 18 patients also underwent reversibility testing with spirometry and MRI. Quantitative MRI measures of ventilation were calculated, VDP and the ventilation heterogeneity index (VH_I), and compared to spirometry, Asthma Control Questionnaire 7 (ACQ7) and blood eosinophil count. Images were reviewed by a multidisciplinary team.

Results

VDP and VH_I correlated with FEV₁, FEV₁/FVC and forced expiratory flow between 25% and 75% of FVC but not with ACQ7 or blood eosinophil count. Discordance of MRI imaging and symptoms and/or pulmonary function tests also occurred, prompting diagnostic re-evaluation in some cases.

Conclusion

Hyperpolarised gas MRI provides a complementary method of assessment in people with difficult to manage asthma in a clinical setting. When used as a tool supporting clinical care in a severe asthma service, occurrences of discordance between symptoms, spirometry and MRI scanning indicate how MRI scanning may add to a management pathway.

This article demonstrates the feasibility of using ¹²⁹Xe MRI in clinical practice. Discordance between symptoms, spirometry and MRI can support the use of further treatment or suggest coexisting breathing control issues or laryngeal disorders.https://bit.ly/3ky4oXP

Bronchial thermoplasty guided by hyperpolarised gas magnetic resonance imaging in adults with severe asthma: a 1-year pilot randomised trial

Patient-specific localisation of ventilation defects using hyperpolarised gas magnetic resonance imaging (MRI) introduces the possibility of regionally targeted bronchial thermoplasty (BT) for the treatment of severe asthma. We aimed to demonstrate that BT guided by MRI to ventilation defects reduces the number of radiofrequency activations while resulting in improved asthma quality-of-life and control scores that are non-inferior to standard BT. In a 1-year pilot randomised controlled trial, 14 patients with severe asthma who were clinically eligible to receive BT underwent hyperpolarised gas MRI to characterise ventilation defects and were randomised to MRI-guided or standard BT. End-points were improved Asthma Quality of Life Questionnaire (AQLQ) and Asthma Control Questionnaire (ACQ) scores, the proportion of AQLQ and ACQ responders and the number of radiofrequency activations and bronchoscopy sessions. Participants who underwent MRI-guided BT received 53% fewer radiofrequency activations than those who had standard BT (p=0.003). At 12 months, the mean improvement from baseline was similar between the MRI-guided group (n=5) and the standard group (n=7) for AQLQ score (MRI-guided: 1.8, 95% CI 0.1-3.5, p=0.04; standard: 0.7, 95% CI -0.9-2.3, p=0.30) (p=0.25) and ACQ-5 score (MRI-guided: -1.4, 95% CI -2.6- -0.2, p=0.03; standard: -0.7, 95% CI -1.3-0.0, p=0.04) (p=0.17). A similar proportion of participants in both groups achieved a clinically relevant improvement in AQLQ score (MRI-guided: 80%; standard: 71%) and ACQ-5 score (MRI-guided: 80%; standard: 57%). Hyperpolarised gas MRI-guided BT reduced the number of radiofrequency activations, and resulted in asthma quality of life and control improvements at 12 months that were non-inferior to standard BT.

Evaluation of sex-based differences in airway size and the physiological implications

Recent evidence suggests healthy females have significantly smaller central conducting airways than males when matched for either height or lung volume during analysis. This anatomical sex-based difference could impact the integrative response to exercise. Our review critically evaluates the literature on direct and indirect techniques to measure central conducting airway size and their limitations. We present multiple sources highlighting the difference between male and female central conducting airway size in both pediatric and adult populations. Following the discussion of measurement techniques and results, we discuss the functional implications of these differences in central conducting airway size, including work of breathing, oxygen cost of breathing, and how these impacts will continue into elderly populations. We then discuss a range of topics for the future direction of airway differences and the benefits they could provide to both healthy and diseased populations. Specially, these sex-differences in central conducting airway size could result in different aerosol deposition or how lung disease manifests. Finally, we detail emerging techniques that uniquely allow for high-resolution imaging to be paired with detailed physiological measures.

Measures of ventilation heterogeneity mapped with hyperpolarized helium-3 MRI demonstrate a T2-high phenotype in asthma

BACKGROUND

Hyperpolarized gas with helium (HHe-3) MR (magnetic resonance) is a noninvasive imaging method which maps and quantifies regions of ventilation heterogeneity (VH) in the lung. VH is an important feature of asthma, but little is known as to how VH informs patient phenotypes.

PURPOSE

To determine if VH indicators quantified by HHe-3 MR imaging (MRI) predict phenotypic characteristics and map to regions of inflammation in children with problematic wheeze or asthma.

METHODS

Sixty children with poorly-controlled wheeze or asthma underwent HHe-3 MRI, including 22 with bronchoalveolar lavage (BAL). The HHe-3 signal intensity defined four ventilation compartments. The non-ventilated and hypoventilated compartments divided by the total lung volume defined a VH index (VHI %).

RESULTS

Children with VHI % in the upper quartile had significantly greater airflow limitation, bronchodilator responsiveness, blood eosinophils, expired nitric oxide (FeNO), and BAL eosinophilic or neutrophilic granulocyte patterns compared to children with VHI % in the lower quartile. Lavage return from hypoventilated bronchial segments had greater eosinophil % than from ventilated segments.

CONCLUSION

In children with asthma, greater VHI % as measured by HHe-3 MRI identifies a severe phenotype with higher type 2 inflammatory markers, and maps to regions of lung eosinophilia. Listed on ClinicalTrials. gov (NCT02577497).

