CN101122633B - Mri脉冲序列的相位校正方法及装置 - Google Patents

Mri脉冲序列的相位校正方法及装置 Download PDF

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CN101122633B
CN101122633B CN200610089180A CN200610089180A CN101122633B CN 101122633 B CN101122633 B CN 101122633B CN 200610089180 A CN200610089180 A CN 200610089180A CN 200610089180 A CN200610089180 A CN 200610089180A CN 101122633 B CN101122633 B CN 101122633B
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贺强
彼得·荷伊博斯
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Abstract

本发明提出一种MRI脉冲序列的相位校正方法，用来计算线性相位和常数相位以对扫描数据进行相位校正，在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，并根据该参考回波计算用于在该扫描中进行相位校正的该常数相位。本发明还提出一种相应的MRI脉冲序列的相位校正装置。使用本发明方法以及装置对扫描数据进行相位校正之后得到了正确的、不受相位跳跃影响的常数相位，进一步对经过相位校正后的数据进行图像重建便可以得到清晰的、无伪影的图像。

Description

MRI脉冲序列的相位校正方法及装置
技术领域
本发明涉及一种磁共振成像(Magnetic Resonance Imaging；MRI)脉冲序列，特别是涉及一种MRI脉冲序列的校正方法及装置。
背景技术
MRI的成像序列，特别是一些快速序列，有时候会因为系统的不完善的性能(如涡电流(eddy current)、射频功率不足等)，以及系统的物理特性(如高场MRI系统的射频穿透(RFpenetration)等)而造成一定的影响。在高场MRI系统中，射频穿透会引起相位跳跃(phasejumping)，从而导致相位校正(Calibration)错误，继而导致图像模糊和产生伪影。
参阅图1，现有方法中对扫描数据进行相位校正的步骤包括：
步骤10：得到扫描数据；
步骤20：对得到的扫描数据进行读出(Readout)傅立叶变换；
步骤30：对变换得到的数据进行相位校正；其中，该相位校正包括对常数相位(Constantphase)和对线性相位(Linear phase)的校正；
步骤40：进行相位编码(Phase encoding)傅立叶变换；以及
步骤50：经过重建得到相位校正后的图像数据。
在实际的现有系统中，一般做法是只对线性相位进行校正，而常常跳过对常数相位的校正。但是由于射频穿透引起的相位跳跃主要是导致常数相位的错误，所以有必要对常数相位进行校正。
发明内容
本发明的目的在于提出一种MRI脉冲序列的相位校正方法及装置，使用正确的常数相位对扫描数据进行相位校正。
为实现上述的目的，本发明提出一种MRI脉冲序列的相位校正方法，用来计算线性相位和常数相位以对扫描数据进行相位校正，其特征在于：在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，并根据该参考回波计算用于在该扫描中进行相位校正的该常数相位。
其中，在该预扫描中获得的数据的行数和在该扫描中获得的数据的行数相等。对该预扫描数据进行线性傅立叶变换得到相应的K空间。从该K空间中减去该线性相位后再根据该参考回波计算该常数相位。优选地选择该预扫描中获得的第一个回波作为该参考回波。通过计算要在该扫描中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性得到该常数相位。
相应地本发明提出一种MRI脉冲序列的相位校正装置，计算线性相位和常数相位用于对扫描数据进行相位校正，包括用来获得扫描数据的扫描模块，该扫描模块具有用来根据线性相位和常数相位校正回波的相位的相位校正模块，其特征在于还包括：预扫描模块，用来在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，通过计算要在该相位校正模块中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性得到该常数相位，并将该常数相位用于该相位校正模块以对相应的回波进行相位校正。
使用本发明方法以及装置对扫描数据进行相位校正之后得到了正确的、不受相位跳跃影响的常数相位，进一步对经过相位校正后的数据进行图像重建便可以得到清晰的、无伪影的图像。
本发明方法可以广泛应用于各种磁共振脉冲序列中，特别是TSE(快速自旋回波)序列，EPI(回波平面成像)序列，或者Trui-IR(翻转恢复)序列中。
附图说明
图1是现有的相位校正方法以及本发明MRI脉冲序列的相位校正方法的流程示意图。
具体实施方式
参阅图1，本发明MRI脉冲序列的相位校正方法包括以下步骤：
步骤20：在扫描开始之前进行相应的无相位编码梯度的预扫描，以获取预扫描数据，在该预扫描中获得的数据的行数和在该扫描中获得的行数相等。也就是说，该预扫描和所述的扫描(正式扫描)的区别仅在于有无相位编码梯度，而二者获得的扫描数据的数量是一样的，在该扫描中有几个回波，在该预扫描中就有几个相应的回波。
步骤21：对该预扫描数据进行线性傅立叶变换得到相应的K空间，所述的线性傅立叶变换优选地为线性快速傅立叶变换。
步骤22：根据下列公式计算该线性相位：
[ Re + iIm ] k = Σ n = 1 N - 1 [ Re ( n + 1 ) + iIm ( n + 1 ) ] k · [ Re ( n ) - iIm ( n ) ] k ; 其中，
k是预扫描中的第k个回波，Re、Im分别为实部和虚部，N是该K空间每行数据的采样点数目，n是该K空间中每行第n个采样点；
从而求得该线性相位为：
Figure G2006100891800D00032
步骤23：从该预扫描数据的K空间中减去该线性相位后再根据该参考回波计算该常数相位。
步骤24：选定该参考回波，在实际的系统中，常选择该预扫描中获得的第一个回波作为该参考回波。
步骤25：计算要在该扫描中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性：
[ Re + iIm ] k , l = Σ n = 1 N [ Re ( n ) + iIm ( n ) ] k · [ Re ( n ) - iIm ( n ) ] l ; 其中，
k是预扫描中的第k个回波，l是该参考回波，Re、Im分别为实部和虚部，N是该K空间每行数据的采样点数目，n是该K空间中每行第n个采样点；
从而求得该常数相位为：
Figure G2006100891800D00034
至此，已经从该预扫描数据中计算得到了该线性相位以及正确的、不受到相位跳跃影响的常数相位，因此可以将它们应用于对应的扫描中进行相位校正。
按照下面的公式对在扫描中得到的相应的回波进行相位校正：
Figure G2006100891800D00035
其中，
A是幅度，
Figure G2006100891800D00036
是相位校正后的数据，
Figure G2006100891800D00037
是相位校正前的数据，和
Figure G2006100891800D00039
分别是在该预扫描中求得的该线性相位和该常数相位。
对经过上述相位校正后的数据进行图像重建便可以得到清晰的、无伪影的图像。
本发明方法可以广泛应用于各种磁共振脉冲序列中，特别是TSE(快速自旋回波)序列，EPI(回波平面成像)序列，或者Trui-IR(翻转恢复)序列中。
相应地，本发明还提出了一种MRI脉冲序列的相位校正装置，计算线性相位和常数相位用于对扫描数据进行相位校正，包括用来获得扫描数据的扫描模块，该扫描模块具有用来根据线性相位和常数相位校正回波的相位的相位校正模块，其特征在于还包括：预扫描模块，用来在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，通过计算要在该相位校正模块中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性得到该常数相位，并将该常数相位用于该相位校正模块以对相应的回波进行相位校正。

