Evaluation of emphysema severity and progression in a rabbit model: Comparison of hyperpolarized ³He and ¹²⁹Xe diffusion MRI with lung morphometry

The apparent diffusion coefficients (ADCs) of hyperpolarized ³He and ¹²⁹Xe gases were measured in the lungs of rabbits with elastase-induced emphysema and correlated against the mean chord length from lung histology. In vivo measurements were performed at baseline and 2, 4, 6, and 8 wk after instillation of elastase (mild and moderate emphysema groups) or saline (control group). ADCs were determined from acquisitions that used two b values. To investigate the effect of b value on the results, b-value pairs of 0 and 1.6 s/cm² and 0 and 4.0 s/cm² were used for ³He, and b-value pairs of 0 and 5.0 s/cm² and 0 and 10.0 s/cm² were used for ¹²⁹Xe. At 8 wk after instillation, the rabbits were euthanized, and the lungs were analyzed histologically and morphometrically. ADCs for the rabbits in the control group did not change significantly from baseline to week 8, whereas ADCs for the rabbits in the emphysema groups increased significantly (P < 0.05) for all gas and b-value combinations except ¹²⁹Xe with the b-value pair of 0 and 5.0 s/cm². The largest percent change in mean ADC from baseline to week 8 (15.3%) occurred with ³He and the b-value pair of 0 and 1.6 s/cm² for rabbits in the moderate emphysema group. ADCs (all b values) were strongly correlated (r = 0.62-0.80, P < 0.001) with mean chord lengths from histology. These results further support the ability of diffusion-weighted MRI with hyperpolarized gases to detect regional and global structural changes of emphysema within the lung.