Novel Thoracic MRI Approaches for the Assessment of Pulmonary Physiology and Inflammation

Excessive pulmonary inflammation can lead to damage of lung tissue, airway remodelling and established structural lung disease. Novel therapeutics that specifically target inflammatory pathways are becoming increasingly common in clinical practice, but there is yet to be a similar stepwise change in pulmonary diagnostic tools. A variety of thoracic magnetic resonance imaging (MRI) tools are currently in development, which may soon fulfil this emerging clinical need for highly sensitive assessments of lung structure and function. Given conventional MRI techniques are poorly suited to lung imaging, alternate strategies have been developed, including the use of inhaled contrast agents, intravenous contrast and specialized lung MR sequences. In this chapter, we discuss technical challenges of performing MRI of the lungs and how they may be overcome. Key thoracic MRI modalities are reviewed, namely, hyperpolarized noble gas MRI, oxygen-enhanced MRI (OE-MRI), ultrashort echo time (UTE) MRI and dynamic contrast-enhanced (DCE) MRI. Finally, we consider potential clinical applications of these techniques including phenotyping of lung disease, evaluation of novel pulmonary therapeutic efficacy and longitudinal assessment of specific patient groups.

State-of-the-art MR Imaging for Thoracic Diseases

Since thoracic MR imaging was first used in a clinical setting, it has been suggested that MR imaging has limited clinical utility for thoracic diseases, especially lung diseases, in comparison with x-ray CT and positron emission tomography (PET)/CT. However, in many countries and states and for specific indications, MR imaging has recently become practicable. In addition, recently developed pulmonary MR imaging with ultra-short TE (UTE) and zero TE (ZTE) has enhanced the utility of MR imaging for thoracic diseases in routine clinical practice. Furthermore, MR imaging has been introduced as being capable of assessing pulmonary function. It should be borne in mind, however, that these applications have so far been academically and clinically used only for healthy volunteers, but not for patients with various pulmonary diseases in Japan or other countries. In 2020, the Fleischner Society published a new report, which provides consensus expert opinions regarding appropriate clinical indications of pulmonary MR imaging for not only oncologic but also pulmonary diseases. This review article presents a brief history of MR imaging for thoracic diseases regarding its technical aspects and major clinical indications in Japan 1) in terms of what is currently available, 2) promising but requiring further validation or evaluation, and 3) developments warranting research investigations in preclinical or patient studies. State-of-the-art MR imaging can non-invasively visualize lung structural and functional abnormalities without ionizing radiation and thus provide an alternative to CT. MR imaging is considered as a tool for providing unique information. Moreover, prospective, randomized, and multi-center trials should be conducted to directly compare MR imaging with conventional methods to determine whether the former has equal or superior clinical relevance. The results of these trials together with continued improvements are expected to update or modify recommendations for the use of MRI in near future.

Pulmonary Functional Imaging: Part 2-State-of-the-Art Clinical Applications and Opportunities for Improved Patient Care

Pulmonary functional imaging may be defined as the regional quantification of lung function by using primarily CT, MRI, and nuclear medicine techniques. The distribution of pulmonary physiologic parameters, including ventilation, perfusion, gas exchange, and biomechanics, can be noninvasively mapped and measured throughout the lungs. This information is not accessible by using conventional pulmonary function tests, which measure total lung function without viewing the regional distribution. The latter is important because of the heterogeneous distribution of virtually all lung disorders. Moreover, techniques such as hyperpolarized xenon 129 and helium 3 MRI can probe lung physiologic structure and microstructure at the level of the alveolar-air and alveolar-red blood cell interface, which is well beyond the spatial resolution of other clinical methods. The opportunities, challenges, and current stage of clinical deployment of pulmonary functional imaging are reviewed, including applications to chronic obstructive pulmonary disease, asthma, interstitial lung disease, pulmonary embolism, and pulmonary hypertension. Among the challenges to the deployment of pulmonary functional imaging in routine clinical practice are the need for further validation, establishment of normal values, standardization of imaging acquisition and analysis, and evidence of patient outcomes benefit. When these challenges are addressed, it is anticipated that pulmonary functional imaging will have an expanding role in the evaluation and management of patients with lung disease.

Evaluating post-bronchodilator response in well-controlled paediatric severe asthma using hyperpolarised 129Xe-MRI: A pilot study

INTRODUCTION

Pulmonary function tests (PFTs) are the main objective measures used to assess asthma in children. However, PFTs provide a global measure of lung function. Hyperpolarised xenon-129 magnetic resonance imaging (129Xe-MRI) can assess lung function spatially. This cross-sectional cohort study aimed to evaluate the use of 129Xe-MRI in detecting ventilation abnormalities in children with well-controlled severe asthma pre- and post-bronchodilator (BD).

METHOD

Six healthy children (aged 11 ± 3) and six with well-controlled severe asthma (14 ± 1) underwent spirometry, multiple breath washout (MBW), and 129Xe-MRI. These tests were repeated post-BD in the asthma cohort. Image analysis was performed in MATLAB. Wilcoxon signed-rank test, repeated measures analysis of variance (ANOVA), and Spearman's rank correlation coefficient were used for statistical analysis.

RESULTS

A significantly higher number of ventilation defects were found in the asthma cohort pre-BD compared to the healthy participants and post-BD within the asthma cohort (p = 0.02 and 0.01). A greater number of wedge-shaped defects were detected in the asthma cohort pre-BD compared to healthy participants and post-BD within the asthma cohort (p = 0.01 and 0.008, respectively). 129Xe ventilation defect percentage (VDP) and coefficient of variation (CoV) were significantly higher in the asthma cohort pre-BD compared to the healthy cohort (p = 0.006 for both). VDP and CoV were reduced significantly post-BD in the asthma cohort, to a level where there was no longer a significant difference between the two cohorts.

