Qi Wenyuan, Yang Yongyi, Niu Xiaofeng, King Michael A
Department of Electrical and Computer Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, USA.
Med Phys. 2012 Aug;39(8):5182-93. doi: 10.1118/1.4738377.
Motion-compensated temporal processing can have a major impact on improving the image quality in gated cardiac single photon emission computed tomography (SPECT). In this work, we investigate the effect of different optical flow estimation methods for motion-compensated temporal processing in gated SPECT. In particular, we explore whether better motion estimation can substantially improve reconstructed image quality, and how the estimated motion would compare to the ideal case of known motion in terms of reconstruction.
We consider the following three methods for obtaining the image motion in 4D reconstruction: (1) the Horn-Schunck optical flow equation (OFE) method, (2) a recently developed periodic OFE method, and (3) known cardiac motion derived from the NURBS-based cardiac-torso (NCAT) phantom. The periodic OFE method is used to exploit the inherent periodic nature in cardiac gated images. In this method, the optical flow in a sequence is modeled by a Fourier harmonic representation, which is then estimated from the image data. We study the impact of temporal processing on 4D reconstructions when the image motion is obtained with the different methods above. For quantitative evaluation, we use simulated imaging with multiple noise realizations from the NCAT phantom, where different patient geometry and lesion sizes are also considered. To quantify the reconstruction results, we use the following measures of reconstruction accuracy and defect detection in the myocardium: (1) overall error level in the myocardium, (2) regional accuracy of the left ventricle (LV) wall, (3) accuracy of regional time activity curves of the LV, and (4) perfusion defect detectability with a channelized Hotelling observer (CHO). In addition, we also examine the effect of noise on the distortion in the reconstructed LV wall shape by detecting its contours. As a preliminary demonstration, these methods are also tested on two sets of clinical acquisitions.
For the different quantitative measures considered, the periodic OFE further improved the reconstruction accuracy of the myocardium compared to OFE in 4D reconstruction; its improvement in reconstruction almost matched that of the known motion. Specifically, the overall mean-squared error in the myocardium was reduced by over 20% with periodic OFE; with noise level fixed at 10%, the regional bias on the LV was reduced from 20% (OFE) to 14% (periodic OFE), compared to 11% by the known motion. In addition, the CHO results show that there was also improvement in lesion detectability with the periodic OFE. The regional time activity curves obtained with the periodic OFE were also observed to be more consistent with the reference; in addition, the contours of the reconstructed LV wall with the periodic OFE were demonstrated to show less degree of variations among different noise realizations. Such improvements were also consistent with the results obtained from the clinical acquisitions.
Use of improved optical flow estimation can further improve the accuracy of reconstructed images in 4D. The periodic OFE method not only can achieve improvements over the traditional OFE, but also can almost match that of the known motion in terms of the several quality measures considered.
运动补偿时间处理对改善门控心脏单光子发射计算机断层扫描（SPECT）的图像质量可能有重大影响。在本研究中，我们调查了不同光流估计方法在门控SPECT运动补偿时间处理中的效果。特别是，我们探讨了更好的运动估计是否能显著提高重建图像质量，以及在重建方面估计的运动与已知运动的理想情况相比如何。
我们考虑以下三种在四维重建中获取图像运动的方法：（1）霍恩 - 舒恩克光流方程（OFE）法，（2）最近开发的周期性OFE法，（3）从基于非均匀有理B样条（NURBS）的心脏躯干（NCAT）模型导出的已知心脏运动。周期性OFE法用于利用心脏门控图像中固有的周期性。在该方法中，序列中的光流由傅里叶谐波表示建模，然后从图像数据中估计。我们研究当使用上述不同方法获得图像运动时，时间处理对四维重建的影响。为了进行定量评估，我们使用来自NCAT模型的具有多个噪声实现的模拟成像，其中还考虑了不同的患者几何形状和病变大小。为了量化重建结果，我们使用以下心肌重建准确性和缺陷检测的指标：（1）心肌中的总体误差水平，（2）左心室（LV）壁的区域准确性，（3）LV的区域时间 - 活性曲线的准确性，以及（4）使用通道化霍特林观察者（CHO）的灌注缺陷可检测性。此外，我们还通过检测其轮廓来检查噪声对重建LV壁形状失真的影响。作为初步演示，这些方法也在两组临床采集中进行了测试。
对于所考虑的不同定量指标，与四维重建中的OFE相比，周期性OFE进一步提高了心肌的重建准确性；其在重建方面的改进几乎与已知运动的改进相匹配。具体而言，使用周期性OFE时，心肌中的总体均方误差降低了超过20％；在噪声水平固定为10％时，LV上的区域偏差从20％（OFE）降至14％（周期性OFE），而已知运动为11％。此外，CHO结果表明，使用周期性OFE时病变可检测性也有所提高。还观察到使用周期性OFE获得的区域时间 - 活性曲线与参考曲线更一致；此外，使用周期性OFE重建的LV壁轮廓在不同噪声实现之间的变化程度较小。这些改进也与临床采集获得的结果一致。
使用改进的光流估计可以进一步提高四维重建图像的准确性。周期性OFE方法不仅可以在传统OFE的基础上实现改进，而且在所考虑的几个质量指标方面几乎可以与已知运动相匹配。
