  State-of-the-art magnetic resonance imaging uses hyperpolarized molecules to
track metabolism in vivo, but large superconducting magnets are required, and
the strong magnetic fields largely preclude measurement in the presence of
conductive materials and magnify problems of magnetic susceptibility
inhomogeneity. Operating at zero and low field circumvents these limitations,
but until now has not been possible due to limited sensitivity. We show that
zero- and low-field nuclear magnetic resonance can be used for probing two
important metabolic reactions: the conversion of hyperpolarized fumarate to
malate and pyruvate to lactate. This work paves the way to a heretofore
unexplored class of biomedical imaging applications.