CONCLUSION

129Xe-MRI is a sensitive marker of ventilation inhomogeneity in paediatric severe asthma and may potentially be used as a biomarker to assess disease progression and therapeutic response.

Exercise-induced alterations in phospholipid hydrolysis, airway surfactant, and eicosanoids and their role in airway hyperresponsiveness in asthma

The mechanisms responsible for driving endogenous airway hyperresponsiveness (AHR) in the form of exercise-induced bronchoconstriction (EIB) are not fully understood. We examined alterations in airway phospholipid hydrolysis, surfactant degradation, and lipid mediator release in relation to AHR severity and changes induced by exercise challenge. Paired induced sputum (n = 18) and bronchoalveolar lavage (BAL) fluid (n = 11) were obtained before and after exercise challenge in asthmatic subjects. Samples were analyzed for phospholipid structure, surfactant function, and levels of eicosanoids and secreted phospholipase A₂ group 10 (sPLA₂-X). A primary epithelial cell culture model was used to model effects of osmotic stress on sPLA₂-X. Exercise challenge resulted in increased surfactant degradation, phospholipase activity, and eicosanoid production in sputum samples of all patients. Subjects with EIB had higher levels of surfactant degradation and phospholipase activity in BAL fluid. Higher basal sputum levels of cysteinyl leukotrienes (CysLTs) and prostaglandin D₂ (PGD₂) were associated with direct AHR, and both the postexercise and absolute change in CysLTs and PGD₂ levels were associated with EIB severity. Surfactant function either was abnormal at baseline or became abnormal after exercise challenge. Baseline levels of sPLA₂-X in sputum and the absolute change in amount of sPLA₂-X with exercise were positively correlated with EIB severity. Osmotic stress ex vivo resulted in movement of water and release of sPLA₂-X to the apical surface. In summary, exercise challenge promotes changes in phospholipid structure and eicosanoid release in asthma, providing two mechanisms that promote bronchoconstriction, particularly in individuals with EIB who have higher basal levels of phospholipid turnover.

Breath Hold Facilitates Targeted Deposition of Aerosolized Droplets in a 3D Printed Bifurcating Airway Tree

The lungs have long been considered a desired route for drug delivery but, there is still a lack of strategies to rationally target delivery sites especially in the presence of heterogeneous airway disease. Furthermore, no standardized system has been proposed to rapidly test different ventilation strategies and how they alter the overall and regional deposition pattern in the airways. In this study, a 3D printed symmetric bifurcating tree model mimicking part of the human airway tree was developed that can be used to quantify the regional deposition patterns of different delivery methodologies. The model is constructed in a novel way that allows for repeated measurements of regional deposition using reusable parts. During ventilation, nebulized ~3-micron-sized fluid droplets were delivered into the model. Regional delivery, quantified by precision weighing individual airways, was highly reproducible. A successful strategy to control regional deposition was achieved by combining an inspiratory wave form with a "breath hold" pause after each inspiration. Specifically, the second generation of the tree was successfully targeted, and deposition was increased by up to four times in generation 2 when compared to a ventilation without the breath hold (p < 0.0001). Breath hold was also demonstrated to facilitate deposition into blocked regions of the model, which mimic airway closure during an asthma that receive no flow during inhalation. Additionally, visualization experiments demonstrated that in the absence of fluid flow, the deposition of 3-micron water droplets is dominated by gravity, which, to our knowledge, has not been confirmed under standard laboratory conditions.

Airway closure is the predominant physiological mechanism of low ventilation seen on hyperpolarized helium-3 MRI lung scans

Hyperpolarized helium-3 MRI (³He MRI) provides detailed visualization of low- (hypo- and non-) ventilated lungs. Physiological measures of gas mixing may be assessed by multiple breath nitrogen washout (MBNW) and of airway closure by a forced oscillation technique (FOT). We hypothesize that in patients with asthma, areas of low-ventilated lung on ³He MRI are the result of airway closure. Ten control subjects, ten asthma subjects with normal spirometry (non-obstructed), and ten asthmatic subjects with reduced baseline lung function (obstructed) attended two testing sessions. On visit one, baseline plethysmography was performed followed by spirometry, MBNW, and FOT assessment pre and post methacholine challenge. On visit two, ³He MRI scans were conducted pre and post methacholine challenge. Post methacholine the volume of low-ventilated lung increased from 8.3% to 13.8% in the non-obstructed group (P = 0.012) and from 13.0% to 23.1% in the obstructed group (P = 0.001). For all subjects, the volume of low ventilation from ³He MRI correlated with a marker of airway closure in obstructive subjects, Xrs (6 Hz) and the marker of ventilation heterogeneity Scond with r² values of 0.61 (P < 0.001) and 0.56 (P < 0.001), respectively. The change in Xrs (6 Hz) correlated well (r² = 0.45, p < 0.001), whereas the change in Scond was largely independent of the change in low ventilation volume (r² = 0.13, P < 0.01). The only significant predictor of low ventilation volume from the multi-variate analysis was Xrs (6 Hz). This is consistent with the concept that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.NEW & NOTEWORTHY This study introduces a novel technique of generating high-resolution 3D ventilation maps from hyperpolarized helium-3 MRI. It is the first study to demonstrate that regions of poor or absent ventilation seen on ³He MRI are primarily the result of airway closure.

