The Spin-Lattice Relaxation of Hyperpolarized $^{89}$Y Complexes

The low sensitivity of NMR can be overcome by dynamic nuclear polarization (DNP). However, a limitation to the use of hyperpolarized materials is the signal decay due to $T_{1}$ relaxation. Among NMR-active nuclei, $^{89}$Y is potentially valuable in medical imaging because in chelated form, pH-sensitive agents can be developed. $^{89}$Y also offers many attractive features -- 100 \% abundance, a 1/2 spin, and a long $T_{1}$, up to 10 min. Yet, developing new $^{89}$Y complexes with even longer $T_{1}$ values is desirable. Designing such complexes relies upon understanding the mechanism(s) responsible for $T_{1}$ relaxation. We report an approach to hyperpolarized $T_{1}$ measurements that enabled an analysis of relaxation mechanisms by selective deuteration of the ligand backbone, the solvent or both. Hyperpolarized $^{89}$Y -- DTPA, DOTA, EDTA, and deuterated EDTA complexes were studied. Results suggest that substitution of low-gamma nuclei on the ligand backbone as opposed to that of the solvent most effectively increase the $^{89}$Y $T_{1}$. These results are encouraging for in vivo applications as the presence of bound water may not dramatically affect the $T_{1}$.