Analysis and design of a coupled coaxial line TEM resonator for magnetic resonance imaging

Benahmed, Nadia; Feham, Mohammed; Khelif, M

doi:10.1088/0031-9155/51/8/009

Cited by 6 publications

(3 citation statements)

References 5 publications

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“…At 7 Tesla, the proton Larmor frequency is 297 MHz, which corresponds to a wavelength of 14 cm in tissue. When conventional transmit RF coils (2,3) are used, standing wave patterns with local signal maxima and minima are observed (4,5). The patterns cause an inhomogeneous RF excitation field and thus lead to a spatially variable image contrast that bears the risk to obscure anatomical or pathological details.…”

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confidence: 99%

Coaxial waveguide MRI

Alt

Müller

Umathum

et al. 2011

Magnetic Resonance in Med

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As ultrahigh-field MR imaging systems suffer from the standing wave problems of conventional coil designs, the use of antenna systems that generate travelling waves was suggested. As a modification to the original approach, we propose the use of a coaxial waveguide configuration with interrupted inner conductor. This concept can focus the radiofrequency energy to the desired imaging region in the human body and can operate at different Larmor frequencies without hardware modifications, as it is not limited by a lower cut-off frequency. We assessed the potential of the method with a hardware prototype setup that was loaded with a tissue equivalent phantom and operated with imaging areas of different size. Signal and flip angle distributions within the phantom were analyzed, and imaging at different Larmor frequencies was performed. Results were compared to a finite difference time domain simulation of the setup that additionally provides information on the spatial distribution of the specific absorption rate load. Furthermore, simulation results with a human model (virtual family) are presented. It was found that the proposed method can be used for MRI at multiple frequencies, achieving transmission efficiencies similar to other travelling wave approaches but still suffers from several limitations due to the used mode of wave propagation. Magn Reson Med 67:1173-1182, 2012. V C 2011 Wiley Periodials, Inc.Key words: MRI; travelling wave MRI; radiofrequency electronics; waveguides Ultrahigh-field whole-body MRI systems operating at field strengths of B 0 ! 7 T offer higher signal-to-noise ratio (SNR), which enables enhanced image resolution and acquisition speed, while the higher susceptibility contrast is favorable for functional MRI (1). A technical challenge at high field strengths is the radiofrequency (RF) wavelength in tissue which is-other than at 1.5 T-in the order of the size of the anatomical structures. At 7 Tesla, the proton Larmor frequency is 297 MHz, which corresponds to a wavelength of 14 cm in tissue.When conventional transmit RF coils (2,3) are used, standing wave patterns with local signal maxima and minima are observed (4,5). The patterns cause an inhomogeneous RF excitation field and thus lead to a spatially variable image contrast that bears the risk to obscure anatomical or pathological details. For this reason, currently no large volume coil (i.e., body coil) concepts for ultrahigh-field MRI systems are available. A more homogeneous RF excitation over a large field-of-view can be achieved with RF coils with multiple transmit channels (parallel transmission) (6-9); however, this requires new RF systems with dedicated transmit coils in combination with multiple transmitters and power amplifiers.To avoid the standing wave problems, Brunner et al. (10) suggested the use of propagating or travelling waves for whole-body MRI at high magnetic fields. Using the bore of the magnet as a hollow circular waveguide, they transmitted the RF energy from a patch antenna at the far end of the bore. RF transmi...

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confidence: 99%

Coaxial waveguide MRI

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Müller

Umathum

et al. 2011

Magnetic Resonance in Med

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“…The fundamental assumption for the determination of these matrices is that the distribution of the electromagnetic field is larger in the orthogonal plane to the length of the resonator. Under this quasi-TEM condition electromagnetic field distribution is identical to that resulting from static excitation of the resonator, which allows the determination of the matrices L, C and G from the solution of Laplace's equation [7,8] in the transverse plane of the resonator. In the case where n+1 conductors are surrounded by a homogeneous medium of conductivityσ , permittivity ε and permeability μ , then L, C and G are related by:…”

Section: Theorymentioning

confidence: 99%

Optimization of a Transverse Electromagnetic Resonator for Magnetic Resonance Imaging at High Field of 9.4 T

Bouhmidi

Benahmed

Aliane

et al. 2012

2012 Spring Congress on Engineering and Technology

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In this paper, we have implemented a numerical tool using finite element method (FEM) for designing an antenna for MRI (Magnetic Resonance Imaging) probe operating at high field. The birdcage antenna type is a transverse electromagnetic (TEM) coupled lines resonator, and the developed model in the absence of a biological load is based on the propagation equations of voltages and currents on the coupled lines of the resonator. This numerical tool simulates the frequency response of the reflection coefficient of the RF (Radio Frequency) excitation port of the resonator, which allows us to estimate the level of adaptation of the antenna for imaging small animals at a field of 9.4 T.

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“…Many efforts have been made to analyze the EM parameters of the slotted-tube resonator in order to show the properties of the probe and design an optimum structure [3], [6]. In Ben Ahmed and Feham [7], [8] rigorous analytical expressions for the EM parameters of the shielded STR with a circular cross section have been obtained using the finite element method (FEM) and method of moments (MoM) in two dimensions, but without taking into account the influence of the thickness of the STR.…”

Section: Introductionmentioning

confidence: 99%

Comparing Saddle, Slotted-tube and Parallel-plate Coils for Magnetic Resonance Imaging

Nespor

Bartušek

Dokoupil

2014

Measurement Science Review

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The paper is concerned with a comparison of the properties of RF coils of three configurations for MRI measurements on small animals. In comparison with the classical saddle coil the proposed modification of slotted-tube coil exhibits identical homogeneity of B 1 field in a larger space. The parallel-plate coil has a satisfactory homogeneity of B 1 field over the whole internal space. The signal-to-noise ratio measured for all three coils is roughly the same and is given by the magnitude of RF pre-amplifier noise. As the slotted-tube and parallel-plate coils have a lower inductance compared with the saddle coil, they can be tuned to resonance on the 200 MHz frequency even at larger dimensions. The results show that the parallel-plate coil has very good properties for the measurement of small animals.

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Analysis and design of a coupled coaxial line TEM resonator for magnetic resonance imaging

Cited by 6 publications

References 5 publications

Coaxial waveguide MRI

Coaxial waveguide MRI

Optimization of a Transverse Electromagnetic Resonator for Magnetic Resonance Imaging at High Field of 9.4 T

Comparing Saddle, Slotted-tube and Parallel-plate Coils for Magnetic Resonance Imaging

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