Department of Radiology, Stanford University, Stanford, CA, USA.
Med Image Anal. 2011 Oct;15(5):708-19. doi: 10.1016/j.media.2011.05.018. Epub 2011 Jun 13.
The tracking and compensation of patient motion during a magnetic resonance imaging (MRI) acquisition is an unsolved problem. For brain MRI, a promising approach recently suggested is to track the patient using an in-bore camera and a checkerboard marker attached to the patient's forehead. However, the possible tracking range of the head pose is limited by the fact that the locally attached marker must be entirely visible inside the camera's narrow field of view (FOV). To overcome this shortcoming, we developed a novel self-encoded marker where each feature on the pattern is augmented with a 2-D barcode. Hence, the marker can be tracked even if it is not completely visible in the camera image. Furthermore, it offers considerable advantages over the checkerboard marker in terms of processing speed, since it makes the correspondence search of feature points and marker-model coordinates, which are required for the pose estimation, redundant. The motion correction with the novel self-encoded marker recovered a rotation of 18° around the principal axis of the cylindrical phantom in-between two scans. After rigid registration of the resulting volumes, we measured a maximal error of 0.39 mm and 0.15° in translation and rotation, respectively. In in vivo experiments, the motion compensated images in scans with large motion during data acquisition indicate a correlation of 0.982 compared to a corresponding motion-free reference.
在磁共振成像（MRI）采集过程中跟踪和补偿患者运动是一个尚未解决的问题。对于脑部 MRI，最近提出的一种很有前途的方法是使用腔内摄像机和贴在患者额头上的棋盘标记来跟踪患者。然而，由于局部附着的标记必须完全在摄像机的窄视场（FOV）内可见，因此头部姿势的可能跟踪范围受到限制。为了克服这一缺点，我们开发了一种新型自编码标记，其中图案上的每个特征都用二维条码增强。因此，即使标记在摄像机图像中不完全可见，也可以对其进行跟踪。此外，与棋盘标记相比，它在处理速度方面具有相当大的优势，因为它使特征点和标记模型坐标的对应搜索（这是姿势估计所必需的）变得多余。使用新型自编码标记进行的运动校正可以在两次扫描之间恢复围绕圆柱状仿射体主轴的 18°旋转。在对结果体积进行刚性配准后，我们分别测量到平移和旋转的最大误差为 0.39 毫米和 0.15°。在体内实验中，在数据采集过程中存在较大运动的扫描中，运动补偿后的图像与相应的无运动参考图像之间的相关性为 0.982。