Peripheral and proximal lung ventilation in asthma: Short-term variation and response to bronchodilator inhalation

BACKGROUND

The relative involvement of the large and small airways in asthma is not clear. Hyperpolarized gas magnetic resonance imaging (MRI) provides high-resolution 3-dimensional images of ventilation distribution that can be quantified by the ventilated volume percentage (VV%) of the lungs.

OBJECTIVE

Our aims were to (1) quantify the baseline reproducibility of VV%, (2) assess the ventilation distribution between the proximal and peripheral lungs, and (3) investigate regional ventilation response to bronchodilator inhalation in a cohort of patients with asthma.

METHODS

A total of 33 patients with poorly controlled, moderate-to-severe asthma were scanned with hyperpolarized ³He MRI. Two image data sets were acquired at baseline, and 1 image data set was acquired after bronchodilator inhalation. Images were divided into proximal and peripheral regions for analysis.

RESULTS

Bland-Altman analysis showed strong reproducibility of VV% (bias = 0.12%; LOA = -1.86% to 2.10%). VV% variation at baseline was greater in the periphery than in the proximal lung. The proximal lung was better ventilated than the peripheral lung. Ventilation increased significantly in response to bronchodilator inhalation, globally and regionally, and the ventilation increase in response to bronchodilator inhalation was greater in the peripheral lung than in the proximal lung. Hyperpolarized gas MRI was more sensitive to changes in response to bronchodilator inhalation (58%) than spirometry (33%).

CONCLUSION

The peripheral lung showed reduced ventilation and a greater response to bronchodilator inhalation than the proximal lung. The high level of baseline reproducibility and sensitivity of hyperpolarized gas MRI to bronchodilator reversibility suggests that it is suitable for low subject number studies of therapy response.

Expanding Applications of Pulmonary MRI in the Clinical Evaluation of Lung Disorders: Fleischner Society Position Paper

Pulmonary MRI provides structural and quantitative functional images of the lungs without ionizing radiation, but it has had limited clinical use due to low signal intensity from the lung parenchyma. The lack of radiation makes pulmonary MRI an ideal modality for pediatric examinations, pregnant women, and patients requiring serial and longitudinal follow-up. Fortunately, recent MRI techniques, including ultrashort echo time and zero echo time, are expanding clinical opportunities for pulmonary MRI. With the use of multicoil parallel acquisitions and acceleration methods, these techniques make pulmonary MRI practical for evaluating lung parenchymal and pulmonary vascular diseases. The purpose of this Fleischner Society position paper is to familiarize radiologists and other interested clinicians with these advances in pulmonary MRI and to stratify the Society recommendations for the clinical use of pulmonary MRI into three categories: (a) suggested for current clinical use, (b) promising but requiring further validation or regulatory approval, and (c) appropriate for research investigations. This position paper also provides recommendations for vendors and infrastructure, identifies methods for hypothesis-driven research, and suggests opportunities for prospective, randomized multicenter trials to investigate and validate lung MRI methods.

Minimizing airflow turbulence in women lowers the work of breathing to levels similar to men

Smaller airways increase resistance and the propensity toward turbulent airflow, both of which are thought to be mechanisms behind greater resistive and total work of breathing (Wb) in females. Previous research examining the effect of airway size on the Wb between the sexes is limited by the inability to experimentally manipulate airway size. Heliox (21% oxygen, balance helium) is less dense than room air, which reduces turbulent airflow and airway resistance. The purpose of our study was to utilize heliox inspiration in women to provide a stimulus physiologically similar to increasing airway size. We hypothesized that when breathing heliox women would have a Wb similar to men breathing room air. Eighteen healthy young subjects (n = 9 women, 9 men) completed two maximal exercise tests on a cycle ergometer over 2 days. Subjects breathed room air for one test and heliox for the other. Wb was assessed with an esophageal balloon catheter. During the room air trial, when ventilations were >65 L/min, women had a significantly greater Wb compared with men (P < 0.05). The greater Wb in women was due to greater resistance to turbulent flow. For both sexes, breathing heliox resulted in increased expiratory flow (+132 ± 18% of room air), an elimination of expiratory flow limitation, and a reduction in Wb (69 ± 12% of room air) (all P < 0.05). When the women were breathing heliox, Wb was not different from that in the men breathing room air. Our findings support the idea that the smaller conducting airways in females are responsible for a greater total and resistive Wb.NEW & NOTEWORTHY When healthy young women breathe heliox gas during exercise, their work of breathing is not different from men breathing room air. Heliox inspiration reduces airway resistance and promotes laminar flow, which is a physiologically similar effect of increasing airway size. Our findings provide experimental evidence that smaller airways in women are responsible for the greater work of breathing during exercise.

Asthma and Lung Mechanics

This article will discuss in detail the pathophysiology of asthma from the point of view of lung mechanics. In particular, we will explain how asthma is more than just airflow limitation resulting from airway narrowing but in fact involves multiple consequences of airway narrowing, including ventilation heterogeneity, airway closure, and airway hyperresponsiveness. In addition, the relationship between the airway and surrounding lung parenchyma is thought to be critically important in asthma, especially as related to the response to deep inspiration. Furthermore, dynamic changes in lung mechanics over time may yield important information about asthma stability, as well as potentially provide a window into future disease control. All of these features of mechanical properties of the lung in asthma will be explained by providing evidence from multiple investigative methods, including not only traditional pulmonary function testing but also more sophisticated techniques such as forced oscillation, multiple breath nitrogen washout, and different imaging modalities. Throughout the article, we will link the lung mechanical features of asthma to clinical manifestations of asthma symptoms, severity, and control. © 2020 American Physiological Society. Compr Physiol 10:975-1007, 2020.

