ANALYSIS OF MACHINE-DEPENDENT AND OBJECT-INDUCED GEOMETRIC DISTORTION IN 2DFT MR IMAGING
ANALYSIS OF MACHINE-DEPENDENT AND OBJECT-INDUCED GEOMETRIC DISTORTION IN 2DFT MR IMAGING
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DOI：
10.1016/0730-725x(92)90011-n
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发表时间：
1992-07-01
影响因子：
2.5
通讯作者：
BEERSMA, R
中科院分区：
文献类型：
--
作者：
BAKKER, CJG;MOERLAND, MA;BEERSMA, R
MR成像中的几何失真主要由静态场的不均匀性和梯度的非线性引起。 本文的目的是分析物体和机器对几何变形的影响，以确定达到规定精度所需的修正。 通过在圆柱形样品管网格上的成像实验中读出梯度的强度、方向和极性的系统变化来测量系统相关的缺陷。 对于本研究中使用的1.5 T系统，观察到静态场相关误差高达7 mm，梯度相关误差高达4 mm（中冠状面，FOV 400 mm，G读数在0.5和3.0 mT/m之间）。 场相关的误差被证明是成反比的梯度强度，而梯度相关的误差原来是几乎独立的梯度强度。 因此，当几何保真度优先于信噪比考虑时，似乎可以始终应用最强的可用选择和读出梯度。 MR图像中的系统相关几何失真的校正可以容易地通过查表来执行。 物体引起的失真的梯度场进行了研究，通过实验的网格上的样品管浸入到一个圆柱形的水浴中的可变盐浓度。 这些实验揭示了物体对梯度误差分布的影响可以忽略不计，并且得出这样的结论：对梯度的非线性的校正仅需要应用系统相关的校正因子。 对象相关的失真的B 0进行了研究，通过传统的SE和脂肪抑制IR实验幻影和人类受试者。 在这些实验中，读出梯度的极性被反转。 减影图像显示出显着的对象引起的不均匀性的静态场在组织-空气界面，并在紧邻的对象被成像。 第一次尝试，以纠正对象相关的B 0的不均匀性是由轮廓分析的源图像。 目前，这种校正仍然必须手动完成。
Geometric distortion in MR imaging predominantly arises from the inhomogeneity of the static field and the nonlinearity of the gradients. It is the purpose of this paper to analyse the object and machine related contributions to geometric distortion in order to determine which corrections are necessary for attaining a specified precision. System related imperfections were measured by systematic variation of the strength, direction, and polarity of the read-out gradient in imaging experiments on a grid of cylindrical sample tubes. For the 1.5-T system used in this study, static field related errors up to 7 mm and gradient related errors up to 4 mm were observed (midcoronal plane, FOV 400-mm, G-read between 0.5 and 3.0 mT/m). Field related errors were shown to be inversely proportional to gradient strength, whereas gradient related errors turned out to be virtually independent of gradient strength. It therefore seems recommendable to always apply the strongest available selection and read-out gradients when geometric fidelity is given preference to signal-to-noise considerations. Correction of system related geometric distortions in MR images can readily be performed by table lookup. Object-induced distortions of the gradient fields were studied by experiments on a grid of sample tubes immersed into a cylindrical water bath of variable saline concentration. These experiments revealed a negligible influence of the object on the gradient error distribution, and lead to the conclusion that correction for the nonlinearity of the gradients only requires the application of system dependent correction factors. Object-related distortions of B0 were studied by conventional SE and fat-suppressed IR experiments on phantoms and human subjects. In these experiments the polarity of the read-out gradient was reversed. Subtraction images showed significant object-induced inhomogeneities of the static field at tissue-air interfaces and in the immediate vicinity of the object being imaged. A first attempt to correct for object related B0 inhomogeneities was made by contour analysis of the source images. At present this correction still has to be done manually.