Large Static Field Research Articles

Theory of superlocalized magnetic nanoparticle hyperthermia: Rotating versus oscillating fields

The main idea of magnetic hyperthermia is to increase locally the temperature of the human body by means of injected superparamagnetic nanoparticles. They absorb energy from a time-dependent external magnetic field and transfer it into their environment. In the so-called superlocalization, the combination of an applied oscillating and a static magnetic field gradient provides even more focused heating since for large enough static field the dissipation is considerably reduced. Similar effect was found in the deterministic study of the rotating field combined with a static field gradient. Here we study theoretically the influence of thermal effects on superlocalization and on heating efficiency. We demonstrate that when time-dependent steady state motions of the magnetization vector are present in the zero temperature limit, then deterministic and stochastic results are very similar to each other. We also show that when steady state motions are absent, the superlocalization is severely reduced by thermal effects. Our most important finding is that in the low frequency range (ω→0) suitable for hyperthermia, the oscillating applied field is shown to result in two times larger intrinsic loss power and specific absorption rate then the rotating one with identical superlocalization ability which has importance in technical realization.

The characteristic analysis of the built-in vector atomic magnetometer in a nuclear magnetic resonance oscillator

We analyze the amplitude-frequency, phase-frequency, signal amplification, linear range, and vector characteristics of the built-in vector atomic magnetometer operating at extreme off-resonance condition in a nuclear magnetic resonance oscillator, which makes possible its performance improvement by a balanced strategy in optimizing the parameters based on the proposed model. The experiment validates our prediction of the amplitude-frequency characteristic, and the numerical simulation indicates that the applied carrier field with following demodulation procedure holds the potential to give one order of magnitude, which is experimentally-validated to have at least twice, signal enhancement and enable the vector characteristic, where a large longitudinal static field and an appropriate transverse relaxation time are preferred to have optimized characteristics depending on different applications.

Nonlinear dielectric features of highly polar glass formers: Derivatives of propylene carbonate.

We have measured the nonlinear dielectric behavior of several highly polar propylene carbonate (PC) derivatives in the vicinity of their glass transition temperatures. Focus is on the effects of a large static electric field on the frequency dependent permittivity and on the cubic susceptibility measured using sinusoidal fields of high amplitude. The case of vinyl-PC shows dielectric saturation as well as an electro-rheological effect, i.e., a field induced increase of dielectric relaxation times, whose magnitude changes linearly with the apparent activation energy. The extent of this shift of the loss profile caused by the field correlates strongly with the peak magnitude of the cubic susceptibility, |χ3|, underlining the notion of a link between the |χ3| "hump" and this electro-rheological behavior. Further support for this picture emerges from the observation that the most polar of these liquids, (S)-(-)-methoxy-PC with εs ≈ 250, lacks both the electro-rheological effect in ε″(ω) and the "hump" typically observed in |χ3(ω)|. The absence of any sensitivity of the dynamics to an electric field is contrary to the expectation that the electro-rheological effect correlates with the field induced entropy change, which is extraordinarily high for this liquid. The results suggest that the dependence of the relaxation time on the electric field is not directly linked to the entropy change.

From a Piano Stool to a Sandwich: A Stepwise Route for Improving the Slow Magnetic Relaxation Properties of Thulium

Two mononuclear TmIII complexes, [Tm(COT)I(THF)2] (1-Tm) and [K(18-crown-6)(THF)2][Tm(COT)2] (2-Tm), display slow relaxation of the magnetization, making these compounds rare examples of non-Kramers TmIII single-molecule magnets (SMMs). Utilizing a stepwise synthetic approach for the installation of cyclooctatetraenide (COT) ligands, we can observe the effect of symmetry optimization at the metal center. This method results in an 85% increase in the energy barrier to magnetization reversal (Ueff) for 2-Tm (Ueff = 53.3 K) over 1-Tm (Ueff = 7.93 K). The increased local symmetry of 2-Tm reduces the need for large static fields, eliciting SMM behavior under a small field of 200 Oe. This illustrates the power of fine-tuning the ligand environment to enhance the magnetic relaxation properties of non-Kramers ions.

