Snider Joseph, Plank Markus, May Larry, Liu Thomas T, Poizner Howard
Institute for Neural Computation, University of California, USA.
J Vis Exp. 2011 Oct 4(56):3364. doi: 10.3791/3364.
Functional magnetic resonance imaging (fMRI) provides excellent functional brain imaging via the BOLD signal with advantages including non-ionizing radiation, millimeter spatial accuracy of anatomical and functional data, and nearly real-time analyses. Haptic robots provide precise measurement and control of position and force of a cursor in a reasonably confined space. Here we combine these two technologies to allow precision experiments involving motor control with haptic/tactile environment interaction such as reaching or grasping. The basic idea is to attach an 8 foot end effecter supported in the center to the robot allowing the subject to use the robot, but shielding it and keeping it out of the most extreme part of the magnetic field from the fMRI machine (Figure 1). The Phantom Premium 3.0, 6DoF, high-force robot (SensAble Technologies, Inc.) is an excellent choice for providing force-feedback in virtual reality experiments, but it is inherently non-MR safe, introduces significant noise to the sensitive fMRI equipment, and its electric motors may be affected by the fMRI's strongly varying magnetic field. We have constructed a table and shielding system that allows the robot to be safely introduced into the fMRI environment and limits both the degradation of the fMRI signal by the electrically noisy motors and the degradation of the electric motor performance by the strongly varying magnetic field of the fMRI. With the shield, the signal to noise ratio (SNR: mean signal/noise standard deviation) of the fMRI goes from a baseline of 380 to ~330, and ~250 without the shielding. The remaining noise appears to be uncorrelated and does not add artifacts to the fMRI of a test sphere (Figure 2). The long, stiff handle allows placement of the robot out of range of the most strongly varying parts of the magnetic field so there is no significant effect of the fMRI on the robot. The effect of the handle on the robot's kinematics is minimal since it is lightweight (2.6 lbs) but extremely stiff 3/4" graphite and well balanced on the 3DoF joint in the middle. The end result is an fMRI compatible, haptic system with about 1 cubic foot of working space, and, when combined with virtual reality, it allows for a new set of experiments to be performed in the fMRI environment including naturalistic reaching, passive displacement of the limb and haptic perception, adaptation learning in varying force fields, or texture identification.
功能磁共振成像（fMRI）通过血氧水平依赖（BOLD）信号提供出色的脑功能成像，具有包括非电离辐射、解剖和功能数据的毫米级空间精度以及近乎实时分析等优点。触觉机器人可在合理受限的空间内精确测量和控制光标位置及力。在此，我们将这两种技术相结合，以进行涉及运动控制与触觉/触感环境交互（如伸手或抓握）的精确实验。基本思路是将一个由中心支撑的8英尺末端执行器连接到机器人上，让受试者使用该机器人，但对其进行屏蔽，并使其远离功能磁共振成像（fMRI）机器磁场最强烈的部分（图1）。幻影高级3.0六自由度高力机器人（SensAble Technologies公司）是在虚拟现实实验中提供力反馈的绝佳选择，但它本质上并非磁共振安全设备，会给灵敏的功能磁共振成像设备引入大量噪声，且其电动机会受到功能磁共振成像强烈变化磁场的影响。我们构建了一个工作台和屏蔽系统，使机器人能够安全地引入功能磁共振成像环境，同时限制电动机会产生电噪声对功能磁共振成像信号的衰减，以及功能磁共振成像强烈变化的磁场对电动机性能的衰减。有了屏蔽装置后，功能磁共振成像的信噪比（SNR：平均信号/噪声标准差）从约380的基线降至约330，无屏蔽时则降至约250。剩余噪声似乎不相关，且不会给测试球体的功能磁共振成像添加伪影（图2）。长而硬的手柄可将机器人放置在磁场变化最强烈部分的范围之外，因此功能磁共振成像对机器人没有显著影响。手柄对机器人运动学的影响极小，因为它很轻（约2.6磅），但由极硬的3/4英寸石墨制成，并且在中间的三自由度关节上平衡良好。最终结果是一个与功能磁共振成像兼容的触觉系统，其工作空间约为1立方英尺，并且与虚拟现实相结合时，它允许在功能磁共振成像环境中进行一系列新的实验，包括自然伸手、肢体被动位移和触觉感知、在不同力场中的适应性学习或纹理识别。
