Design considerations and coil comparisons for 7 T brain imaging

The development of 300 MHz radio-frequency (RF) head coils analogous to those used at field strengths of 1.5 and 3 T is complicated by increased dissipative losses in conductive tissue, effects arising from the short RF wavelength in biological tissue (about 13 cm at 300 MHz), and the constraints imposed by the use of head gradient sets desirable for mitigating increased static field susceptibility effects. In this study, five RF head coils were constructed and tested on a 7 T scanner including 2 TEM designs, 2 birdcage designs and a local receive-only array. Signal-to-noise ratio, coil reception profiles and interactions between the coil and dielectric head were examined. Particular attention was placed on the coil’s reception in the neck and shoulders, where the head gradient is unable to spatially encode the image. With the use of conductive shields and distributed capacitance, all of the coil designs could be made to image effectively at high field, but each design was found to have subtle differences in field distribution, interaction with the dielectric boundary conditions of the head and fringe fields in the neck and shoulders. In particular, the birdcage and array coils were found to have reducedB ₁ reception field profiles in the neck and shoulders which helped reduce signal detection outside the linear region of the head gradient coil. Although the TEM coils exhibited higher signal detection in the neck and shoulders, all the coils picked up enough signal from these regions to produce artifacts in the brain. These artifacts could be mitigated through use of a conductive shield or by small local dephasing shims sewn into the shoulders of a jacket worn by the subject. Although homogeneous in low-dielectric-constant phantoms, the volume coil’sB ₁ profile was strongly peaked in the center of the head, rendering them spatially complementary to that observed in the surface coil array. The image profile of the surface coil was found to be less dramatically changed from patterns observed at lower field strength. Its dielectric brightening pattern was found to depend on the orientation of the coil with respect to the head.     

An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (≥1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B₁ field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.     

The effects of coupled B1 fields in B1 encoded TRASE MRI - A simulation study

Transmit Array Spatial Encoding (TRASE) is a novel MRI technique that encodes spatial information by introducing phase gradients in the transmit RF (B₁) magnetic field. Since TRASE relies on the use of multiple RF fields (B₁ fields with different phase gradients) for k-space traversal, a TRASE pulse sequence requires RF pulses that are produced by switching between the transmit coils (B₁ fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B₁ fields that impair the spatial encoding. Therefore, TRASE is sensitive to B₁ field perturbations arising from inductive coupling among the RF transmit coils and any B₁ field isolation (coil decoupling) technique requires an understanding of the effects of the B₁ field interactions. The purpose of this study was to investigate the effects of B₁ field coupling using Bloch equation based simulations and to determine the acceptable level of B₁ field interactions for 2D TRASE imaging. The simulations show that 2D TRASE MRI (using a 3-coil setup) displays ideal performance for pairwise coupling constant lower than k = 0.01 while having acceptable performance up to k = 0.1. This translates into S₁₂ measurements of range ~(− 50 dB to −30 dB) required for successful 2D TRASE MRI in this study. This result is of crucial importance for designers of practical TRASE transmit array systems.     

The Microwave Spectrum of m-Tolunitrile: Methyl Internal Rotation and N Nuclear Quadrupole Coupling

The microwave spectrum of m-tolunitrile (3-methylbenzonitrile, m-C₆H₄CH₃CN) has been investigated in the frequency range from 1 to 4 and 8 to 26.5 GHz. The spectra in the two lowest states of internal methyl rotation (m = 0, ±1) were recorded by means of pulsed molecular beam Fourier transform microwave (MB-FTMW) spectrometers. The interpretation of the spectra was based on an asymmetric frame–symmetric top Hamiltonian with inclusion of centrifugal distortion terms and first-order contributions from ¹⁴N nuclear quadrupole coupling. A least-squares analysis yielded the rotational constants A = 3295.9103(10) MHz, B = 1199.1188(2) MHz, C = 883.9223(1) MHz, all elements of the nuclear quadrupole coupling tensor χ_aa = −3.626(1) MHz, χ_bb = 1.684(1) MHz, χ_cc = 1.943(1) MHz, and χ_ab = −1.870(3) MHz, as well as the threefold barrier to internal rotation, V₃ = 14.2 cm⁻¹, and the angle between the internal rotor axis and the principal moment of inertia a axis, θ = 42.66°, using fixed values for the sixfold barrier term V₆ (−11 cm⁻¹) and the moment of inertia of the methyl top I_α (3.16 u Ų).     

