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The design and development of dedicated radiofrequency (RF) coils is a fundamental task to maximize the signal-to-noise ratio (SNR) in nuclear magnetic resonance (NMR) applications. Coil resistance reduces the SNR and should be minimized by employing conductors of appropriate shape and cross section. At RF, the conductor resistance is increased due...
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Flexible form-fitting radiofrequency coils provide high signal-to-noise ratio during MRI, and in array configuration large anatomical areas of interest can be covered. We propose a modular system-"ModFlex"-of flexible lightweight 4-channel coaxial coil arrays for 3T MRI. We investigated the performance difference between commercial reference coils...
Citations
... e noise voltage (V N ) was calculated using equations (7)-(10) [1,27]. e sample noise (R S ) was calculated using equation (7) with σ being the sample conductivity and the electric field per unit current. ...
Purpose:
Although full-wave simulations could be used to aid in RF coil design, the algorithms may be too slow for an iterative optimization algorithm. If quasistatic simulations are accurate within the design tolerance, then their use could reduce simulation time by orders of magnitude compared to full-wave simulations. This paper examines the accuracy of quasistatic and full-wave simulations at 3 Tesla.
Methods:
Three sets of eight coils ranging from 3-10 cm (24 total) were used to measure SNR on three phantoms with conductivities of 0.3, 0.6, and 0.9 S/m. The phantom conductivities were chosen to represent those typically found in human tissues. The range of coil element sizes represents the sizes of coil elements seen in typical coil designs. SNR was determined using the magnetic and electric fields calculated by quasistatic and full-wave simulations. Each simulated SNR dataset was scaled to minimize the root mean squared error (RMSE) when compared against measured SNR data. In addition, the noise values calculated by each simulation were compared against benchtop measured noise values.
Results:
The RMSE was 0.285 and 0.087 for the quasistatic and full-wave simulations, respectively. The maximum and minimum quotient values, when taking the ratio of simulated to measured SNR values, were 1.69 and 0.20 for the quasistatic simulations and 1.29 and 0.75 for the full-wave simulations, respectively. The ratio ranges, for the calculated quasistatic and full-wave total noise values compared to benchtop measured noise values, were 0.83-1.06 and 0.27-3.02, respectively.
Conclusions:
Full-wave simulations were on average 3x more accurate than the quasistatic simulations. Full-wave simulations were more accurate in characterizing the wave effects within the sample, though they were not able to fully account for the skin effect when calculating coil noise.
... In connection with this technology, different concepts have been proposed, which can be classified into two methods: The magnetic induction method (MIM) and the magnetic resonance method (MRM) [1,3,4,6,8,[14][15][16]. The MIM transmits electric power using a magnetic field induced in a coil [8,14]. ...
... Presently, the MIM is applied for the wireless charging of portable devices and is also used for the wireless charging of some electric vehicles [8,14]. The MRM transfers energy using the resonance phenomenon between coils [1,3,4,6,[14][15][16]. The MRM is similar to the MIM in that the magnetic fields generated from the current flowing through the primary coil pass through the secondary coil to induce the induction current, but the resonance frequency of the primary coil and the resonance frequency of the secondary coil are the same. ...
... However, in order to ensure high Q, the size of each coil must be large compared to the MIM systems. Such a MRM can be applied to various fields, compared to the MIM [14][15][16]. ...
In this study, a wireless power transmission (WPT) system for high power was developed to supply the wirelessly powered transfer cart for a clean environment (such as liquid crystal display (LCD), semiconductor, and flat panel display (FPD) device industries) to improve the cleanliness of related industrial production lines and save energy. The power transmission method of WPT and the core design were optimized, and a shortened track was fabricated to enable WPT via short power lines for diverse applications in a small space-constrained workshop. In realizing the shortened Litz wire system, the amount of heat generated increased due to the increased resistance in the system, and efforts were made to improve the thermal performance. A simple approach was also proposed to estimate the skin depth caused by the skin effects in a cable made up of multiple strands of multiple wires, validated through thermal analysis by using ANSYS software in terms of heat generation by an electric field. Structure designs were implemented to improve the heat transfer performance, and the experimental results of WPT systems at a power level of 21.54 kW demonstrate that the power transfer distance of WPT was above 15 mm with a charging efficiency above 83.24%.
