Ultra-low Field Magnetic Resonance Imaging Research Articles

High-sensitivity in vivo contrast for ultra-low field magnetic resonance imaging using superparamagnetic iron oxide nanoparticles.

Magnetic resonance imaging (MRI) scanners operating at ultra-low magnetic fields (ULF; <10 mT) are uniquely positioned to reduce the cost and expand the clinical accessibility of MRI. A fundamental challenge for ULF MRI is obtaining high-contrast images without compromising acquisition sensitivity to the point that scan times become clinically unacceptable. Here, we demonstrate that the high magnetization of superparamagnetic iron oxide nanoparticles (SPIONs) at ULF makes possible relaxivity- and susceptibility-based effects unachievable with conventional contrast agents (CAs). We leverage these effects to acquire high-contrast images of SPIONs in a rat model with ULF MRI using short scan times. This work overcomes a key limitation of ULF MRI by enabling in vivo imaging of biocompatible CAs. These results open a new clinical translation pathway for ULF MRI and have broader implications for disease detection with low-field portable MRI scanners.

Ultra-low noise graphene/copper/nylon fabric for electromagnetic interference shielding in ultra-low field magnetic resonance imaging

In ultra-low-field magnetic resonance imaging (ULF MRI) working in the micro-tesla magnetic field range, the superconducting quantum interference device (SQUID) as the signal detector is very susceptible to electromagnetic interference (EMI) so that the system normally works in a shielded room. However, the leakage of EMI in the shielded room may still seriously reduce the system performance. In order to improve the electromagnetic compatibility of the system, we designed a microwave absorbing composite, graphene/Cu/nylon fabric (GCN fabric). In this design, high shielding effectiveness and low-noise performance of the EMI shielding material are equally crucial due to the extremely sensitive detection with SQUID. The shielding effectiveness of 5-layer fabric ranges between 30 dB and 67 dB from 30 MHz to 3 GHz and its maximum appears at 60 MHz. Furthermore, GCN fabric introduces little extra system noise when applied in the ULF MRI system with magnetic field noise of 0.8 fT/Hz at 5 kHz. The SQUID unlocked tuned signal is thus increased by 33% and the signal-to-noise ratio of MRI image is increased by a factor of 4.3. In future, portable and inexpensive unshielded ULF MRI with low-noise might be realized by potential optimization on the component and preparation technology of GCN fabric.

Simulation and Measurements of Transient Fields From Conductive Plates of Shielded Room for SQUID-Based Ultralow Field Magnetic Resonance Imaging

In order to enhance the signal-to-noise ratio (SNR) of ultralow field magnetic resonance imaging (MRI) measurements, a prepolarization ( B p) coil is usually introduced to generate a magnetic field with magnitude in the order of tens of mT or even stronger to prepolarize the sample before MRI sequences. The macromagnetization M of the sample is now directly determined by the B p strength, thus the SNR is effectively improved. However, the rapid turn-off of B p current induces eddy currents in the conductive shielded room and the surrounding conductors, which in turn causes unwanted transient magnetic fields in the imaging area. In this paper, according to the relative position of the B p coil and the shielded room, two physical models of the conductive shielded room are described to simulate the transient fields at the sample position: the B p coil plane parallel to the ceiling and floor of the shielded room, and the coil plane perpendicular to the walls. The simulation is in good agreement with the experimental results. This paper will help us design the structure of the conductive shielded room as well as develop the eddy current cancellation technique.

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

The signal amplification by reversible exchange (SABRE) technique is a very promising method for increasing magnetic resonance (MR) signals. SABRE can play a particularly large role in studies with a low or ultralow magnetic field because they suffer from a low signal-to-noise ratio. In this work, we conducted real-time superconducting quantum interference device (SQUID)-based nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI) studies in a microtesla-range magnetic field using the SABRE technique after designing a bubble-separated phantom. A maximum enhancement of 2658 for 1H was obtained for pyridine in the SABRE-NMR experiment. A clear SABRE-enhanced MR image of the bubble-separated phantom, in which the para-hydrogen gas was bubbling at only the margin, was successfully obtained at 34.3 μT. The results show that SABRE can be successfully incorporated into an ultralow-field MRI system, which enables new SQUID-based MRI applications. SABRE can shorten the MRI operation time by more than 6 orders of magnitude and establish a firm basis for future low-field MRI applications.

