Direct detection of SABRE-SHEATH hyperpolarization and spin-lattice relaxation of [1-¹³C]pyruvate

Nuclear magnetism is typically investigated by perturbing the spin system with radio frequency pulses, but low polarization and detection using induction coils limit direct access to the longitudinal magnetization. The hyperpolarization technique SABRE-SHEATH requires ultra-low magnetic fields for spin order transfer; consequently, SQUID sensors with a frequency-independent sensitivity are well-suited for unperturbed detection in this regime. We demonstrate direct observation of hyperpolarization build up (T_B) and spin lattice relaxation (T₁) in [1-¹³C]pyruvate, hyperpolarized with SABRE-SHEATH at 150 nT and 500 nT. The values for T_B of 36 s and 26 s and T₁ of 40 s and 43 s, respectively, suggests a shift in dominant polarization transfer efficacy or complexes, highlighting the method's merit in characterizing hyperpolarization pathways. Moreover, as demand for hyperpolarized probes in metabolic imaging continues to grow, the exceptional time resolution makes direct detection a valuable tool for understanding and optimizing polarization dynamics and reactor designs.