... This estimation only considers the "classical" skin effect and neglects the lateral skin effect contributions for flat strip conductors, which tend to dominate especially at higher frequencies [24][25][26]. In addition, the conductor losses of multiturn and multi-loop coils can be increased due to the proximity effect [27], which constrains the current distribution to an even smaller region than the skin effect. ...
Magnetic resonance imaging (MRI) is a major imaging modality, giving access to anatomical and functional information with high diagnostic value. To achieve high-quality images, optimization of the radio-frequency coil that detects the MR signal is of utmost importance. A widely applied strategy is to use arrays of small coils in parallel on MR scanners equipped with multiple receive channels that achieve high local detection sensitivity over an extended lateral coverage while allowing for accelerated acquisition and SNR optimization by proper signal weighting of the channels. However, the development of high-density coil arrays gives rise to several challenges due to the increased complexity with respect to mutual decoupling as well as electronic circuitry required for coil interfacing. In this work, we investigate a novel single-element coil design composed of small loops in series, referred to as “multi-loop coil (MLC).” The MLC concept exploits the high sensitivity of small coils while reducing sample induced noise together with an extended field of view, similar to arrays. The expected sensitivity improvement using the MLC principle is first roughly estimated using analytical formulae. The proof of concept is then established through fullwave 3D electromagnetic simulations and validated by B1 mapping in MR experiments on phantom. Investigations were performed using two MLCs, each composed of 19 loops, targeting MRI at high (3 T) and at ultra-high field strength (7 T). The 3 T and 7 T MLCs have an overall diameter of 12 and 6 cm, respectively. For all investigated MLCs, we demonstrate a sensitivity improvement as compared to single loop coils. For small distances inside the sample, i.e., close to the coil, a sensitivity gain by a factor between 2 and 4 was obtained experimentally depending on the set-up. Further away inside the sample, the performance of MLCs is comparable to single loop coils. The MLC principle brings additional degrees of freedom for coil design and sensitivity optimization and appears advantageous for the development of single coils but also individual elements of arrays, especially for applications with a larger area and shallow target depth, such as skin imaging or high-resolution MRI of brain slices.
... However, strip conductors provide higher losses with respect to circular wire, due to reduced current density homogeneity in their cross-sectional section, which affect RF coil's overall performance in the frequency range routinely used in MR clinical scanners. Therefore, the total coil resistance for a Helmholtz coil constituted by wire conductors can be calculated with Eq. (4), while for strip coil it can be evaluated by taking into account both classical and lateral skin effects [17]. ...
Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are non-invasive techniques for tissue characterization. MRI/MRS in small phantoms with a clinical magnetic resonance scanner requires the design and development of dedicated radiofrequency coils. This paper describes the simulation, design, and application of a ¹H transmit/receive Helmholtz coil, suitable for MRI/MRS studies in small phantoms with a clinical 3T scanner. Coil inductance and resistance were analytically calculated by taking into account the conductors cross geometry while magnetic field and sample-induced resistance were calculated with magnetostatic approaches. Finally, the coil sensitivity was measured with the perturbing sphere method. Successively, a coil prototype was built and tested on the workbench and by acquisition of MRI and MRS data. Results show that such coil could provide a low cost and easy to build device for MRI/MRS experiments with a clinical scanner in small specimens.
... The underlying mechanism is related to the so-called "skin effect." [15] The current becomes increasingly concentrated toward the outer surface or "skin" as the frequency of the current flowing through a conductor, such as the solenoid coil, increases. For copper, the skin depth is 0.206 mm at 100 kHz, rising to 0.065 mm at 1 MHz. ...
In this chapter, the basic elements of the usual instrumentation for characterizing materials for magnetic hyperthermia and for applying this therapy will be explained, with a special emphasis on coil design and temperature measurement. First, the importance of the shape of the coil and its cooling is discussed considering the dimension of the sample, the field intensity and its geometry, and whether the exploitation of the magnetic field takes place inside or outside the coil. Then, some temperature measurement methods are described with a special attention to the harmonics of the magnetization of magnetic nanoparticles (MNPs) as a thermal property. Following, the two main methodologies for specific absorption rate (SAR) determination—the magnetic and the calorimetric method, the last one in its adiabatic and nonadiabatic configurations—and the instrumentation used for both are also explained.
