Signal enhancement of hyperpolarized (15)N sites in solution—increase in solid‐state polarization at 3.35 T and prolongation of relaxation in deuterated water mixtures

Hyperpolarized (15)N sites have been found to be promising for generating long‐lived hyperpolarized states in solution, and present a promising approach for utilizing dissolution‐dynamic nuclear polarization (dDNP)‐driven hyperpolarized MRI for imaging in biology and medicine. Specifically, (15)N sites with directly bound protons were shown to be useful when dissolved in D(2)O. The purpose of the current study was to further characterize and increase the visibility of such (15)N sites in solutions that mimic an intravenous injection during the first cardiac pass in terms of their H(2)O:D(2)O composition. The T (1) values of hyperpolarized (15)N in [(15)N(2)]urea and [(15)N]NH(4)Cl demonstrated similar dependences on the H(2)O:D(2)O composition of the solution, with a T (1) of about 140 s in 100% D(2)O, about twofold shortening in 90% and 80% D(2)O, and about threefold shortening in 50% D(2)O. [(13)C]urea was found to be a useful solid‐state (13)C marker for qualitative monitoring of the (15)N polarization process in a commercial pre‐clinical dDNP device. Adding trace amounts of Gd(3+) to the polarization formulation led to higher solid‐state polarization of [(13)C]urea and to higher polarization levels of [(15)N(2)]urea in solution.