Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T
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Effective radiated power
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Abstract Birdcage coils are widely used as a radiofrequency (RF) resonator in magnetic resonance imaging (MRI) because of their capability to Produce a highly homogeneous B 1 field Over a large volume within the coil. When they are employed for high‐frequency MRI, the interaction between the electromagnetic field and the object to be imaged deteriorates the B 1 ‐field homogeneity and increases the specific absorption rate (SAR) in the object. To investigate this problem, a finite‐element method (FEM) is developed to analyze the SAR and the B 1 field in a two‐dimensional (2D) model of a birdcage coil loaded with a 2D model of a human head. The electric field, magnetic field, and SAR distributions are shown, and a comprehensive study is carried out for both linear and quadrature birdcage coils at 64, 128, 171, and 256 MHz. It is that to generate the same value of the B 1 field, the SAR is increased significantly with the frequency, and for the same imaging method the SAR produced by a quadrature coil is significantly lower than that of a linear coil. It is also shown that the B 1 ‐field inhomogeneity is increased significantly with the frequency.
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In this paper we are demonstrating a novel method of designing and implementing a modified birdcage type Radio Frequency (RF) coil for small animal Nuclear Magnetic Resonance (NMR) imaging. This RF coil is basically a band pass type birdcage coil which is specifically designed to perform the whole body NMR imaging of small animal at 1.5T MRI systems. The designed RF coil contains the saw tooth shaped pattern as the leg conductors. The magnetic field produced at 63.85 MHz resonance frequency by this designed saw toothed shape leg pattern RF coil is significantly stronger than the magnetic field produced by a conventional straight leg band pass type birdcage coil designed with the same dimension. A full wave 3D electromagnetic simulation is carried out to optimize the RF coil dimensions, capacitor values and to study the RF coil electromagnetic characteristics.
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In numerical analyses of radiofrequency (RF) fields for MRI, RF power is often permitted to radiate out of the problem region. In reality, RF power will be confined by the magnet bore and RF screen enclosing the magnet room. We present numerical calculations at different frequencies for various surface and volume coils, with samples from simple spheres to the human body in environments from free space to a shielded RF room. Results for calculations within a limited problem region show radiated power increases with frequency. When the magnet room RF screen is included, nearly all the power is dissipated in the human subject. For limited problem regions, inclusion of a term for radiation loss results in an underestimation of transmit efficiency compared to results including the complete bore and RF screen. If the term for radiated power is not included, calculated coil efficiencies are slightly overestimated compared to the complete case.
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This paper reviews the field of multiple or parallel radiofrequency (RF) transmission for magnetic resonance imaging (MRI). Currently the use of ultra-high field (UHF) MRI at 7 tesla and above is gaining popularity, yet faces challenges with non-uniformity of the RF field and higher RF power deposition. Since its introduction in the early 2000s, parallel transmission (pTx) has been recognized as a powerful tool for accelerating spatially selective RF pulses and combating the challenges associated with RF inhomogeneity at UHF. We provide a survey of the types of dedicated RF coils used commonly for pTx and the important modeling of the coil behavior by electromagnetic (EM) field simulations. We also discuss the additional safety considerations involved with pTx such as the specific absorption rate (SAR) and how to manage them. We then describe the application of pTx with RF pulse design, including a practical guide to popular methods. Finally, we conclude with a description of the current and future prospects for pTx, particularly its potential for routine clinical use.
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Receiver radio frequency coil (RF coil) is one of the important components in MRI system, which it operates by receiving RF signal that emitted from the excited body part. In this paper, a novel design of dual resonant RF phased array coil for MRI 3T and 7T system is proposed. Phased array is chosen due to its advantages for parallel imaging, high SNR and large field of view. The design of phased array coil is numerically simulated in order to be able to operate at 127.8 MHz (for 3T MRI) and 298.2 MHz (for 7T MRI). The simulated result shows that the magnitude of the reflection coefficient is less than -10 dB for both operating frequencies. The magnetic field distribution is also simulated to confirm its homogeneity at 127.8 MHz and 298.2 MHz. As a result of the magnetic field distribution, it is uniformly seen in particular at 127.8 MHz. Moreover, the simulated specific absorption rate (SAR) by 0.577 W/kg at 127.8 MHz and 6.680 W/kg at 298.2 MHz is obtained when the RF coil is excited by 50W of input power.
