Abstract
Lung function tests are commonly used to monitor lung disease in cystic fibrosis (CF). While practical, they cannot locate the exact origin of functional impairment. Contemporary magnetic resonance imaging (MRI) techniques provide information on the location of disease but the need for contrast agents constrains their repeated application. We examined the correlation between functional MRI, performed without administration of contrast agent, and lung clearance index (LCI) from nitrogen multiple-breath washout (N2-MBW).
40 children with CF (median (range) age 12.0 (6–18) years) and 12 healthy age-matched controls underwent functional and structural MRI and lung function tests on the same day. Functional MRI provided semiquantitative measures of perfusion (RQ) and ventilation (RFV) impairment as percentages of affected lung volume. Morphological MRI was evaluated using CF-specific scores. LCI measured global ventilation inhomogeneity.
MRI detected functional impairment in CF: RFV 19–38% and RQ 16–35%. RFV and RQ correlated strongly with LCI (r=0.76, p<0.0001 and r=0.85, p<0.0001, respectively), as did total morphology score (r=0.81, p<0.0001). All indices differed significantly between patients with CF and healthy controls (p<0.001).
Noninvasive functional MRI is a promising method to detect and visualise perfusion and ventilation impairment in CF without the need for contrast agents.
Abstract
New functional MRI without the need for contrast agents correlates strongly with ventilation inhomogeneity in CF http://ow.ly/emo930eQJ67
Introduction
Chronic lung diseases are highly prevalent in both children and adults. One of the most severe chronic lung diseases in childhood is cystic fibrosis (CF). CF is the most common autosomal recessive genetic disease in Caucasians [1]. Early diagnosis by newborn screening and improvement of therapy has raised interest in diagnostic tools to detect lung function impairment early and prior to irreversible lung damage [2–4]. Current techniques such as nitrogen multiple-breath washout (N2-MBW) or high-resolution computer tomography (HRCT) allow monitoring of early lung disease [5, 6]. However, both techniques are constrained by methodological drawbacks. N2-MBW estimates global ventilation inhomogeneity by the lung clearance index (LCI), but does not provide the exact location of underlying changes and more specific indices for ventilation inhomogeneities are not yet widely used [7, 8]. The cumulative radiation exposure of HRCT is low [9–11], but problematic for routine follow-up, and breathing manoeuvres are demanding for younger children.
In this regard, magnetic resonance imaging (MRI) is a rapidly evolving, radiation-free imaging technique to investigate the morphology and function of the lungs [12, 13]. Nonproton MRI techniques with inhaled hyperpolarised gaseous tracers such as 3He and 129Xe have been shown to provide the best static and dynamic ventilation image quality [14–16]. Nevertheless, the clinical application of these techniques is impeded by limited availability and the high costs of gases, hardware and trained personnel. Alternatively, pulmonary ventilation can also be assessed by proton MRI using T1-shortening oxygen as a gaseous tracer [17]. A recent study using promising contrast enhancement MRI [18, 19] showed a close correlation between lung function and MRI outcomes [20]. A drawback of this method is the need for a contrast agent and possible adverse effects, e.g. painful procedures, allergic reactions or neuronal tissue deposition of contrast media [21, 22].
To overcome those drawbacks, a new MRI technique, termed matrix pencil (MP) decomposition, a derivative of the Fourier decomposition (FD) method, was developed for evaluation of regional fractional ventilation and relative perfusion of the lung [23]. This novel MRI technique provides visual and numeric information of functional deficits in perfusion and ventilation domains of the lung. Importantly, intravenous contrast, inhaled tracers or breath-holding manoeuvres are not required; the technique is applicable during free tidal breathing.
The aim of this study was to assess whether indices from functional MRI differ between healthy controls and patients with CF. We further analysed the correlation between this new functional MRI method and N2-MBW in children with CF. We hypothesised that functional MRI detects fractional ventilation and relative perfusion defects in CF lung disease and that these indices relate to lung function.
Methods
Study design
This was a prospective, cross-sectional, single-centre, observational study at the University Children's Hospital of Bern, Bern, Switzerland, conducted between February 2016 and May 2017. Participants underwent N2-MBW, spirometry, body plethysmography and MRI on the same day, in that order.