Effects of Anti-T2 Biologic Treatment on Lung Ventilation Evaluated by MRI in Adults With Prednisone-Dependent Asthma

BACKGROUND

The functional consequence of airway obstruction in asthma can be regionally measured using inhaled gas MRI. Ventilation defects visualized by MRI persist post-bronchodilator in patients with severe asthma with uncontrolled sputum eosinophilia and may be due to eosinophil-driven airway pathology that is responsive to "anti-T2" therapy.

RESEARCH QUESTION

Do anti-T2 therapies that clear eosinophils from the airway lumen decrease ventilation defects, measured by inhaled gas MRI, in adults with prednisone-dependent asthma?

STUDY DESIGN AND METHODS

Inhaled hyperpolarized gas MRI was performed before and after bronchodilation in 10 prednisone-dependent patients with asthma with uncontrolled eosinophilic bronchitis (sputum eosinophils ≥3%) at baseline and 558 (100-995) days later when their eosinophilic bronchitis had been controlled (sputum eosinophils <3%) by additional anti-T2 therapy. The effect of anti-T2 therapy on ventilation defects, quantified as the MRI ventilation-defect-percent (VDP), was evaluated before and after bronchodilation for all patients and compared between patients dichotomized based on the median percentage of sputum eosinophils at baseline (15.8%).

RESULTS

MRI VDP was improved pre- (ΔVDP_+anti-T2: -3% ± 4%, P = .02) and post-bronchodilator (ΔVDP_+anti-T2: -3% ± 4%; P = .04) after additional anti-T2 therapy that controlled eosinophilic bronchitis (n = 2 mepolizumab, n = 2 reslizumab, n = 3 benralizumab, n = 1 dupilumab, n = 2 increased daily prednisone). A greater post-bronchodilator ΔVDP_+anti-T2 was observed in those patients with median or higher percentage of sputum eosinophils at baseline (≥15.8%; P = .01). In 7 of 10 patients with asthma, residual ventilation defects persisted despite bronchodilator and anti-T2 therapy.

INTERPRETATION

Controlling sputum eosinophilia with anti-T2 therapies improves ventilation defects, measured by inhaled gas MRI, in adults with prednisone-dependent asthma.

Correlation between fractional exhaled nitric oxide and Asthma Control Test score and spirometry parameters in on-treatment-asthmatics in Ho Chi Minh City

Background

Although fractional exhaled nitric oxide (FeNO) is a reliable and easily applied marker of airway inflammation in asthma, the relationship between FeNO and indicators of asthma control [Asthma Control Test (ACT) score] and/or severity (spirometry parameters) remains unclear. This study aims to determine possible correlations between FeNO and ACT score; and between FeNO and spirometry parameters.

Methods

A cross-sectional study with convenience sampling was conducted among ambulatory patients in the Asthma & COPD clinic at the University Medical Center, Ho Chi Minh City from March 2016 to March 2017. Using measurement of FeNO, the ACT questionnaire and a spirometry test, correlations were determined between FeNO and the ACT score and spirometry parameters.

Results

Four hundred and ten asthmatic patients (mean age 42 years; 65% female) were included and analyzed; their mean time since onset of asthma was 9.5 years. All patients were treated following step 2 to 4 of GINA guidelines. Mean (SD) FeNO was 29.5 (24.4) parts per billion (ppb) and mean (SD) ACT score was 20.5 (40). A significant difference in FeNO values was found among the three groups with different asthma control levels categorized according to the ACT score (P=0.001) but was not found among the three groups with different asthma treatment levels (P=0.425). FeNO was significantly inversely correlated with the ACT score (Spearman’s r =−0.224, P<0.001) and with spirometry parameters indicate airway obstruction such as predicted FEV1, FEV1/FVC, predicted PEF and predicted FEF25–75% with Spearman’s r were −0.187; −0.143; −0.091 and −0.195, respectively (all P<0.05), whereas no correlation between FeNO and FVC—an indicator of airway restriction—was found.

Conclusions

In these asthmatic patients in Vietnam, an inverse correlation was found between FeNO and the ACT score and between FeNO and spirometry indicators of airway obstruction. Therefore, FeNO may be a useful tool in asthma management.

Reactance and elastance as measures of small airways response to bronchodilator in asthma

Bronchodilation alters both respiratory system resistance (Rrs) and reactance (Xrs) in asthma, but how changes in Rrs and Xrs compare, and respond differently in health and asthma, in reflecting the contributions from the large and small airways has not been assessed. We assessed reversibility using spirometry and oscillometry in healthy and asthma subjects. Using a multibranch airway-tree model with the mechanics of upper airway shunt, we compared the effects of airway dilation and small airways recruitment to explain the changes in Rrs and Xrs. Bronchodilator decreased Rrs by 23.0 (19.0)% in 18 asthma subjects and by 13.5 (19.5)% in 18 healthy subjects. Estimated respiratory system elastance (Ers) decreased by 23.2 (21.4)% in asthma, with no significant decrease in healthy subjects. With the use of the model, airway recruitment of 15% across a generation of the small airways could explain the changes in Ers in asthma with no recruitment in healthy subjects. In asthma, recruitment accounted for 40% of the changes in Rrs, with the remaining explained by airway dilation of 6.8% attributable largely to the central airways. Interestingly, the same dilation magnitude explained the changes in Rrs in healthy subjects. Shunt only affected Rrs of the model. Ers was unaltered in health and unaffected by shunt in both groups. In asthma, Ers changed comparably to Rrs and could be attributed to small airways, while the change in Rrs was split between large and small airways. This implies that in asthma Ers sensed through Xrs may be a more effective measure of small airways obstruction and recruitment than Rrs.NEW & NOTEWORTHY This is the first study to quantify to relative contributions of small and large airways to bronchodilator response in healthy subjects and patients with asthma. The response of the central airways to bronchodilator was similar in magnitude in both study groups, whereas the response of the small airways was significant among patients with asthma. These results suggest that low-frequency reactance and derived elastance are both sensitive measures of small airway function in asthma.

