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Dynamic nuclear polarization (DNP) allows measuring enhanced nuclear magnetic resonance (NMR) signals. Though the efficiency of DNP has been known to increase at low fields, the usefulness of DNP has not been throughly investigated yet. Here, using a superconducting quantum interference device-based NMR system, we performed a series of DNP experiments with a nitroxide radical and measured DNP spectra at several magnetic fields down to sub-microtesla. In the DNP spectra, the large overlap of two peaks having opposite signs results in net enhancement factors, which are significantly lower than theoretical expectations [30] and nearly invariant with respect to magnetic fields below the Earth’s field. The numerical analysis based on the radical’s Hamiltonian provides qualitative explanations of such features. The net enhancement factor reached 325 at maximum experimentally, but our analysis reveals that the local enhancement factor at the center of the rf coil is 575, which is unaffected by detection schemes. We conclude that DNP in the hyperfine-field-dominant region yields sufficiently enhanced NMR signals at magnetic fields above 1 μ T. [ABSTRACT FROM AUTHOR]

• Circularly polarised RF selectively induces either positive or negative enhancements. • O-DNP with circularly polarised RF renders an enhancement factor of around 150,000 at 1.2 μT. • Images with resolution of close to 1 mm were obtained using O-DNP in microtesla fields. Magnetic resonance imaging in ultra-low fields is often limited by mediocre signal-to-noise ratio hindering a higher resolution. Overhauser dynamic nuclear polarisation (O-DNP) using nitroxide radicals has been an efficient solution for enhancing the thermal nuclear polarisation. However, the concurrence of positive and negative polarisation enhancements arises in ultra-low fields resulting in a significantly reduced net enhancement, making O-DNP far less attractive. Here, we address this issue by applying circularly polarised RF. O-DNP with circularly polarised RF renders a considerably improved enhancement factor of around 150,000 at 1.2 μ T. A birdcage coil was adopted into an ultra-low field MRI system to generate the circularly polarised RF field homogeneously over a large volume. We acquired an MR image of a nitroxide radical solution with an average in-plane resolution of 1 mm. De-noising through compressive sensing further improved the image quality. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The first in-situ Overhauser-enhanced nuclear magnetic resonance experiment using an atomic magnetometer at ultra-low field. • Signal-to-noise ratio was 32 after sixteen averages. • On the Larmor precession of 1 H spins, a decaying oscillation was superimposed. • We attribute it to a transient 87 Rb spin precession in response to a non-adiabatic field variation. • Atomic magnetometry technique shows a new capability of zero and ultralow-field NMR. Abstract The development of atomic magnetometers has led to nuclear magnetic resonance (NMR) in zero and ultralow magnetic fields without using cryogenic sensors. However, in-situ detection, meaning that a sample locates in the detection space beside a vapor cell, has been conducted only with parahydrogen-induced polarization. Other hyperpolarization techniques remain unexplored yet. In this work, we demonstrate that Overhauser dynamic nuclear polarization allows in-situ NMR detection with an atomic magnetometer at less than 1 μ T. The 1 H NMR signal of a nitroxide radical solution was observed at 13.83 Hz, which corresponds to 325 nT. Signal-to-noise ratio was 32 after sixteen averages. On the Larmor precession of 1 H spins, a decaying oscillation was superimposed. We attribute it to a transient 87 Rb spin precession in response to a non-adiabatic field variation. This work shows a new capability of zero- and ultralow-field NMR. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) spectroscopy is one of the most significant applications of nuclear magnetic resonance (NMR). Here, we demonstrate that the 2D NMR can be performed even at a low magnetic field of less than 5 μT, which is ten times less than the Earth’s magnetic field. The pulses used in the experiment were composed of circularly polarized fields for coherent as well as wideband excitations. Since the excitation band covers the entire spectral range, the simplest two-pulse sequence delivered the full 2D spectrum. At 5 μT, methanol with 13 C enriched up to 99% belongs to a strongly coupled regime, and thus its 2D spectrum exhibits complicated spectral correlations, which can be exploited as a fingerprint in chemical analysis. In addition, we show that, with compressive sensing, the acquisition of the 2D spectrum can be accelerated to take only 45% of the overall duration. [ABSTRACT FROM AUTHOR]

Highlights: [•] A pulse of circular polarization can rapidly and precisely drive nuclear spins, even in the regime beyond the rotating wave approximation. [ABSTRACT FROM AUTHOR]

Highlights: [•] The feasibility of neuronal current detection with ULF NMR was investigated. [•] We built an ULF NMR system with polarizing fields of 50mT and a noise . [•] Evoked activity was emulated with a current dipole phantom and realistic parameters. [•] The AC effect is not feasible with current ULF NMR technology. [•] The DC effect is feasible with current ULF NMR technology. [ABSTRACT FROM AUTHOR]