Gang Grace J, Russ Tom, Ma Yiqun, Toennes Christian, Siewerdsen Jeffrey H, Schad Lothar R, Stayman J Webster
Johns Hopkins University, Baltimore, MD, USA.
Universität Heidelberg, Mannheim, Germany.
Conf Proc Int Conf Image Form Xray Comput Tomogr. 2020 Aug;2020:400-403.
Metal artifacts are a major confounding factor for image quality in CT, especially in image-guided surgery scenarios where surgical tools and implants frequently occur in the field-of-view. Traditional metal artifact correction methods typically use algorithmic solutions to interpolate over the highly attenuated projection measurements where metal is present but cannot recover the missing information obstructed by the metal. In this work, we treat metal artifacts as a missing data problem and employ noncircular orbits to maximize data completeness in the presence of metal. We first implement a local data completeness metric based on Tuy's condition as the percentage of great circles sampled by a particular orbit and accounted for the presence of metal by discounting any rays that pass through metal. We then compute the metric over many locations and many possible metal locations to reflect data completeness for arbitrary metal placements within a volume of interest. We used this metric to evaluate the effectiveness of sinusoidal orbits of different magnitudes and frequencies in metal artifact reduction. We also evaluated noncircular orbits in two imaging systems for phantoms with different metal objects and metal arrangements. Among a circular, tilted circular, and a sinusoidal orbit of two cycles per rotation, the latter is shown to most effectively remove metal artifacts. The noncircular orbit not only reduce the extent of streaks, but allows better visualization of spatial frequencies that cannot be recovered by metal artifact correction algorithms. These results illustrate the potential of relatively simple noncircular orbits to be robust against metal implants which ordinarily present significant challenges in interventional imaging.
金属伪影是CT图像质量的一个主要干扰因素，尤其是在图像引导手术场景中，手术工具和植入物经常出现在视野范围内。传统的金属伪影校正方法通常使用算法解决方案对存在金属的高衰减投影测量值进行插值，但无法恢复被金属遮挡的缺失信息。在这项工作中，我们将金属伪影视为一个数据缺失问题，并采用非圆形轨道来在存在金属的情况下最大化数据完整性。我们首先基于图伊条件实现了一种局部数据完整性度量，即特定轨道采样的大圆百分比，并通过剔除任何穿过金属的射线来考虑金属的存在。然后，我们在许多位置和许多可能的金属位置上计算该度量，以反映感兴趣体积内任意金属放置的数据完整性。我们使用这个度量来评估不同幅度和频率的正弦轨道在减少金属伪影方面的有效性。我们还在两个成像系统中评估了非圆形轨道对具有不同金属物体和金属排列的体模的效果。在圆形、倾斜圆形和每旋转两圈的正弦轨道中，后者被证明能最有效地去除金属伪影。非圆形轨道不仅减少了条纹的程度，还能更好地显示金属伪影校正算法无法恢复的空间频率。这些结果说明了相对简单的非圆形轨道在抵抗金属植入物方面的潜力，金属植入物通常在介入成像中带来重大挑战。