Study population
We enrolled 40 unselected children with CF (supplementary table E1) across a wide age range (6–18 years), irrespective of bacterial colonisation or antibiotic use, to ensure a broad range of disease severity. Eligibility criteria were a confirmed diagnosis of CF, the ability to perform pulmonary function tests and MRI, and not requiring supplemental oxygen therapy. Healthy age-matched controls had neither a history of chronic lung disease nor acute respiratory infection in the 4 weeks prior to the investigations. The study was approved by the Ethics Committee of Bern (EKNZ 2015-326 and KEK 2017-00279). We obtained written informed consent from parents and participants, if older than 14 years.
MRI data acquisition and evaluation
Examinations were performed on a 1.5 T whole-body MRI scanner (MAGNETOM Aera; Siemens Healthineers, Erlangen, Germany) using a 12-channel thorax and a 24-channel spine receiver coil array. Sedation was not applied. Parents or caregivers were allowed to accompany children in the scanner cabin during imaging. Functional MRI scans were followed by morphological MRI scans. Regional fractional lung ventilation and relative perfusion were assessed using MP-MRI to acquire multiple coronal slices to cover the whole lung volume with a time-resolved ultra-fast steady-state free precession pulse sequence [24]. Details of scan parameters are given in supplementary table E2. Voxel distributions were used to estimate functional impairment. A similar thresholding method was used previously in a study comparing FD-MRI with dynamic contrast-enhanced MRI in CF patients [25]. For sensitivity analysis we additionally used 60% deviation as cut-off. The primary outcomes were percentage of the lung volume with impaired fractional ventilation RFV and relative perfusion RQ (supplementary figure E1), and morphological assessment based on a protocol previously described by Eichinger et al. [26] for MRI in CF (supplementary table E3). Secondary outcomes were diagnostic quality, measurement duration (supplementary material) and interobserver agreement between the two readers.
Lung function testing
N2-MBW was performed with an unmodified device (Exhalyzer D; Eco Medics, Duernten, Switzerland) according to consensus [27]. Primary outcome was the LCI, calculated as cumulative expired volume over functional residual capacity, for global ventilation inhomogeneity. Secondary outcomes were phase III slope indices Scond and Sacin for convection- and diffusion convection-dependent ventilation inhomogeneity, respectively [27].
Spirometry and body plethysmography (Jaeger MasterScreen; CareFusion, Hochberg, Germany) were performed after N2-MBW, according to European Respiratory Society/American Thoracic Society guidelines [28]. Outcomes were forced expiratory volume in 1 s (FEV1) and forced vital capacity from spirometry, and the ratio of residual volume and total lung capacity (RV/TLC).
Statistical analysis
We first assessed differences in MRI and lung function indices between patients with CF and healthy controls. In a second step we examined a possible correlation between functional indices from MRI and lung function. In a third step we examined the correlation between the morphological pathologies and lung function. For this, we used the mean of the morphological scores from both readers. Upper limit of normal was calculated using the mean+1.64sd of equipment-specific reference values for N2-MBW [29] and data from healthy age-matched controls for functional MRI indices. Continuous variables were compared with the Wilcoxon–Mann–Whitney test, as appropriate. All correlations were described with Spearman's rank correlation coefficient (r). Interobserver agreement was calculated using the intraclass correlation coefficient (ICC). In general, ICC was defined as very good (>0.80), good (0.6–0.80) or moderate (0.4–0.59) [30]. p-values <0.05 were considered statistically significant. As appropriate, Bonferroni correction was used for multiple comparisons. Analyses were performed using Stata version 13 (StataCorp, College Station, TX, USA) and Matlab version 2012b (MathWorks, Natick, MA, USA). Additional details and the sample size calculation are provided in the supplementary material.
Results
Participants
MRI scans, N2-MBW, spirometry and body plethysmography were successfully performed in all 40 patients with CF (median (range) age 12.0 (6–18) years) and all 12 healthy controls (median (range) age 12.0 (5–17) years). Characteristics of study participants are presented in supplementary table E1.
Data quality and measurement duration
Fractional ventilation and relative perfusion images were feasible and generated for all datasets obtained. In patients with CF, the mean (range) duration of the functional MRI scans was 5.6 (4.4–8.1) min and of the morphological MRI scans was 18.2 (13.2–24.6) min (supplementary table E4). Values were similar in healthy controls (supplementary table E5). Functional and morphological images from one healthy participant, one patient with a mild course of CF and one patient with a severe course of CF (based on FEV1 values) are shown in figures 1, 2 and 3, respectively.