Claims (10)

1.一种MRI脉冲序列的相位校正方法，用来计算线性相位和常数相位以对扫描数据进行相位校正，其特征在于：在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，并根据该参考回波计算用于在该扫描中进行相位校正的该常数相位；
其中，在该预扫描中获得的数据的行数和在该扫描中获得的数据的行数相等。
2.根据权利要求1的方法，其特征在于：对该预扫描数据进行线性傅立叶变换得到相应的K空间。
3.根据权利要求2的方法，其特征在于：所述的线性傅立叶变换是线性快速傅立叶变换。
4.根据权利要求2的方法，其特征在于：该线性相位根据下列公式计算：
[ Re + iIm ] k = Σ n = 1 N - 1 [ Re ( n + 1 ) + iIm ( n + 1 ) ] k · [ Re ( n ) - iIm ( n ) ] k ; 其中，
k是预扫描中的第k个回波，Re、Im分别为实部和虚部，N是该K空间每行数据的采样点数目，n是该K空间中每行第n个采样点；
从而求得该线性相位为：
Figure F2006100891800C00012
5.根据权利要求4的方法，其特征在于：从该K空间中减去该线性相位后再根据该参考回波计算该常数相位。
6.根据权利要求4的方法，其特征在于：选择该预扫描中获得的第一个回波作为该参考回波。
7.根据权利要求5或6的方法，其特征在于：计算要在该扫描中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性：
[ Re + iIm ] k , l = Σ n = 1 N [ Re ( n ) + iIm ( n ) ] k · [ Re ( n ) - iIm ( n ) ] l ; 其中，
k是预扫描中的第k个回波，l是该参考回波，Re、Im分别为实部和虚部，N是该K空间每行数据的采样点数目，n是该K空间中每行第n个采样点；
从而求得该常数相位为：
8.根据权利要求7的方法，其特征在于：在该扫描中对回波进行相位校正：
Figure F2006100891800C00022
其中，
A是幅度，
Figure F2006100891800C00023
是相位校正后的数据，
Figure F2006100891800C00024
是相位校正前的数据，
Figure F2006100891800C00025
和
Figure F2006100891800C00026
分别是在该预扫描中求得的该线性相位和该常数相位。
9.根据权利要求1的方法，其特征在于：所述的脉冲序列是TSE序列，EPI序列，或者Trui-IR序列。
10.一种MRI脉冲序列的相位校正装置，计算线性相位和常数相位用于对扫描数据进行相位校正，包括用来获得扫描数据的扫描模块，该扫描模块具有用来根据线性相位和常数相位校正回波的相位的相位校正模块，其特征在于还包括：
预扫描模块，用来在扫描开始之前进行相应的无相位编码梯度的预扫描，在该预扫描得到的回波中选取一个作为参考回波，通过计算要在该相位校正模块中进行相位校正的回波在该预扫描中对应的回波与该参考回波之间的相关性得到该常数相位，并将该常数相位用于该相位校正模块以对相应的回波进行相位校正；其中，在该预扫描中获得的数据的行数和在该扫描中获得的数据的行数相等。
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