Computational fluid dynamics modeling of Bacillus anthracis spore deposition in rabbit and human respiratory airways.

Three-dimensional computational fluid dynamics and Lagrangian particle deposition models were developed to compare the deposition of aerosolized Bacillus anthracis spores in the respiratory airways of a human with that of the rabbit, a species commonly used in the study of anthrax disease. The respiratory airway geometries for each species were derived respectively from computed tomography (CT) and µCT images. Both models encompassed airways that extended from the external nose to the lung with a total of 272 outlets in the human model and 2878 outlets in the rabbit model. All simulations of spore deposition were conducted under transient, inhalation-exhalation breathing conditions using average species-specific minute volumes. Two different exposure scenarios were modeled in the rabbit based upon experimental inhalation studies. For comparison, human simulations were conducted at the highest exposure concentration used during the rabbit experimental exposures. Results demonstrated that regional spore deposition patterns were sensitive to airway geometry and ventilation profiles. Due to the complex airway geometries in the rabbit nose, higher spore deposition efficiency was predicted in the nasal sinus compared to the human at the same air concentration of anthrax spores. In contrast, higher spore deposition was predicted in the lower conducting airways of the human compared to the rabbit lung due to differences in airway branching pattern. This information can be used to refine published and ongoing biokinetic models of inhalation anthrax spore exposures, which currently estimate deposited spore concentrations based solely upon exposure concentrations and inhaled doses that do not factor in species-specific anatomy and physiology for deposition.

3D 3He 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 microl 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.

Wall relaxation of 3He in spin-exchange cells.

The 3He longitudinal spin-relaxation rate T1-1 is crucial for production of highly polarized 3He by spin-exchange optical pumping. We show that T1-1 is increased by a factor of 2-20 solely by exposure of spin-exchange cells to a few-kG magnetic field. The original T1-1 can be restored by degaussing the cell. The effect is attributed to magnetic surface sites and has been observed in both Pyrex and aluminosilicate-glass cells. Our results both advance the understanding of wall relaxation and demonstrate the use of 3He as an extremely sensitive probe of surface magnetism.