Bing Keong Li

Focused, eight-element transceive phased array coil for parallel magnetic resonance imaging of the chest--theoretical considerations.

A new transceive system for chest imaging for MRI applications is presented. A focused, eight‐element transceive torso phased array coil is designed to investigate transmitting a focused radiofrequency field deep within the torso and to enhance signal homogeneity in the heart region. The system is used in conjunction with the SENSE reconstruction technique to enable focused parallel imaging. A hybrid finite‐difference‐time‐domain/method‐of‐moments method is used to accurately predict the radiofrequency behavior inside the human torso. The simulation results reported herein demonstrate the feasibility of the design concept, which shows that radiofrequency field focusing with SENSE reconstruction is theoretically achievable. Magn Reson Med 53:1251–1257, 2005.

Hybrid numerical techniques for the modelling of radiofrequency coils in MRI.

Radiofrequency (RF) coils for use in MRI can have a significant effect on both the signal‐to‐noise‐ratio of MR images and the specific absorption rate inside the biological sample. In the past, prototypes were constructed and tested to investigate the performance of the RF coils and often required several iterations to achieve an acceptable result. However, with the advancement in computational electromagnetic techniques, RF coil modelling has now become the modus operandi of coil design because it can produce accurate numerical results, thus reducing the time and effort spent in designing and prototyping RF coils. Two hybrid methods –method of moments (MoM)/finite difference time domain (FDTD) and MoM/finite element method (FEM) – for RF coil modelling are presented herein. The paper provides a brief overview of FDTD, FEM and MoM. It discusses the hybridisation of these methods and how they are integrated to form versatile techniques. The numerical results obtained from these hybrid methods are compared with experimental results from prototype coils over a range of operating frequencies. The methods are then applied to the design of a new type of phased‐array coil – the rotary phased array. From these comparisons, it can be seen that the numerical methods provide a useful aid for the design and optimisation of RF coils for use in MRI. Copyright

A New Decoupling Method for Quadrature Coils in Magnetic Resonance Imaging

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

High-Field Magnetic Resonance Imaging With Reduced Field/Tissue RF Artefacts—A Modeling Study Using Hybrid MoM/FEM and FDTD Technique

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

Image reconstructions with the rotating RF coil

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.

Stacked Phased Array Coils for Increasing the Signal-to-Noise Ratio in Magnetic Resonance Imaging

A new concept of using a stacked phased coil array to increase the signal-to-circuit noise ratio (SCNR) in magnetic resonance imaging (MRI) is introduced. Unlike conventional phased coil arrays, the proposed stacked phased coil array is constructed by stacking the coil elements closely together in the vertical direction. Through a proper combination of the coil terminal voltages, the SCNR is shown to increase with the square root of the number of coil elements. A prototype two-element array is constructed and an experimental method is designed to determine the combiner coefficients in a simulated MRI electromagnetic field environment. The experimental results show that the mutual coupling effect among the array coils can be totally removed and the combiner output voltage increases with the number of coil elements. This demonstrates the feasibility of the proposed method.A new concept of using a stacked phased coil array to increase the signal-to-circuit noise ratio (SCNR) in magnetic resonance imaging (MRI) is introduced. Unlike conventional phased coil arrays, the proposed stacked phased coil array is constructed by stacking the coil elements closely together in the vertical direction. Through a proper combination of the coil terminal voltages, the SCNR is shown to increase with the square root of the number of coil elements. A prototype two-element array is constructed and an experimental method is designed to determine the combiner coefficients in a simulated MRI electromagnetic field environment. The experimental results show that the mutual coupling effect among the array coils can be totally removed and the combiner output voltage increases with the number of coil elements. This demonstrates the feasibility of the proposed method.

Rotational magnetic induction tomography

In magnetic induction tomography (MIT), an array of excitation coils is typically used to apply time-varying magnetic fields to induce eddy currents in the material to be studied. The magnetic fields from the eddy currents are then detected by an array of sensing coils to form an image of passive electromagnetic properties (i.e. conductivity, permittivity and permeability). Increasing the number of transmitters and receivers can provide a better image quality at the expense of a larger and more expensive MIT system. Instead of increasing the number of coils, this study investigates the possibility of rotating a single transmit–receive coil to image the electrical properties of the sample, by emulating an array of 200 transmit–receive coils by time-division multiplexing. Engineering details on the electromechanical design and development of a rotating MIT system are presented. The experimental results indicate that representative images of conductive samples can be obtained at 5 MHz by rotating a single transmit–receive coil.

A ultra high field multi-element transceive volume array for small animal MRI

In this work, a dedicated, 9.4T shielded 8-element transceive volume-array for large rat MRI applications is designed and the prototype constructed. Angularly oriented radiating structured coil elements are used. It is shown that by orienting the radiating conductors of the coil element angularly, the RF penetration depth can be increased and the mutual coupling effect can be regulated to minimum. In addition, experimental results show that the prototype can produce homogenous B1 field and is well suited for the Transmit SENSE application and the GRAPPA parallel imaging technique.

Parallel Solvers for Finite-Difference Modeling of Large-Scale, High-Resolution Electromagnetic Problems in MRI

With the movement of magnetic resonance imaging (MRI) technology towards higher field (and therefore frequency) systems, the interaction of the fields generated by the system with patients, healthcare workers, and internally within the system is attracting more attention. Due to the complexity of the interactions, computational modeling plays an essential role in the analysis, design, and development of modern MRI systems. As a result of the large computational scale associated with most of the MRI models, numerical schemes that rely on a single computer processing unit often require a significant amount of memory and long computational times, which makes modeling of these problems quite inefficient. This paper presents dedicated message passing interface (MPI), OPENMP parallel computing solvers for finite-difference time-domain (FDTD), and quasistatic finite-difference (QSFD) schemes. The FDTD and QSFD methods have been widely used to model/ analyze the induction of electric fields/ currents in voxel phantoms and MRI system components at high and low frequencies, respectively. The power of the optimized parallel computing architectures is illustrated by distinct, large-scale field calculation problems and shows significant computational advantages over conventional single processing platforms.

On the Accurate Modeling of a Complex Antenna for Breast Tumor Detection Using a Hybrid MOM/FDTD Approach

Accurate, computationally intensive numerical algorithms have become necessary to handle the increasing complexity of electromagnetic problems in biological applications. In this work, a high performance hybrid- MoM/FDTD approach is presented for electromagnetic analysis and design applications in microwave breast tumor detection. The proposed algorithm is based on Huygens equivalent surface methods. The simulation study of a bow-tie antenna-breast phantom interaction demonstrates the efficacy of the proposed algorithm and suggests that, with further development, this hybrid scheme would be a useful tool in the continued development of microwave imaging for breast cancer.