Neutrons, nephrology and nuclear tracers: magnetic resonance with millions fewer nuclei

Abstract:  The technique of spin-exchange optical pumping has found extensive use in both fundamental research and a variety of applications. For example, high-density polarized He-3 targets have played an important role in elucidating the spin structure of the nucleon, and more recently, enabling form-factor measurements at very high momentum transfer. Phenomena such as the role of quark orbital angular momentum, and the importance of diquark-like structures, are among the physics that has resulted from such work. MRI using polarized He-3 and Xe-129 have provided the highest resolution images of the gas space of lungs ever produced. And very recently, polarized Xe-129, dissolved into the blood following inhalation, has been used to try to visualize, in real time, the function of the kidney. However, the strategy of somehow combining magnetic resonance with the use of tiny quantities of a tracer is limited by the poor signal-to-noise that results when very few nuclei are involved. I will describe a new technique we call Polarized Nuclear Imaging (PNI) in which magnetic resonance techniques play a key role, but imaging data are acquired not through detecting weak electromagnetic signals, but through the detection of gamma rays. The net effect is to increase the sensitivity of magnetic resonance by factors of between a million and a billion. The techniques described may also be useful in the study of exotic nuclei.