T1-Weighted Image by Ultra-Low Field SQUID-MRI

We are developing an ultra-low field (ULF) magnetic resonance imaging (MRI) system with a high-temperature superconductor superconducting quantum interference device (SQUID) for the purpose of food contaminant inspection. Our previous ULF SQUID-MRI system for food inspection used a resonant circuit to detect a magnetic field, which was tuned at the Larmor frequency. However, when the MR signal frequency decreases, the signal bandwidth becomes narrower. Since the line width of the nuclear magnetic resonance spectrum of food with a short relaxation time becomes broad, it is difficult to measure the MR signal. Therefore, to address this issue, we employed a non-resonant method to widen the bandwidth. In this paper, we describe a ULF SQUID-MRI system using a non-resonant Cu wound flux transformer to image food with a shorter relaxation time. As a result, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -weighted images of samples containing water and oil could be acquired by varying the polarizing time.

Temperature determination of the pre-polarising process of an earth’s magnetic field MRI system

Magnetic resonance imaging is a low inherent sensitivity technique with a low nuclear polarisation at thermal equilibrium. Low sensitivity is a major problem for ultralow field magnetic resonance imaging, but it can be overcome by improving the polarising process. This procedure may increase the temperature of the sample to be imaged affecting its integrity. We investigate the temperature increase generated by a pre-polarisation process for an ultralow field nuclear magnetic resonance system. We measured the pre-polarising field with a magnetic field sensor inside the system to study the possible effects on it. Temperature readings inside the system were also obtained varying the polarising time. A linear fitting was then calculated showing a low rate of 0.0086°C/s, and a total increment of 5°C for around 10 minutes. This is an important temperature increment that it should be considered when running imaging experiments of biological subjects.

Demonstration of full tensor current density imaging using ultra-low field MRI

Direct imaging of impressed dc currents inside the head can provide valuable conductivity information, possibly improving electro-magnetic neuroimaging. Ultra-low field magnetic resonance imaging (ULF MRI) at μT Larmor fields can be utilized for current density imaging (CDI). Here, a measurable impact of the magnetic field BJ, generated by the impressed current density J, on the MR signal is probed using specialized sequences. In contrast to high-field MRI, the full tensor of BJ can be derived without rotation of the subject in the scanner, due to a larger flexibility in the sequence design.We present an ULF MRI setup based on a superconducting quantum interference device (SQUID), which is operating at a noise level of 380 aT Hz−1/2 and capable of switching all imaging fields within a pulse sequence. Thereby, the system enables zero-field encoding, where the full tensor of BJ is probed in the absence of other magnetic fields. 3D CDI is demonstrated on phantoms with different geometries carrying currents of approximately 2 mA corresponding to current densities between 0.45 and 8 A/m2. By comparison to an in vivo acquired head image, we provide insights to necessary improvements in signal-to-noise ratio.

3D-Spatial encoding with permanent magnets for ultra-low field magnetic resonance imaging

We describe with a theoretical and numerical analysis the use of small permanent magnets moving along prescribed helical paths for 3D spatial encoding and imaging without sample adjustment in ultra-low field magnetic resonance imaging (ULF-MRI). With our developed method the optimal magnet path and orientation for a given encoding magnet number and instrument architecture can be determined. As a proof-of-concept, we studied simple helical magnet paths and lengths for one and two encoding magnets to evaluate the imaging efficiency for a mechanically operated ULF-MRI instrument with permanent magnets. We demonstrate that a single encoding magnet moving around the sample in a single revolution suffices for the generation of a 3D image by back projection.

