Pulmonary ventilation visualized using hyperpolarized helium-3 and xenon-129 magnetic resonance imaging: differences in COPD and relationship to emphysema
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abstract
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 (ADCHX) and by 3He gas only (ADCHO). CT wall area percentage and the lowest 15th percentile point of the CT lung density histogram (HU15%) were also evaluated. For lung regions accessed by 3He gas only, mean 3He ADCHO was significantly greater than for regions accessed by both gases (ADCHO = 0.503 ± 0.119 cm2/s, ADCHX = 0.470 ± 0.125 cm2/s, P < 0.0001). The difference between 3He and 129Xe ventilation volume was significantly correlated with CT HU15% ( r = −65, P = 0.04) and 3He ADCHO ( 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.
