Introduction
Lung volume subdivisions can provide important prognostic and diagnostic information about lung health in people with cystic fibrosis (pwCF). Inspiratory capacity (IC) is a subtle marker of lung function decline1 and exercise ability,2 total lung capacity (TLC) an important factor in donor/recipient matching and in understanding breathing patterns associated with transplantation,3 and functional residual capacity (FRC) may be a sensitive marker of early lung disease.4
However, the traditional method of lung volume measurement involves the assessment of flow over time, using forced manoeuvres which are not representative of real-world breathing, and involves the use of mouthpieces, face masks or enclosed chambers that alter normal ventilation.5–8 Whole-body plethysmography (WBP) can be problematic in claustrophobic individuals, spirometry requires complex repeated forced manoeuvres and both may be hazardous in those with transmissible respiratory infections.9 This has led to the exploration of other methods for measuring lung volumes, including plain chest radiographs,10–12 CT,13–15 MRI14 and chest wall kinematics.16 However, plain chest radiographs cannot measure lung volume subdivisions; although thoracic CT is capable of calculating accurate lung volumes,17 it confers an increased radiation dose18 19 and requires non-physiological supine positioning; MRI is time-consuming and difficult to acquire in claustrophobic or dyspnoeic individuals. Chest wall kinematics can reliably estimate lung volumes3 but is impractical for routine use.
Dynamic chest radiography (DCR) is a low-dose, real-time digital imaging system that visualises the thorax over 10–20 s throughout the breathing cycle. A high-resolution flat panel detector (FPD) maximises image quality and field of view through digital reconstruction with minimal ionising radiation. DCR produces a ‘moving chest radiograph’, where thoracic structures can be observed throughout the breathing cycle. Using automatic detection of visible lung borders in the posteroanterior (PA) projection,20 the projected lung area (PLA) can be traced during different breathing phases. This technology has already been applied to diaphragm motion analysis in healthy volunteers,21 suspected diaphragm palsy,22 chronic obstructive pulmonary disease23 24 and individuals taking CF transmembrane conductance regulator (CFTR)-modifying drugs.25 PLA has been shown to correlate well with vital capacity in healthy volunteers26 and those with interstitial lung disease,27 but to our knowledge, this relationship has not been explored in pwCF.
DCR offers several advantages: first, unlike spirometry or plethysmography, it mirrors normal breathing, and since no mouthpiece or nose clip is required,20 it can be used in those with altered airway anatomy such as the presence of a tracheostomy. Second, image acquisition is rapid. Finally, DCR images are acquired at a distance from the performing radiographer, making it an appealing option in individuals with transmissible respiratory infections. The technology is Conformité Européenne marked for cineradiographic imaging in the UK and European Union and licensed for use in the USA.
The ability to combine chest imaging with rapid, non-aerosol-generating and physiological pulmonary function testing (PFT) may make DCR a useful adjunct to traditional measures of lung health in pwCF, a condition in which subtle markers of disease progression and treatment response are needed,28 especially in the era of CFTR modulator therapies.29 30 The aims of this study were to assess the feasibility of using PA and lateral DCR to calculate lung volume subdivisions in pwCF, explore the correlation between DCR lung areas and PFTs performed by WBP, and develop models to calculate lung volume subdivisions from DCR lung areas.