Characterization of Nuclear Magnetism at Ultralow and Zero Field using SQUIDs

TL;DR

This paper demonstrates the use of SQUIDs to detect nuclear magnetism at ultralow fields, achieving high hyperpolarization of [1-13C]pyruvate for enhanced MRI diagnostics, with potential for direct observation of hyperpolarized signals.

Contribution

It introduces a novel hyperpolarization setup combined with SQUID detection at ultralow magnetic fields, enabling unprecedented sensitivity in nuclear magnetic measurements.

Findings

Achieved 0.4% 13C polarization with hyperpolarization technique.

Realized a signal enhancement of approximately 100 million times over thermal equilibrium.

Detected a 13C signal of 6.20±0.34 pT in ultralow noise conditions.

Abstract

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.