Large T1 contrast enhancement using superparamagnetic nanoparticles in ultra-low field MRI

Superparamagnetic iron oxide nanoparticles (SPIONs) are widely investigated and utilized as magnetic resonance imaging (MRI) contrast and therapy agents due to their large magnetic moments. Local field inhomogeneities caused by these high magnetic moments are used to generate T2 contrast in clinical high-field MRI, resulting in signal loss (darker contrast). Here we present strong T1 contrast enhancement (brighter contrast) from SPIONs (diameters from 11 nm to 22 nm) as observed in the ultra-low field (ULF) MRI at 0.13 mT. We have achieved a high longitudinal relaxivity for 18 nm SPION solutions, r1 = 615 s−1 mM−1, which is two orders of magnitude larger than typical commercial Gd-based T1 contrast agents operating at high fields (1.5 T and 3 T). The significantly enhanced r1 value at ultra-low fields is attributed to the coupling of proton spins with SPION magnetic fluctuations (Brownian and Néel) associated with a low frequency peak in the imaginary part of AC susceptibility (χ”). SPION-based T1-weighted ULF MRI has the advantages of enhanced signal, shorter imaging times, and iron-oxide-based nontoxic biocompatible agents. This approach shows promise to become a functional imaging technique, similar to PET, where low spatial resolution is compensated for by important functional information.

Adaptive suppression of power line interference in ultra-low field magnetic resonance imaging in an unshielded environment

Power-line harmonic interference and fixed-frequency noise peaks may cause stripe-artifacts in ultra-low field (ULF) magnetic resonance imaging (MRI) in an unshielded environment and in a conductively shielded room. In this paper we describe an adaptive suppression method to eliminate these artifacts in MRI images. This technique utilizes spatial correlation of the interference from different positions, and is realized by subtracting the outputs of the reference channel(s) from those of the signal channel(s) using wavelet analysis and the least squares method. The adaptive suppression method is first implemented to remove the image artifacts in simulation. We then experimentally demonstrate the feasibility of this technique by adding three orthogonal superconducting quantum interference device (SQUID) magnetometers as reference channels to compensate the output of one 2nd-order gradiometer. The experimental results show great improvement in the imaging quality in both 1D and 2D MRI images at two common imaging frequencies, 1.3 kHz and 4.8 kHz. At both frequencies, the effective compensation bandwidth is as high as 2 kHz. Furthermore, we examine the longitudinal relaxation times of the same sample before and after compensation, and show that the MRI properties of the sample did not change after applying adaptive suppression. This technique can effectively increase the imaging bandwidth and be applied to ULF MRI detected by either SQUIDs or Faraday coil in both an unshielded environment and a conductively shielded room.

Development of Compact Ultra-Low-Field MRI System Using an Induction Coil

Ultra-low-field magnetic resonance imaging (ULF-MRI) is attractive for MRI applications such as a concurrent system of magnetoencephalography (MEG) and MRI. We are developing a compact ULF-MRI system that can be combined with a small-animal MEG system. In this paper, we developed a detection method that uses an induction coil instead of a superconducting quantum interference device for the ULF-MRI. The induction coil has advantages such as being more robust, easy to handle, and flexible for designing. We fabricated an induction coil from a copper wire with dimensions of 29 mm $\times39$ mm $\times29$ mm (inner diameter $\times $ outer diameter $\times $ length). A capacitor was connected to increase the sensitivity by the LC resonance effect. The resonance frequency was 3 kHz, and the corresponding magnetic field density for MRI measurement was $70~\mu \text{T}$ . The detection coil was integrated with the compact ULF-MRI system and placed inside a magnetically shielded box. MRI measurement was demonstrated by using water phantoms with a total volume of $5.5 \times 10^{3}$ mm $^{3}$ . The size of the obtained image was $32 \times 32$ pixels for 30.2 mm $\times30.2$ mm. A clear shape of the water phantom was taken with this ULF-MRI system. This result shows the induction coil can be used for the compact ULF-MRI system.