Sputum Eosinophilia and Magnetic Resonance Imaging Ventilation Heterogeneity in Severe Asthma

RATIONALE

Inflammation and smooth muscle dysfunction are integral components of severe asthma that contribute to luminal obstruction causing airflow limitation, ventilation heterogeneity, and symptoms. This is important for guiding treatment decisions directed at the inflammatory (e.g., anti-T-helper cell type 2 monoclonal antibodies) and noninflammatory, smooth muscle-mediated (e.g., bronchial thermoplasty) components of severe asthma.

OBJECTIVES

To investigate the contribution of eosinophilic bronchitis and smooth muscle dysfunction to magnetic resonance imaging (MRI) ventilation heterogeneity in patients with severe asthma.

METHODS

We measured the inhaled hyperpolarized gas MRI response to salbutamol as a marker of smooth muscle dysfunction, and sputum eosinophils as a marker of airway inflammation, and their contributions to ventilation heterogeneity (quantified as the ventilation defect percent [VDP]) in 27 patients with severe asthma. Spirometry and forced oscillation airway resistance measurements were also acquired pre- and postsalbutamol. Patients were dichotomized on the basis of sputum eosinophilia, and pre- and postsalbutamol VDP and physiological measurements were evaluated.

MEASUREMENTS AND MAIN RESULTS

MRI VDP improved with salbutamol inhalation in patients in whom sputum eosinophilia was uncontrolled (≥3%, n = 16) (P = 0.002) and in those in whom it was controlled (<3%, n = 11) (P = 0.02), independent of improvements in FEV₁, indicating smooth muscle response. In those patients in whom sputum eosinophilia was uncontrolled, greater VDP persisted postsalbutamol (P = 0.004). Postsalbutamol VDP correlated with sputum eosinophils (r = 0.63; P = 0.005).

CONCLUSIONS

In patients with severe asthma, MRI regionally identifies the inflammatory and noninflammatory components of airway disease. Ventilation heterogeneity persists postsalbutamol in patients with uncontrolled eosinophilic bronchitis, which may be the functional consequence of airway inflammation.

A Comparison of Two Hyperpolarized 129Xe MRI Ventilation Quantification Pipelines: The Effect of Signal to Noise Ratio

RATIONALE

Hyperpolarized ¹²⁹Xe MRI enables quantitative evaluation of regional ventilation. To this end, multiple classifiers have been proposed to determine ventilation defect percentage (VDP) as well as other cluster populations. However, consensus has not yet been reached regarding which of these methods to deploy for multicenter clinical trials. Here, we compare two published classification techniques-linear-binning and adaptive K-means-to establish their limits of agreement and their robustness against reduced signal-to-noise ratio (SNR).

METHODS

A total of 29 subjects (age: 38.4 ± 19.0 years) were retrospectively identified for inter-method comparison. For each ¹²⁹Xe ventilation image, 7 images with reduced SNR were generated with equal decrements relative to the native SNR. All 8 sets of images were then analyzed using both methods independently to classify all lung voxels into four clusters: VDP, low-, medium-, and high-ventilation-percentage (LVP, MVP and HVP). For each cluster, the percentage of the lung it comprised was compared between the two methods, as well as how these values persisted as SNR was degraded.

RESULTS

The limits of agreement for calculating VDP were [+0.2%, +4.0%] with a +1.5% bias for binning relative to K-means. However, the inter-method agreement for the other clusters was moderate, with biases of -5.7%, 8.1%, and -4.0% for LVP, MVP, and HVP, respectively. As SNR decreased below ∼4, both methods began reporting values that deviated substantially from the native image. By requiring VDP to remain within ≤1.8% of that calculated from the native image, the minimum tolerable SNR values were 2.4 ± 1.0 for the linear-binning, and 3.5 ± 1.5 for the K-means.

CONCLUSIONS

Both methods agree well in quantifying VDP, but agreement for LVP and MVP remains variable. We suggest a required SNR threshold be two standard deviations above the minimum value of 3.5 ± 1.5 for robust determination of VDP, suggesting a minimum SNR of 6.6. However, robust quantification of the ventilated clusters required an SNR of 13.4.

Dismantling the pathophysiology of asthma using imaging

Asthma remains an important disease worldwide, causing high burden to patients and healthcare systems and presenting a need for better management and ultimately prevention and cure. Asthma is a very heterogeneous condition, with many different pathophysiological processes. Better measurement of those pathophysiological processes are needed to better phenotype disease, and to go beyond the current, highly limited measurements that are currently used: spirometry and symptoms. Sophisticated three-dimensional lung imaging using computed tomography and ventilation imaging (single photon emission computed tomography and positron emission tomography) and magnetic resonance imaging and methods of lung imaging applicable to asthma research are now highly developed. The body of current evidence suggests that abnormalities in structure and ventilatory function measured by imaging are clinically relevant, given their associations with disease severity, exacerbation risk and airflow obstruction. Therefore, lung imaging is ready for more widespread use in clinical trials and to become part of routine clinical assessment of asthma.

