Longitudinal gradient coil optimization in the presence of transient eddy currents.

The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. Shielded and unshielded longitudinal gradient coils are used to exemplify this novel approach. To assist in the evaluation of transient eddy currents induced within a realistic cryostat vessel, a low-frequency finite-difference time-domain (FDTD) method using the total-field scattered-field (TFSF) scheme was performed. The simulations demonstrate the effectiveness of the proposed method for optimizing longitudinal gradient fields while taking into account the spatial and temporal behavior of the eddy currents.     

优化的贴片型测量探头及其生物医学应用研究

采用时域有限差分（FDTD）法结合遗传算法（GA）优化设计了适用于浅表生物组织高频测量的宽带同轴贴片探头，并用该探头测量了人体背部1～7GHz频带内的反射特性。优化设计出的圆形同轴贴片探头具有轴对称特点，辐射近场在生物组织中透人深、反射系数的频率响应特性好。所获得的人体背部反射特性的测量结果将为背部浅表组织电特性的重建提供有利的依据。     

1 × 4 Wavelength Demultiplexer C-Band Using Cascaded Multimode Interference on SiN Buried Waveguide Structure

Back reflection losses are a key problem that limits the performances of optical communication systems that work on wavelength division multiplexing (WDM) technology based on silicon (Si) Multimode Interference (MMI) waveguides. In order to overcome this problem, we propose a novel design for a 1 × 4 optical demultiplexer based on the MMI in silicon nitride (SiN) buried waveguide structure that operates at the C-band spectrum. The simulation results show that the proposed device can transmit four channels with a 10 nm spacing between them that work in the C-band with a low power loss range of 1.98–2.35 dB, large bandwidth of 7.68–8.08 nm, and good crosstalk of 20.9–23.6 dB. Thanks to the low refractive index of SiN, a very low back reflection of 40.57 dB is obtained without using a special angled MMI design, which is usually required, using Si MMI technology. Thus, this SiN demultiplexer MMI technology can be used in WDM technique for obtaining a high data bitrate alongside a low back reflection in optical communication systems.     

体外冲击波碎石焦点区域的时域有限差分法数值仿真

体外冲击波碎（Extracorporeal shock wave lithotropisy，ESWL）形成的焦点区域形状、大小、位置和声压分布等是影响其治疗效果的主要因素之一。本文利用笔者等以前提出FDTD（Finite Difference Time Domain）超声波非线性传播的仿真法，以实际Reichenberger的ESWL实验为例建立仿真模型，数值仿真TESWL超声波非线性传播过程，研究ESWL焦点附近的声压分布、焦点区域的大小、形状、位置及人体组织对焦点区域的影响。     

小腿微波医学成像三维分析模型

通过对人体小腿上部微波散射特性的分析及其成像，可以利用微波探测白血病及监测白血病的治疗过程。本文首次建立了该探测系统系统的三维分析模型，采用柱坐标系下的时域有限差分法分析人体小腿上部微波散射特性。     

Manipulation of image intensity distribution at 7.0 T: passive RF shimming and focusing with dielectric materials

PURPOSE: To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach. MATERIALS AND METHODS: The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B(1) field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations. RESULTS: MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain. CONCLUSION: The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B(1) field inside the human head of a given RF coil can be adjusted to reduce the B(1) field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing).     

FDTD simulations to assess the performance of CFMA-434 applicators for superficial hyperthermia

Introduction: Contact flexible microstrip applicators (CFMA), operating at 434 MHz, are applied at the Academic Medical Center (AMC) for superficial hyperthermia (e.g. chest wall recurrences and melanoma). This paper investigates the performance of CFMA, evaluating the stability of the specific absorption rate (SAR) distribution, effective heating depth (EHD) and effective field size (EFS) under different conditions.

Methods: Simulations were performed using finite differences and were compared to existing measurement data, performed using a rectangular phantom with a superficial fat-equivalent layer of 1 cm and filled with saline solution. The electrode plates of the applicators measure ～7 × 20, 29 × 21 and 20 × 29 cm². Bolus thickness varied between 1 and 2 cm. The impact of the presence of possible air layers between the rubber frame and the electrodes on the SAR distribution was investigated.

Results: The EHD was ～1.4 cm and the EFS ranged between ～60 and ～300 cm², depending on the applicator type. Both measurements and simulations showed a split-up of the SAR focus with a 2 cm water bolus. The extent and location of air layers has a strong influence on the shape and size of the iso-SAR contours with a value higher than 50%, but the impact on EFS and EHD is limited.

Conclusion: Simulations, confirmed by measurements, showed that the presence of air between the rubber and the electrodes changes the iso-SAR contours, but the impact on the EFS and EHD is limited.