Comparison between healthy controls and patients with CF
Fractional ventilation impairment from functional MRI differed significantly between healthy controls and patients with CF (median (IQR) 17.1 (14.6–20.6) versus 29.4 (24.9–31.9); p<0.001) (supplementary figure E2). Likewise relative perfusion impairment differed significantly between both groups (median (IQR) 15.1 (13.4–17.4) versus 27.1 (22.9–29.2); p<0.001) (supplementary figure E3). Morphological MRI score revealed minimal structural changes in only four healthy controls (for both readers total score ranged from 0 to 3), while showing a wide range of structural changes in patients with CF (supplementary table E4). Indices from N2-MBW differed significantly between groups, as to a lesser degree did some outcomes from spirometry and body plethysmography (supplementary table E6).
Interestingly, two patients with CF had results within the normal range in all functional modalities, two patients with CF showed compromised relative perfusion and fractional ventilation while LCI was within normal limits, and 17 patients with CF had normal FEV1 results, but compromised fractional ventilation and relative perfusion assessed by functional MRI (figures 4a and b, and supplementary figures E4 and E5).
Example of a healthy child: a 7-year-old healthy boy. a) Coronal and b) axial 5-mm T2-weighted BLADE and HASTE (half-Fourier acquisition single-shot turbo spin-echo) images, respectively. Chest magnetic resonance imaging (MRI) was normal. c, d) Functional MRI shows fairly homogeneous distribution of c) fractional ventilation as well as d) relative perfusion. Small regions in both lungs were classified as hypoperfused as well as peripheral regions in the middle lobe of the right lung as hypoventilated. e, f) Masks overlaid on morphological images present e) fractional ventilation defects (5% of the slice area) and f) perfusion defects (4% of the slice area). Both lung clearance index (5.7 lung turnovers) and forced expiratory volume in 1 s (−0.30 z-scores) were normal.
Example of a child with mild lung disease: a 9-year-old boy with cystic fibrosis. a) Coronal and b) axial 5-mm T2-weighted HASTE (half-Fourier acquisition single-shot turbo spin-echo) sequences. Mild bronchial wall thickening (b, arrows) is noted in the right lower lobe (Eichinger Score 1 (R1)). Otherwise normal chest magnetic resonance imaging (MRI). c) Functional MRI shows two main areas with reduced fractional ventilation located in the upper lobe of the right lung and basal part of the left lung. d) Relative perfusion map shows several regions with reduced perfusion in both lungs. e, f) Masks overlaid on morphological images present e) fractional ventilation defects (16% of the slice area) and f) perfusion defects (19% of the slice area). Lung clearance index was at the upper limit of normal (8.4 lung turnovers) and forced expiratory volume in 1 s was normal (0.46 z-scores).
Example of a child with advanced lung disease: a 12-year-old girl with cystic fibrosis. a) Coronal HASTE (half-Fourier acquisition single-shot turbo spin-echo) sequence shows mild bronchiectasis and peribronchial infiltrates in the right upper lobe (arrow) (Eichinger Score 10 (R1)). b) Axial T2-weighted BLADE sequence shows moderate bronchial wall thickening in the right upper lobe (short arrow) and mucus plugging in the right upper lobe (long arrow). c, d) Functional magnetic resonance imaging shows c) large regions with reduced fractional ventilation, which are spatially matched with d) areas of decreased relative perfusion. e, f) Masks overlaid on morphological images present e) fractional ventilation defects (36% of the slice area) and f) relative perfusion defects (31% of the slice area). Both lung clearance index (13.4 lung turnovers) and forced expiratory volume in 1 s (−3.1 z-scores) showed pathological values.
Correlation between lung clearance index (LCI) and impairment of the lung for patients with cystic fibrosis (CF) and healthy age-matched controls: a) fractional ventilation (RFV) and b) relative perfusion (RQ). Upper limit of normal is given as dashed lines for both modalities: LCI 8.0 lung turnovers, RFV 24.2% and RQ 19.3%.
Correlation between functional MRI and lung function in patients with CF
In patients with CF, the extent of impaired ventilation relative to lung volume (RFV) quantified by functional MRI was between 19% and 38% (supplementary table E4). We found a strong correlation between RFV and LCI (r=0.76, p<0.0001), as well as a correlation between RFV, Scond and Sacin (table 1 and figure 4a). We found a strong correlation between RFV and FEV1 (r= −0.64, p<0.0001) (supplementary figure E4). The RV/TLC ratio correlated with indices of functional MRI (table 1).