Ultra-Low Field SQUID-NMR using LN2 Cooled Cu Polarizing Field coil

We are developing an Ultra-Low Field (ULF) Magnetic Resonance Imaging (MRI) system using a High-Temperature Superconductor superconducting quantum interference device (HTS rf-SQUID) for food inspection. The advantages of the ULF-NMR (Nuclear Magnetic Resonance) / MRI as compared with a conventional high field MRI are that they are compact and of low cost. In this study, we developed a ULF SQUID-NMR system using a polarizing coil to measure fat of which relaxation time T1 is shorter. The handmade polarizing coil was cooled by liquid nitrogen to reduce the resistance and accordingly increase the allowable current. The measured decay time of the polarizing field was 40 ms. The measurement system consisted of the liquid nitrogen cooled polarizing coil, a SQUID, a Cu wound flux transformer, a measurement field coil for the field of 47 μT, and an AC pulse coil for a 90°pulse field. The NMR measurements were performed in a magnetically shielded room to reduce the environmental magnetic field. The size of the sample was ϕ35 mm × L80 mm. After applying a polarizing field and a 90°pulse, an NMR signal was detected by the SQUID through the flux transformer. As a result, the NMR spectra of fat samples were obtained at 2.0 kHz corresponding to the measurement field Bm of 47 μT. The T1 relaxation time of the mineral oil measured in Bm was 45 ms. These results suggested that the ULF-NMR/MRI system has potential for food inspection.

AC Defluxing of SQUIDs

Magnetic flux trapping is a very serious problem in SQUID-based instrumentation and superconducting electronics. A critical consideration for avoiding flux trapping is a prevention of SQUID sensors or circuits from being exposed to magnetic fields. Unfortunately, this measure cannot be applied, if sensors must be exposed to a strong magnetic field or transients. For example, unavoidable exposures take place in ultralow field magnetic resonance imaging and superparamagnetic relaxation measurements using unshielded thin-film planar SQUID sensors. SQUID sensors stop working after being exposed to magnetic fields of only a few mT in strength. Earlier, we reported successful results of defluxing of planar LTS SQUID-gradiometers using strong decaying ac magnetic pulses. In this paper, we present further experimental results and discuss a possible mechanism to explain the observed defluxing effects. The new ac defluxing technique is much faster and dissipates much less energy than a conventional thermal cycling.

スピン交換光ポンピングによる&lt;sup&gt;131&lt;/sup&gt;Xeの超偏極と極低磁場における核磁気共鳴信号の増大

Magnetic resonance imaging (MRI) has been used in clinical as major tomographic inspection with CT scans. Compared with CT, MRI has a number of advantages for example no radiation exposure. However, MRI has also disadvantages e.g. large size, high cost and never use to patients who have implant devices or metals because have to use strong field. Therefore, ultra-low-field MRI (ULF-MRI) has attracted attention because of usage of ultra-low fields such as earth field about 40∼50 μT. But there is a fatal weakness of ULF-MRI which SN ratio decreases. In this paper we show that spin-exchange optical pumping (SEOP) can increase 131Xe nuclear magnetic resonance (NMR) signals. We also show that SEOP is possible at very low power LASER with field in pulse sequence of NMR measurement.

Thin-Film-Based Ultralow Noise SQUID Magnetometer

We report on the development of an ultralow noise thin-film based SQUID magnetometer. A niobium thin-film pickup coil is connected to the input coil of a SQUID current sensor. The low capacitance of the used sub-micrometer cross-type Josephson junctions enable superior noise performance of the device. Application scenarios e.g. in geophysics and ultra-low field magnetic resonance imaging are discussed.

Field Dependence Study of Commercial Gd Chelates With SQUID Detection

The contrast agents (CAs), which can enhance the image contrast by changing the relaxation times in the target locations, are widely used in clinical high-field magnetic resonance imaging (MRI). However, the commercial clinical CAs have attracted little attention in ultralow field (ULF) MRI, whose measurement field is four orders of magnitude lower than that of the clinical high-field counterpart. In this paper, we measure the relaxivity levels of three gadolinium (Gd)-based commercial CAs (Gd-DTPA, Gd-EOB-DTPA, and Gd-DTPA-BMA) with our four-channel magnetically unshielded ULF-MRI system using low- $T_{{c}}$ superconducting quantum interference devices as detectors. The system has a 0.65-T permanent-magnet pair as prepolarization field $B_{{p}}$ and applies a static $B_{0}$ field of 129 uT. We compare the longitudinal relaxivity levels of the CAs at $B_{{p}}$ and $B_{0}$ field strengths, as well as the reported values at 1.5 T. The results show that the relaxivity levels of three commercial CAs at 129 uT are all about twice those at high fields, which demonstrates one particular advantage of ULF-MRI.

Rotatable Small Permanent Magnet Array for Ultra-Low Field Nuclear Magnetic Resonance Instrumentation: A Concept Study.

