A Tool Box to Evaluate the Phased Array Coil Performance Using Retrospective 3D Coil Modeling

PURPOSE: To efficiently evaluate phased array coil performance using a software tool box with which we can make visual comparison of the sensitivity of every coil element between the real experiment and EM simulation. MATERIALS AND METHODS: We have developed a C++- and MATLAB-based software tool called Phased Array Coil Evaluator (PACE). PACE has the following functions: Building 3D models of the coil elements, importing the FDTD simulation results, and visualizing the coil sensitivity of each coil element on the ordinary Cartesian coordinate and the relative coil position coordinate. To build a 3D model of the phased array coil, we used an electromagnetic 3D tracker in a stylus form. After making the 3D model, we imported the 3D model into the FDTD electromagnetic field simulation tool. RESULTS: An accurate comparison between the coil sensitivity simulation and real experiment on the tool box platform has been made through fine matching of the simulation and real experiment with aids of the 3D tracker. In the simulation and experiment, we used a 36-channel helmet-style phased array coil. At the 3D MRI data acquisition using the spoiled gradient echo sequence, we used the uniform cylindrical phantom that had the same geometry as the one in the FDTD simulation. In the tool box, we can conveniently choose the coil element of interest and we can compare the coil sensitivities element-by-element of the phased array coil. CONCLUSION: We expect the tool box can be greatly used for developing phased array coils of new geometry or for periodic maintenance of phased array coils in a more accurate and consistent manner.

Ultrasonic guided waves in long bones based on finite-difference time-domain method / 医用生物力学

Objective To discuss the effects of elastic modulus on propagation characteristics of ultrasonic guided waves in long bones based on finite-difference time-domain (FDTD) method, so as to provide the theoretical references for evaluating bone fatigue damage at the early stage. Methods A cylinder was used to model the long bone, and FDTD method was used to simulate the long bone with different elastic modulus. Then, the propagation characteristics of different guided wave modes were calculated, including the phase velocity, the group velocity, the central frequency and the energy. ResultsThe elastic modulus of long bones was closely related with the propagation characteristics of ultrasonic guided waves. The phase velocity, the group velocity, the central frequency and the energy were all reduced with the decrease in elastic modulus, and the variation tendency of L(0,5) mode was the most obvious. Conclusions The propagation of ultrasound guided waves can reflect the variation of elastic modulus of long bones, which provides a possible way to evaluate the fatigue damage at the early stage in long bones.

The EM energy absorption in different rat model exposed to microwave irradiation / 中华放射医学与防护杂志

Objective To find out the relationship between the weight and whole body averaged SAR. and the relationship between the weight and the EM energy absorption in rat model exposed to 3 GHz microwave irradiation.Methods Based on 3D rat NMR images,a rat EM model consisting of 45 tissues was established, then by changing the size of model cell,16 models of which the sizes varied from 9 cm to 24 cm, and the weights from 16 g tO 334 g correspondingly were established too.Using FDTD method,the induced electric fields in these rat models were calculated,and then SAR and EM energy absorption have been obtained accordingly.Results The relationship between the weight and the whole body averaged SAR of rat model exposed to 3 GHz microwave irradiation can be divided into three sections,and each of them can be fitted into one line.In small rat section, SAR changed sharply;in middle rat section,SAR changed moderately;in large rat section,SAR changed litflely.The relationship between the weight and the EM energy absorption of rat model were fitted into one quadratic polynomial curve.The EM energy absorption was almost identical when the model is small,with the increasing of the weight,the EM energy absorption also increases,but the increasing ratio of the energy is less than that of the weight.Conclusions Larger rat absorbs more EM energy,but gets less SAR.Exposed to the same irradiation,the dose differences of larger rat is much less than that of smaller rat because of the changes in body weight,so the larger rat is to be recommended.

THE STUDY ON MEASURING TECHNIQUE OF PARTIAL DISCHARGE IN GAS INSULATED SWITCHGEAR USING ULTRA HIGH FREQUENCY METHOD WITH EXTERNAL SENSORS / 药物分析学报

Objective In order to find early latent faults and prevent catastrophic failures, diagnosis of insulation condition by measuring technique of partial discharge(PD) in gas insulated switchgear (GIS) is applied in this paper, which is one of the most basic ways for diagnosis of insulation condition. Methods Ultra high frequency(UHF) PD detection method by using internal sensors has been proved efficient, because it may avoid the disturbance of corona, but the sensor installation of this method will be limited by the structure and operation condition of GIS.There are some of electromagnetic (E-M) waves leak from the place of insulation spacer, therefore, the external sensors UHF measuring PD technique is applied, which isn't limited by the operation condition of GIS. Results This paper analyzes propagated electromagnetic (E-M) waves of partial discharge pulse excited by using the finite-difference time-domain (FDTD) method. The signal collected at the outer point is more complex than that of the inner point, and the signals' amplitude of outer is about half of the inner, because it propagates through spacer and insulation slot. Set up UHF PD measuring system. The typical PD in 252kV GIS bus bar was measured using PD detection UHF technique with external sensors. Finally, compare the results of UHF measuring technique using external sensors with the results of FDTD method simulation and the traditional IEC60270 method detection. Conclusion The results of experiment shows that the UHF technique can realize the diagnosis of insulation condition, the results of FDTD method simulation and the result UHF method detection can demonstrate each other, which gives references to further researches and application for UHF PD measuring technique.

Analysis of Endcap Effect for MRI Birdcage RF Coil by FDTD Method

PURPOSE: B1 field of birdcage RF (radiofrequency) coil that is used most for brain imaging in magnetic resonance imaging (MRI) decreases toward endring from the coil center. We investigated how much RF B1 homogeneity effect the endcap shield brings form the coil center as it towards to endcap region. MATERIALS AND METHODS: We compared RF B1 field distribution by each finite difference time domain (FDTD) simulations for lowpass, highpass and hybrid birdcage RF coils. We selected the highpass birdcage RF coil that was the highest RF B1 field condition as simulation result, and studied how much RF B1 homogeneity effect was occurred when endcap shield was applied to endring area. RESULTS: B1 field of the highpass birdcage RF coil was higher than other birdcage RF coil types as simulation result. However, the RF B1 homogeneity was lower than other coil types. RF B1 field of highpass birdcage RF coil with endcap shield is similar with RF B1 field of hybrid birdcage RF coil and the overall RF B1 homogeneity in sagittal direction was better. CONCLUSION: In this paper, proposed method can apply improving RF B1 homogeneity of RF coil in clinical examination.