To determine whether alveolar collapse, defined as an abnormal increase in CT attenuation during expiration, can be detected using paired attenuation histograms in inspiration and expiration and used as a potential predictive marker in IPF patients.
 
Methods
Sixty-six individuals with IPF obtained CT scans during inspiration and expiration. Density histograms were created and analyzed. After each respective 3-year observation period, the patient population was split into two subgroups according to their status (endpoints: death/transplantation vs still under clinical surveillance). An independent t-test was used to compare the CT-derived histogram parameters of attenuation between the two subgroups (ratio of mean attenuation in expiration/inspiration (meanHUratio) and SD, skewness and kurtosis in expiration).
 
Results
After the individual observation period of 3 years, 37 patients were still under clinical surveillance while 29 had died or received a transplantation. All baseline characteristics (meanHUratio and SD, skewness and kurtosis of expiratory attenuation histograms) were significantly different between the subgroups (p = 0.004, p = 0.009, p < 0.001 and p < 0.001, respectively).

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Conclusion
Expiratory attenuation histogram analysis can be used to demonstrate the concept of alveolar collapse as a potential prognostic marker in IPF patients.