Superconductor Analog-to-Digital Converter for High-Resolution Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) requires a sensitive radio receiver to detect the weak resonant magnetic field from nuclear spins, usually hydrogen in biological tissues. In a typical static magnetic field of 1.5 T, a transient RF magnetic field at 63 MHz must be measured to fT amplitudes. This must be amplified and then digitized with an analog-to-digital converter (ADC) with a high dynamic range (typically 16 bits), in order to obtain high image resolution. As the magnetic field is increased in order to obtain even finer resolution, the required dynamic range is expected to increase further. A superconductor ADC (oversampling at 20 GHz) has previously been demonstrated to exhibit very high dynamic range (up to 24 bits) as part of a receiver for broadband digital-RF communication. Here, we present preliminary results on a similar ADC applied to a 4 T commercial small-animal MRI system, substituted for the built-in 16-bit ADC. The superconductor ADC was mounted on a 4 K cryocooler in a magnetically shielded container outside the MRI room and functioned well despite the presence nearby of a large static field and a high-power RF transmitter. Images were obtained either using direct digitization of the 170-MHz signals or, alternatively, using a low-noise analog mixer and digitization of a 1-MHz intermediate frequency signal. While the results demonstrated the superior dynamic range of the superconductor ADC, further improvement was limited by the thermal noise from the room-temperature pickup coil. Current work is investigating the use of a cryocooled coil (which may be superconducting) and a cold low-noise preamplifier in order to take full advantage of the large dynamic range of the superconductor ADC. This should permit improved spatial resolution, allowing one to “zoom in” to a region of interest.

Wannier-Stark states in double-periodic lattices. II. Two-dimensional lattices

We analyze the Wannier-Stark spectrum of a quantum particle in tilted two-dimensional lattices with the Bloch spectrum consisting of two subbands, which could be either separated by a finite gap or connected by the Dirac points. For rational orientations of the static field given by an arbitrary superposition of the translation vectors, the spectrum is a ladder of energy bands. We obtain asymptotic expressions for the energy bands in the limit of large and weak static fields and study them numerically for intermediate field strength. We show that the structure of energy bands determines the rate of spreading of localized wave packets, which is the quantity measured in laboratory experiments. It is shown that wave-packet dispersion becomes a fractal function of the field orientation in the long-time regime of ballistic spreading.

A TDPAC study of static and dynamic magnetic behaviour

The a-FexHf100−x system is used to explore the application of TDPAC (the time differential perturbed γ–γ angular correlation technique) to non-trivial anisotropic magnetic relaxation. The effect of fluctuations in this system is primarily to cause a decay of the zero-frequency component, which is characterized by the phenomenological decay rate λ. The zero-field magnetic phase diagram, constructed from both static and dynamic features of the data, and the temperature dependence of λ are both fully consistent with the physics of partial bond frustration. The results demonstrate that the magnetic fluctuations are meaningfully characterized by simple spectrum features, and are not obscured by large static fields or severe disorder.

Estimation of error fields from ferromagnetic parts in ITER

Error fields in tokamaks are small departures from the exact axisymmetry of the ideal magnetic field configuration. Their reduction below a threshold value by the error field correction coils is essential since sufficiently large static error fields lead to discharge disruption.The error fields are originated not only by magnets fabrication and installation tolerances, by the joints and by the busbars, but also by the presence of ferromagnetic elements. It was shown that superconducting joints, feeders and busbars play a secondary effect; however in order to estimate of the importance of each possible error field source, rough evaluations can be very useful because it can provide an order of magnitude of the correspondent effect and, therefore, a ranking in the request for in depth analysis. The paper proposes a two steps procedure. The first step aims to get the approximate magnetization state of ferromagnetic parts; the second aims to estimate the full 3D error field over the whole volume using equivalent sources for magnetic masses and taking advantage from well assessed approximate closed form expressions, well suited for the far distance effects.

ITER non-axisymmetric error fields induced by its magnet system

Error fields in Tokamaks are small departures of the exact axisymmetry of the ideal magnetic field configuration. Their reduction beyond a threshold value by the error field correction coils is essential since sufficiently large static error fields lead to discharge disruption. The error fields are originated not only by coils fabrication and installation alignment tolerances, joints and busbars but also due to the presence of ferromagnetic elements. The start of plasma current flattop with relatively low plasma density is considered as a critical state of the 15 MA scenario for the onset of locked modes causing disruptions. A figure of merit, the ‘3-mode’ criterion, based on the lowest error field harmonics [(1,1); (2,1); (3,1)] was chosen to assess the error fields expected in ITER. Analysis performed last years in an independent way by CREATE (EU) and Efremov Inst. (RF) groups has allowed a deep understanding of the error fields induced by all the possible sources. Both groups were successfully benchmarked with the estimation of the error fields induced by a given deformed shape of a TF coil obtaining a perfect match of the results up to 2 orders of magnitude smaller than the defined threshold of B 3-mode/ B to < 5 × 10 −5. Three different sets of independent variables based on a 3D rigid body movement of the coils have been provided for the 6 CS modules, 18 TF coils and 6 PF coils tolerances, which have allowed a clear understanding of the weight of each of the variables in the induced error fields. The results obtained in 2008 with a realistic set of magnets tolerances concluded that the system of correction coils provides effective suppression of the error fields with margin to correct possible impact of other sources. In particular, it has been shown that superconducting joints, feeders and busbars play a secondary effect; however the radial position of the TF coils and the tilt and radial shift of the CS stack would have a relevant influence. The ensuing recent sets of variables studied aimed at deepening the understanding of the operational limits. The present paper addresses the impact of the fabrication tolerances and installation misalignments of the magnet system; the impact of ferromagnetic inserts and busbars as well as the design of the correction coils will be thoroughly covered elsewhere (Amoskov et al., in press [1]).

