Ultra-low Field Nuclear Magnetic Resonance Research Articles

Detection of nuclear magnetic resonance in the microtesla range using a high Tc dc-SQUID

We have detected the ultra-low field nuclear magnetic resonance signal from water samples using a high-Tc dc-SQUID sensor. The measurements were carried out in a homemade magnetically shielded room. Resonance spectra of 1H from tap water and other substance samples were obtained in the field range from 7-110μT corresponding to resonance frequency 300-4.68kHz. Two kind of experimental systems were built, the first one is a directly coupled system, its signal to noise ratio in a single-shot measurement is around 4 for about 15 ml water. The second one used a Cu coil to transfer the flux to the SQUID sensor. Signal to noise ratio was improved to about 20 in a single-shot measurement for 5ml water, which benefits from the improvement of coupling efficiency. The effect of residual gradient in the magnetically shielded room was also investigated. J-coupling of 2,2,2-Trifluoroethyl alcohol was measured, the peaks are consistent with high field results.

Time-Domain Frequency Correction Method for Averaging Low-Field NMR Signals Acquired in Urban Laboratory Environment

Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector, ultra-low-field nuclear magnetic resonance (ULF-NMR) signals of protons are recorded in an urban environment without magnetic shielding. The homogeneity and stability of the measurement field are investigated. NMR signals of protons are studied at night and during working hours. The Larmor frequency variation caused by the fluctuation of the external magnetic field during daytime reaches around 5 Hz when performing multiple measurements for about 10 min, which seriously affects the results of averaging. In order to improve the performance of the averaged data, we suggest the use of a data processor, i.e. the so-called time-domain frequency correction (TFC). For a 50-times averaged signal spectrum, the signal-to-noise ratio is enhanced from 30 to 120 when applying TFC while preserving the NMR spectrum linewidth. The TFC is also applied successfully to the measurement data of the hetero-nuclear J-coupling in 2,2,2-trifluoroethanol.

A Superconducting Quantum Interference Device Measurement System for Ultra Low-Field Nuclear Magnetic Resonance

We describe a superconducting quantum interference device (SQUID)-based nuclear magnetic resonance (NMR) spectrometer operating at ultralow magnetic fields far below the Earth’s field. The spectrometer consists of a helium-cooled magnetic sensor system and two Helmholtz coils, one for pre-polarizing the sample by fields of up to 5 mT, and one for the detection in fields of the nanotesla and microtesla range. The spectrometer represents the current state of the art in ultralow-field NMR and enables the observation of phenomena that are difficult or impossible to achieve by a conventional NMR setting. In particular, one can obtain broad band spectra covering different nuclei, such as 1H and 31P, with a frequency resolution in the millihertz range, observe the variation of their heteronuclear coupling with the detection field strength, and investigate relaxation processes that reflect molecular dynamics in the millisecond range.

Liquid-State Nuclear Spin Comagnetometers

We discuss nuclear spin comagnetometers based on ultralow-field nuclear magnetic resonance in mixtures of miscible solvents, each rich in a different nuclear spin. In one version thereof, Larmor precession of protons and 19F nuclei in a mixture of thermally polarized pentane and hexafluorobenzene is monitored via a sensitive alkali-vapor magnetometer. We realize transverse relaxation times in excess of 20 s and suppression of magnetic field fluctuations by a factor of 3400. We estimate it should be possible to achieve single-shot sensitivity of about 5×10(-9) Hz, or about 5×10(-11) Hz in ≈1 day of integration. In a second version, spin precession of protons and 129Xe nuclei in a mixture of pentane and hyperpolarized liquid xenon is monitored using superconducting quantum interference devices. Application to spin-gravity experiments, electric dipole moment experiments, and sensitive gyroscopes is discussed.