Coils for ingestible capsules: Near-field magnetic induction link

Monolayer- and compact-multilayer-stacked ingestible TX coils are investigated for ingestible capsule systems. The inductive link through the human body is modeled. The efficiency of the near-field magnetic induction link budget is evaluated in the air, in the homogeneous human body and in the three-layer human body. The variations of the position and the orientation of the TX capsule coil are taken into account to evaluate the coupling response between the TX ingestible and RX on-body coils.     

Polarization spectroscopy of ion-pair states of ICl

Molecular constants for the E0⁺(³P₂) and 1(³P₂) ion-pair states of ICl vapor have been determined using sequential two-photon polarization-labeling spectroscopy. The two states are coupled by a heterogeneous perturbation which is analyzed in some detail for low-lying vibrational levels of 1(³P₂). The I³⁵Cl potential constants for the 1(³P₂) state and the rotation-vibration constants for the set of f sublevels—i.e., the constants unaffected by coupling with the E state—are (in cm⁻¹) 1(³P₂): Y_0,0= 39103.814(32), Y_1,0= 170.213(15), Y_2,0= −0.4528(22), Y_3,0= −7.0(12) × 10⁻⁴, Y_4,0= −1.48(24) × 10⁻⁵ and Y_5,0= −6.6(19) × 10⁻⁸, Y^(f)_0,1= 5.6878(17) × 10⁻² Y^(f)_1,1= −2.110(24) × 10⁻⁴, Y^(f)_2,1= −1.23(62) × 10⁻⁷, and Y^(f)_0,1= −3.08(22) × 10⁻⁸Likewise, the I³⁵Cl constants determined for the E 0⁺(³P₂) state are E 0⁺(³P₂: Y_0,0= 39054.38(61), Y_1,0= 166.96(10), Y_2,0 = −0.3995(42), Y_0,1= 5.738(31) × 10⁻², and Y_1,1= −1.67(26) × 10⁻⁴Practical constraints in pumping the sequence E 0⁺ ← B 0⁺ ← × 0⁺ restrict the analysis of the E state to levels v = 9–15. Given the long extrapolation to the equilibrium state the 3σ statistical uncertainties quoted for these constants should be treated with caution.     

Novel optical film sensor design based on p-polarized reflectance

A new scheme of optical film sensor is presented. The sensor is based on p-polarized reflectance, consisting of a sensing coated substrate, is easily optimized for maximum sensitivity in different applications. The resolutions of refractive index n_f, extinction coefficient k_f and thickness h_f of the sensitive films are predicted to be 10⁻⁷, 10⁻⁵ and 10⁻³ nm, respectively. Experimentally, we selected the sol–gel derived SnO₂ films as gas-sensitive films and conducted preliminary gas-sensing test. The results indicate that novel optical film sensor scheme has higher sensitivity, and the detection sensitivity is available to 10⁻¹ ppm on the condition of optimum optical parameters and incident angle.     

Preparation of CoFe alloy nanoparticles with tunable electromagnetic wave absorption performance

CoFe alloy nanoparticles with different particle sizes were successfully prepared by hydrogen-thermal reduction of CoFe₂O₄ microspheres at different temperatures. It was found that the real part of the permeability of the CoFe alloy nanoparticles increased with the decrease of the particle size, and accordingly the reflection loss (RL) peak moved towards the low-frequency region. The maximum RL of the coating based on CoFe alloy nanoparticles exceeded −23 dB, and a wide effective absorption band width (RL≤−10 dB) of about 6 GHz was simultaneously achieved. The EMA performance was confirmed to be tunable by the control of the absorber's particle size, and thus a new clue for the design of the EMA applications was supplied.     

The microwave spectrum and rotational structure of the Δ and Σ electronic states of sulfur monoxide

The spectrum of ¹Δ and ³Σ SO has been studied in the millimeter and submillimeter region of the microwave spectrum. This expanded spectral coverage has made possible the measurement of twenty-two previously unobserved transitions, several of which are necessary for an accurate calculation of the energy levels. As a result, it is now possible to calculate the rotational transitions between energy levels for which J ≤ 10 in both the ground ³Σ electronic state and the excited ¹Δ electronic state to an accuracy comparable to that of the microwave measurements themselves ( 1 MHz). Among the molecular constants calculated are; for the ¹Δ state: B₀ = 21 295.405 MHz, D₀ = 0.0350 MHz, ω_e = 1108 cm⁻¹, and r₀ = 1.4920 Å; and for the ³Σ state: B₀ = 21 523.561 MHz, D₀ = 0.03399 MHz, λ₀ = 158 254.387 MHz, γ₀ = −168.342 MHz, ₀ = 0.305 MHz, r₀ = 1.4840 Å, B_e = 21 609.552 MHz, λ_e = 157 779.2 MHz, and r_e = 1.4811 Å.     

