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Mark Conradi

Washington University in St. Louis
Laser-Polarized Nuclear Spins for Imaging Human Lungs
Optical pumping techniques developed over the last 30 years, together with high power diode lasers, now allow liter quantities of He-3 gas to achieve nuclear spin polarizations of 40-50%. These polarizations and the resulting magnetic resonance sensitivities are about 100,000 times larger than typical thermal equilibrium values, allowing for the first time MR images to be made of the air spaces of the lung. The optical pumping physics will be discussed in detail. Our research has focused on the diffusion of He-3 in the lung, which is restricted by collisions with the airway and alveolar walls. In emphysema, airway expansion and tissue destruction lessen this restriction, resulting in an increase in apparent diffusivity by about a factor of 3 compared to healthy lungs. Measurements of diffusivity are being used to characterize the extent and location of disease in patients scheduled for lung volume reduction surgery. The fundamentals of restricted diffusion will be presented. The diffusion of He-3 gas in the lung is anisotropic, with greater diffusion along the airway axis than across it. Despite the airway axes in any volume element being distributed over all directions, the separate longitudinal and transverse diffusivities can be determined. In particular, the transverse diffusion is a direct measure of the mean radius of the airways; in healthy humans good agreement is found between the He-3 MR-measured radius and the radius measured previously by microscopic examination of excised lungs. In emphysematous lungs, the radius is substantially increased.
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