Non-cryogenic anatomical imaging in ultra-low field regime: hand MRI demonstration

Ultra-low field (ULF) MRI with a pulsed prepolarization is a promising method with potential for applications where conventional high-, mid-, and low-field medical MRI cannot be used due to cost, weight, or other restrictions. Previously, successful ULF demonstrations of anatomical imaging were made using liquid helium-cooled SQUIDs and conducted inside a magnetically shielded room. The Larmor frequency for these demonstrations was ～3 kHz. In order to make ULF MRI more accessible, portable, and inexpensive, we have recently developed a non-cryogenic system. To eliminate the requirement for a magnetically shielded room and improve the detection sensitivity, we increased the frequency to 83.6 kHz. While the background noise at these frequencies is greatly reduced, this is still within the ULF regime and most of its advantages such as simplicity in magnetic field generation hardware, and less stringent requirements for uniform fields, remaining. In this paper we demonstrate use of this system to image a human hand with up to 1.5mm resolution. The signal-to-noise ratio was sufficient to reveal anatomical features within a scan time of less than 7 min. This prototype can be scaled up for constructing head and full body scanners, and work is in progress toward demonstration of head imaging.     

Two-dimensional NMR spectroscopy in Earth's magnetic field

We demonstrate the first two-dimensional correlation NMR (COSY) spectra obtained at ultra low frequencies (ULF) using the Earth's magnetic field. Using a specially developed spectrometer with multiple audio-frequency pulses under controlled pulse phase, we observe magnetisation transfer arising from heteronuclear J-couplings in trifluoroethanol and para-difluorobenzene. The 2D COSY spectra exhibit all diagonal and off-diagonal multiplets consistent with known J-couplings in these molecules.     

On concomitant gradients in low-field MRI

Growing interest in magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, approximately muT fields) has been motivated by several advantages over its counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes, reduced imaging artifacts from susceptibility variations within a sample, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. Acquisition of MRI at ULF must, however, account for concomitant gradients that would otherwise result in severe image distortions. In this paper, we introduce the general theoretical framework that describes concomitant gradients, explain why such gradients are more problematic at low field, and present possible approaches to correct for these unavoidable gradients in the context of a non-slice-selective MRI protocol.     

Studying Specific Features of the Resonance Structure of the Background Noise Spectrum in the Frequency Range 1–10 Hz with Allowance for the Slope of the Earth’s Magnetic Field

We present new data on midlatitude features of the resonance structure of the magnetic ultralow-frequency noise spectrum in the frequency range 0.1–10 Hz, namely, the different frequency scale and frequency shift of the resonance-structure maxima for the East-West and North-South components. Resonance spectra of the magnetic components and polarization parameter of magnetic noise for the plane-stratified ionosphere model are numerically simulated with allowance for the inclination of the Earth's magnetic field on the basis of the International Reference Ionosphere IRI-2001 standard. Dependence of the calculated parameters of the resonance structure in the magnetic-component spectra on the source direction and inclination angle of the Earth's magnetic field is explored. The calculations make it possible to explain the different behavior of resonant oscillations in the linear-component spectra and the features of the resonance structure in the background-noise polarization parameter. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 575–584, July 2008.     

Excitation of Electromagnetic Waves in a Plane Waveguide with Anisotropic Upper Wall

We solve the problem of propagation of electromagnetic ULF waves in a plane waveguide with anisotropic upper wall from an arbitrary source with allowance for an arbitrary inclination of the magnetic field. The procedure for calculating magnetic components of the signal radiated by horizontal and vertical dipoles is developed in detail. The solution is presented in the form of the Fourier–Bessel transform of the desired field components. Impedance-type boundary conditions were used for an arbitrary inclination of the magnetic field. The obtained solutions can be used for interpreting experiments on measuring the spectra of ULF electromagnetic noise in various geographical regions and calculating parameters of ULF signals from controlled sources.     

Ultra-low field NMR measurements of liquids and gases with short relaxation times

Interest in nuclear magnetic resonance measurements at ultra-low magnetic fields (ULF, approximately microT fields) has been motivated by various benefits and novel applications including narrow NMR peak-width, negligible susceptibility artifacts, imaging of samples inside metal containers, and possibility of directly imaging neuronal currents. ULF NMR/MRI is also compatible with simultaneous measurements of biomagnetic signals. However the most widely used technique in ULF NMR-prepolarization at high field and measurement at lower field-results in large transient signals which distort the free induction decay signal. This is especially severe for the measurement of signals from samples and materials with short T1 time. We have devised an approach that largely cancels the transient signals. The technique was successfully used to measure NMR signals from liquids and gases with T1 in the range 1-4 ms.     

Mobile high resolution xenon nuclear magnetic resonance spectroscopy in the earth's magnetic field

Conventional high resolution nuclear magnetic resonance (NMR) spectra are usually measured in homogeneous, high magnetic fields (>1 T), which are produced by expensive and immobile superconducting magnets. We show that chemically resolved xenon (Xe) NMR spectroscopy of liquid samples can be measured in the Earth's magnetic field (5 x 10(-5) T) with a continuous flow of hyperpolarized Xe gas. It was found that the measured normalized Xe frequency shifts are significantly modified by the Xe polarization density, which causes different dipolar magnetic fields in the liquid and in the gas phases.     