MRI methodological development of intervertebral disc degeneration: a rabbit in vivo study at 9.4 T

Intervertebral disc (IVD) degeneration is a complex process characterized by biochemical and structural changes in both the nucleus pulposus and the anulus fibrosus. In this study, we were able to obtain in vivo magnetic resonance (MR) images of the rabbit spine, with several MR imaging (MRI) contrasts (ρ, T₁ and T₂). We quantified several parameters (T₂, apparent diffusion coefficient, disc height and area) to differentiate between healthy and degenerative IVDs and to characterize the degeneration process. To our knowledge, there has not been any previous in vivo study of rabbit IVDs at high-field MRI (9.4 T).A custom radio frequency (RF) coil for 9.4 T was designed to match rabbit IVD morphology, to study the degeneration in vivo on a model of human lumbar disease. Our new probe, a custom half-birdcage-type coil, obtains the necessary exploration depth while meeting the requirements for signal homogeneity and sensitivity of the study. This design addresses some of the difficulties with constructing RF coils at high field strengths.     

Infrared and microwave spectra, molecular conformation and electric dipole moment of methyl thiolformate

The microwave spectrum (41-10 GHz) and the infrared spectrum (4000-50 cm⁻¹) of methyl thiolformate have been obtained and analyzed. The spectra are consistent with a single molecular conformation having a planar array of heavy atoms and with the alkyl group cis to the carbonyl group. The measured rotational constants are: A, 11042.22 MHz; B, 5118.27 MHz; C, 3562.03 MHz (κ = −0.5839). No internal rotation doublets were observed in the microwave spectrum for the ground vibrational state, which implies that the barrier hindering internal rotation of the methyl group is either much larger or much smaller than the corresponding value for methyl formate. If the former is true then a lower limit of 10.5 kJ mol⁻¹ may be placed on the barrier height.The dipole moment of methyl thiolformate was measured using the Stark effect to be 1.58 ± 0.05 Debyes (μ_A = 1.52 D; μ_B = 0.43 D) for the vapor, and for dilute solutions in benzene at 295 K the value of 1.6 ± 0.1 D was found from capacitance measurements.SCF computations using minimal basis sets of STO/3G atomic orbitals and extended basis sets of STO/4.31G atomic orbitals have been carried out for methyl thiolformate and methyl formate. Energy differences between rotational isomers and estimates of barrier heights are given together with the calculated dipole moments.     

Development of surface-coil-type resonators consisting of an irradiation and receiver coil system for EPR measurements at 700 MHz

Surface-coil-type resonators (SCRs) consisting of irradiation and receiver coils that are highly isolated from each other were developed for electron paramagnetic resonance (EPR) measurements. Their sample space was open to free space. For these coils, a circular single-turn one-loop receiver coil and a square single-turn twin-loop irradiation coil were fabricated. The transmission lines were set to resonate at about 700 MHz. A phantom (agar, including a nitroxide radical and physiological saline solution) is located on the receiver coil, and the irradiation coil is under the receiver coil. In this condition, the isolation between the receiver and irradiation SCRs was about 40 dB at the resonant frequency. When radiowaves that were divided from the line to the irradiation SCR were applied to the line from the receiver SCR at the appropriate phase and power to cancel the direct coupling between both SCRs, the isolation increased to more than 70 dB. In the conventional SCR, the noise level increased at high incident power. Because such an increase in the noise was not observed in the irradiation-receiver SCR system, high sensitivity at high incident power was obtained.     

Molecular structure, nuclear quadrupole coupling constant and dipole moment of nitrogen trichloride from microwave spectroscopy

The rotational constants of four isotopic species of nitrogen trichloride have been obtained from transitions in the millimeter region. Two r_s structures have been obtained with the following average values of the parameters. rN−C1=1.7535 ± 0.0020 A.The Stark effect of the J = 3 ← 2 transition was analyzed to obtaine the value 0.39 ± 0.01 D for the dipole moment of NCl₃. The measurement of the separation of the two strongest hyperfine components of the J = 2 ← 1 transition yielded the value of −108 ± 3 MHz for the N---Cl bond axis quadrupole coupling constant.     