Coronary MRA at 3 T using 3d multi-interleaved multi-echo acquisition with varpro fat-water separation

Introduction Coronary MR Angiography is a valuable tool for non-invasive assessment of coronary arteries. Presently, contrastenhanced, fat-saturated, ECG-triggered and navigatorgated 3D spoiled gradient-echo sequence is employed for whole-heart Coronary MRA at 3 T[1]. However, large static field variations at 3 T frequently result in non-uniform fat-suppression over the field-of-view (FOV), obscuring the delineation of coronary arteries. Multi-echo Dixon approaches utilizing iterative decomposition have been shown to provide robust fat-water separation even in the presence of large field inhomogeneities. In this study, an ECG-triggered navigator-gated 3D spoiled gradientecho multi-interleaved multi-echo (GRE-MEMI) pulse sequence is introduced which utilizes VARPRO[2] fatwater separation to achieve reliable fat-suppression and provides enhanced visualization of coronary arteries.

Theoretical Study on the Considerable Second-Order Nonlinear Optical Properties of Naphthylimido-Substituted Hexamolybdates

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 x 10(-30) esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest betao(o) value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to thenitrogen atom, could generate a large static electronic field and give the large contribution to NLO response.The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives.

ＥＰＩにおける画像の歪みに影響する撮像パラメータの検討

Echo planar imaging (EPI) is highly sensitive to static magnetic field inhomogeneities. The degree of local image compression and stretching is a function of the static field gradient in the phase-encoding direction. This is caused by the accumulation of a phase shift. Any static field shift will lead to a position shift in the image, and it is the regions with large static fields that are the most difficult to correct. We reduce image distortion by SENSE with an array coil. However, we often use a surface coil because we cannot use an array coil in clinical studies. In this case, image distortion becomes greater, and reduction of distortion is very important. For the purpose of this study, we examined the relation between imaging parameters and image distortion. Image distortion of EPI is unrelated to the following parameters: number of phase encodings, half scan, echo time, and diffusion b-value. However, the following parameters influenced image distortion: FOV, number of frequency encodings, rectangle FOV, and multi-shot imaging. Image distortion of EPI is decided by the area of the phase-encoding gradient and the interval of readout gradients. We hope that many institutions will find these data useful.

EPI image reconstruction with correction of distortion and signal losses

To derive and implement a method for correcting geometric distortions and recovering magnetic resonance imaging (MRI) signal losses caused by susceptibility-induced magnetic field gradients (SFGs) in regions with large static field inhomogeneities in echo-planar imaging (EPI). Factors to account for field inhomogeneities and SFGs were added in a traditional EPI equation that was a simple Fourier transform (FT) for expressing the actual k-space data of an EPI scan. The inverse calculation of this "distorted EPI" equation was used as a kernel to correct geometric distortions and reductions in intensity during reconstruction. A step-by-step EPI reconstruction method was developed to prevent complicated phase unwrapping problems. Some EPI images of phantom and human brains were reconstructed from standard EPI k-spaces. All images were reconstructed using the proposed multistep method. Geometric distortions were corrected and SFG-induced MRI signal losses were recovered. Results suggest that applying our method for reconstructing EPI images to reduce distortions and MRI signal losses is feasible.

A [formula omitted]SR study of the unusual magnetism of [formula omitted

We present the results of low longitudinal field μ SR experiments in the intermetallic magnetic system LaCrSb 3 . In the low field phase ( < 250 G ) , we find evidence of at least two magnetically inequivalent muon sites characterized by large static internal fields which are not changed appreciably by the spin-reorientation transition at about 90 K. Other characteristics of the muon spin relaxation function that do reflect the reorientation are discussed.

GAMMA simulations of stray field responses: slice thickness and pulse calibration.

A stray field (STRAFI) module has been added to the GAMMA magnetic resonance simulation platform in order to facilitate computational investigations of NMR experiments in large static field gradients that are on the order of 50 T/m. The package has been used to examine system response during echo trains generated by the application of shaped pulses. The associated echo amplitude maxima and effective slice thickness are presented. A new accurate method for STRAFI pulse calibration based on relative echo amplitudes is proposed.

