A unified scatter rejection and correction method for cone beam computed tomography.
A unified scatter rejection and correction method for cone beam computed tomography.
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一种用于锥束计算机断层扫描的统一散射抑制与校正方法
DOI：
10.1002/mp.14681
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发表时间：
2021-03
期刊：
影响因子：
3.8
通讯作者：
Gopal A
中科院分区：
文献类型：
--
作者：
Altunbas C;Park Y;Yu Z;Gopal A
散射辐射是基于平板探测器的锥形束CT（CBCT）中图像质量下降的主要原因。虽然最近推出的二维防散射栅格能够阻挡大部分散射通量，但仍有小部分散射通量传输到探测器，这对于在CBCT中实施双能成像等定量成像技术仍然是一个重大挑战。此外，这种残留散射也是栅格诱导伪影的主要来源，这阻碍了二维栅格在CBCT中的应用。
因此，我们提出了一种使用二维防散射栅格实现有效散射抑制和残留散射校正的新方法；通过这种方法，我们将二维栅格的作用从单纯的散射抑制装置扩展到散射测量装置。
在我们的方法中，二维栅格的不透射线隔片模拟放置在探测器上的微束挡阵列，从而产生空间周期性的隔片阴影。通过选择足够薄的栅格隔片，可以在保持散射强度均匀性的同时降低主射线强度。这使我们能够将隔片阴影中调制的像素信号强度与局部散射强度相关联。然后我们的方法利用这种相关性来测量并从投影中去除残留散射强度。对于被成像的物体不做任何假设。我们将此称为基于栅格的散射采样（GSS）。
在这项工作中，我们通过模拟和测量评估了栅格隔片信号调制的原理、散射估计的准确性以及GSS方法对图像质量的影响。我们还使用二维栅格原型通过实验实施了GSS方法。
我们的结果表明，GSS方法提高了CT值的准确性，并减少了与散射相关的图像伪影。在使用二维栅格和残留散射校正的情况下，与仅使用二维栅格相比，在骨盆大小的体模中，HU不均匀性从65 HU降低到30 HU，并且由于体模尺寸变化导致的HU变化从59 HU降低到20 HU。通过GSS方法进行残留散射校正后，栅格诱导的环形伪影得到抑制，噪声降低了41%。在抑制环形伪影前后，调制传递函数（MTF）的形状得以保持。
我们基于栅格的散射采样方法能够将二维栅格用作散射测量和校正装置。这种方法显著提高了CBCT中的定量准确性，进一步缩小了CBCT和多排探测器CT之间的图像质量差距。
通过使用所提出的方法校正残留散射，投影中的栅格诱导线状伪影以及CBCT图像中的相关环形伪影也得到了抑制，同时空间分辨率没有受到影响。
Scattered radiation is the primary cause of image quality degradation in flat panel detector-based cone beam CT (CBCT). While recently introduced 2D antiscatter grids reject the majority of scatter fluence, the small percentage of scatter fluence still transmitted to the detector remains a major challenge for implementation of quantitative imaging techniques such as dual energy imaging in CBCT. Additionally, this residual scatter is also a major source of grid-induced artifacts, which impedes implementation of 2D grids in CBCT. We therefore present a new method to achieve both robust scatter rejection and residual scatter correction using a 2D antiscatter grid; in doing so, we expand the role of 2D grids from mere scatter rejection devices to scatter measurement devices. In our method, the radiopaque septa of the 2D grid emulate a micro array of beam-stops placed on the detector which introduce spatially periodic septal shadows. By selecting sufficiently thin grid septa, primary intensity can be reduced while preserving uniformity of scatter intensity. This enables us to correlate the modulated pixel signal intensity in septal shadows with local scatter intensity. Our method then exploits this correlation to measure and remove residual scatter intensity from projections. No assumptions are made about the object being imaged. We refer to this as Grid-based Scatter Sampling (GSS). In this work, we evaluate the principle of signal modulation with grid septa, the accuracy of scatter estimates, and the effect of the GSS method on image quality using simulations and measurements. We also implement the GSS method experimentally using a 2D grid prototype. Our results demonstrate that the GSS method increased CT number accuracy and reduced image artifacts associated with scatter. With 2D grid and residual scatter correction, HU nonuniformity was reduced from 65 HU to 30 HU in pelvis sized phantoms, and HU variations due to change in phantom size were reduced from 59 HU to 20 HU, when compared to use of only a 2D grid. With residual scatter correction via GSS method, grid-induced ring artifacts were suppressed, leading to a 41% reduction in noise. The shape of the modulation transfer function (MTF) was preserved before and after suppression of ring artifacts. Our grid-based scatter sampling method enables utilization of a 2D grid as a scatter measurement and correction device. This method significantly improves quantitative accuracy in CBCT, further reducing the image quality gap between CBCT and multi-detector CT. By correcting residual scatter with the proposed method, grid-induced line artifacts in projections and associated ring artifacts in CBCT images were also suppressed with no compromise of spatial resolution.
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