Quantifying gravity-dependent and gravity-independent ventilation distribution by hyperpolarized ¹²⁹Xe MRI in...

Hyperpolarized ¹²⁹XeMRI can be used to quantify ventilation heterogeneity. The radio-frequency bias correction method has been shown to preserve the physiological ventilation gradient in healthy individuals. In this study, we tested the hypothesis that idiopathic pulmonary fibrosis (IPF) and obese asthma would have worsened gravity-dependent and gravity-independent ventilation distribution gradients.

We analyzed 3-dimensional HPXeMRI ventilation images from our databank using both the conventional N4ITK and the RF bias correction methods. We divided the lung regions equally into apical/middle/basal compartments (gravity independent) and anterior/middle/posterior compartments (gravity dependent). We averaged the ventilation signals within each compartment and calculated ventilation differences (Δ $\dot{V}$ ) between basal and apical compartments (Δ $\dot{V}$ _B-A) and between posterior and anterior compartments (Δ $\dot{V}$ _P-A).

We included 10 younger (age: 23 ± 3 years), 10 older healthy subjects (age: 62 ± 7 years), 30 patients with idiopathic pulmonary fibrosis (IPF) (age: 74 ± 6 years) and 10 obese asthma patients (age: 40 ± 10 years). Younger and older healthy subjects had greater ventilation in the basal compartment (Δ $\dot{V}$ _B-A=0.20 ± 0.17 and 0.11 ± 0.15 respectively) and the posterior compartment (Δ $\dot{V}$ _P-A=0.08 ± 0.06 and 0.13 ± 0.16 respectively). Twenty-five IPF patients had positive Δ $\dot{V}$ _B-A (0.17 ± 0.08) and Δ $\dot{V}$ _P-A (0.11 ± 0.07). Five patients had negative Δ $\dot{V}$ _P-A (-0.17 ± 0.09, p<0.0001) with three also having negative Δ $\dot{V}$ _B-A (-0.04 ± 0.01, p=0.049). Six obese asthma patients had positive Δ $\dot{V}$ _P-A (0.13 ± 0.08). Four patients had negative Δ $\dot{V}$ _P-A (-0.11 ± 0.15, p=0.017) with two also having negative Δ $\dot{V}$ _B-A (-0.17 and -0.09).

With the RF depolarization bias correction method, we showed physiological ventilation gradients in healthy subjects and detected abnormal (inverse) ventilation gradients in a subgroup of patients with IPF and obese asthma, despite low ventilation defect percentage (VDP), and mild or no spirometry abnormalities. The abnormal ventilation gradients may reflect the heterogeneity of the disease that may be associated with different treatment response and prognosis. These will need to be explored in future studies with a different design including larger populations.