Same-session Repeatability of 129Xe MRI/MRS Measures of Gas Exchange in Idiopathic Pulmonary Fibrosis

Rationale. Patients with idiopathy pulmonary fibrosis (IPF) are known to have altered patterns of 129Xe gas exchange MRI and spectroscopy, including a reduced red blood cell to membrane ratio (RBC:M), reduced RBC chemical shift, and increased membrane high percent (MHP). On spectroscopy, the RBC:M is believed to reflect the efficiency of gas exchange between the interstitial membrane and the RBCs, while RBC chemical shift reflects pulmonary capillary oxygenation. On imaging, the MHP is believed to reflect interstitial membrane and thickening and has been associated with therapy response and poor outcomes in IPF. Previous work by Hahn et al. has characterized the repeatability of regional 129Xe gas exchange metrics in healthy subjects. Here we evaluate the repeatability of these measurements in IPF to differentiate normal measurement variation from meaningful changes reflecting disease progression or therapy response. Methods. Participants with diagnosed IPF (N=11, 1F/10M, age 70.55 ± 6.31yrs) underwent hyperpolarized 129Xe MRI. All underwent scans at study baseline using a 1L dose bag of xenon mixture, with a subset also undergoing scans at follow-up visits at 6 months (N=8) and one year (N=5) after baseline, for a total of 24 visits in all. At each visit, two sets of 129Xe MRI/MRS dynamic spectroscopy were performed with a brief break between each pair of scans. Repeatability was assessed using the Bland-Altman analysis and inter-class correlation coefficient (ICC). Results. RBC:M was most repeatable, with Bland-Altman limits of agreement (LOA) of [-0.062 – 0.057] and an ICC of 0.89. RBC chemical shift was also repeatable, with an LOA of [-0.500 – 0.435 ppm] and an ICC of 0.83. The MHP measurement had an LOA of [-23.850% – 23.250%] and an ICC of 0.7. There was negligible bias between the first and second scan across all three measurements. Conclusion. These results suggest that the RBC:M measurement is particularly repeatable in IPF, as well as the RBC chemical shift. The repeatability of the membrane high percent (MHP) was more modest, perhaps due in part to sensitivity to lung inflation volume. However, this marker exhibits large changes in interstitial lung diseases, is sensitive to both disease progression and therapy response, and has potential as a treatable trait in IPF. Studies using tailored dose bag volumes and consistent patient coaching are ongoing and expected to improve repeatability further. Overall, these findings provide a foundational assessment of repeatability of 129Xe MRI and spectroscopy in IPF.

Duke Scholars

Author Bastiaan Driehuys Radiology

Author Robert Matthew Tighe Medicine, Pulmonary, Allergy, and Critical Care Medicine