Quantitative analysis of hyperpolarized ¹²⁹ Xe gas transfer MRI

Hyperpolarized ¹²⁹ Xe magnetic resonance imaging (MRI) using Dixon-based decomposition enables single-breath imaging of ¹²⁹ Xe in the airspaces, interstitial barrier tissues, and red blood cells (RBCs). However, methods to quantitatively visualize information from these images of pulmonary gas transfer are lacking. Here, we introduce a novel method to transform these data into quantitative maps of pulmonary ventilation, and ¹²⁹ Xe gas transfer to barrier and RBC compartments.

A total of 13 healthy subjects and 12 idiopathic pulmonary fibrosis (IPF) subjects underwent thoracic ¹ H MRI and hyperpolarized ¹²⁹ Xe MRI with one-point Dixon decomposition to obtain images of ¹²⁹ Xe in airspaces, barrier and red blood cells (RBCs). ¹²⁹ Xe images were processed into quantitative binning maps of all three compartments using thresholds based on the mean and standard deviations of distributions derived from the healthy reference cohort. Binning maps were analyzed to derive quantitative measures of ventilation, barrier uptake, and RBC transfer. This method was also used to illustrate different ventilation and gas transfer patterns in a patient with emphysema and one with pulmonary arterial hypertension (PAH).

In the healthy reference cohort, the mean normalized signals were 0.51 ± 0.19 for ventilation, 4.9 ± 1.5 x 10^-3 for barrier uptake and 2.6 ± 1.0 × 10^-3 for RBC (transfer). In IPF patients, ventilation was similarly homogenous to healthy subjects, although shifted toward slightly lower values (0.43 ± 0.19). However, mean barrier uptake in IPF patients was nearly 2× higher than in healthy subjects, with 47% of voxels classified as high, compared to 3% in healthy controls. Moreover, in IPF, RBC transfer was reduced, mainly in the basal lung with 41% of voxels classified as low. In healthy volunteers, only 15% of RBC transfer was classified as low and these voxels were typically in the anterior, gravitationally nondependent lung.

This study demonstrates a straightforward means to generate semiquantitative binning maps depicting ¹²⁹ Xe ventilation and gas transfer to barrier and RBC compartments. These initial results suggest that the method could be valuable for characterizing both normal physiology and pathophysiology associated with a wide range of pulmonary disorders.