Functional magnetic resonance imaging (MRI) versus lung function outcomes in the 40 patients with cystic fibrosis
The extent of impaired perfusion relative to lung volume (RQ) was between 16% and 35% (supplementary table E4). RQ showed a significant, excellent correlation with LCI (r=0.85, p<0.0001) (table 1 and figure 4b). Scond also correlated with RQ (r=0.62, p<0.0001) in comparison with a correlation between Sacin and RQ (r=0.44, p=0.0049). We found a correlation between FEV1 and RQ (r= −0.67, p<0.0001) (supplementary figure E5). Changes in threshold by using 60% of the median value of the voxel distribution did not change the correlation between LCI, RFV and RQ (r=0.77, p<0.0001 and r=0.8, p<0.0001, respectively).
Correlation between morphological MRI score and lung function
The total morphology score in patients with CF ranged from 0 to 31 for both readers. For detailed subscores, see supplementary table E4. We found excellent correlation between LCI and total morphology scores (r=0.81, p<0.0001), as well as the subscores for bronchiectasis and bronchial wall thickening (r=0.81, p<0.0001) and mucus plugging (r=0.79, p<0.0001). We also found a correlation between the total morphology scores and Scond (r=0.70, p<0.0001), as well as subscores. FEV1 correlated with the total morphology score (r= −0.66, p<0.0001) (table 2).
Correlation between lung function parameters with magnetic resonance imaging (MRI)-defined morphological pathologies in lung structure in the 40 patients with cystic fibrosis
Inter-reader correlation in patients with CF
Correlation between both readers was strong for total morphology score (r=0.84, p<0.0001), bronchiectasis/air wall thickening (r=0.73, p<0.0001) and mucus plugging (r=0.85, p<0.0001) (supplementary table E4). Calculation of the correlation between the scores of each reader separately and lung function values resulted in similar findings for reader1 and reader2, e.g. total morphology score and LCI (r=0.77 for reader1 and r=0.77 for reader2; p<0.0001). Agreement (ICC) between readers was good, with 0.64 for total morphology score and 0.63 for airway wall thickening and bronchiectasis (supplementary table E4).
Discussion
Main findings
Functional MRI reliably detected ventilation and perfusion defects in children at various stages of CF lung disease. The findings differed significantly compared with the group of healthy controls. This is the first study to examine a function–function correlation between two modalities using the combination of a novel proton-based functional MP-MRI technique and N2-MBW in patients with CF and healthy controls. The extent of ventilation and perfusion impairment detected by functional MRI is physiologically plausible as the correlation with LCI is strong. The main technical advantage of the new MRI technique is the contrast-agent-free application during normal tidal breathing. Structure–function correlation was also strong between an established morphological CF score and LCI [20]. Furthermore, we revealed a good agreement between both readers for morphological MRI scores. The short duration required to obtain functional MRI is especially attractive for application in children, and when considering time constraints in radiology departments and outpatient clinics. This novel MRI technique appears applicable to most lung diseases that are not primarily characterised by interstitial remodelling.
Comparison with previous studies
Technical considerations
Lung imaging for clinical routine or clinical trial outcome assessment should be sensitive, fast, safe, noninvasive and easily applicable even in children, as structural and functional airway pathologies already occur in asymptomatic infants and children with CF [1, 4, 13]. In this study, we performed measurements with a non-contrast-enhanced functional MRI using MP decomposition. This technique has been validated against different modalities, e.g. CT, dynamic contrast enhancement MRI and hyperpolarised noble gas MRI, in human and animal models [25, 31–33]. These studies determined a good correlation with dynamic contrast enhancement and hyperpolarised noble gas MRI, and a high test reproducibility. Furthermore, a good reproducibility of the applied functional MRI method in healthy volunteers was shown [34]. Recently, a study using a FD-based self-gated non-contrast-enhanced functional lung imaging technique called SENCEFUL in patients with CF was reported by Veldhoen et al. [35]. Results are not directly comparable, as Veldhoen et al. [35] also studied adult patients with more severe lung disease and they only assessed impairment of ventilation. Still, they also report a significant difference in ventilation efficiency between patients and controls. Furthermore, despite the slightly more severe course of disease in their population and the different MRI techniques, associations between impairment of ventilation and FEV1 are surprisingly comparable between studies (r=0.65 and 0.64, respectively). Of note, MP-MRI as used in our study does not measure the information on regional ventilation and perfusion directly, but by the extraction of signal modulations in the lung caused by the change of lung parenchyma density and regional blood flow.