ObjectWe studied the feasibility of generating the variable magnetic fields required for ultra-low field nuclear magnetic resonance relaxometry with dynamically adjustable permanent magnets. Our motivation was to substitute traditional electromagnets by distributed permanent magnets, increasing system portability.Materials and MethodsThe finite element method (COMSOL®) was employed for the numerical study of a small permanent magnet array to calculate achievable magnetic field strength, homogeneity, switching time and magnetic forces. A manually operated prototype was simulated and constructed to validate the numerical approach and to verify the generated magnetic field.ResultsA concentric small permanent magnet array can be used to generate strong sample pre-polarisation and variable measurement fields for ultra-low field relaxometry via simple prescribed magnet rotations. Using the array, it is possible to achieve a pre-polarisation field strength above 100 mT and variable measurement fields ranging from 20–50 μT with 200 ppm absolute field homogeneity within a field-of-view of 5 x 5 x 5 cubic centimetres.ConclusionsA dynamic small permanent magnet array can generate multiple highly homogeneous magnetic fields required in ultra-low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) instruments. This design can significantly reduce the volume and energy requirements of traditional systems based on electromagnets, improving portability considerably.

Ultralow-Field High-Tc SQUID NMR/MRI System With a 77-K Cooled Copper Flux Transformer

We propose a high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting quantum interference device ultralow-field magnetic resonance imaging (ULF MRI) system for food inspection. Although the ULF MRI is compact and low cost, compared with a conventional high-field MRI, the detected signal is extremely weak. Thus, a nuclear magnetic resonance (NMR) system was constructed and optimized before realizing the MRI system. The purpose of the study is to improve the signal-to-noise ratio (SNR) of the NMR signal. A copper-wound flux transformer cooled at 77 K by liquid nitrogen and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mbox{LC}$</tex-math></inline-formula> resonance were employed and tested to improve the SNR. As a result, the measured SNR was approximately equal to 236 with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mbox{LC}$</tex-math></inline-formula> resonance, improving by a factor of 7.8.

Ultra-low field MRI food inspection system prototype

We develop an ultra-low field (ULF) magnetic resonance imaging (MRI) system using a high-temperature superconducting quantum interference device (HTS-SQUID) for food inspection. A two-dimensional (2D)-MR image is reconstructed from the grid processing raw data using the 2D fast Fourier transform method. In a previous study, we combined an LC resonator with the ULF-MRI system to improve the detection area of the HTS-SQUID. The sensitivity was improved, but since the experiments were performed in a semi-open magnetically shielded room (MSR), external noise was a problem. In this study, we develop a compact magnetically shielded box (CMSB), which has a small open window for transfer of a pre-polarized sample. Experiments were performed in the CMSB and 2D-MR images were compared with images taken in the semi-open MSR. A clear image of a disk-shaped water sample is obtained, with an outer dimension closer to that of the real sample than in the image taken in the semi-open MSR. Furthermore, the 2D-MR image of a multiple cell water sample is clearly reconstructed. These results show the applicability of the ULF-MRI system in food inspection.

Multichannel ULF-MRI Study in Magnetic Unshielded Urban Laboratory Environment

Ultralow field (ULF) magnetic resonance imaging (MRI), which obtains images in the static magnetic field typically on the orders of tens to hundreds of microteslas, exhibits some potential advantages. However, the ULF-MRI system faces the challenge of poor signal-to-noise ratio (SNR). In addition to the introduction of a prepolarization technique and the usage of extremely sensitive superconducting quantum interference devices (SQUIDs), the multisensor parallel imaging technique can also improve the SNR and allow accelerated image acquisition. In this paper, a four-channel ULF-MRI system was built with low-T c SQUID-based second-order axial gradiometer in an urban laboratory environment without magnetic shield. To suppress the temporal field fluctuation and to balance the environment gradient field tensor, an active compensation and a gradient field shimming system were developed. Using the Fourier imaging method and weighted image superposition, the 2-D and 3-D four-channel MRI images of pepper pieces were obtained at a measurement field of 129 μT, i.e., a Larmor frequency of 5.5 kHz with an in-plane resolution of about 2 mm × 2 mm. These results demonstrated the feasibility of performing 2-D and 3-D multisensor parallel ULF MRI in the unshielded urban environment.