3D 3 He diffusion MRI as a local in vivo morphometric tool to evaluate emphysematous rat lungs

In this work, we investigate 3 He magnetic resonance imaging as a noninvasive morphometric tool to assess emphysematous disease state on a local level. Emphysema was induced intratracheally in rats with 25 U/100 g body wt of porcine pancreatic elastase dissolved in 200 μl saline. Rats were then paired with saline-dosed controls. Nine three-dimensional (3D) 3 He diffusion-weighted images were acquired at 1, 2, or 3 wk postdose, after which the lungs were harvested and prepared for histological analysis. Recently introduced indexes sensitive to the heterogeneity of the air space size distribution were calculated. These indexes, D 1 and D 2 , were derived from the moments of the mean equivalent airway diameters. Averaged over the entire lung, it is shown that the average 3 He diffusivity ( D ave ) correlates well with histology ( R = 0.85, P < 0.0001). By matching small (0.046 cm 2 ) regions in 3 He images with corresponding regions in histological slices, D ave correlates significantly with both D 1 and D 2 ( R = 0.88 and R = 0.90, respectively, with P < 0.0001). It is concluded that 3 He MRI is a viable noninvasive morphometric tool for localized in vivo emphysema assessment.