无关的误定位事件对自由活动动物的运动校正脑单光子发射计算机断层扫描（SPECT）图像的影响。
Impact of extraneous mispositioned events on motion-corrected brain SPECT images of freely moving animals.
作者信息
Angelis Georgios I, Ryder William J, Bashar Rezaul, Fulton Roger R, Meikle Steven R
机构信息
Faculty of Health Sciences and Brain and Mind Research Institute, The University of Sydney, Sydney, NSW 2006, Australia.
Faculty of Health Sciences and Brain and Mind Research Institute, The University of Sydney, Sydney, NSW 2006, Australia; School of Physics, University of Sydney, Sydney, NSW 2006, Australia; and Department of Medical Physics, Westmead Hospital, Sydney, NSW 2145, Australia.
出版信息
Med Phys. 2014 Sep;41(9):092502. doi: 10.1118/1.4892931.
PURPOSE
Single photon emission computed tomography (SPECT) brain imaging of freely moving small animals would allow a wide range of important neurological processes and behaviors to be studied, which are normally inhibited by anesthetic drugs or precluded due to the animal being restrained. While rigid body motion of the head can be tracked and accounted for in the reconstruction, activity in the torso may confound brain measurements, especially since motion of the torso is more complex (i.e., nonrigid) and not well correlated with that of the head. The authors investigated the impact of mispositioned events and attenuation due to the torso on the accuracy of motion corrected brain images of freely moving mice.
METHODS
Monte Carlo simulations of a realistic voxelized mouse phantom and a dual compartment phantom were performed. Each phantom comprised a target and an extraneous compartment which were able to move independently of each other. Motion correction was performed based on the known motion of the target compartment only. Two SPECT camera geometries were investigated: a rotating single head detector and a stationary full ring detector. The effects of motion, detector geometry, and energy of the emitted photons (hence, attenuation) on bias and noise in reconstructed brain regions were evaluated.
RESULTS
The authors observed two main sources of bias: (a) motion-related inconsistencies in the projection data and (b) the mismatch between attenuation and emission. Both effects are caused by the assumption that the orientation of the torso is difficult to track and model, and therefore cannot be conveniently corrected for. The motion induced bias in some regions was up to 12% when no attenuation effects were considered, while it reached 40% when also combined with attenuation related inconsistencies. The detector geometry (i.e., rotating vs full ring) has a big impact on the accuracy of the reconstructed images, with the full ring detector being more advantageous.
CONCLUSIONS
Motion-induced inconsistencies in the projection data and attenuation/emission mismatch are the two main causes of bias in reconstructed brain images when there is complex motion. It appears that these two factors have a synergistic effect on the qualitative and quantitative accuracy of the reconstructed images.
目的
对自由活动的小动物进行单光子发射计算机断层扫描（SPECT）脑成像，将能够研究一系列重要的神经过程和行为，而这些通常会受到麻醉药物的抑制，或者由于动物被束缚而无法进行研究。虽然头部的刚体运动可以在重建过程中被跟踪和校正，但躯干的活动可能会干扰脑部测量，特别是因为躯干的运动更为复杂（即非刚体运动），且与头部运动的相关性不佳。作者研究了由于躯干导致的事件误定位和衰减对自由活动小鼠运动校正脑图像准确性的影响。
方法
对逼真的体素化小鼠模型和双隔室模型进行了蒙特卡罗模拟。每个模型都包括一个目标隔室和一个外部隔室，它们能够彼此独立移动。仅基于目标隔室的已知运动进行运动校正。研究了两种SPECT相机几何结构：旋转单头探测器和固定全环探测器。评估了运动、探测器几何结构以及发射光子的能量（因此还有衰减）对重建脑区偏差和噪声的影响。
结果
作者观察到偏差的两个主要来源：（a）投影数据中与运动相关的不一致性，以及（b）衰减与发射之间的不匹配。这两种效应都是由于难以跟踪和建模躯干的方向这一假设导致的，因此无法方便地进行校正。在不考虑衰减效应时，某些区域的运动引起的偏差高达12%，而当同时考虑与衰减相关的不一致性时，偏差达到40%。探测器几何结构（即旋转式与全环式）对重建图像的准确性有很大影响，全环探测器更具优势。
结论
当存在复杂运动时，投影数据中运动引起的不一致性以及衰减/发射不匹配是重建脑图像偏差的两个主要原因。看来这两个因素对重建图像的定性和定量准确性具有协同作用。
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