The magnetization in hyperpolarized gas (HP) MRI is generated by laser polarization that is independent of the magnet and imaging process. As a consequence, there is no equilibrium magnetization during the image acquisition, The competing processes of gas inflow and depolarization of the spins lead to large changes in signal as one samples k-space, A model is developed of dynamic changes in polarization of hyperpolarized He-3 during infusion and in vivo imaging of the lung and verified experimentally in a live guinea pig, Projection encoding is used to measure the view-to-view variation with temporal resolution <4 ms, Large excitation angles effectively sample the magnetization in the early stages of inflow, highlighting larger airways, while smaller excitation angles produce images of the more distal spaces, The work provides a basis for pulse sequences designed to effectively exploit HP MRI in the lung.
