Magnetic resonance imaging simulation with spin-lock preparations to detect tiny oscillatory magnetic fields

Detection of tiny oscillatory magnetic fields using low-field MRI: A combined phantom and simulation study

Extracting quantitative information of neuronal signals by non-invasive imaging is an outstanding challenge for understanding brain function and pathology. However, state-of-the-art techniques offer low sensitivity to deep electrical sources. Stimulus induced rotary saturation is a recently proposed magnetic resonance imaging sequence that detects oscillatory magnetic fields using a spin-lock preparation. Phantom experiments and simulations proved its efficiency and sensitivity, but the susceptibility of the method to field inhomogeneities is still not well understood. In this study, we simulated and analyzed the dynamic of three spin-lock preparations and their response to field inhomogeneities in the presence of a resonant oscillating field. We show that the composite spin-lock preparation is more robust against field variations within the double resonance effect. In addition, we tested the capability of the chosen composite spin-lock preparation to recover information about the spectral components of a composite signal. This study sets the bases to move one step further towards the clinical application of MR-based neuronal current imaging.

Functional magnetic resonance imaging (fMRI) is a unique window to the brain, enabling scientists to follow changes in brain activity in response to hormones, ageing, environment, drugs of abuse and other stimuli. There are two features that make fMRI unique when compared with other imaging modalities used in behavioural neuroscience. First, it can be entirely noninvasive: each animal can serve as its own control over the natural course of its life, vital for following neuroadaptation and other developmental processes critical to understanding behaviour. Second, fMRI has the spatial and temporal resolution to observe patterns of neuronal activity across the entire brain in less than a minute. Although fMRI does not have the cellular spatial resolution of immunostaining, nor the millisecond temporal resolution of electrophysiology, synchronised changes in neuronal activity across multiple brain areas seen with functional MRI can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories and emotions for particular behaviours. Thus, fMRI affords a systems approach to the study of the brain, complementing and building from other neurobiological techniques to understand how behaviour is organised across multiple brain regions. In this review, we present a general background to fMRI and the different imaging modalities that can be used in fMRI studies. Included are examples of the application of fMRI in behavioural neuroscience research, along with discussion of the advantages and disadvantages of this technology.

Influence of alternating magnetic field on non-newtonian blood perfusion and transport of nanoparticles in tissues with embedded blood vessel during hyperthermia

Brain activation is known to lead to enhanced perfusion which, together with other physiological changes (e.g. blood volume, oxygen consumption), results in a decreased deoxyhaemoglobin concentration in regions of neuronal activity (Ogawa et al. 1990). This effect is referred to as the BOLD (blood oxygenation level dependent) effect and is by far the most commonly used method for fMRI studies and manifests itself as a slight increase in the strongly T2* (or T2) weighted MR images used for fMRI studies. BOLD signal changes occur both in the tissue (extra-vascular BOLD effect) and within the vasculature (intra-vascular BOLD effect). Non-BOLD related intra-vascular signal changes can also occur within larger vessels due to in-flow effects.

Functional magnetic resonance imaging at 0.2 Tesla

Magnetic resonance imaging of meniscal degeneration in asymptomatic knees

The use of personal monitors for the assessment of exposure to radiofrequency fields and radiation in potential future epidemiological studies of occupationally exposed populations has been investigated. Data loggers have been developed for use with a commercially available personal monitor and these allowed personal exposure records consisting of time-tagged measurements of electric and magnetic field strength to be accrued over extended periods of the working day. The instrumentation was worn by workers carrying out tasks representative of some of their typical daily activities at a variety of radio sites. The results indicated significant differences in the exposures of workers in various RF environments. A number of measures of exposure have been examined with a view to assessing possible exposure metrics for epidemiological studies. There was generally a good correlation between a given measure of electric field strength and the same measure of magnetic field strength.

Recent observations of the neutral Galactic interstellar medium showed that filamentary structures of H i clouds are aligned with the interstellar magnetic field. Many interesting applications are proposed based on the alignment, such as measurement of magnetic field strength through the Chandrasekhar–Fermi method and removal of foreground dust emissions for the detection of inflationary polarized emission in the cosmic microwave background radiation. However, the physical origin of the alignment remains to be explained. To understand the mechanism, we examine the formation of H i clouds triggered by shock compression of the diffuse warm neutral medium using three-dimensional magnetohydrodynamic simulations. We show that the shock-compressed medium of density cm−3 evolves into H i clouds with cm−3 via thermal instability consistent with previous studies. We apply a machine vision transformation developed by Clark et al. to the simulated column density structures to measure angle correlation between filamentary structures of H i clouds and magnetic field. We find that the orientation of H i filaments depends on the environmental turbulent velocity field, particularly on the strength of shear strain in the direction of the magnetic field, which is controlled by the angle between the shock propagation direction and upstream magnetic field. When the strain along the magnetic field is weak, filamentary components of H i clouds lie perpendicular to the magnetic field. However, the filaments have come to align with the magnetic field, if we enhance the turbulent strain along the magnetic field or if we set turbulence in the preshock medium.