Pseudomodulation: A computer-based strategy for resolution enhancement

Pseudomodulation is defined as computer simulation of the effect on a magnetic resonance spectrum that would be obtained if a sinusoidal field modulation were applied followed by phase-sensitive detection at the fundamental frequency or at one of its harmonics. Since it is done by computer, it is not necessary that it actually be possible to modulate the field. The algorithm is developed here for EPR spectroscopy, but can be applied to any function or to any digitized signal, including an NMR signal, and there can be more than one independent parameter, as, for example, an MR image or a 2D NMR spectrum where there are two. Pseudomodulation transforms and filters a digitized spectrum. Subtraction or addition of various amounts of the even harmonics of the digitized spectral data from or to the original spectrum results in resolution enhancement. Other resolution-enhancement algorithms that use pseudomodulation are also presented. Resolution enhancement by pseudomodulation is tested on simulated spectra to which computer-generated noise has been added and is applied to the EPR S-band spectrum of the blue copper protein azurin.     

Intracavity mode locked laser magnetometer

Intracavity Phase Interferometry is applied to the detection of magnetic fields. A magnetic field induced change in index is converted into a beat frequency between two optical pulse trains. Because the detected frequency is not dependent on the pulse train intensities, a signal to noise improvement over standard amplitude measurements (for instance Faraday rotation) of at least a factor 1000 is achieved. The method applies to any magnetic detection method where the magnetic field can be converted into a change in refractive index.     

High-resolution NMR spectra in inhomogeneous and unstable fields via the three-pulse method

In modern solution nuclear magnetic resonance (NMR), the spectral resolution is mainly dependent on the spatial homogeneity and temporal stability of the magnetic field. The spectral linewidths are usually proportional to the overall field homogeneity and the stability experienced by the sample. Many high-resolution NMR methods have been developed, but few are applicable in inhomogeneous and unstable fields. In this paper, a high-resolution three-pulse method based on intermolecular zero-quantum coherences (iZQCs) is proposed. Since this method is insensitive to field inhomogeneity and instability, spectral information such as the chemical shift can be retained in the resulting spectra. In comparison with the CPMG-HOMOGENIZED method, the new method provides almost pure solvent–solute iZQC signals.     

Noise figure characterization of preamplifiers at NMR frequencies

A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy. At high radiofrequencies, such impedance variation is typically achieved with transmission-line tuners, which however are not available for the relatively low range of typical NMR frequencies. To solve this issue, this work describes an alternative approach that relies on lumped-element circuits for impedance manipulation. It is shown that, using a fixed-impedance noise source and suitable ENR correction, this approach permits noise figure characterization for NMR and MRI purposes. The method is demonstrated for two preamplifiers, a generic BF998 MOSFET module and an MRI-dedicated, integrated preamplifier, which were both studied at 128MHz, i.e., at the Larmor frequency of protons at 3 Tesla. Variations in noise figure of 0.01dB or less over repeated measurements reflect high precision even for small noise figures in the order of 0.4dB. For validation, large sets of measured noise figure values are shown to be consistent with the general noise-parameter model of linear two-ports. Finally, the measured noise characteristics of the superior preamplifier are illustrated by SNR measurements in MRI data.     

Numerical solution to the Bloch equations:paramagnetic solutions under wideband continuous radio frequency irradiation in a pulsed magnetic field

A novel nuclear magnetic resonance(NMR) experimental scheme,called wideband continuous wave NMR(WB-CW-NMR),is presented in this article.This experimental scheme has promising applications in pulsed magnetic fields,and can dramatically improve the utilization of the pulsed field.The feasibility of WB-CW-NMR scheme is verified by numerically solving modified Bloch equations.In the numerical simulation,the applied magnetic field is a pulsed magnetic field up to 80 T,and the wideband continuous radio frequency(RF) excitation is a band-limited(0.68-3.40 GHz) white noise.Furthermore,the influences of some experimental parameters,such as relaxation time,applied magnetic field strength and wideband continuous RF power,on the WB-CW-NMR signal are analyzed briefly.Finally,a multi-channel system framework for transmitting and receiving ultra wideband signals is proposed,and the basic requirements of this experimental system are discussed.Meanwhile,the amplitude of the NMR signal,the level of noise and RF interference in WB-CW-NMR experiments are estimated,and a preliminary adaptive cancellation plan is given for detecting WB-CW-NMR signal from large background interference.     