... The flat strip surface conductor in Fig. 1a and three parallel round wires in Fig. 1b are set with the same width of 20 mm. The current density distribution on the flat strip calculated by the numerical solution, as shown in Fig. 1a, was consistent with the theoretical calculation, using an exact formula given by Giovannetti et al. [9]. In addition, Fig. 1a shows that the current density distribution on flat strip surface conductors is not uniform and mainly distributed at the marginal division. ...
Copper foil has been widely employed in conventional radio frequency (RF) birdcage coils for magnetic resonance imaging (MRI). However, for ultrahigh-field (UHF) MRI, current density distribution on the copper foil is concentrated on the surface and the edge due to proximity effect. This increases the effective resistance and distorts the circumferential sinusoidal current distribution on the birdcage coils, resulting in low signal-to-noise ratio (SNR) and inhomogeneous distribution of RF magnetic (B1) field. In this context, multiple parallel round wires were proposed as legs of a birdcage coil to optimize current density distribution and to improve the SNR and the B1 field homogeneity. The design was compared with three conventional birdcage coils with different width flat strip surface legs for a 9.4 T (T) MRI system, e.g., narrow-leg birdcage coil (NL), medium-leg birdcage coil (ML), broad-leg birdcage coil (BL) and the multiple parallel round wire-leg birdcage coil (WL). Studies were carried out in in vitro saline phantom as well as in vivo mouse brain. WL showed higher coil quality factor Q and more homogeneous B1 field distribution compared to the other three conventional birdcage coils. Furthermore, WL showed 12, 10 and 13% SNR increase, respectively, compared to NL, ML and BL. It was proposed that conductor’s shape optimization could be an effective approach to improve RF coil performance for UHF MRI.
... A good summary of the different methods for estimating conductor losses in RF coils for NMR applications is included in a recent review. 10 However, no closed-form expression for strip conductors' resistance, taking into account both classical and lateral skin effects, is available. ...
... The analyzer was set in averaging mode (64 averages) for improving measurements sensitivity, and its resolution was 10 mO. Tables 1 and 2 show, respectively, circular wire and strip coil simulation results obtained by HFSS FEM at different frequencies and a comparison against previous results available in the literature for the circular wire 5 and strip 10,11,13 coil. ...
... Conversely, R irr for strip coil was approximately the same of the circular wire coil. If we compare the results to the "hybrid" approach, 10,11,13 we can conclude that HFSS predicted the losses within the strip conductors with a relative difference below 12.5% up to 85.2 MHz, whereas this difference increased to 23% at 127.8 MHz. However, it is important to underline that strip conductor resistances estimated in Refs [11,13] have been obtained with the approximation that the term R extra (see Equation 12) was the same for both strip and circular wire coils, by neglecting differences in capacitor and soldering losses. ...
An accurate coil design is a fundamental task to maximize signal-to-noise ratio in magnetic resonance applications. Coil design techniques take advantage of computer simulations especially when coil size is comparable to the radiofrequency (RF) wavelength. In particular, the estimation of the losses within the conductors as well as the radiative losses, both as a function of frequency, is instrumental to a complete coil performance characterization. However, the cross-sectional shape of the conductors strongly affects the radiofrequency coil's performance, especially at those frequencies where conductor losses represent the dominant power dissipation mechanism. Indeed, at radiofrequencies, the current flowing in the conductor is distributed in the proximity of its surface instead of being uniformly distributed over the cross section; it follows that an accurate conductor losses estimation can be performed only in the case of wire conductors by using analytical formulations. For strip conductors, although different theoretical approaches have been proposed in literature by taking into account the losses, no closed-form expression for conductors resistance is available which takes into account both classical and lateral skin effects. In this work, finite element method (FEM) simulations have been performed for estimating conductor and radiative losses in planar surface loops made of strips and circular wires; the results have been compared against analytical formulations and literature data. Workbench tests performed on two circular coil prototypes, the first one constituted by a strip and the second one by circular wire conductors, tuned at 63.9 MHz and 127.8 MHz, showed a good agreement with FEM simulations.