SQUID-based systems for co-registration of ultra-low field nuclear magnetic resonance images and magnetoencephalography

The ability to perform magnetic resonance imaging (MRI) in ultra-low magnetic fields (ULF) of ∼100μT, using superconducting quantum interference device (SQUID) detection, has enabled a new class of magnetoencephalography (MEG) instrumentation capable of recording both anatomical (via the ULF MRI) and functional (biomagnetic) information about the brain. The combined ULF MRI/MEG instrument allows both structural and functional information to be co-registered to a single coordinate system and acquired in a single device. In this paper we discuss the considerations and challenges required to develop a combined ULF MRI/MEG device, including pulse sequence development, magnetic field generation, SQUID operation in an environment of pulsed pre-polarization, and optimization of pick-up coil geometries for MRI in different noise environments. We also discuss the design of a “hybrid” ULF MRI/MEG system under development in our laboratory that uses SQUID pick-up coils separately optimized for MEG and ULF MRI.

ULF-NMR system using HTS-SQUID and permanent magnet

We have constructed an ultra-low field (ULF) nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) system using an HTS-rf-SQUID and room-temperature electromagnets in a magnetically shielded room (MSR). In this study, in order to improve the signal to noise ratio (S/N) of the system, we introduced a permanent magnet instead of the electromagnet for pre-polarizing the sample to enhance the pre-polarizing field (Bp). The cylindrical permanent magnet of 270mT was used to magnetize a water sample for several seconds outside the MSR and about 1.5m away from the SQUID. We constructed an instrument to transfer the magnetized sample from the permanent magnet to under the SQUID in 0.5s. Since the non-adiabatic condition cannot be kept in such sample transfer scheme, an AC pulse coil to apply an AC pulse field BAC to rotate the magnetization moments for π/2 was introduced to measure a free induction decay (FID) signal from the sample. By this system, we obtained an NMR signal from the water sample of 10ml while applying a static field of 45μT and π/2 pulse after the transfer. The S/N of the NMR spectrum was about 100 by a single shot, which was 10 times larger than that obtained with the electromagnet of 32mT. In addition, we demonstrated the measurements of the longitudinal relaxation time (T1) and the spin echo signal of the water sample by the system.

Development of signal to noise ratio of HTS-rf-SQUID for ultra-low field NMR / MRI by 77K LC resonant circuit

We constructed an ultra-low field (ULF) nuclear magnetic resonance (NMR) / magnetic resonance imaging (MRI) system with a high temperature superconductor (HTS) radio-frequency (rf) superconducting quantum interference device (SQUID) coupled with a LC circuit inside a cryostat filled with liquid nitrogen. Several LC resonant circuits with 139 to 1500 turns coils made from copper wires of 0.2, 0.4, and 0.6 mm in diameter were tested to obtain the optimum configuration for the circuit. For each coil, a capacitance was selected to fix the resonance frequency at around 10 kHz. The LC resonant circuit using a 500 turns coil made from 0.2 mm diameter wire offered the largest signal-to-noise ratio (S/N) of 11.9 with a quality factor of 53.3 at 10.2 kHz and 77 K. With this circuit, the S/N of free induction decay (FID) signals was tested using the ULF SQUID-NMR/MRI system.

Magnetic Field Improved ULF-NMR Measurement in an Unshielded Laboratory Using a Low-Tc SQUID

Abstract Ultra Low Field Nuclear Magnetic Resonance (ULF-NMR) measurements were performed in an urban laboratory environment using a 2 nd order low- T C SQUID gradiometer. To improve the magnetic environment around the sample, two kinds of compensation methods were developed. Firstly, a spatially correlated active compensation system which includes two separated sets of squared Helmholtz coils, a 3-axis fluxgate and a home-made Proportional Integral Differential (PID) feedback electronics were developed to suppress the low frequency fluctuation. The low frequency disturbance of environmental magnetic field can be suppressed over two orders of amplitude. Secondly, linear gradient compensation in three directions was introduced to improve the homogeneity of the environmental magnetic field. Combining the two methods mentioned above, the relaxation time T 2* of proton was measured to be about 0.45s, and the signal-to-noise ratio of spectrum was improved by a factor of 9 for a 50 times averaged signal.