New Zealand Developments in Earth's Field NMR

We review here two decades of development of Earth's field nuclear magnetic resonance (NMR), starting with the first demonstration of resonant refocusing and the generation of spin echoes, the first measurement of diffusion using Earth's field NMR, and its application to Antarctic research, the adaptation of the apparatus to allow its use indoors, in a conventional laboratory setting, and finally, the construction of a flexible laboratory-based Earth's field NMR system capable of not only a range of relaxation and spectroscopy applications but also the straightforward demonstration of NMR Imaging. Authors' address: Paul T. Callaghan, MacDiarmid Institute, Victoria University of Wellington, PO Box 600, Wellington, New Zealand     

Ultralow-frequency variations of the magnetic field during the propagation of acoustic-gravitaty waves in the ionosphere

The effect of the generation of ULF variations of the magnetic field during the propagation of acoustic-gravitaty waves (AGW) in the ionosphere is analyzed within a simple model. It is shown that AGW-induced ionospheric irregularities can significantly contribute to the ground magnetic field variations. Some changes in ULF variations associated with seismic events are considered as application of this mechanism.     

Implementation of active noise control in a multi-modal spray dryer exhaust stack

Tonal noise emitted from large-diameter spray dryer exhaust stacks used in the dairy industry can give rise to complaints from nearby communities. In many cases, the tone at the fan blade passing frequency is characterized by a frequency above the first mode cut on frequency of the exhaust stack and both its amplitude and the frequency are time varying. The variation in amplitude is a result of turbulence and temperature variations in the duct which cause angular variations in the nodal plane of modes with diametrical nodes. This in turn results in large fluctuations in sound pressure with time at any specified location in the duct, thus presenting a significant challenge for an ANC system with fixed control source and error sensor locations. In many food processing industries, the use of sound absorptive materials in silencers is not acceptable and, particularly when the fan speed is variable, it is difficult to achieve an acceptable passive solution at a reasonable cost. Here, the design and implementation of an active noise control system for tonal noise propagating above the cut-on frequency of the first higher order mode in large size cylindrical industrial exhaust stack is discussed, where the frequency and amplitude vary significantly and relatively rapidly with time. Physical system design principles and control algorithm optimization for a practical active noise control system are presented. Finally, real time control results which were achieved by a prototype installation on a large-diameter, in-service exhaust stack are given. Significant noise reductions were achieved in the community.     

300 MHz核磁共振频率源的研制

介绍了基于Analog Device公司AD9954设计的数字化频率源,可以实现宽带高频（5 MHz~125 MHz、280 MHz~301 MHz）、高稳定度（1×10^-9）、低相位噪声（301 MHz,≤-127 dBc/Hz@250 kHz offset）频率输出以及频率、相位和幅度的高速切换. 该数字化频率源可应用于300 MHz核磁共振谱仪和磁共振成像仪,可部分取代昂贵的进口频率源. 本文最后给出核磁实验结果.     

Detection of NMR signals with a radio-frequency atomic magnetometer

We demonstrate detection of proton NMR signals with a radio-frequency (rf) atomic magnetometer tuned to the NMR frequency of 62 kHz. High-frequency operation of the atomic magnetometer makes it relatively insensitive to ambient magnetic field noise. We obtain magnetic field sensitivity of 7 fT/Hz1/2 using only a thin aluminum shield. We also derive an expression for the fundamental sensitivity limit of a surface inductive pick-up coil as a function of frequency and find that an atomic rf magnetometer is intrinsically more sensitive than a coil of comparable size for frequencies below about 50 MHz.     

NMR phase noise in bitter magnets

We have studied the temporal instability of a high field resistive Bitter magnet through nuclear magnetic resonance (NMR). This instability leads to transverse spin decoherence in repeated and accumulated NMR experiments as is normally performed during signal averaging. We demonstrate this effect via Hahn echo and Carr--Purcell--Meiboom--Gill (CPMG) transverse relaxation experiments in a 23-T resistive magnet. Quantitative analysis was found to be consistent with separate measurements of the magnetic field frequency fluctuation spectrum, as well as with independent NMR experiments performed in a magnetic field with a controlled instability. Finally, the CPMG sequence with short pulse delays is shown to be successful in recovering the intrinsic spin--spin relaxation even in the presence of magnetic field temporal instability.     

First Experiments on Generating and Receiving Artificial ULF (0.3–12 Hz) Emissions at a Distance of 1500 km

We present first results on generation and reception of an artificial ULF signal at distances of 800 and 1500 km in the frequency range 0.3–12 Hz. The high-sensitivity receiving equipment and a stable current in the antenna exciting the ULF field in the Earth-ionosphere cavity allowed us to detect the signal with high accuracy. It was found that the amplitude-frequency dependences and the polarization of an artificial signal are strongly different in the daytime and at night, which is related to the influence of the near-Earth waveguide and resonance structures on the detected signal. We calculate the signal amplitude for different models of the ionosphere. It is shown that the model of an anisotropic, vertically inhomogeneous ionosphere describes adequately the amplitude-frequency dependences of the measured ULF signal. We calculate the apparent resistance of the underlying surface at the reception point using two measured orthogonal components H_x and E_y of an electromagnetic field. It is shown that the calculation accuracy of the apparent resistance obtained using a controlled ULF source exceeds significantly the accuracy achieved using natural electromagnetic fields.