Magnetization and diffusion effects in NMR imaging of hyperpolarized substances

The special magnetization characteristics of hyperpolarized noble gases have led to an interest in using these agents for new MRI applications. In this note, the magnetization effects and NMR signal dependence of two noble gases, ³He and ¹²⁹Xe, are modeled across a range of gradient-echo imaging parameters. Pulse-sequence analysis shows a wide variation in optimum flip angles between imaging of gas (e.g., ³He or ¹²⁹Xe) in air spaces (e.g., trachea and lung) and in blood vessels. To optimize imaging of the air spaces, it is also necessary to reduce the otherwise substantial signal losses from diffusion effects by increasing voxel size. The possibility of using hyperpolarized ¹²⁹Xe for functional MRI (fMRI) is discussed in view of the results from the blood flow analysis. The short-lived nature of the hyperpolarization opens up new possibilities, as well as new technical challenges, in its potential application as a blood-flow tracer.