Schad L, Lott S, Schmitt F, Sturm V, Lorenz W J
J Comput Assist Tomogr. 1987 May-Jun;11(3):499-505. doi: 10.1097/00004728-198705000-00025.
With magnetic resonance (MR) imaging, accurate spatial information--critical for effective stereotaxy--demands a homogeneous static field and linear gradients. Inhomogeneities and nonlinearities induced by eddy currents during the pulse sequences distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. These errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "two-dimensional phantom" displaying "pincushion" distortion in the image (i.e., x, y) plane, and the "three-dimensional phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 5 to 1-2 mm) by calculations based on modeling the distortion as a fourth order two-dimensional polynomial. Based on these corrected images, errors in the z coordinate and tilt of image planes may be corrected by adjustment of the gradient shimming currents. Such correction not only implements stereotaxy under MR guidance but also provides for the accurate transfer of anatomic/pathologic information between MR and CT images.
对于磁共振（MR）成像而言，精确的空间信息（这对有效的立体定向至关重要）需要均匀的静磁场和线性梯度。脉冲序列期间由涡流引起的不均匀性和非线性会使图像失真，并在x - y平面和z轴上产生立体定向坐标的伪位移。位置上的这些误差可以通过放置在立体定向引导系统内的两个体模来评估——一个“二维体模”，在图像（即x、y）平面中显示“枕形”失真，以及“三维体模”，显示图像平面本身的位移、扭曲和倾斜。通过将失真建模为四阶二维多项式进行计算，可以“校正”枕形失真（将位移从5毫米减少到1 - 2毫米）。基于这些校正后的图像，可以通过调整梯度匀场电流来校正z坐标和图像平面倾斜的误差。这种校正不仅能在MR引导下实现立体定向，还能在MR图像和CT图像之间准确传递解剖/病理信息。
