Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

In this work, the opportunities and challenges for the use of parallel transmission in combination with 2D RF pulses designed on EPI-based excitation trajectories for diffusion-weighted imaging (DWI) with reduced FOV are presented and analyzed in detail. The use of localized excitation allows for shortening of the EPI read-out, which is especially important for EPI applications outside of the brain. DWI is chosen as a practically important and relevant example demonstrating the key aspects of 2D spatial selection. The properties of accelerated pulses are explored experimentally in phantoms for two different schemes, in which the thickness of the excited limited slices is encoded either along the frequency or phase encoding directions of the excitation trajectory. The feasibility of application of parallel transmission for MR imaging in humans is analyzed based on several pilot experiments. Although the parallel transmission acceleration is demonstrated to work in some examples in the spinal cord and abdomen, the results also uncover a number of challenges. Nonetheless, the reduction of FOV in the phase encoding direction of the read-out train along with the associated substantial shortening of the minimum echo train length and reduction of geometric distortions motivates further search for an advantageous use of the parallel transmit technology in EPI applications. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 153–173, 2015

Halbach magnet is a type of permanent magnet generating a relatively high and homogeneous magnetic field. It is suitable for Nuclear Magnetic Resonance (NMR) studies of small volume chemical or biological samples. In this article, the model of a Halbach magnet made from an odd number of cylindrical layers is proposed for the first time. Then after the optimization of interlayer distances for odd layers Halbach cylinders, the model is verified by the simulation with a magnet inner radius of 30 mm and an outer radius of 49 mm. Moreover, the disturbance of uniformity in 5 mm DSV (Diameter of Spherical Volume) is presented with errors in magnetic strength and angular variation. As a result, a minimum uniformity of 46 ppm inside a 5 mm DSV is achieved, while it increases practically in the presence of magnetic blocks errors. The good performance of the Halbach magnet with odd layers may find potential applications in NMR. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 134–141, 2015

In most clinical magnetic resonance imaging systems, only commercial receive coils with the appropriate connector and encoding can be plugged in. When willing to use a dedicated receive coil for a specific study which cannot be achieved with commercial coils, the researcher faces the connecting issue related to the specificity of the proprietary connector. In this work, a universal device is proposed which allows for the connection of any single channel dedicated coil on any magnetic resonance (MR) system, as long as it is provided with at least one commercial receive coil. Technical feasibility of the universal connecting device was demonstrated on a 3 T MR clinical imager. The device included an independent active decoupling circuit while signal transmission to the data cabinet was achieved by electromagnetic coupling with a commercial receive coil plugged to the MR device. The universal connecting device was notably characterized in terms of signal-to-noise ratio (SNR) and compared to the standard connection. Image SNR was comparable using both means of connection. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 125–133, 2015

Echo-planar imaging (EPI) can suffer from geometrical distortion due to magnetic field inhomogeneity. To correct the geometric distortions in EPI, a magnetic field map is used. Our purpose was to develop a novel image-based method for estimating the field inhomogeneity map from the distorted EPI image and T1-weighted image of the brain using k-space textures. Based on magnetic resonance imaging physics, our method synthesizes the distorted image to match the measured EPI image through the generating process of EPI image by updating the estimated field inhomogeneity map. The estimation process was performed to minimize the cost function, which was defined by the synthesized EPI image and the measured EPI image with geometric distortion, using an iterative conjugate gradient algorithm. The proposed method was applied to simulation and human data. To evaluate the performance of the proposed method quantitatively, we used the normalized root mean square error (NRMSE) between the ground truth and the results estimated by our proposed method. In simulation data, the values of the NRMSE between the ground truth and the estimated field inhomogeneity map were <0.08. In both simulation and human data, the estimated EPI images were very similar to input EPI images, and the NRMSE values between them were <0.09. The results of the simulated and human data demonstrated that our method produced a reasonable estimation of the field inhomogeneity map. The estimated map could be used for distortion correction in EPI images. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 45B: 142–152, 2015