Physiological considerations
Interestingly, we found that lung function changes measured at the mouth based on ventilation distribution efficiency of the whole lung are well associated with impairment in ventilation and perfusion detected by local imaging of the lung. Our data thereby confirm the concept of LCI being a sensitive marker of global ventilation distribution.
To capture the large range of variety in functional disease in CF phenotypes, which may even fluctuate over time, it is of great importance to use multimodal diagnostic technologies [36]. Functional MRI may detect clinically silent pathological processes earlier than even the already very sensitive N2-MBW. Before morphological changes occur, ventilation/perfusion in the lung is likely to be affected, but may be missed by previous imaging or lung function techniques. The quantification of ventilation/perfusion allows estimating early changes of the latter. This is particularly true with regard to the Euler–Liljestrand reflex, inducing vasoconstriction in unventilated peripheral lung compartments [37]. We chose LCI as the “gold standard” comparator for functional MRI indices because of its high repeatability, specificity and sensitivity to detect global ventilation inhomogeneity, a proxy for both central and peripheral airway involvement in CF [38]. However, global ventilation inhomogeneity is only detected if lung compartments are at least slowly ventilated. Scond, a parameter for convection-dependent ventilation inhomogeneity, strongly correlated with perfusion impairment, bronchiectasis and mucus plugging in the lung, and was moderately correlated with fractional ventilation impairment. A previous study already showed the high prevalence of elevated Scond values in CF, but did not find a correlation between lung function outcomes. This was explained by the theory that in the presence of a complete obliteration of small airways, Scond will not contribute to convective ventilation [39]. Whether these findings are epiphenomena or a reflection of a true structure–function interaction in small airways needs to be determined in longitudinal studies.
Strengths and limitations
Our findings are strengthened by the rigorously performed study design. We always adhered to the full protocol within a clinical setting and on the same day. The order was thus respected (i.e. first functional then morphological sequences). All data were obtained using standard commercially available equipment provided by the manufacturer. Morphological scores are reliable as agreement between both readers was good. However, short- and long-term repeatability require further study. Interestingly, function–function associations using two different but very sensitive techniques seem quite linear over a wide range of disease severity. This cannot be extrapolated to end-stage lung disease and very likely different outcome measures are more suitable in more severe disease.
The functional and morphological MRI protocols required up to 30 min compared with a HRCT scan examination which takes only 5 s. HRCT is currently the “gold standard” for imaging morphological changes of lung disease. Nevertheless, HRCT only provides detailed information on morphological impairment, whereas estimating trapped gas requires repeated scans and breathing manoeuvres. The free tidal breathing in our protocol could have influenced the estimation of the amount of air trapping in the patients [40]. Our study overcomes some risks from previous imaging modalities in CF, e.g. allergic sensitisation or anaphylactic reactions to contrast agents, deposition of contrast agents in tissue and cumulative ionising radiation dose, which is especially important as survival is steadily increasing in patients with CF [9, 21, 22].
Clinical relevance
With this study we emphasise the applicability of functional lung MRI in paediatric patients with CF. All MRI measurements were feasible and of good quality. The non-contrast-enhanced protocol, the free tidal breathing during the measurement, the nonionising radiation and the short time to perform functional MRI demonstrate the great advantages compared with commonly used imaging protocols and techniques. The “new era” of less severe disease, heralded by new treatment targets and better therapeutic options, demands sensitive physiological and structural outcomes to prevent irreversible airway changes. Personalised diagnostic tools enable personalised treatment, which currently plays an increasing role in clinical practice. Potentially reversible functional impairment can now be detected before irreversible morphological changes occur in the lungs. We want to draw attention to new functional imaging modalities and their combination with sensitive lung function indices to determine fractional ventilation impairment in the lung. The potential role of this MRI technique for other lung diseases, such as primary ciliary dyskinesia, chronic obstructive pulmonary disease or asthma, has to be investigated in future studies.
Conclusions
Functional MRI of the lung is feasible in children over 5 years of age and a promising diagnostic technique. Fractional ventilation and relative perfusion impairment of the lung correlate strongly with the LCI in patients with CF. Functional MRI has the potential to detect fractional ventilation and relative perfusion inhomogeneities, which may be the earliest changes of the airways and potentially still reversible. While confirmation in larger cohorts seems to be required, our results may promote the application of MRI as a quantitative morphological and functional test in chronic lung diseases such as CF.