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Ultra-high field MRI has many advantages such as increasing spatial resolution and exploiting contrast never before seen in-vivo. This contrast has been shown to be beneficial for many applications such as monitoring early and late effect to radiation therapy and transient changes during disease to name a few. However, at higher field strengths the RF wave, needed to for transmitting and receiving signal, approaches that of the head. This leads to constructive and deconstructive interference and a non -uniform flip angle over the volume being imaged. A transmit or transceive RF surface coil arrays is currently a method of choice to overcome this problem; however, mutual inductance between elements poses a significant challenge for the designer. A method to decouple elements in such an array is by using circumferential shielding; however, the potential benefits and/or disadvantages have not been investigated. This abstract primarily focuses on understanding power deposition — measured through Specific Absorption Rate — in the sample using circumferentially shielded RF coils. Various geometries of circumferentially shielded coils are explored to determine the behaviour of shield width and its effect on required transmit power and power deposition to the sample. Our results indicate that there is an optimization on shield width depending on the imaging depth. Additionally, the circumferential shield focuses the field more than unshielded coils, meaning that slight SAR may even be lower for circumferential shielded RF coils in array.
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A local radio frequency (rf) shielding consisting of a Cu plate and an LC balun circuit has been developed for a compact magnetic resonance imaging (MRI) system with a 0.3 T permanent magnet. Performance of the local rf shielding was evaluated using an artificial external noise source irradiating a human subject whose hand was inserted into the rf coil of the MRI system. Power spectra of the rf signal detected through the rf coil demonstrated that the local rf shield achieved 30.1 dB external noise suppression. With the local rf shielding, a MRI of the subject's hand was performed using a three-dimensional gradient-echo sequence. Anatomical structures of the subject's hand were clearly visualized. It was concluded that the local rf shielding could be used for the compact MRI system instead of a rf shielded room.
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Introduction Functional MRI requires excellent system stability as a prerequisite to successful experimentation. The transmit performance of the RF coil and the power amplifier subsystem are especially important aspects of achieving sufficient stability for fMRI. At 3T the demands placed on system components are even more stringent. Commercial MRI scanners at 1.5T are equipped with a whole body RF coil and a head RF coil. However, there are two approaches to the operation of the head RF coils. Mode A -the head coil is a receive only RF coil and the whole body coil is used to transmit the RF excitation or, alternatively, mode B the head coil is a transmit and receive RF coil. Both of these configurations have been used for fMRI at 1.5T although a comparison of the relative merits of each may not have been performed. In general the main advantage of mode A is that:(1) A more homogeneous RF excitation is possible particular in the superior-inferior direction. This can be used to increase the field of view of the coil or reduce the length of the receive RF coil and increase the signal to noise ratio (SNR). (2) RF spin lableling outside of the head RF coil, for example in the neck, can be performed. (3) The RF amplifier is always connected to the same transmit coil so the RF electronics can be optimised to this coil. The main advantages of mode B are that:(1) Lower transmit RF power is required for head imaging. (2) Only the head is in the RF field so less total RF power is deposited in the body. (3) The design has been proven at high field (3T). Whole body RF coils at 3T have only recently been available (1,2) so the choice of head coil mode for 3T MRI scanners has always been mode B. The purpose of this abstract is to compare the two head coil modes for BOLD fMRI at 3T. In particular the effect of coil mode choice on signal stability is examined. Additional problems may be expected in mode A because physiological noise is an important source of noise in fMRI (3) and in mode A the entire body is coupled to the RF coil system. This may lead to loading fluctuations due to breathing or movement which in turn lead to RF flip angle variations. Another potential problem is that much greater power is required in mode A and the RF amplifier output may be less stable.
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