Characterizing longitudinal and transverse relaxation rates of ferrofluids in microtesla magnetic fields

Shortening spin-lattice relaxation rates (1/T1) or spin-spin relaxation rates (1/T2) is the purpose of magnetic resonance imaging contrast agents. In this work, an ultralow field nuclear magnetic resonance spectrometer and imager are set up to characterize the spin relaxation rates of Fe3O4 superparamagnetic iron oxide (SPIO) for image contrast. It was found that both 1/T1 and 1/T2 increase linearly when the magnetic susceptibility χ of SPIO increases by increasing the concentration of SPIO dispersed in water. In an applied field, magnetic moments of SPIO generate microscopic field gradients that weaken the field homogeneity, in turn de-phasing the proton’s nuclear spin and enhancing the relaxation rates. A T1-contrast image is demonstrated, using SPIO as the contrast agent and high-Tc superconducting quantum interference devices as the detector. T1-contrast imaging in microtesla fields might provide a potential modality for discriminating cancer.

Relaxation Behavior Study of Ultrasmall Superparamagnetic Iron Oxide Nanoparticles at Ultralow and Ultrahigh Magnetic Fields

Ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) have attracted attention because of their current and potential usefulness as contrast agents for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). USPIOs are usually used for their significant capacity to produce predominant proton relaxation effects, which result in signal reduction. However, most previous studies that utilized USPIOs have been focused on the relaxation behavior at commonly used magnetic fields of clinical MRI systems (typically 1-3 T). In this paper, magnetic relaxation processes of protons in water surrounding the USPIOs are studied at ultralow (≤10 mT) and ultrahigh magnetic fields (14.1 T). USPIOs used in our experiments were synthesized with a core size of 6 nm, and transferred from organic to water by ligand exchange. The proton spin-lattice relaxation time (T(1)) and spin-spin relaxation time (T(2)) were investigated at ultralow (212 μT for T(2) and 10 mT for T(1)) and at 14.1 T with different iron concentrations. At all of the fields, there is a linear relationship between the inverse of relaxation times and the iron concentration. The spin-spin relaxivity (r(2)) at 14.1 T is much larger than that value of the ultralow field. At ultralow field, however, the spin-lattice relaxivity (r(1)) is larger than the r(1) at ultrahigh field. The results provide a perspective on potential in vivo and in vitro applications of USPIOs in ultralow and ultrahigh field NMR and MRI.

SQUIDs vs. Induction Coils for Ultra-Low Field Nuclear Magnetic Resonance: Experimental and Simulation Comparison

Nuclear magnetic resonance (NMR) is widely used in medicine, chemistry and industry. One application area is magnetic resonance imaging (MRI). Recently it has become possible to perform NMR and MRI in the ultra-low field (ULF) regime requiring measurement field strengths of the order of only 1 Gauss. This technique exploits the advantages offered by superconducting quantum interference devices or SQUIDs. Our group has built SQUID based MRI systems for brain imaging and for liquid explosives detection at airport security checkpoints. The requirement for liquid helium cooling limits potential applications of ULF MRI for liquid identification and security purposes. Our experimental comparative investigation shows that room temperature inductive magnetometers may provide enough sensitivity in the 3-10 kHz range and can be used for fast liquid explosives detection based on ULF NMR technique. We describe experimental and computer-simulation results comparing multichannel SQUID based and induction coils based instruments that are capable of performing ULF MRI for liquid identification.