CT and Functional MRI to Evaluate Airway Mucus in Severe Asthma

BACKGROUND

Intraluminal contributor(s) to airflow obstruction in severe asthma are patient-specific and must be evaluated to personalize treatment. The occurrence and functional consequence of airway mucus in the presence or absence of airway eosinophils remain undetermined.

OBJECTIVE

The objective of this study was to understand the functional consequence of airway mucus in the presence or absence of eosinophils and to identify biomarkers of mucus-related airflow obstruction.

METHODS

Mucus plugs were quantified on CT scans, and their contribution to ventilation heterogeneity (using MRI ventilation defect percent [VDP]) was evaluated in 27 patients with severe asthma. Patients were dichotomized based on sputum eosinophilia such that the relationship between mucus, eosinophilia, and ventilation heterogeneity could be investigated. Fractional exhaled nitric oxide (Feno) and related cytokines in sputum were measured.

RESULTS

Mucus plugging was present in 100% of asthma patients with sputum eosinophils and 36% of those without sputum eosinophils (P = .0006) and was correlated with MRI VDP prebronchodilator (r = 0.68; P = .0001) and postbronchodilator (r = 0.72; P < .0001). In a multivariable regression, both mucus and eosinophils contributed to the prediction of postbronchodilator MRI VDP (R² = 0.75; P < .0001). Patients with asthma in whom the mucus score was high had raised Feno (P = .03) and IL-4 (P = .02) values. Mucus plugging correlated with Feno (r = 0.63; P = .005).

CONCLUSIONS

Both airway eosinophils and mucus can contribute to ventilation heterogeneity in patients with severe asthma. Patients in whom mucus is the dominant cause of airway obstruction have evidence of an upregulated IL-4/IL-13 pathway that could be identified according to increased Feno level.

Older age and obesity are associated with increased airway closure in response to methacholine in patients with asthma

BACKGROUND AND OBJECTIVE

The reduction of forced expiratory volume in 1 s (FEV₁ ) in response to methacholine challenge in asthma may reflect two components: airway narrowing, assessed by the change in FEV₁ /forced vital capacity (FVC), and airway closure, assessed by the change in FVC. The purpose of this study was to determine the degree and determinants of airway closure in response to methacholine in a large group of asthmatic patients participating in studies conducted by the American Lung Association-Airways Clinical Research Centers (ALA-ACRC).

METHODS

We used the methacholine challenge data from participants in five studies of the ALA-ACRC to determine the closing index, defined as the contribution of airway closure to the decrease in FEV₁ , and calculated as %ΔFVC/%ΔFEV₁ .

RESULTS

There were a total of 936 participants with asthma, among whom the median closing index was 0.67 relative to that of a published healthy population of 0.54. A higher closing index was associated with increased age (10-year increments) (0.04, 95% CI = 0.02, 0.05, P < 0.005) and obesity (0.07, 95% CI = 0.03, 0.10, P < 0.001). There was no association between the closing index and asthma control.

CONCLUSION

Our findings confirm that airway closure in response to methacholine occurs in a large, diverse population of asthmatic participants, and that increased airway closure is associated with older age and obesity. These findings suggest that therapies targeting airway closure may be important in patients with a high closing index.

Airway Transmural Pressures in an Airway Tree During Bronchoconstriction in Asthma

Airway transmural pressure in healthy homogeneous lungs with dilated airways is approximately equal to the difference between intraluminal and pleural pressure. However, bronchoconstriction causes airway narrowing, parenchymal distortion, dynamic hyperinflation, and the emergence of ventilation defects (VDefs) affecting transmural pressure. This study aimed to investigate the changes in transmural pressure caused by bronchoconstriction in a bronchial tree. Transmural pressures before and during bronchoconstriction were estimated using an integrative computational model of bronchoconstriction. Briefly, this model incorporates a 12-generation symmetric bronchial tree, and the Anafi and Wilson model for the individual airways of the tree. Bronchoconstriction lead to the emergence of VDefs and a relative increase in peak transmural pressures of up to 84% compared to baseline. The highest increase in peak transmural pressure occurred in a central airway outside of VDefs, and the lowest increase was 27% in an airway within VDefs illustrating the heterogeneity in peak transmural pressures within a bronchial tree. Mechanisms contributing to the increase in peak transmural pressures include increased regional ventilation and dynamic hyperinflation both leading to increased alveolar pressures compared to baseline. Pressure differences between intraluminal and alveolar pressure increased driven by the increased airway resistance and its contribution to total transmural pressure reached up to 24%. In conclusion, peak transmural pressure in lungs with VDefs during bronchoconstriction can be substantially increased compared to dilated airways in healthy homogeneous lungs and is highly heterogeneous. Further insights will depend on the experimental studies taking these conditions into account.

The Strain on Airway Smooth Muscle During a Deep Inspiration to Total Lung Capacity

The deep inspiration (DI) maneuver entices a great deal of interest because of its ability to temporarily ease the flow of air into the lungs. This salutary effect of a DI is proposed to be mediated, at least partially, by momentarily increasing the operating length of airway smooth muscle (ASM). Concerningly, this premise is largely derived from a growing body of in vitro studies investigating the effect of stretching ASM by different magnitudes on its contractility. The relevance of these in vitro findings remains uncertain, as the real range of strains ASM undergoes in vivo during a DI is somewhat elusive. In order to understand the regulation of ASM contractility by a DI and to infer on its putative contribution to the bronchodilator effect of a DI, it is imperative that in vitro studies incorporate levels of strains that are physiologically relevant. This review summarizes the methods that may be used in vivo in humans to estimate the strain experienced by ASM during a DI from functional residual capacity (FRC) to total lung capacity (TLC). The strengths and limitations of each method, as well as the potential confounders, are also discussed. A rough estimated range of ASM strains is provided for the purpose of guiding future in vitro studies that aim at quantifying the regulatory effect of DI on ASM contractility. However, it is emphasized that, owing to the many limitations and confounders, more studies will be needed to reach conclusive statements.