We investigate the electron states and optical absorption in square- and hexagonal-shaped two-dimensional (2D) HgTe quantum dots and quantum rings in the presence of a perpendicular magnetic field. The electronic structure is modeled by means of the $s{p}^{3}{d}^{5}{s}^{*}$ tight-binding method within the nearest-neighbor approximation. Both bulklike and edge states appear in the energy spectrum. The bulklike states in quantum rings exhibit Aharonov-Bohm oscillations in magnetic field, whereas no such oscillations are found in quantum dots, which is ascribed to the different topology of the two systems. When magnetic field varies, all the edge states in square quantum dots appear as quasibands composed of almost fully flat levels, whereas some edge states in quantum rings are found to oscillate with magnetic field. However, the edge states in hexagonal quantum dots are localized like in rings. The absorption spectra of all the structures consist of numerous absorption lines, which substantially overlap even for small line broadening. The absorption lines in the infrared are found to originate from transitions between edge states. It is shown that the magnetic field can be used to efficiently tune the optical absorption of HgTe 2D quantum dot and quantum ring systems.

Zeeman splitting of conduction band in HgTe quantum wells near the Dirac point

Measurement of magnetic fields in dense molecular clouds is essential for understanding the fragmentation process prior to star formation. Radio interferometric observations of CCS 22.3 GHz emission, from the starless core TMC-1C, have been carried out with the Karl G. Jansky Very Large Array to search for Zeeman splitting of the line in order to constrain the magnetic field strength. Toward a region offset from the dust peak, we report a detection of the Zeeman splitting of the CCS 21–10 transition, with an inferred magnetic field of ∼2 mG. If we interpret the dust peak to be the core of TMC-1C, and the region where we have made a detection of the magnetic field to be the envelope, then our observed value for the magnetic field is consistent with a subcritical mass-to-flux ratio envelope around a core with supercritical mass-to-flux ratio. The ambipolar diffusion time-scale for the formation of the core is consistent with the relevant time-scale based on chemical modelling of the TMC-1C core. This work demonstrates the potential of deep CCS observation to carry out future measurements of magnetic field strengths in dense molecular clouds and, in turn, understand the role of the magnetic field in star formation.

In the present paper, by using the method of the effective Hamiltonian, we study Bloch's electrons in the presence of a uniform electric and magnetic field. We show that all wave functions are periodic in time. This result is the fully quantum-mechanical analog of the well-known Bloch oscillations predicted by quasi-classical dynamics. Furthermore, using the githt-binding method for a single band of an electron in a simple cubic lattice in the presence of a uniform electric and magnetic field, we obtain an equation of Harper's form. From this equation we take exactly the points at which Bragg reflection occurs for the subbands of Landau.

Very few spacecraft observations of magnetic Pc 3–4 pulsations at L < 4 have been reported in the past despite the fact that Pc 3–4 pulsations are a common occurrence on the ground at L < 4. In order to fill this observational gap in the magnetosphere, we have initiated an analysis of magnetic field data acquired by the AMPTE CCE spacecraft. A procedure has been developed to reduce the artificial modulation and noise in magnetic field measurements, with the resultant data useful for studying magnetic pulsations with amplitude less than 1 nT in the 1000 nT (L ∼ 3) background and periods longer than 12 s. From a survey of 41 CCE passes in L = 2–6 and in the postnoon sector, we find that the most commonly observed pulsations in this region are the east‐west oscillations of magnetic field with clearly L‐dependent frequencies. These are identified to be the toroidal mode standing Alfvén waves. From a comparison of CCE and GOES 5 magnetic field measurements there is evidence that the resonances can be excited by the same mechanism as those previously found at geostationary orbit or beyond. Abrupt changes in resonance frequency, as a function of L, are not usually seen, supporting previous reports that the mass or number density profile on the dayside does not have a steep plasmapause structure.

Taking into consideration the Coulomb collisions, a kinetic wave equation for homogeneous plasma potential oscillations in magnetic fields is derived. It is shown that, if allowance is made for collisions, this removes the divergences in the matrix elements describing nonlinear wave interactions. The nonlinear stage of kinetic instability (Buneman instability) excited in non-isothermal plasma (Ti>>Te) by current flowing along the magnetic field, is discussed.