Spin dynamics of [1,2-13C2]pyruvate hyperpolarization by parahydrogen in reversible exchange at micro Tesla fields

J Coupling Constants of <1 Hz Enable 13C Hyperpolarization of Pyruvate via Reversible Exchange of Parahydrogen

Observing pyruvate metabolism in vivo has become a focal point of molecular magnetic resonance imaging. Signal amplification by reversible exchange (SABRE) has recently emerged as a versatile hyperpolarization technique. Tuning of the spin order transfer (SOT) in SABRE is challenging as the small 1H-13C J couplings, in the 13C-pyruvate case, result in SOT being not readily discernible. We demonstrate an experimental method using frequency-selective excitation of parahydrogen-derived polarization SOT sequence (SEPP-SPINEPT); its application led to up to 5700-fold 13C signal gain. In this way, we estimated the lifetime of two Ir-pyruvate SABRE complexes alongside the individual probing of eight small 1H-13C J couplings that connect the hydride protons in these complexes to 1- and 2-13C pyruvate spins, affording values between 0 and 2.69 Hz. Using electronic structure calculations, we define the low-energy structure of the corresponding complexes. Hence, this study demonstrates a novel approach to analyzing the spin topology of short-lived organometallic complexes.

Delivering Robust Proton-Only Sensing of Hyperpolarized [1,2-13C2]-Pyruvate Using Broad-Spectral-Range Nuclear Magnetic Resonance Pulse Sequences

Hyperpolarized [1-13C]pyruvate is the leading hyperpolarized injectable contrast agent and is currently under evaluation in clinical trials for molecular imaging of metabolic diseases, including cardiovascular disease and cancer. One aspect limiting broad scalability of the technique is that hyperpolarized 13C MRI requires specialized 13C hardware and software that are not generally available on clinical MRI scanners, which employ proton-only detection. Here, we present an approach that uses pulse sequences to transfer 13C hyperpolarization to methyl protons for detection of the 13C-13C pyruvate singlet, employing proton-only excitation and detection only. The new pulse sequences are robust to the B1 and B0 magnetic field inhomogeneities. The work focuses on singlet-to-magnetization (S2M) and rotor-synchronized (R) pulses, both relying on trains of hard pulses with broad spectral width coverage designed to effectively transform hyperpolarized 13C2-singlet hyperpolarization to 1H polarization on the CH3 group of [1,2-13C2]pyruvate. This approach may enable a broader adoption of hyperpolarized MRI as a molecular imaging technique.