High permeability and low power loss of Ti and Zn substitution lithium ferrite in high frequency range

The effect of Zn and Ti on the magnetic, power loss and structural properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in step of 0.05)+0.5 wt% Bi₂O_3, prepared by standard ceramic technique, has been investigated. Complex permeability (μ*=μ′−jμ″) has been analyzed at room temperature in frequency range from 1 to 10³ MHz. It was found an enhancement in permeability with Ti and Zn concentration in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ and exhibits the maximum value 106 for x=0.20 sample. Complex permeability of these ferrites exhibits stable frequency response up to 7 MHz beyond which the real part decreases sharply and imaginary part increases to have a peak at the relaxation frequency. Power loss measurements have been carried out in induction condition (B=10 mT) in frequency range of 50 kHz to 3 MHz. Power loss has been found to be quite low with the substitution of Ti and Zn in lithium ferrite.     

Microwave and low-frequency vibrational spectra of bullvalene

The microwave spectrum of bullvalene has been investigated in the region 18–40 GHz. In addition to transitions in the ground vibrational state, transitions arising from five excited vibrational states below 600 cm⁻¹ have also been observed. A combination of microwave intensity measurements and infrared and Raman data has been utilized to assign these vibrations. Three of the vibrations are E-type modes at 241, 355, and 588 cm⁻¹. One is an A₁-type mode at 445 cm⁻¹, and another is an A₂-type at 266 cm⁻¹. The microwave spectrum indicates the presence of a first-order Coriolis interaction for the E modes at 241 and 588 cm⁻¹. The first-order Coriolis coupling constant q = 0.557 MHz for the 241 cm⁻¹ vibration. The spectral results are consistent with C_3v symmetry for bullvalene.     

Millimeter-Wave and High-Resolution Infrared Spectra of Monoisotopic SCSe: Equilibrium, Ground State, and ν1, mν2, and nν3 Rovibrational Parameters

The Fourier transform infrared spectrum of monoisotopic SC⁸⁰Se has been investigated in the ν₂, ν₃, 2ν₂, 2ν₃, and ν₁ regions with a resolution between 3 and 4 × 10⁻³ cm⁻¹. In addition, the millimeter-wave spectrum has been studied in the region 150 to 320 GHz, and ground and ν₂ = 1 excited state transitions have been measured. Ground state constants, B₀ = 2043.285 4(4) MHz and D₀ = 146.53(5) Hz, have been determined from a merge of millimeter-wave data and ground state combination differences spanning J values up to 77 and 143, respectively. The band centers ν₂ = 352.341 075(9) cm⁻¹ and ν₃ = 505.480 06(5)cm⁻¹ have been determined. The rovibrational parameters of numerous overtone and combination levels (ν₁ν^l2₂ν₃) = 02⁰0, 02²0, 03¹0, 03³0, 04⁰0, 04²0, 00⁰2, and 00⁰3 have been obtained from polynomial analyses whose standard deviations ranged from 0.7 to 3.5 × 10⁻⁴ cm⁻¹. The 10⁰0 level, ν_eff 1435.840 cm⁻¹, is anharmonically perturbed by the 04⁰0 level, with an avoided crossing at J = 55, and W₁₂₂₂₂ = 0.963 09(1) cm⁻¹. Transitions to both the upper (E⁺) and lower (E⁻) sublevels of the dyad were observed for 1 ≤ J′ ≤ 117 and 4 ≤ J′ ≤ 171, respectively, and the deperturbed wavenumbers ν₁ = 1435.542 76(2) and 4ν⁰₂ = 1432.725 00(3) cm⁻¹ were derived. Furthermore, a local crossing of the E⁻ and 04²0 levels involving l-type resonance was observed at J = 91.     

