Pulmonary ventilation visualized using hyperpolarized helium-3 and xenon-129 magnetic resonance imaging: Differences in COPD and relationship to emphysema

In subjects with chronic obstructive pulmonary disease (COPD), hyperpolarized xenon-129 (¹²⁹Xe) magnetic resonance imaging (MRI) reveals significantly greater ventilation defects than hyperpolarized helium-3 (³He) MRI. The physiological and/or morphological determinants of ventilation defects and the differences observed between hyperpolarized ³He and ¹²⁹Xe MRI are not yet understood. Here we aimed to determine the structural basis for the differences in ventilation observed between ³He and ¹²⁹Xe MRI in subjects with COPD using apparent diffusion coefficients (ADC) and computed tomography (CT). Ten COPD ex-smokers provided written, informed consent and underwent MRI, CT, spirometry, and plethysmography. ³He and ¹²⁹Xe MRI ventilation volume was generated using semiautomated segmentation, and ADC maps were registered to generate ADC values for lung regions of interest ventilated by both gases (ADC_HX) and by ³He gas only (ADC_HO). CT wall area percentage and the lowest 15th percentile point of the CT lung density histogram (HU_15%) were also evaluated. For lung regions accessed by ³He gas only, mean ³He ADC_HO was significantly greater than for regions accessed by both gases (ADC_HO = 0.503 ± 0.119 cm²/s, ADC_HX = 0.470 ± 0.125 cm ²/s, P< 0.0001). The difference between ³He and ¹²⁹Xe ventilation volume was significantly correlated with CT HU _15% (r = -65, P = 0.04) and ³He ADC_HO (r = 0.70, P = 0.02), but not CT wall area percentage (r= =0.34, P = 0.33). In conclusion, in this small study in COPD subjects, we observed significantly decreased ¹²⁹Xe MRI ventilation compared with ³He MRI, and these regions of decreased ¹²⁹Xe ventilation were spatially and significantly correlated with regions of increased pulmonary emphysema, but not airway wall thickness.