Early onset of airway derecruitment assessed using the forced oscillation technique in subjects with asthma

Derecruitment of air spaces in the lung occurs when airways close during exhalation and is related to ventilation heterogeneity and symptoms in asthma. The forced oscillation technique has been used to identify surrogate measures of airway closure via the reactance (Xrs) versus lung volume relationship. This study used a new algorithm to identify derecruitment from the Xrs versus lung volume relationship from a slow vital capacity maneuver. We aimed to compare two derecruitment markers on the Xrs versus volume curve, the onset reduction of Xrs (DR1_vol) and the onset of more rapid reduction of Xrs (DR2_vol), between control and asthmatic subjects. We hypothesized that the onset of DR1_vol and DR2_vol occurred at higher lung volume in asthmatic subjects. DR1_vol and DR2_vol were measured in 18 subjects with asthma and 18 healthy controls, and their relationships with age and height were examined using linear regression. In the control group, DR1_vol and DR2_vol increased with age (r² = 0.68, P < 0.001 and r² = 0.71, P < 0.001, respectively). DR1_vol and DR2_vol in subjects with asthma [76.58% of total lung capacity (TLC) and 56.79%TLC, respectively] were at higher lung volume compared with control subjects (46.1 and 37.69%TLC, respectively) (P < 0.001). DR2_vol correlated with predicted values of closing capacity (r = 0.94, P < 0.001). This study demonstrates that derecruitment occurs at two points along the Xrs-volume relationship. Both derecruitment points occurred at significantly higher lung volumes in subjects with asthma compared with healthy control subjects. This technique offers a novel way to measure the effects of changes in airways/lung mechanics. NEW & NOTEWORTHY This study demonstrates that the forced oscillation technique can be used to identify two lung volume points where lung derecruitment occurs: 1) where derecruitment is initiated and 2) where onset of rapid derecruitment commences. Measurements of derecruitment increase with age. The onset of rapid derecruitment was highly correlated with predicted closing capacity. Also, the initiation and rate of derecruitment are significantly altered in subjects with asthma.

Mechanisms and Biomarkers of Exercise-Induced Bronchoconstriction

Exercise is a common trigger of bronchoconstriction. In recent years, there has been increased understanding of the pathophysiology of exercise-induced bronchoconstriction. Although evaporative water loss and thermal changes have been recognized stimuli for exercise-induced bronchoconstriction, accumulating evidence points toward a pivotal role for the airway epithelium in orchestrating the inflammatory response linked to exercise-induced bronchoconstriction. Overproduction of inflammatory mediators, underproduction of protective lipid mediators, and infiltration of the airways with eosinophils and mast cells are all established contributors to exercise-induced bronchoconstriction. Sensory nerve activation and release of neuropeptides maybe important in exercise-induced bronchoconstriction, but further research is warranted.

COPD biomarkers and phenotypes: opportunities for better outcomes with precision imaging

A number of chronic diseases have benefited from both imaging and personalised medicine, but unfortunately, for patients with chronic obstructive pulmonary disease (COPD), there has been little clinical uptake or recognition of the key advances in thoracic imaging that might help detect disease early, or, perhaps more importantly, might help develop and phenotype patients for novel or personalised therapies that may halt disease progression. We outline our vision for how computed tomography and magnetic resonance imaging may be used to better inform COPD patient care, and, perhaps more importantly, how these may be used to help develop new therapies directed at early disease. We think that imaging and precision medicine should be considered and used together as "precision imaging" at specific stages of COPD when the major pathologies may be more responsive to therapy. While "precision medicine" is the tailoring of medical treatment to individual patients, we define "precision imaging" as the tailoring of specific therapies and interventions to individual patients with a detailed quantitative understanding of their specific imaging phenotypes and measurements. Finally, we stress the importance of "seeing" the pathology, because without this understanding, you can neither treat nor cure patients with COPD.

SPECT Ventilation Imaging in Asthma

Uneven distribution of ventilation, or ventilation heterogeneity, has been observed in asthma for over 60 years using multiple breath nitrogen washout (MBNW) studies. Ventilation heterogeneity has been known to predict airway hyperresponsiveness (the ability of the airways to constrict too easily and by too much) in asthma, which is a core physiological characteristic of this disease. SPECT ventilation imaging allows topographical analysis of changes in ventilation distribution. Technegas as a SPECT ventilation agent has a key advantage as it remains fixed after inhalation, which allows imaging of upright ventilation distribution, analogous of pulmonary function tests. Recent studies using Technegas ventilation SPECT have shown spatial imaging markers also relate to airway hyperresponsiveness in asthma, and are predicted by a MBNW index of peripheral ventilation heterogeneity. It has also been shown that low-ventilation regions induced by bronchoconstriction were also related to peripheral ventilation heterogeneity. Furthermore, this suggests that the function of peripheral airways may determine the topographical pattern of airway narrowing with a more widespread distribution of narrowing. SPECT ventilation adds spatial characterisation information and it should be included in research protocols to enhance the understanding of complex physiological mechanisms in asthma.