Chapter 23: Beyond Spin I=1/2; Hyperpolarized ¹³¹Xe and ⁸³Kr Magnetic Resonance

The large chemical shift range of ¹²⁹Xe is the basis of many of the captivating studies described in the various chapters of this book. The only other stable noble gas isotope with nuclear spin I=1/2, ³He, lacks any appreciable chemical shift range. Beyond spin I=1/2, there are two stable noble gas isotopes, ⁸³Kr (I=9/2) and ¹³¹Xe (I=3/2), that occur at sufficiently high natural abundance for practical applications. Unfortunately the usable chemical shift ranges associated with these isotopes are inferior to that of ¹²⁹Xe. However, spin I>1/2 isotopes have a nuclear electric quadrupole moment that enables a host of supplementary magnetic resonance studies. The spin I>1/2 noble gas isotopes can all be hyperpolarized but their ‘quadrupolar’ relaxation times are fairly short causing rapid depolarization compared to hp ¹²⁹Xe. This chapter reviews how this apparent disadvantage can be turned into a useful probe for surfaces in material sciences and biomedical research. This unique probe can ‘sense’ surfaces at far lower surface to volume ratios (S/V) than the ¹²⁹Xe chemical shift. The underlying physical concept will be illustrated in the first part of the chapter using the ‘simpler’ I=3/2 isotope ¹³¹Xe that has 4 Zeeman energy levels rather than the 10 levels of ⁸³Kr. The second part will focus on ⁸³Kr, which strikes the best balance between achievable signal intensity and utility for novel, surface indicative MRI contrast.