Background‐Free Proton NMR Spectroscopy with Radiofrequency Amplification by Stimulated Emission Radiation

Abstract

We report on the utility of Radiofrequency Amplification by Stimulated Emission Radiation (RASER) for background‐free proton detection of hyperpolarized biomolecules. We performed hyperpolarization of ≈0.3 M ethyl acetate via pairwise parahydrogen addition to vinyl acetate. A proton NMR signal with signal‐to‐noise ratio exceeding 100 000 was detected without radio‐frequency excitation at the clinically relevant magnetic field of 1.4 T using a standard (non‐cryogenic) inductive detector with quality factor of Q=68. No proton background signal was observed from protonated solvent (methanol) or other added co‐solvents such as ethanol, water or bovine serum. Moreover, we demonstrate RASER detection without radio‐frequency excitation of a bolus of hyperpolarized contrast agent in biological fluid. Completely background‐free proton detection of hyperpolarized contrast agents in biological media paves the way to new applications in the areas of high‐resolution NMR spectroscopy and in vivo spectroscopy and imaging.

RASER MRI: Magnetic resonance images formed spontaneously exploiting cooperative nonlinear interaction

“…At low magnetic fields (4 and 8 mT are demonstrated here), the RASER signal is many orders of magnitude larger compared to the signal of the more abundant background protons with low Boltzmann polarization. At higher magnetic fields (1.4 T) in RASER NMR spectroscopy, no proton background signals of water were observed because the RF from the RASER active protons does not excite the chemically shifted water protons ( 46 ). Further potential advantages are the absence of external RF excitation ( 27 ) (e.g., caused by the transmission coil), which imply minimal specific absorption rate, avoiding unintended heat deposition.…”