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ABSTRACT: Recent studies have shown that rotating a single RF transceive coil (RRFC) provides a uniform coverage of the object and brings a number of hardware advantages (i.e. requires only one RF channel, averts coil-coil coupling interactions and facilitates large-scale multi-nuclear imaging). Motion of the RF coil sensitivity profile however violates the standard Fourier Transform definition of a time-invariant signal, and the images reconstructed in this conventional manner can be degraded by ghosting artifacts. To overcome this problem, this paper presents Time Division Multiplexed-Sensitivity Encoding (TDM-SENSE), as a new image reconstruction scheme that exploits the rotation of the RF coil sensitivity profile to facilitate ghost-free image reconstructions and reductions in image acquisition time. A transceive RRFC system for head imaging at 2 Tesla was constructed and applied in a number of in vivo experiments. In this initial study, alias-free head images were obtained in half the usual scan time. It is hoped that new sequences and methods will be developed by taking advantage of coil motion.
Journal of Magnetic Resonance 09/2009; 201(2):186-98. · 2.14 Impact Factor
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ABSTRACT: A new decoupling method for magnetic resonance imaging (MRI) phased arrays is studied by experimental measurements. A laboratory measurement setup is built to characterise the signal coupling paths and their coupling strengths. A new concept, the receiving mutual impedance, is introduced to measure the coupled signals between the phased array elements. Measured values of the receiving mutual impedances for a typical two-element surface-coil array are obtained and used in other experiments to find the uncoupled voltages from the received voltages. Results show that the new decoupling method is both accurate and robust over a wide frequency range. Comparison of the uncoupled voltages with the actual ideal uncoupled voltages confirms that if the position of the signal source is known, almost error-free uncoupled voltages can be obtained. The errors resulted from a change of the position of the signal source are also measured and it is found that they generally increase with the deviation of the signal source from its position where the receiving mutual impedances are measured. The maximum % error of the uncoupled voltages is found to be below 10% when the signal source changes its position over a distance of half the length of a surface coil. Over this distance change, the signal isolation between the two surface coils is found to be at least 20 dB, whereas the maximum is more than 300 dB. The results demonstrate the effectiveness and the feasibility of the new decoupling method for use in MRI phased arrays.
IET Science Measurement ? Technology 10/2008; · 0.60 Impact Factor
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ABSTRACT: This paper presents a stabilized Bi-conjugate gradient algorithm that can significantly improve the performance of the impedance method, which has been widely applied to model low-frequency field induction phenomena in voxel phantoms. The improved quasi-static impedance method offers remarkable computational advantages in terms of convergence performance and memory consumption over the conventional, stationary iterative method-based algorithm. The scheme has been validated against other numerical/analytical solutions on a lossy, multilayered sphere phantom with an ideal coil loop excitation. To further demonstrate the computational performance and application capability of the developed algorithm, the induced fields inside a whole body human phantom responding to hyperthermia device was evaluated. The simulation results show the numerical accuracy and superior performance of the method.
Engineering in Medicine and Biology Society, 2008. EMBS 2008. 30th Annual International Conference of the IEEE; 09/2008
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ABSTRACT: Due to complex field/tissue interactions, high-field magnetic resonance (MR) images suffer significant image distortions that result in compromised diagnostic quality. A new method that attempts to remove these distortions is proposed in this paper and is based on the use of transceiver-phased arrays. The proposed system uses, in the examples presented herein, a shielded four-element transceive-phased array head coil and involves performing two separate scans of the same slice with each scan using different excitations during transmission. By optimizing the amplitudes and phases for each scan, antipodal signal profiles can be obtained, and by combining both the images together, the image distortion can be reduced several fold. A combined hybrid method of moments (MoM)/finite element method (FEM) and finite-difference time-domain (FDTD) technique is proposed and used to elucidate the concept of the new method and to accurately evaluate the electromagnetic field (EMF) in a human head model. In addition, the proposed method is used in conjunction with the generalized auto-calibrating partially parallel acquisitions (GRAPPA) reconstruction technique to enable rapid imaging of the two scans. Simulation results reported herein for 11-T (470-MHz) brain imaging applications show that the new method with GRAPPA reconstruction theoretically results in improved image quality and that the proposed combined hybrid MoM/FEM and FDTD technique is suitable for high-field magnetic resonance imaging (MRI) numerical analysis
IEEE Transactions on Electromagnetic Compatibility 12/2006; · 1.18 Impact Factor
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ABSTRACT: A powerful decoupling method is introduced to obtain decoupled signal voltages from quadrature coils in magnetic resonance imaging (MRI). The new method uses the knowledge of the position of the signal source in MRI, the active slice, to define a new mutual impedance which accurately quantifies the coupling voltages and enables them to be removed almost completely. Results show that by using the new decoupling method, the percentage errors in the decoupled voltages are of the order of 10(-7) % and isolations between two coils are more than 170 dB.
IEEE Transactions on Biomedical Engineering 11/2006; 53(10):2114-6. · 2.28 Impact Factor
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ABSTRACT: In this work, a new design concept in chest imaging for MRI application is presented. A focused, 8-element transceive torso phased array coil is designed to investigate transmitting focused B<sup>1</sup> field deep within the torso to enhance signal intensity and use in conjunction with SENSE reconstruction technique. Hybrid FDTD/MOM method is used to accurately predict the RF behavior inside the human torso. The simulation results reported herein demonstrate the feasibility of the design concept which shows that B<sub>1</sub> field focusing with SENSE reconstruction is achievable, and the 8-element transceive torso phased array coil has the advantage to be used in transmit and receive mode for optimum and fast chest imaging.
Engineering in Medicine and Biology Society, 2004. IEMBS '04. 26th Annual International Conference of the IEEE; 10/2004
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ABSTRACT: An inverse methodology to assist in the design of radio-frequency (RF) head coils for high field MRI application is described in this work. Free space time-harmonic electromagnetic Green's functions and preemphasized B<sub>1</sub> field are used to calculate the current density on the coil cylinder. With B<sub>1</sub> field preemphasized and lowered in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the EM field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T is calculated using inverse methodology with preemphasized B<sub>1</sub> fields. FDTD is employed to calculate B<sub>1</sub> field and signal intensity inside a homogenous cylindrical phantom and human head. A comparison with conventional RF birdcage coil is reported here and demonstrated that inverse-method designed coil with preemphasized B<sub>1</sub> field can help in decreasing the notorious bright region caused by EM field/tissue interactions in the human head images at 4 T.
Engineering in Medicine and Biology Society, 2004. IEMBS '04. 26th Annual International Conference of the IEEE; 10/2004
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ABSTRACT: An inverse methodology to assist in the design of radio-frequency (RF) head coils for high field MRI application is described in this work. Free space time-harmonic electromagnetic Green's functions and preemphasized B/sub 1/ field are used to calculate the current density on the coil cylinder. With B/sub 1/ field preemphasized and lowered in the middle of the RF transverse plane, the calculated current distribution can generate an internal magnetic field that can reduce the EM field/tissue interactions at high frequencies. The current distribution of a head coil operating at 4 T is calculated using inverse methodology with preemphasized B/sub 1/ fields. FDTD is employed to calculate B/sub 1/ field and signal intensity inside a homogenous cylindrical phantom and human head. A comparison with conventional RF birdcage coil is reported here and demonstrated that inverse-method designed coil with preemphasized B/sub 1/ field can help in decreasing the notorious bright region caused by EM field/tissue interactions in the human head images at 4 T.
Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 02/2004; 2:1128-31.
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