Seong-min Hwang

Pre-polarization enhancement by dynamic nuclear polarization in SQUID-based ultra-low-field nuclear magnetic resonance

We achieved enhanced pre-polarization in a superconducting quantum interference device (SQUID)-based microtesla nuclear magnetic resonance (NMR) experiment by using dynamic nuclear polarization (DNP). The pre-polarization field is necessary to provide enough signal to noise to perform SQUID-based ultra-low-field (ULF) NMR/magnetic resonance imaging (MRI) experiments. However, it is quite tricky to deal with the strong transient magnetic field when operating the SQUID in a magnetically shielded room (MSR); besides the direct interference with the sensitive SQUID sensor, the strong magnetic field and its abrupt change generate magnetization in local areas in the MSR and eddy currents along the wall, which makes the NMR measurement difficult. The enhanced 1H NMR signals of water in TEMPOL and TEMPO solutions were obtained with a relatively weak radio-frequency (rf) field and double-relaxation oscillation SQUIDs (DROS) at a few mT pre-polarization fields. In our experimental condition, the enhancement factor was near ten in spite of the rf power far below the saturation in both samples.

Two-dimensional NMR spectroscopy of 13C methanol at less than 5 μT

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 5μT, which is ten times less than the Earths 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 5μT, 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.

Strong pulsed excitations using circularly polarized fields for ultra-low field NMR

A pulse, which is produced by a single coil and thereby has a linear polarization, cannot coherently drive nuclear spins if the pulse is stronger than the static field B0. The inaccuracy of the pulse, which arises from the failure of the rotating wave approximation, has been an obstacle in adopting multiple pulse techniques in ultra-low field NMR where B0 is less than a few μT. Here, we show that such a limitation can be overcome by applying pulses of circular polarization using two orthogonal coils. The sinusoidal nutation of the nuclear spins was experimentally obtained, which indicates that coherent and precise controls of the nuclear spins can be achieved with circularly polarized pulses. Additional demonstration of the Carl-Purcell-Meiboom-Gill sequence verifies the feasibility of adopting multiple pulse sequences to ultra-low field NMR studies.

Toward a brain functional connectivity mapping modality by simultaneous imaging of coherent brainwaves

Matching the proton-magnetic-resonance frequency to the frequency of a periodic neural oscillation (e.g., alpha or gamma band waves) by magnetic resonance imaging techniques, enables direct visualization of brain functional connectivity. Functional connectivity has been studied by analyzing the correlation between coherent neural oscillations in different areas of the brain. In electro- or magneto-encephalography, coherent source reconstruction in a source-space is very tricky due to power leaking from the correlation among the sources. For this reason, most studies have been limited to sensor-space analyses, which give doubtful results because of volume current mixing. The direct visualization of coherent brain oscillations can circumvent this problem. The feasibility of this idea was demonstrated by conducting phantom experiments with a SQUID-based, micro-Tesla NMR/MRI system. We introduce an experimental trick, an effective step-up of the measurement B-field in a pulse sequence, to decouple the magnetic resonance signal from the strong magneto-encephalographic signal at the same frequency.

Dynamic nuclear polarization in the hyperfine-field-dominant region

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 and nearly invariant with respect to magnetic fields below the Earths field. The numerical analysis based on the radicals 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 μT.

Magnetic resonance imaging without field cycling at less than earth's magnetic field

A strong pre-polarization field, usually tenths of a milli-tesla in magnitude, is used to increase the signal-to-noise ratio in ordinary superconducting quantum interference device-based nuclear magnetic resonance/magnetic resonance imaging experiments. Here, we introduce an experimental approach using two techniques to remove the need for the pre-polarization field. A dynamic nuclear polarization (DNP) technique enables us to measure an enhanced resonance signal. In combination with a π/2 pulse to avoid the Bloch-Siegert effect in a micro-tesla field, we obtained an enhanced magnetic resonance image by using DNP technique with a 34.5 μT static external magnetic field without field cycling. In this approach, the problems of eddy current and flux trapping in the superconducting pickup coil, both due to the strong pre-polarization field, become negligible.

Toward a magnetic resonance electrical impedance tomography in ultra-low field: A direct magnetic resonance imaging method by an external alternating current

Measuring the electrical impedance of biological tissues in a low frequency range is challenging. Here, we have conducted a superconducting quantum interference device-based microtesla magnetic resonance (MR) imaging study. To obtain an MR image caused by an injected alternating current (ac), we utilized the direct resonance method in which the nuclear spins resonate with the ac magnetic field generated by the external ac current. This method requires an adiabatic pulse and non-adiabatic step-down pulse techniques. The experimental and simulation results agree well with each other and show the feasibility of low-frequency magnetic resonance electrical impedance tomography in the kHz range.Measuring the electrical impedance of biological tissues in a low frequency range is challenging. Here, we have conducted a superconducting quantum interference device-based microtesla magnetic resonance (MR) imaging study. To obtain an MR image caused by an injected alternating current (ac), we utilized the direct resonance method in which the nuclear spins resonate with the ac magnetic field generated by the external ac current. This method requires an adiabatic pulse and non-adiabatic step-down pulse techniques. The experimental and simulation results agree well with each other and show the feasibility of low-frequency magnetic resonance electrical impedance tomography in the kHz range.