The residual ruthenium catalyst concentration in a 1 mL purified fumarate solution was 16 µM. For comparison, the phase extraction used for preclinical experiments with PHIP-polarized pyruvate produces solutions contaminated with ~30 μM residual rhodium catalyst . The toxicity of other ruthenium(II) complexes has been investigated in mice and the LD50 was found to be >2000 mg kg -1 [ 28 ], and the IC50 in human nontumour breast cells was found to be 1-20 μM . These results indicate that the purity of our solutions from the catalyst should allow for in vitro and preclinical studies since the bolus of hyperpolarized solution will be further diluted in the blood. It will be necessary to investigate the toxicity of the catalyst, precursor, and reaction side product molecules, which are currently unknown.