相位分区和局部多项式曲面拟合的相位解缠绕  

New phase-unwrapping method based on phase partition and local polynomial surface fitting

作　　者：程军营[1,2] 王常青 冯衍秋 陈武凡 Cheng Junying;Wang Changqing;Feng Yanqiu;Chen Wufan(School of Automation Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China;School of Biomedical Engineering, Southern Medical University, Guangzhou 510515, China)

机构地区：[1]电子科技大学自动化工程学院,成都611731 [2]南方医科大学生物医学工程学院,广州510515

出　　处：《中国图象图形学报》2018年第6期906-916,共11页Journal of Image and Graphics

基　　金：国家自然科学基金项目(61471188)~~

摘　　要：目的准确的解缠绕相位是两点或三点Dixon技术等在磁共振临床应用的前提和关键,然而当相位图像中存在严重噪声、快速相位变换或不连通区域时,当前许多已经提出的相位解缠绕算法将会失败。为此本文提出一种基于相位分区和局部多项式曲面拟合的相位解缠绕新方法,该新方法在相位图像存在严重噪声、快速相位变换或不连通区域的情况下仍可以稳定可靠的工作。方法首先将获得的相位图像分成连通块,块内相位都在给定的相位区间内,把像素个数小于给定阈值的块归类为残余像素。然后利用局域增长的局部多项式曲面拟合方法依次进行块与块之间相位解缠绕和残余像素相位解缠绕。最后使用仿真与真实磁共振Dixon数据来评价提出方法,并与PRELUDE(phase region expanding labeler for unwrapping discrete estimates)方法进行了比较。结果在不同信噪比、快速相位变换或存在不连通区域的仿真实验中,即使当数据中存在信噪比为0.5、相邻相位变换大于π弧度或不连通区域时,提出方法的平均错误率不大于0.51%。对于100层真实的磁共振膝关节和踝关节水与脂肪分离图像,提出方法生成结果中发生明显解缠绕错误及水与脂肪互换的比率为6.00%,而PRELUDE却为42.00%。结论本文提出了一种磁共振相位解缠绕算法,利用相位分区方法,可靠的实现相位图像分块;利用局部多项式曲面拟合方法,准确的实现相位解缠绕。提出方法能够更加鲁棒的实现相位解缠绕,这将有益于相位相关的磁共振临床的应用,如两点和三点Dixon水脂分离技术、磁敏感加权成像和人脑相位成像等。Objective An accurate phase map is crucial for magnetic resonance imaging（ MRI）-based clinical applications,such as two-point and three-point Dixon techniques,susceptibility-weighted imaging,and human-brain phase imaging. However,the phase calculated from the complex MR signal is commonly wrapped back into the range of（-π,π]for the arctangent operation. Phase unwrapping is required to recover the underlying actual phase. A certain number of phase unwrapping methods have been proposed under the assumption that the underlying actual phase difference betweenadjacent pixels should not be larger than π. If the wrapped phase map contains severe noise,rapid phase change,or even disconnected regions in the interesting region,then the underlying actual phase difference between adjacent pixels may be larger than π; thus,this assumption becomes invalid. A new phase-unwrapping method is proposed; this method is based on phase partition and local polynomial surface fitting,which works robustly under the situation of severe noise,rapid phase changes,and disconnected regions. Method The proposed method first exploits the phase partition method to split the acquired phase map into the connected blocks. The phase inside each block remains within a given interval,and the wrapped phase difference between adjacent pixels is less than π. Therefore,no phase wrap exists inside each block. To reduce the affection of noise,the blocks with pixel number of less than a given threshold（ such as 50） are clustered into residual pixels. Then,the proposed method sequentially performs inter-block phase unwrapping and residual-pixel phase unwrapping by a region-growing local polynomial surface fitting approach. Three simulated datasets were separately generated with signalto-noise ratios（ SNRs） varying from 6. 5 to 0. 5,phase height changing from 100 rad to 200 rad,and disconnected regions to test the performance of the proposed method on the data under the situation of severe noise,different phase change levels and disconnected

关 键 词：磁共振成像 相位解缠绕 相位分区 局部多项式曲面拟合 水与脂肪分离 

分 类 号：TP391[自动化与计算机技术—计算机应用技术]