MRI of [2‐ 13 C]Lactate without J‐coupling artifacts

Purpose Imaging of [2‐ 13 C]lactate, a metabolic product of [2‐ 13 C]pyruvate, is over considerable interest in hyperpolarized 13 C studies. However, artifact‐free imaging of a J‐coupled nuclear spin species can be challenging due to the peak‐splitting induced by the spin‐spin interactions. In this work, two new techniques resolving these J‐modulated artifacts are presented. Theory and Methods The Product Operator Formalism (POF) of density matrix theory is used to both numerically and analytically derive the coherences arising during radiofrequency excitation and readout of a J‐coupled spin system. A combination of computer simulations and experiments with [2‐ 13 C]lactate and 13 C‐formate phantoms are then used to verify the performance of two imaging methods. In the first approach, a quadrature imaging technique is used to eliminate scalar coupling artifacts via the combination of in‐phase and quadrature images acquired at echo times differing by 1/2J with an echoplanar readout. The second approach employs a highly narrowband RF excitation pulse to image a single peak from the J‐coupled doublet. Results Simulations using a numerical Shepp‐Logan phantom, in vitro experiments using thermally polarized [2‐ 13 C]lactate, thermally and hyperpolarized 13 C‐formate phantoms, and in vivo imaging of [2‐ 13 C]lactate produced in rat brain following injection of hyperpolarized [2‐ 13 C]pyruvate show artifact‐free images and demonstrate potential utility of these methods. Conclusion The quadrature imaging and the narrowband excitation techniques resolve the J‐coupling induced ghosting and blurring artifacts present with conventional MRI of J‐coupled signals such as [2‐ 13 C]lactate.