Signal intensity correction for multichannel MR images using radon transformation

The purpose of this study is to correct signal intensity at low‐field MRI system with multichannel receiver coils using Radon transformation and filtered backprojection (FBP) method. An open‐type 0.32 T MRI system and a body size phantom were used to acquire the MR images. We used various types of coils from 2‐channels to 4‐channels, which minimized the loss of signal. In the intensity correction process, Radon transform was used for the images of each channel and low‐pass filtering was applied to reduce noise. After that FBP was used for the space transform again from the Radon space to the image space. We also made changes to the projection ranges and their intervals, and then confirmed them to evaluate the optimal parameters. All the intensity corrected results were compared with its original sum‐of‐square (SOS) images, and the corrected images showed more uniform and homogeneous intensities than the images without correction. In addition, these results were also shown in the quantitative values through the signal intensity variations according to the cut view along the horizontal lines of the images. The feasibility of our approach and results for signal intensity correction may be useful and helpful for the researchers of low‐field system with multichannel coils.     

The investigation of gas holdup distribution in a two-phase bubble column using ultrasonic computed tomography

In this study, time-averaged gas holdup distributions were investigated in a 16 cm diameter bubble column for two-phase dispersed system of air–water and air–glycerol solution of 10 wt% by using ultrasonic computed tomography (UCT). A quantitative result of UCT – as a coupling of the ultrasonic transmission method and the iterative filtered backprojection (IFBP) image reconstruction technique – is presented. The UCT results are in a good agreement with those by the bed expansion method. A higher gas holdup in the air–glycerol 10 wt% solution than in the air–water system was observed. The distribution of gas holdup in the column with an attached baffle is also investigated by UCT.     

用逼近定理解决有限角图象重建问题

本文提出了有限角图象重建的新方法。Ｗｅｉｅｒｓｔｒａｓｓ三角多项式逼近定理用来估计缺失角外的投影数据，补全投影数据后再用常用的滤波反投影算法重建图象。文章给出并证明了投影值函数连续的一个充分条件，把该方法与已有的方法作了比较，说明了它的优良性能和应用前景。     

聚束SAR的快速卷积反投影成像算法

本文对应用于聚束SAR成像的卷积反投影(CBP)算法进行了详细研究,采用子孔径处理的方法,使得这种算法的计算量得到明显的降低.同时,对计算过程的分析表明CBP算法中所涉及的数量巨大的乘法运算可以用加法运算代替,加法操作又可以简单有效地利用可编程逻辑器件进行并行处理.通过上述改进,CBP算法成像处理在计算量降低的同时,硬件结构也得到了很大的简化.论文同时给出了大前斜视角情况下的处理方法、点目标仿真和利用快速CBP算法对E-SAR原始数据的成像结果.     

二相流场光学层析成像的研究

本文对气固二相流场进行光学层析投影，得出了固相质量浓度与它的消光率之间的线性关系，并利用这一关系和反向投影算法重建了气固二相流场的截面分布。为进一步研究二相流场的光学层析成像提供了基础。     

一种改进的SAR反投影图像相位梯度自聚焦方法

反投影(BP) 算法是一类经典的合成孔径雷达(SAR)成像处理方法。BP在已知航迹条件下可以实现数据的高精度聚焦。但是,BP重建图像中,运动误差导致的目标散焦通常沿着不同的方向存在,残留距离徙动不能严格限制在一个距离分辨率单元内,运动补偿困难。文中提出了一种修正投影栅格的子带宽相位梯度自聚焦(PGA)处理方案。其中,基于数据采集平面的非均匀栅格重建图像,完全去除了目标散焦方向的空变特性。然后,对子带宽分解的反投影数据进行PGA运动补偿并拼接得到全分辨率图像。最后,点目标仿真验证了该方法的有效性。     

结合全变差最小化的双能CT重建*

双能CT能够用来检测物质的有效原子序数和电子密度,可区别密度相近而原子序数不同的物质。物质的有效原子序数和电子密度的精确性是双能CT重建的关键。针对得到的有效原子序数和电子密度图中的噪声,将全变差(TV)最小化应用到双能CT中。衰减系数分解模型选用基材料分解模型,首先对高低能投影进行投影匹配,得到两组基材料分解系数投影,然后用滤波反投影(FBP)重建算法重建得到分解系数,接着得到有效原子序数和电子密度图,最后用基于TV最小化的算法处理以上两组数据。仿真实验结果表明,双能CT可以检测出物质的有效原子序数和电子密度,引入TV的双能CT重建算法能得到质量较好的物质的电子密度和有效原子序数图,更利于物质的检测。     

混合软影绘制算法

实时软影映射算法能够使用一张阴影映射图绘制出复杂动态场景的真实的软影效果.结合物体空间和图像空间的算法,提出一种在物体空间中提取面光源轮廓边的方法;然后利用面光源轮廓边的属性,提出一个在图像空间中确定轮廓边像素走向的方法;最后反投影拟合的轮廓边计算最终的几何可见性.实验结果表明,本文算法减少了提取轮廓边的消耗,提高了绘制效率;增加了拟合轮廓边的精度,提高了绘制效果.     

等角扇形束投影的CT重建算法研究

通过线阵探测器CT系统CD-300BX获得C/C复合材料多个角度下的扇形束CT扫描投影数据,设计并实现了滤波反投影算法。讨论了常用滤波函数和内插方式对CT重建质量的影响,采用峰值信噪比和平均梯度对CT图像重建质量进行客观评价,并与CT系统算法重建结果进行对比。研究结果表明:选用R-L滤波和紧邻内插时CT图像重建质量最好,本文开发的滤波反投影算法重建质量达到了CT系统的重建质量,可满足扇形束投影的CT重建需求。