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Abstract

Ultralow-field (ULF) nuclear magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) are promising methods allowing for, e.g., the simultaneous detection of multiple nuclei or imaging in the vicinity of metals. To overcome the inherently low signal-to-noise ratio that usually hampers a wider application, we present an alternative approach to prepolarized ULF MRS, employing hyperpolarization techniques like signal amplification by reversible exchange (SABRE) or Overhauser dynamic nuclear polarization (ODNP). Both techniques allow continuous hyperpolarization of 1H and other MR-active nuclei. For implementation, a superconducting quantum interference device (SQUID)-based ULF MRS/MRI detection scheme was constructed. Due to their very low intrinsic noise level, SQUIDs are superior to conventional Faraday detection coils at ULFs. The noise level of the here presented system is ~ 1 fT/Hz-1/2. Additionally, the broadband characteristics of SQUIDs enable them to simultaneously detect the MR signal of different nuclei such as 13C, 19F, or 1H. The MR signal can be measured in absolute units, which allows quantitative investigations of the hyperpolarization techniques without a reference. The detection field can be changed without the need of tuning or matching the SQUID-based detection coil. The whole system was constructed with the goal of providing an easily accessible sample volume. Therefore, the setup sits inside a three-layer shielding chamber consisting of two layers of mu metal for shielding DC and low-frequency magnetic field noise and one layer of aluminum for shielding high-frequency noise.
As a demonstration of the performance of the system, first quantitative two-dimensional NMR results from SABRE-enhanced samples are presented. These measurements enabled us to understand the underlying physical mechanisms of SABRE in so far unprecedented detail (effects of solvent and heteronuclei). In addition ODNP-enhanced MRI was performed on a dead rat. 3D images with an isotropic resolution in the mm range could be acquired with this method.