Air trapping on HRCT assessed by quantitative image analysis as an early predictor of bronchiolitis obliterans syndrome in lung transplant recipients

RATIONALE: Current diagnosis of bronchiolitis obliterans syndrome (BOS) in lung transplant recipients by spirometry identifies the disease process after the point at which treatment appears to be effective. Earlier identification of BOS may allow improved outcomes. We aim to identify early predictors of BOS by quantitatively assessing air trapping in computed tomography scans (HRCT) of transplanted lungs. Previous studies of air trapping by visual analysis of HRCT were poor at early identification of BOS. METHODS: The clinical HRCT images of eight lung transplant recipients who met spirometric criteria for BOS by FEV1, and who had the absence of infection and rejection confirmed by bronchoalveolar lavage and transbronchial biopsy within one month of spirometry, were reviewed. For each patient, the HRCT within one month of the spirometric diagnosis of BOS (visit 2) was evaluated. A previous HRCT (visit 1) at least two months prior to visit 2 and six months after transplantation was compared. Semi-automatic lung and lobar segmentation followed by quantitative image analysis were performed on the images of the transplanted lung at residual volume (RV) and total lung capacity (TLC). For each scan, previously published quantitative measures of air trapping were assessed: lobar volume at RV (RVCT), whole lung RV/TLC ratio (RV/TLCCT), density masks between -950 and -860 HU at RV (DM-950-860), median HU at RV, and 10th percentile HU at RV were computed. The changes in these measurements between visits 1 and 2 were compared using a t-test. RESULTS: Visit 1 scans were an average of 144 (range 42-243) days prior to visit 2 scans. The mean RVCT increased 52.53 cc (p=.48) and the mean change in RV/TLCCT increased 3.35% (p=0.47) at visit 2 when compared to visit 1. The DM-950-860 of the expiratory images increased by 0.48 % (p=0.78) from visit 1 to visit 2. Between the two visits, the average median HU at RV and 10th percentile of HU at RV decreased by 0.24 % (p=0.95) and 0.32 % (p=0.85), respectively. CONCLUSION: In our small sample of lung transplant patients, we found that quantitative measures of air trapping seen at the time of spirometric BOS diagnosis were also seen approximately five months earlier. These measures may serve as an earlier marker of chronic lung transplant rejection; however, additional analysis of CT images over an extended period of time is necessary.