Comparison of 12 Quadrature Birdcage Coils with Different Leg Shapes at 9.4 T

The purpose of this study was to analyse the relationship between the radio frequency (RF) coil performance and conductor surface shape for ultra-high field (UHF) magnetic resonance imaging. Twelve different leg-shaped quadrature birdcage coils were modeled and built, e.g., 4 mm-width-leg conventional birdcage coil, 7 mm-width-leg conventional birdcage, 10 mm-width-leg conventional birdcage coil, 13 mm-width-leg conventional birdcage coil, inside arc-shape-leg birdcage coil, outward arc-shape-leg birdcage coil, inside right angle-shape-leg birdcage coil, outward right angle-shape-leg birdcage coil, vertical 4 mm-width-leg vertical birdcage, 6 mm-width-leg vertical birdcage, 8 mm-width-leg vertical birdcage and 10 mm-width-leg vertical birdcage. Studies were carried out in both electromagnetic simulations with finite element method as well as in vitro saline phantom experiments at 9.4 T. Both the results of simulation and experiment showed that conventional birdcage coil produces the highest signal-to-noise ratio (SNR) while the vertical birdcage coil produces the most homogeneous RF magnetic (B ₁) field at UHF. For conventional birdcage coils, as well as the vertical birdcage coils, only the proper width of legs results in the best performance (e.g., B ₁ homogeneous and SNR). For vertical birdcage coils, the wider the leg size, the higher RF magnetic (B ₁) field intensity distribution.     

The design of a body RF coil for low-field open MRI using pseudo electric dipole radiation and simulated annealing

The paper presents a new body RF coil design scheme for a low-field open MRI system. The RF coil is composed of four rectangular loops which are made of wide copper strips located near the surfaces of the bottom and top pole faces of the permanent magnet. The body RF coil has been designed by using the pseudo electric dipole radiation (PEDPR) method with the Metropolis algorithm. In the calculation of the RF fields via the finite difference time domain (FDTD) method, the computational time increases as the RF frequency becomes lower. Moreover, the computational process using the FDTD method takes a very long time when the RF coil is optimized. The optimization requires varying the configuration of the RF coil system and performing successive calculations of field strength and field homogeneity. When we perform these successive calculations, the computational time can be reduced by using the PEDPR method, where the segmented current elements of the RF coil are treated as pseudo electric dipole radiation sources. Because the RF coil is made of wide strips, the variation of the current density on the strip has been considered in the B₁-field calculation. For each configuration of the RF coil system, the current distribution is calculated via circuit analysis, where each copper strip is considered as a parallel combination of current element lines. The preliminary field calculation study by the FDTD method verifies both the circuit analysis method for the current distribution and the PEDPR method for the radiation field strength. The optimization of the RF coil configuration is performed by the Simulated Annealing (SA) process using the Metropolis algorithm. Simulations have been performed for a 10 MHz RF frequency. The optimized RF coil has four rectangular loops of 37 cm × 100 cm with 6.5 cm wide strips which are separated vertically 49 cm and horizontally center-to-center 63 cm. In the 25 cm diameter of spherical volume (DSV), the design results show a good field inhomogeneity of the B₁-field below 0.49 dB (5.8%).     

Characteristics of 980/1550 nm WDM coupler based on planar curved waveguides

A wavelength division multiplexer (WDM) for 980/1550 nm based on planar curved waveguide coupler (CWC) is proposed. Compared with conventional parallel straight waveguide coupler (SWC), this structure has more flexibility with two variable parameters of bending radius R and minimum edge-to-edge spacing d₀, which are the two main parameters for the splitting ratio of coupler and decrease the complexity of device design and fabrication. Based on coupled mode theory (CMT) and waveguide theory, R and d₀ of the WDM CWC are designed to be R=13.28 m and d₀=4.39 μm. The contrast ratio (CR) and insertion loss (IL) for 980 and 1550 nm are CR₁=24.62 dB, CR₂=24.56 dB and IL₁=0.014 dB, IL₂=0.015 dB, respectively. The 3D beam propagation method (BPM) is used to verify the validity of the design result. The influence of R and d₀ variations on the device performance is analyzed. For CR>20 dB, the variation ranges of R and d₀ should be within −0.10 to +0.44 m and −0.05 to +0.02 μm, respectively.     

Speeds of sound and isentropic compressibilities of (2-ethoxyethanol + ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol) binary mixtures at 298.15 K

The speed of sound (u) has been obtained at a frequency of 8.3 MHz in {CH₃CH₂OCH₂CH₂OH + HOCH₂CH₂(OCH₂CH₂)_nOH}for n = 0, 1, 2, and 3 over the whole composition range of studied binary liquid mixtures, at T = 298.15 K. The speed of sound values were combined with those of our previous results for densities and viscosities to obtain isentropic compressibility (κ_s), free volume (V_f), and intermolecular free length (L_f). From all these data excess isentropic compressibility (κ_s^E), excess free volume (V_f^E) and excess intermolecular free length (L_f^E) as well as the deviations of the speed of sound (Δu) were obtained. The results are interpreted in terms of molecular interactions occurring in the solutions.