WS07.03Expiratory lung proton magnetic resonance imaging is a simple sensitive method to detect gas trapping in cystic fibrosis?

Abstract

Objectives

End-expiratory narrowing and obstruction of smaller airways in the lungs of people with Cystic Fibrosis (pwCF) causes “gas trapping” distal to airway closure. Early detection of CF lung disease is crucial for long-term lung health. Lung proton Magnetic Resonance Imaging (¹H-MRI) with a standard spoiled gradient echo recalled (SPGR) sequence offers a non-ionising alternative to Computed Tomography (CT) for visually assessing gas trapping. We aimed to develop a method to quantify gas trapping and compare it to other clinical metrics of lung function, including hyperpolarised xenon (¹²⁹Xe) ventilation MRI, body plethysmography, and spirometry.

Methods

Fifteen healthy children and 27 pwCF underwent 3D SPGR ¹H-MRI at approximately residual volume (RV) and total lung capacity (TLC) during a short breath hold. The lung cavity was segmented in all images using a semi-automated approach. A threshold was derived from the TLC images, based on the assumption that gas trapping regions in the RV images should have similar MR signal intensities (SIs) to the lungs at TLC, as both are filled with air. This threshold was applied to the RV images to determine the proportion of lung below it, enabling quantification of gas-trapping volume (GTV%) as a percentage of total lung volume. The upper limit of normal (ULN) was defined at mean +1.64SD of healthy cohort GTV%.

Results

Healthy participants (61.5% M) had a median (range) age of 10.64 (8.71-11.92) years and GTV% 1.52% (0.21-2.42%). PwCF (40.7% M) had a median (range) age of 16.90 (6.40-47.50) years, GTV% 20.45% (4.22-83.09%) and FEV₁ z-score -1.14 (-5.43, 2.17), with 14 having normal FEV₁ z-scores. Healthy participants had homogeneous SIs throughout the lungs, while pwCF had heterogeneous areas of increased and decreased SIs, with darker regions corresponding to gas trapping. In pwCF, GTV% strongly correlated with FEV₁ z-scores (p<0.001, ρ=-0.7851) and ventilation defect percentage (VDP) (p<0.001, ρ=0.9223), derived from ¹²⁹Xe-MRI. GTV% correlated with gas trapping measured during body plethysmography (RV/TLC%) (p<0.001, ρ=0.7594). The ULN of GTV% was 2.32%. All pwCF exceeded this, including those with normal FEV₁, highlighting that GTV% is a sensitive metric of early lung function impairment.

Conclusions

Using a standard, easily implementable 3D SPGR ¹H-MRI breath-hold sequence, gas trapping in CF is visually apparent and can be quantified to detect lung function impairment with high sensitivity, even when FEV1 is normal. These images can be obtained in children aged >5 years and have the potential to be routinely utilised to assess for early lung disease.