Picosecond large angle reorientation of the magnetization in Ni81Fe19 circular thin-film elements

Large angle picosecond reorientation of the magnetization has been studied in circular Ni81Fe19 thin-film elements of 30 μm diameter and 500 Å thickness by means of an optical pump–probe technique. The sample was pumped by an optically triggered magnetic field pulse and probed by a time resolved magneto-optical Kerr effect measurement. The temporal profile of the pulsed field and the in-plane uniaxial anisotropy of the element were first determined from measurements made in large static fields where the magnetization exhibited small amplitude ferromagnetic resonance oscillations. Measurements of large amplitude oscillations were then made in a smaller static field that was still larger than the in-plane uniaxial anisotropy field and sufficient to saturate the sample. Using the measured temporal profile of the pulsed field, the Landau–Lifshitz–Gilbert equation was used to model the motion of the magnetization as a coherent rotation process. The same values of the anisotropy and damping constants provided an adequate simulation of both the high and low field data. The magnetization was found to move through an angle of up to about 30° on subnanosecond time scales. The dependence of the reorientation upon the direction of the static applied field and observed deviations from the coherent precession model are discussed.

Single-shot EPI with signal recovery from the susceptibility-induced losses.

A major problem in the gradient-recalled echo-planar imaging (EPI) method that also uses a long echo time (TE) is the severe signal loss in regions with large static field inhomogeneities. These regions include the ventral frontal, medial temporal, and inferior temporal lobes, which experience inhomogeneities induced by susceptibility effects commonly found near air/tissue interfaces. For functional magnetic resonance imaging (fMRI) studies that use both gradient-recalled EPI at relatively long TE and high-field scanners, this signal loss is severe, preventing investigation of certain human cognitive processes that involve these regions, such as memory and attention. Methods have been developed to recover this signal loss; however, most of them require multiple excitations and thus compromise temporal resolution. In this report, a new technique is described which achieves good signal recovery within a single excitation. It is anticipated that this technique will prove useful for fMRI studies in inhomogeneous areas that require high temporal resolution.

Conformal radiotherapy of challenging paraspinal tumors using a multiple arc segment technique

Purpose: Target volumes that wrap around the spinal cord are difficult to treat. We present and evaluate a refined multiple arc segment (MAS) technique that is applicable with standard three-dimensional (3D) radiotherapy equipment and may be a solution for facilities that do not yet have full access to intensity-modulated radiotherapy (IMRT). Methods and Materials: The presented technique consisted of 13 isocentric coplanar beam’s eye view shaped fields, delivered as 20° or 10° arc segments with an integrated multileaf collimator (MLC) using automatic sequential field delivery. Dose–volume histograms (DVH) for this technique were compared to a modified bar-arc technique (MBA) modeled as 30 static fields and to an inverse planned IMRT technique using 7 coplanar, equispaced beams delivered with the same MLC. Results: Compared to the MBA technique, maximum dose and target coverage were similar when using 80% of the maximum dose as the reference dose. However, the MAS technique reduced the maximum doses (to ≥ 1% of the organs at risk [OAR]) by 9% for the spinal cord, 17% for the esophagus, and 25% for the trachea, as well as the mean doses. Although inverse planned IMRT could further reduce exposure of OAR except for the spinal cord and improve target coverage, our forward planned MAS technique seems to achieve clinically comparable results. Conclusion: Substituting a series of small split-field arc segments for large static fields and using additional narrow paraspinal segments significantly improves the sparing of organs at risk for paraspinal targets. Although these results are not quite as good as those achieved with IMRT, for facilities not yet equipped with inverse treatment planning capability, the presented technique enables dose escalation for primary paraspinal tumors and retreatment of recurrent lesions.

Electric field effects on the helium dimer

Three methods of perturbing the ground state of the He4 dimer by electric fields are treated: the Stark shift by large static electric fields; dissociation by a pulsed electric field; and the coupling of a plane wave to the interaction-induced quadrupole moment of the dimer. The second process, with coupling to the interaction-induced polarizability anisotropy of the dimer, appears to offer the best prospect for observable perturbation of the dimer. The estimates are based on results of ab initio calculations of properties of the He–He system.

Demonstrating the Spatial Resolution of Field Gradient NMR

In recent years, field gradient NMR has become a method of increasing importance in measuring very small dislocations of molecules. Rough estimations indicate that, by utilizing large field gradients, one should be able to detect motions down to the 10-nm scale. However, this limit has not yet been experimentally detected. In this paper, we present a method that allows the direct measurement of the spatial resolution of field gradient NMR. For our experimental setup, utilizing an extremely large static field gradient of about 180 T m−1, we find a lower limit of 7 nm, thus for the first time confirming the expected resolution. Furthermore, we discuss the effect of unwanted vibrations as a limiting factor of the method.