Characterization of Nuclear Magnetism at Ultralow and Zero Field using SQUIDs

Nuclear magnetism underpins areas such as medicine in magnetic resonance imaging (MRI). Hyperpolarization of nuclei enhances the quantity and quality of information that can be determined from these techniques by increasing their signal to noise ratios by orders of magnitude. However, some of these hyperpolarization techniques rely on the use of low to ultralow magnetic fields (ULF) (nTs-mTs). The broadband character and ultrasensitive field sensitivity of superconducting quantum interference devices (SQUID) allow for probing nuclear magnetism at these fields, where other magnetometers, such as Faraday coils and flux gates do not. To this end, we designed a reactor to hyperpolarize [1-$^{13}$C]pyruvate with the technique, signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). Hyperpolarized pyruvate has been shown to be very powerful for the diagnosis of tumours with MRI as its metabolism is associated with various pathologies. We were able to characterize the field sensitivity of our setup by simulating the filled reactor in relation to its placement in our ultralow noise, ULF MRI setup. Using the simulations, we determined that our hyperpolarization setup results in a $^{13}$C polarization of 0.4 %, a signal enhancement of $\sim$100~000~000 versus the predicted thermal equilibrium signal at earth field ($\sim$50 $\mu$T). This results in a $^{13}$C signal of 6.20$\pm$0.34~pT, which with our ultralow noise setup, opens the possibility for direct observation of the hyperpolarization and the subsequent spin-lattice relaxation without system perturbation.