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

Abstract

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 p=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 lT 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.Currently, a strong pre-polarization field (B p ) is indispensable for increasing the signal-to-noise ratio (SNR) in most superconducting quantum interference device (SQUID)-based ultra-low field (ULF) nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) studies. 1-20 The field should be applied for a while before acquisition, because a static magnetic field (B m ) operating in the micro-tesla range cannot generate sufficient nuclear spin polarization. There are some challenges involved in producing a strong B p , which is one of the critical issues in ULF-NMR/MRI. In order to create a stronger B p , various types of B p coils have been used, such as a permanent magnet 1 and coils with the following cooling agents: forced air, 2 water, 3,4 a fluorine-based industrial coolant, 5 liquid helium, 6,7 and liquid nitrogen. [8][9][10][11][12] In spite of these efforts, the strength of B p has been limited to below approximately 150 mT. 3 Increasing the magnitude of the B p has also yielded unwanted effects: flux trapping in the superconducting pickup coil 7 and the generation of eddy currents on the walls of a magnetically shielded room (MSR). 4,11-13 These side effects can deteriorate the NMR/MRI signals.There is an alternate method to increase the SNR that does not involve increasing B p. Nuclear polarization can be enhanced by using the dynamic nuclear polarization (DNP) method. When unpaired electron spins are resonated with the magnetic field by applying an external radio-frequency (rf) field (B rf ), creating electron spin resonance (ESR), nuclear spins have non-equilibrium polarization due to the transfer of polarization from the excited unpaired electron spins to the nuclear spins via cross-relaxation. The nuclear spin polarization in the non-equilibrium state is much larger than that in the thermal equilibrium state for a given magnetic field. 21 Several DNP experiments with small B p that use the SQUID-based NMR/MRI system have been reported. 14-16 However, we have demonstrated an experimental approach without B p. We have obtained a clear DNP MR image by a circularly polarized pulse technique 17,18 at 34.5 lT B m. The problems of eddy currents and...

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

“…However, we demonstrated that circularly polarised RF results in a substantially higher enhancement factor of over 150,000 at 1.2 µT and can be implemented with a birdcage coil. The magnetic field applied for O-DNP was only 1.2 µT (B p ), which is approximately 30 times lower than 34.5 µT in our previous work [29]. However, the quality of the image ( Fig.…”

“…The total acquisition time constituted to 77 min. The phantom in the present work is the same as in our previous study [29] and was filled with a 2 mM TEM-POL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) solution which seemed to achieve the optimal result in the microtesla field range [30,31]. It has the dimensions of 54 mm * 43.5 mm * 40 mm, featuring a letter K in the middle as well as rectangles and circles with varying sizes (top 10 mm, middle 5 mm, bottom 3 mm) on the left and right sides, respectively.…”

SQUID-based ultralow-field MRI of a hyperpolarized material using signal amplification by reversible exchange

“…In addition, to obtain a good MR image in SQUID-based microtesla MRI, a pre-polarization field ( B p ) is required before the MR signal from the sample is detected. Therefore, some attempts 17,22,23 were made to obtain a strong B p , including optimizing the B p coils and cooling these coils to reduce noise. Increasing B p using a coil also yielded critical effects: the generation of eddy currents on the walls of a magnetically shielded room (MSR) 24,25 and flux trapping in a pickup coil 26 .…”

“…The hyperpolarization technique can solve this pre-polarization problem to enable the polarization of materials with an extremely high order of magnitude. There have been several recent reports 18,22,27–30 on this strategy. The most well-known way of achieved hyperpolarization, DNP, which is used to transfer polarization from electrons to nuclei, was harnessed to obtain a high-contrast microtesla MR image 18,22 .…”

“…There have been several recent reports 18,22,27–30 on this strategy. The most well-known way of achieved hyperpolarization, DNP, which is used to transfer polarization from electrons to nuclei, was harnessed to obtain a high-contrast microtesla MR image 18,22 . Alternatively, SABRE, which induces hyperpolarization on a variety of substrates via low-cost, high-throughput hyperpolarization without complex devices under normal conditions, utilizes para-hydrogen ( p -H 2 ) by forming a metal complex with some target substrates and transfers hyperpolarization from a polarized proton to a weakly bound substrate 31–33 .…”