pH Dependence of T1 for 13C‐Labelled Small Molecules Commonly Used for Hyperpolarized Magnetic Resonance Imaging

Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin‐lattice relaxation time (T1), which is on the order of a few tens of seconds. Here, we analyzed the pH‐dependence of T1 for commonly used HP 13C‐labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T1 of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B0=1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B0=1 T). Our data shows that increasing hydronium ion concentrations shorten the T1 of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T1 shortening. In addition, enhanced proton exchange and chemical reactions at the pKa appear to be detrimental for the HP‐state. Hyperpolarization enhances the nuclear magnetic resonance signal artificially by up to five orders of magnitude. However, the hyperpolarized state decays quickly with the longitudinal relaxation time T1. The authors analyze the pH dependence of T1 for several 13C‐labelled small molecules commonly used for hyperpolarized magnetic resonance imaging.