Hyperpolarized ³He and ¹²⁹Xe MRI: Differences in asthma before bronchodilation

Purpose: To compare hyperpolarized helium-3 (³He) and xenon-129 (¹²⁹Xe) MRI in asthmatics before and after salbutamol inhalation. Materials and Methods: Seven asthmatics provided written informed consent and underwent spirometry, plethysmography, and MRI before and after salbutamol inhalation. ³He and ¹²⁹Xe ventilation defect percent (VDP) and ventilation coefficient of variation (COV) were measured. To characterize the airways spatially related to ventilation defects, wall area percent (WA%) and lumen area (LA) were evaluated for two subjects who had thoracic x-ray computed tomography (CT) acquired 1 year before MRI. Results: Before salbutamol inhalation, ¹²⁹Xe VDP (8 ± 5%) was significantly greater than ³He VDP (6 ± 5%, P = 0.003). Post-salbutamol, there was a significant improvement in both ¹²⁹Xe (5 ± 4%, P < 0.0001) and ³He (4 ± 3%, P = 0.001) VDP, and the improvement in ¹²⁹Xe VDP was significantly greater (P = 0.008). ¹²⁹Xe MRI COV (Pre: 0.309 ± 0.028, Post: 0.296 ± 0.036) was significantly greater than ³He MRI COV (Pre: 0.282 ± 0.018, Post: 0.269 ± 0.024), pre- (P < 0.0001) and post-salbutamol (P < 0.0001) and the decrease in COV post-salbutamol was significant (¹²⁹Xe, P = 0.002; ³He, P < 0.0001). For a single asthmatic, a sub-segmental ¹²⁹Xe MRI ventilation defect that was visible only before salbutamol inhalation but not visible using ³He MRI was spatially related to a remodeled fourth generation sub-segmental airway (WA% = 78%, LA = 2.9 mm ²). Conclusion: In asthma, hyperpolarized ¹²⁹Xe MRI may help reveal ventilation abnormalities before bronchodilation that are not observed using hyperpolarized ³He MRI.