Supplementary material
Supplementary Material
Please note: supplementary material is not edited by the Editorial Office, and is uploaded as it has been supplied by the author.
Supplementary material ERJ-01464-2017_Supplement
Figure E1. Data acquisition scheme. (A) Data acquisition scheme based on time-resolved imaging of multiple coronal slices (S1-S6) covering the chest volume. (B) Analysis of the voxel distribution extracted from segmented fractional ventilation and perfusion maps (on the left side), which were generated from the acquired MR data using the matrix pencil decomposition method. A threshold value equal to 0.75 of the median of the signal intensity distribution is calculated for every functional map separately. Lung regions with voxel values below the calculated threshold are regarded as areas of functional impairment and are visualized as a mask overlaid on the morphological image. Representative masks of fractional ventilation and perfusion defects are shown on the right side. ERJ-01464-2017_Figure_E1
Figure E2. Box plot of impairment of ventilation. Box plot of fractional impairment of ventilation from healthy controls (HC) and patients with cystic fibrosis (CF). ERJ-01464-2017_Figure_E2
Figure E3. Box plot of impairment of perfusion. Box plot of relative impairment of perfusion from healthy controls (HC) and patients with cystic fibrosis (CF). ERJ-01464-2017_Figure_E3
Figure E4. Correlation between FEV1 and fractional impairment of ventilation. Correlation between FEV1 and fractional impairment of ventilation (RFV) of the lung given in percentage. For illustration scale for FEV1 was reversed. Closed circles denote patients with CF and open circles illustrate healthy aged matched controls. For illustration we reversed the scale of FEV1. ULN is given in dashed lines for both modalities FEV1 (-1.64 z-scores), RFV (24.2%). ERJ-01464-2017_Figure_E4
Figure E5. Correlation between FEV1 and relative impairment of perfusion. Correlation between FEV1 and relative impairment of perfusion (RQ) of the lung given in percentage. For illustration scale for FEV1 was reversed. Closed circles denote patients with CF and open circles illustrate healthy aged matched controls. For illustration we reversed the scale of FEV1. ULN is given in dashed lines for both modalities FEV1 (-1.64 z-scores), RQ (19.3%). ERJ-01464-2017_Figure_E5
Disclosures
Supplementary Material
G. Bauman ERJ-01464-2017_Bauman
O. Bieri ERJ-01464-2017_Bieri
P. Latzin ERJ-01464-2017_Latzin
S. Nyilas ERJ-01464-2017_Nyilas
O. Pusterla ERJ-01464-2017_Pusterla
Acknowledgements
The authors would like to thank all of the children and their families for their participation in the study. The authors would also like to express their thanks especially to Sharon Schmidt, Linn Krueger, Tobias Ehrenreich, Sophie Yammine, Chiara Abbas (Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland); Bernd Jung, Michael Ith (Dept of Interventional and Pediatric Radiology, Inselspital, Bern University Hospital, Bern, Switzerland); Tanja Haas (Dept of Radiology, Division of Radiological Physics, University of Basel Hospital, Basel, Switzerland); and Philippe C. Cattin, Robin Sandkühler (Dept of Biomedical Engineering, University of Basel, Allschwil, Switzerland); and to all the study nurses and medical-technical assistants from the Dept of Radiology (Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland) for their patient care, support in measurements and recruitment of children.
Author contributions: S. Nyilas, G. Bauman, O. Pusterla, U. Frey, C. Casaulta, O. Bieri and P. Latzin designed the study concept. S. Nyilas collected study data. S. Nyilas, G. Bauman, G. Sommer and E. Stranzinger analysed the data. S. Nyilas, G. Bauman, G. Sommer, E. Stranzinger, O. Pusterla, U. Frey, I. Korten, F. Singer, C. Casaulta, O. Bieri and P. Latzin interpreted the data. S. Nyilas and G. Bauman drafted the manuscript. All authors revised the manuscript.
Footnotes
This article has supplementary material available from erj.ersjournals.com
Support statement: The work for this report was funded by the Swiss National Foundation (grant SNF 320030_149576), Swiss Cystic Fibrosis Foundation (CFCH) and Botnar Foundation. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. Funding information for this article has been deposited with the Crossref Funder Registry.
Conflict of interest: Disclosures can be found alongside this article at erj.ersjournals.com
- Received January 13, 2017.
- Accepted August 31, 2017.
- Copyright ©ERS 2017
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