Cepek Jeremy, Chronik Blaine A, Fenster Aaron
Robarts Research Institute, The University of Western Ontario, London, Ontario N6A 5B7, Canada and Biomedical Engineering, The University of Western Ontario, London, Ontario N6A 5B9, Canada.
Robarts Research Institute, The University of Western Ontario, London, Ontario N6A 5B7, Canada and Department of Physics and Astronomy, The University of Western Ontario, London, Ontario N6A 3K7, Canada.
Med Phys. 2014 May;41(5):052301. doi: 10.1118/1.4870961.
The interventional magnetic resonance (MR) imaging environment presents many challenges for the accurate localization of interventional devices. In particular, geometric distortion of the static magnetic field may be both appreciable and unpredictable. This paper aims to quantify the sensitivity of localization error of various passive device localization frames to static magnetic field distortion in MR.
Three localization frames were considered based on having distinctly different methods of encoding position and orientation in MR images. For each frame, the effects of static field distortion were modeled, allowing rotational and translational errors to be computed as functions of the level of distortion, which was modeled using a first order approximation. Validation of the model was performed by imaging the localization frames in a 3T clinical MR scanner, and simulating the effects of static field distortion by varying the scanner's center frequency and gradient shim values.
Plots of the rotational and translational components of error in localization frame position and orientation estimates are provided for ranges of uniform static field distortions of 1-100 μT and static field distortion gradients of 0.01-1 mT/m in all three directions. The theoretical estimates are in good agreement with the results obtained by imaging.
The error in position and orientation estimation of passive localization frames in MR can be sensitive to static magnetic field distortions. The level of sensitivity, the type of error (i.e., rotational or translational), and the direction of error are dependent on the frame's design and the method used to image it. If 2D gradient echo imaging is employed, frames with position and orientation estimate sensitivity to slice-select error (such as the z-frame) should be avoided, since this source of error is not easily correctable. Accurate frame position and orientation estimates that are insensitive to static field distortion can be achieved using 2D gradient echo imaging if: (a) the method of determining position and orientation only uses in-plane measurements of marker positions, (b) the in-plane marker positions in images are not sensitive to slice-select error, and (c) methods of correcting in-plane error in the frequency-encoded direction are employed.
介入磁共振（MR）成像环境给介入设备的精确定位带来了诸多挑战。特别是，静磁场的几何畸变可能既显著又不可预测。本文旨在量化各种无源设备定位框架在MR中对静磁场畸变的定位误差敏感性。
基于在MR图像中编码位置和方向的方法明显不同，考虑了三种定位框架。对于每个框架，对静磁场畸变的影响进行建模，从而能够将旋转和平移误差计算为畸变水平的函数，畸变水平使用一阶近似进行建模。通过在3T临床MR扫描仪中对定位框架进行成像，并通过改变扫描仪的中心频率和梯度匀场值来模拟静磁场畸变的影响，对模型进行验证。
给出了在1 - 100 μT的均匀静磁场畸变范围以及所有三个方向上0.01 - 1 mT/m的静磁场畸变梯度范围内，定位框架位置和方向估计中误差的旋转和平移分量的曲线图。理论估计与成像获得的结果高度一致。
MR中无源定位框架的位置和方向估计误差可能对静磁场畸变敏感。敏感程度、误差类型（即旋转或平移）以及误差方向取决于框架的设计及其成像方法。如果采用二维梯度回波成像，应避免使用对层面选择误差（如z框架）的位置和方向估计敏感的框架，因为这种误差源不易校正。如果满足以下条件，使用二维梯度回波成像可以实现对静磁场畸变不敏感的准确框架位置和方向估计：（a）确定位置和方向的方法仅使用标记位置的平面内测量；（b）图像中的平面内标记位置对层面选择误差不敏感；（c）采用在频率编码方向上校正平面内误差的方法。
