Quadrature lowpass birdcage coil for low field open MRI scanner

Abstract: The main goal of this research was to demonstrate the utility of using a quadrature birdcage coil in lowpass version for magnetic resonance imaging (MRI) in open scanner at very low static field. When used in horizontal bore MRI systems, the birdcage coil is able to produce circular polarized field using quadrature excitation, thus reaching SNR improvement of 2 as maximum value. The birdcage coil in high-pass version can be employed even in a vertical bore MRI scanner, by combining the sinusoidal and the end-ring resonant modes. In this paper we investigated the lowpass birdcage coil configuration to operate with a vertical bore MRI system, producing two 1B fields components perpendicular to the vertical B0 field. Its implementation is convenient because it requires one half capacitors with respect to the high-pass design. Moreover, at low field the high near-electric field losses, that represent the main disadvantage of the low pass design, can be neglected. Experiments performed on a quadrature birdcage coil prototype mounted on an open MRI scanner, showed that both SNR and homogeneity degree increase with respect to the linear birdcage

Improvement of Magnetic Resonance Imaging at Low Field using a Birdcage Coil

The main goal of this research was to demonstrate the utility of using a transmitter-receiver birdcage coil for magnetic resonance imaging at very low static field (0.18 T). As well known, the SNR decreases with frequency, thus reducing the image quality at very low field. Moreover, we would expect that the birdcage coil Q factor reduces with fre-quency. But, from experimental evidence, we proved that at low frequency (7.66 MHz), the Q factor for a well-designed birdcage coil reverses the trend, reach-ing unexpected high values. It is a prerequisite to use low static magnetic field in microimaging applica-tions for small animals experiments

A fast and accurate simulator for the design of birdcage coils in MRI

The birdcage coils are extensively used in MRI systems since they introduce a high signal to noise ratio and a high radiofrequency magnetic field homogeneity that guarantee a large field of view. The present article describes the implementation of a birdcage coil simulator, operating in high-pass and low-pass modes, using magnetostatic analysis of the coil. Respect to other simulators described in literature, our simulator allows to obtain in short time not only the dominant frequency mode, but also the complete resonant frequency spectrum and the relevant magnetic field pattern with high accuracy. Our simulator accounts for all the inductances including the mutual inductances between conductors. Moreover, the inductance calculation includes an accurately birdcage geometry description and the effect of a radiofrequency shield. The knowledge of all the resonance modes introduced by a birdcage coil is twofold useful during birdcage coil design: - higher order modes should be pushed far from the fundamental one, - for particular applications, it is necessary to localize other resonant modes (as the Helmholtz mode) jointly to the dominant mode. The knowledge of the magnetic field pattern allows to a priori verify the field homogeneity created inside the coil, when varying the coil dimension and mainly the number of the coil legs. The coil is analyzed using equivalent circuit method. Finally, the simulator is validated by implementing a low-pass birdcage coil and comparing our data with the literature

Reduction of the Radiofrequency Heating of Metallic Devices Using a Dual-Drive Birdcage Coil

Cataloged from PDF version of article.In this work, it is demonstrated that a dual-drive birdcage coil can be used to reduce the radiofrequency heating of metallic devices during magnetic resonance imaging. By controlling the excitation currents of the two channels of a birdcage coil, the radiofrequency current that is induced near the lead tip could be set to zero. To monitor the current, the image artifacts near the lead tips were measured. The electric field distribution was controlled using a dual-drive birdcage coil. With this method, the lead currents and the lead tip temperatures were reduced substantially [<0.3 C for an applied 4.4 W/kg SAR compared to >4.9 C using quadrature excitation], as demonstrated by phantom and animal experiments. The homogeneity of the flip angle distribution was preserved, as shown by volunteer experiments. The normalized root-mean-square error of the flip angle distribution was less than 10% for all excitations. The average specific absorption rate increased as a trade-off for using different excitation patterns. Magn Reson Med 69:845–852, 2013. VC 2012 Wiley Periodicals, In

Non-calorimetric determination of absorbed power during magnetic nanoparticle based hyperthermia

Nanomagnetic hyperthermia (NMH) is intensively studied with the prospect of cancer therapy. A major challenge is to determine the dissipated power during in vivo conditions and conventional methods are either invasive or inaccurate. We present a non-calorimetric method which yields the heat absorbed during hyperthermia: it is based on accurately measuring the quality factor change of a resonant radio frequency circuit which is employed for the irradiation. The approach provides the absorbed power in real-time, without the need to monitor the sample temperature as a function of time. As such, it is free from the problems caused by the non-adiabatic heating conditions of the usual calorimetry. We validate the method by comparing the dissipated power with a conventional calorimetric measurement. We present the validation for two types of resonators with very different filling factors: a solenoid and a so-called birdcage coil. The latter is a volume coil, which is generally used in magnetic resonance imaging (MRI) under in vivo condition. The presented method therefore allows to effectively combine MRI and thermotherapy and is thus readily adaptable to existing imaging hardware.Comment: 7 pages, 3 figures+Supplementary Material (2 pages, 3 figures

Wireless power transfer in magnetic resonance imaging at a higher-order mode of a birdcage coil

Magnetic resonance imaging (MRI) is a crucial tool for medical visualization. In many cases, performing a scanning procedure requires the use of additional equipment, which can be powered by wires as well as via wireless power transfer (WPT) or wireless energy harvesting. In this Letter, we propose a novel scheme for WPT that uses a higher-order mode of the MRI scanner's birdcage coil for energy transmission. In contrast to the existing WPT solutions, our approach does not require additional transmitting coils. Compared to the energy harvesting, the proposed method allows supplying significantly more power. We perform numerical simulations demonstrating that one can use the fundamental mode of the birdcage coil to perform a scanning procedure while transmitting the energy to the receiver at a higher-order mode without any interference with the scanning signal or violation of safety constraints, as guaranteed by the mode structure of the birdcage. Also, we evaluate the specific absorption rate along with the energy transfer efficiency and verify our numerical model by a direct comparison with an experimental setup featuring a birdcage coil of a 1.5T MRI scanner.Comment: 6 pages, 5 figures + Supplementary Material 10 pages, 7 figure

Accurate calculation of mutual inductance and magnetic fields in a birdcage coil

We present a detailed and complete analysis of a birdcage coil designed for MRI experiment through an equivalent circuit model. We derive analytic equations which can be used to predict a priori the full mutual inductances between conductors and the complete resonant spectrum for unshielded coils with high accuracy. The equations are valid for lowpass, highpass, and bandpass structures. We show that the approximation of a sinusoidal current pattern--which is usually used in the literature--is justified. Moreover, we reduce Neumann formula to the computation of two integrals and present an accurate algorithm to compute them. We also compute the magnetic field pattern of a birdcage coil. Our model is validated through numerical simulations which are compared to experimental results

Design and construction of an actively frequency-switchable RF coil for field-dependent Magnetisation Transfer Contrast MRI with fast field-cycling

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