Magnetoinductive metasurface of capacitively-loaded split rings for local field homogenization in a 7 T MRI birdcage: A simulation study.

The transmit field B1+ in a 7 T birdcage is inherently inhomogeneous due to the effects of wavelengths on tissue. This work investigates the homogenization of this field through metasurfaces that consist of a two-dimensional planar array of capacitively loaded conducting rings. The metasurfaces are placed in the intermediate space between the head and the birdcage on either side of the head. The periodical structure of this type of metasurface supports magnetoinductive waves because of the mutual inductive coupling existing between the elements of the array. The analysis takes advantage of this coupling and exploits the excitation of a standing magnetoinductive wave across the arrays, which creates a strong local field that contributes to locally homogenize the field of the birdcage. The presence of the arrays does not detune the birdcage, so that they can be used with commercial birdcages that operate both to transmit and to receive.

Metasurfaces of capacitively loaded metallic rings for magnetic resonance imaging surface coils.

This work investigates the use of a metasurface made up of a two-dimensional array of capacitively loaded metallic rings to enhance the signal-to-noise ratio of magnetic resonance imaging surface coils and to tailor the magnetic near-field radio frequency pattern of the coils. It is found that the signal-to-noise ratio is increased if the coupling between the capacitively loaded metallic rings in the array is increased. The input resistance and the radiofrequency magnetic field of the metasurface loaded coil are numerically analyzed by means of an efficient algorithm termed the discrete model to determine the signal-to-noise ratio. Standing surface waves or magnetoinductive waves supported by the metasurface introduce resonances in the frequency dependence of the input resistance. The signal-to-noise ratio is found to be optimal at the frequency corresponding to a local minimum existing between these resonances.The discrete model is used in an optimization procedure to fit the structural parameters of a metasurface to enhance the signal-to-noise ratio at the frequency corresponding to this local minimum in the input resistance. It is found that the signal-to-noise ratio can be greatly improved if the mutual coupling between the capacitively loaded metallic rings of the array is made stronger by bringing them closer or by using rings of squared shape instead of circular. These conclusions derived from the numerical results provided by the discrete model are double-checked by means of numerical simulations provided by the commercial electromagnetic solver Simulia CST and by experimental results. Numerical results provided by CST are also shown to demonstrate that the surface impedance of the array of elements can be adjusted to provide a more homogeneous magnetic near-field radio frequency pattern that ultimately leads to a more uniform magnetic resonance image at a desired slice. This is achieved by preventing the reflection of propagating magnetoinductive waves at the edges of the array by matching the elements arranged at the edges of the array with capacitors of suitable value.

The Lorentz force on ions in membrane channels of neurons as a mechanism for transcranial static magnetic stimulation.

Transcranial static magnetic stimulation is a novel noninvasive method of reduction of the cortical excitability in certain neurological diseases that makes use of static magnetic fields generated by permanent magnets. By contrast, ordinary transcranial magnetic stimulation makes use of pulsed magnetic fields generated by strong currents. Whereas the physical principle underlying ordinary transcranial magnetic stimulation is well known, that is, the Faraday´s law, the physical mechanism that explains the interaction between neurons and static magnetic fields in transcranial static magnetic stimulation remains unclear. In the present work, it is discussed the possibility that this mechanism might be the Lorentz force exerted on the ions flowing along the membrane channels of neurons. The overall effect of the static magnetic field would be to introduce an additional friction between the ions and the walls of the membrane channels, thus reducing its conductance. Calculations performed by using a Hodgkin-Huxley model demonstrate that even a slight reduction of the conductance of the membrane channels can lead to the suppression of the action potential, thus inhibiting neuronal activity.

Classical Physics and the Bounds of Quantum Correlations.

A unifying principle explaining the numerical bounds of quantum correlations remains elusive, despite the efforts devoted to identifying it. Here, we show that these bounds are indeed not exclusive to quantum theory: for any abstract correlation scenario with compatible measurements, models based on classical waves produce probability distributions indistinguishable from those of quantum theory and, therefore, share the same bounds. We demonstrate this finding by implementing classical microwaves that propagate along meter-size transmission-line circuits and reproduce the probabilities of three emblematic quantum experiments. Our results show that the "quantum" bounds would also occur in a classical universe without quanta. The implications of this observation are discussed.

