Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

The simulation and the design of radiofrequency (RF) coils are fundamental tasks to maximize Signal-to-Noise Ratio (SNR) in Magnetic Resonance (MR) applications. The estimation of coil resistance, that is, the losses within the coil conductors, which depends on tuning frequency, allows the prediction of coil performance and data SNR. At RF, the conductor resistance is increased due to the skin effect, which distributes the current primarily near the conductor surface instead of uniformly over the cross section. Moreover, the radiative losses estimation as a function of tuning frequency permits a total coil performance characterization, especially for high-frequency tuned coils when this loss mechanism could be the dominant one. In this work we compared Finite Element Method (FEM) simulations with analytical calculations performed in wire loop RF coils for MR applications. Our results showed that FEM can predict the losses within the coil conductors at 5.7 MHz with a relative difference of <3% compared to analytical calculation, while the relative difference increased to 58% at 127.8 MHz. Concerning the radiative losses, the relative difference between analytical formulation and FEM was lower than 3% at 5.7 MHz, and increasing to 44% at 127.8 MHz. Experimental measurements on a circular coil prototype were also performed at 85.2 MHz and 127.8 MHz, showing a better agreement with FEM simulations than with analytical calculations.

Full-wave electromagnetic simulations have been employed to analyze the design approaches that allow for solving the problems preventing inductive performance of micro-solenoidal RF probes and formation of strong and homogeneous magnetic fields inside probes at the studies of millimeter-range sized objects in 14 T MRI scanners. In particular, the effects of non-uniform coil wrapping on field homogeneity inside extended coils and of partitioning the coils by dielectric separators on coil self-resonances have been investigated. The possibility to utilize tunable C-C matching circuits with the coils and to mitigate the effects of sample insertion on the probe resonance frequency has been demonstrated. Challenges of coil fabrication have been also addressed.

Pole plate in permanent magnet MRI system leads to serious impact on the strength, linearity, and inductance of gradient coils due to the induced current of its pure iron material. In this article, a modified mirror current (MC) model of pole plate based on z gradient coil is established, aiming at improving gradient coil performance. Levenberg-Marquardt (LM) algorithm is applied for the gradient coil optimization with and without constraint of the current density coefficient that determines the coil complexity and structure. With the optimal constraint radius 4.466 and 4.942, the minimum square of gradient field errors are obtained as and for longitudinal and transverse gradient coils, respectively. Optimization results combining linearity, inductance and field error ameliorate all three parameters prominently comparing with the original MC model, which proves the efficiency of the modified MC model and LM optimization algorithm in gradient coil construction to compensate the pole plate effect.

Performing multinuclear experiments requires one or more radiofrequency (RF) coils operating at both the proton and second-nucleus frequencies; however, inductive coupling between coils must be mitigated to retain proton sensitivity and coil tuning stability. The inclusion of trap circuits simplifies placement of multinuclear RF coils while maintaining inter-element isolation. Of the commonly investigated non-proton nuclei, perhaps the most technically demanding is carbon-13, particularly when applying a proton decoupling scheme to improve the resulting spectra. This work presents experimental data for trap circuits withstanding high-power broadband proton decoupling of carbon-13 at 7 T. The advantages and challenges of building trap circuits with various inductor and capacitor components are discussed. Multiple trap designs are evaluated on the bench and utilized on an RF coil at 7 T to detect broadband proton-decoupled carbon-13 spectra from a lipid phantom. A particular trap design, built from a coaxial stub inductor and high-voltage ceramic chip capacitors, is highlighted owing to both its performance and adaptability for planar array coil elements with diverse spatial orientations.

A spectrometer was designed and constructed to facilitate measurements of T₁, T₂, spin echo signal-to-noise, and resonator quality factor, Q, between about 400 and 1000 MHz. Pulse patterns are generated at 250 MHz and mixed with the output from a second source to perform excitation and detection. A cross-loop resonator was constructed in which the same sample could be measured in the same resonator over the full range of frequencies. An air-core, four-coil, water-cooled electromagnet with a large experimental volume was built.

Radiofrequency receiver coils in magnetic resonance (MR) systems are used to pick up the signals emitted by the nuclei with high signal-to-noise ratio (SNR) in a small region of sensitivity. The quality of obtained images strongly depends upon the correct choice of the coils geometry and size. The simplest design of such coils is circular and square loops, both producing in the central region-of-interest a magnetic field perpendicular to the coil plane, with an amplitude that decreases along the coil axis. This work reviews a method for coil SNR model development employing an equivalent electric circuit and applies it for circular and square loop design. Coil inductance and resistance were analitically calculated by taking into account for the conductors cross-geometry and the magnetic field pattern was estimated using Biot–Savart law, while the sample-induced resistance was calculated with a method employing a quasi-static approach. Coil performance prediction permitted to compare circular and square loops and demonstrated that when a simple relationship between loops size is satisfied, the performance of both coils resulted to be very similar in terms of SNR. Since the theoretical approach formulation is largely detailed, this article could be interesting for graduate students and researchers working in the field of MR coil design and development.