Liquid-State Nuclear Magnetic Resonance Measurements for Imaging Using HTS-rf-SQUID in Ultra-Low Field

In our study, we constructed a ultra-low field (ULF) nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) system employing a high-temperature superconductor (HTS)-rf-SQUID and room-temperature coils. In the system, we employed a pulsed polarizing field Bp perpendicular to a measurement field Bm; a high-speed switching circuit with an optical FET was used to switch off Bp of approximately 30 mT. When a field gradient dBz/dz of approximately 10 nT/cm was applied to a sample, which was designed to contain water in two compartments, a 1H-NMR spectrum with two peaks was observed. The frequency difference between the two peaks roughly corresponded to the distance between each center of the water in the two parts. We measured the 1H -NMR signals for water and mineral oil and found a clear difference in their signal amplitudes owing to their different molecular compositions. The longitudinal relaxation times T1 of water and the oil were also estimated by changing the polarizing time of Bp. From these results, it was confirmed that 1H- and T1-weighted contrast imaging could be realized by using this system.

Development of Ultra Low Field Nuclear Magnetic Resonance Imaging System Using HTS rf SQUID

We constructed and studied an ultra low-field nuclear magnetic resonance imaging system using a HTS rf SQUID and room-temperature coils. We recorded the free induction decay signals of 1H by employing a measuring field, Bm, of 44.8 μT and a polarizing field, Bp, of 36.8 mT; in the presence of ∂BZ/∂z and ∂BZ/∂y gradients of the order of 0.8 μT/m, we were able to obtain a quasi two-dimensional 4-pixels image of a simple water phantom.

Low-Noise Pulsed Pre-Polarization Magnet Systems for Ultra-Low Field NMR

A liquid cooled, pulsed electromagnet of solenoid configuration suitable for duty in an ultra-low field nuclear magnetic resonance system has been designed, fabricated and successfully operated. The magnet design minimizes Johnson noise, minimizes the hydrogen signal and incorporates minimal metal and no ferromagnetic materials. In addition, an acoustically quiet cooling system permitting 50% duty cycle operation was achieved by designing for single-phase, laminar flow, forced convection cooling. Winding, conductor splicing and epoxy impregnation techniques were successfully developed to produce a coil winding body with integral cooling passageways and adequate structural integrity. Issues of material compatibility, housing, coolant flow system and heat rejection system design will be discussed. Additionally, this pulsed electromagnet design has been extended to produce a boiling liquid cooled version in a paired solenoid configuration suitable for duty in an ultra-low field nuclear magnetic resonance system. This pair of liquid nitrogen cooled coils is currently being tested and commissioned. Issues of material compatibility, thermal insulation, thermal contraction, housing and coolant flow design are discussed.

Ultra-Low Field NMR of ${\rm UF}_{6}$ for $^{235}{\rm U}$ Detection and Characterization

We have demonstrated the first ultra-low field (ULF) nuclear magnetic resonance measurements of uranium hexafluoride (UF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ), which is used in the uranium enrichment process. A sensitive non-invasive detection system would have an important role in non-proliferation surveillance. A two-frequency technique was employed to remove the transients induced by rapidly switching off the 50 mT pre-polarization field. A mean transverse relaxation time T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of 24 ms was estimated for the un-enriched UF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> sample measured at a mean temperature of 80degC.

Detection of 3He spins with ultra-low field nuclear magnetic resonance employing SQUIDs for application to a neutron electric dipole moment experiment

The precession of 3He spins is detected with ultra-low field NMR. The absolute strength of the NMR signal is accurately measured and agrees closely with theoretical calculations. The sensitivity is analyzed for applications to a neutron electric dipole moment (nEDM) fundamental symmetry experiment under development.

Using ultra-low field nuclear magnetic resonance for direct neural current measurements

Abstract. We present the physical basis and experimental evidence for a new technique to detect the direct consequences of neural activity. While ultra-low field (ULF) NMR may be more sensitive for measuring the phase effect that was proposed by other researchers for high-field MRI, yet more compelling is the fact that the proton Larmor frequency at ULF can be tuned to overlap with a band of the neural activity spectrum. We hypothesize that resonant mechanisms at ULF would significantly enhance the effect of neural activity on the NMR signal that can then be imaged using standard MRI methods.