Analysis of the noise correlation in MRI coil arrays loaded with metamaterial magnetoinductive lenses.

A numerical method is shown for calculating the noise correlation coefficient in arrays of magnetic resonance imaging (MRI) coils loaded with capacitively-loaded ring metamaterial lenses, and in the presence of a conducting half-space resembling a sample. This numerical method is validated by comparison with experimental results obtained in two different experimental procedures for double check: noise resistance measurements with a network analyzer and noise correlation measurements in an MRI system. It is found that, for practical array configurations such as overlapping coils or capacitively-decoupled coils, the noise correlation coefficient turns negative for coils loaded with metamaterial lenses. In particular, the analysis is carried out with metamaterial structures known as magnetoinductive lenses, which have been demonstrated in previous works to improve the signal-to-noise ratio of MRI coils. Results are also shown to demonstrate that negative noise correlations have as an effect the improvement of the g-factor in coil arrays for parallel MRI.

Metamaterial magnetoinductive lens performance as a function of field strength.

Metamaterials are artificial composites that exhibit exotic electromagnetic properties, as the ability of metamaterial slabs to behave like lenses with sub-wavelength resolution for the electric or the magnetic field. In previous works, the authors investigated magnetic resonance imaging (MRI) applications of metamaterial slabs that behave like lenses for the radiofrequency magnetic field. In particular, the authors investigated the ability of MRI metamaterial lenses to increase the signal-to-noise ratio (SNR) of surface coils, and to localize the field of view (FOV) of the coils, which is of interest for parallel MRI (pMRI) applications. A metamaterial lens placed between a surface coil and the tissue enhances the sensitivity of the coil. Although the metamaterial lens introduces losses which add to the losses of the tissue, the enhancement of the sensitivity can compensate these additional losses and the SNR of the coil is increased. In a previous work, an optimization procedure was followed to find a metamaterial structure with minimum losses that will maximize the SNR. This structure was termed magnetoinductive (MI) lens by the authors. The properties of surface coils in the presence of MI lenses were investigated in previous works at the proton frequency of 1.5 T systems. The different frequency dependence of the losses in both the MI lenses and the tissue encouraged us to investigate the performance of MI lenses at different frequencies. Thus, in the present work, the SNR and the pMRI ability of MI lenses are investigated as a function of field strength. A numerical analysis is carried out with an algorithm developed by the authors to predict the SNR behavior of a surface coil loaded with a MI lens at the proton frequencies of 0.5 T, 1.5 T and 3 T systems. The results show that, at 0.5 T, there is a gain in the SNR for short distances, but the SNR is highly degraded at deeper distances. However, at 1.5 T and 3T, the MI lenses provide a gain in the SNR up to a certain penetration depth, which is deeper at 3T, and do not degrade the SNR at deeper distances. These numerical results are checked by means of an experiment. Moreover, a second experiment developed with two-channel arrays of surface coils loaded with MI lenses shows that the pMRI ability of the lenses also improves from 1.5 T to 3 T. This improvement was quantified by means of the calculation of the GRAPPA g-factor.

A broadside-split-ring resonator-based coil for MRI at 7 T.

A coil design termed as broadside-coupled loop (BCL) coil and based on the broadside-coupled split ring resonator (BC-SRR) is proposed as an alternative to a conventional loop design at 7T. The BCL coil has an inherent uniform current which assures the rotational symmetry of the radio-frequency field around the coil axis. A comparative analysis of the signal-to-noise ratio provided by BCL coils and conventional coils has been carried out by means of numerical simulations and experiments in a 7T whole body system.

On the applications of micror=-1 metamaterial lenses for magnetic resonance imaging.

In this work some possible applications of negative permeability magnetic metamaterial lenses for magnetic resonance imaging (MRI) are analyzed. It is shown that using magnetic metamaterials lenses it is possible to manipulate the spatial distribution of the radio-frequency (RF) field used in MR systems and, under some circumstances, improve the sensitivity of surface coils. Furthermore a collimation of the RF field, phenomenon that may find application in parallel imaging, is presented. MR images of real tissues are shown in order to prove the suitability of the theoretical analysis for practical applications.