Campbell Warren G, Wells Derek M, Jirasek Andrew
Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia V8P 5C2, Canada.
British Columbia Cancer Agency, Vancouver Island Centre, Victoria, British Columbia V8R 6V5, Canada.
Med Phys. 2014 Nov;41(11):112102. doi: 10.1118/1.4897245.
The objective of this work is to demonstrate imaging artifacts that can occur during the optical computed tomography (CT) scanning of polymer gel dosimeters due to radiation-induced refractive index (RI) changes in polyacrylamide gels.
A 1 L cylindrical polyacrylamide gel dosimeter was irradiated with 3 × 3 cm(2) square beams of 6 MV photons. A prototype fan-beam optical CT scanner was used to image the dosimeter. Investigative optical CT scans were performed to examine two types of rayline bending: (i) bending within the plane of the fan-beam and (ii) bending out the plane of the fan-beam. To address structured errors, an iterative Savitzky-Golay (ISG) filtering routine was designed to filter 2D projections in sinogram space. For comparison, 2D projections were alternatively filtered using an adaptive-mean (AM) filter.
In-plane rayline bending was most notably observed in optical CT projections where rays of the fan-beam confronted a sustained dose gradient that was perpendicular to their trajectory but within the fan-beam plane. These errors caused distinct streaking artifacts in image reconstructions due to the refraction of higher intensity rays toward more opaque regions of the dosimeter. Out-of-plane rayline bending was observed in slices of the dosimeter that featured dose gradients perpendicular to the plane of the fan-beam. These errors caused widespread, severe overestimations of dose in image reconstructions due to the higher-than-actual opacity that is perceived by the scanner when light is bent off of the detector array. The ISG filtering routine outperformed AM filtering for both in-plane and out-of-plane rayline errors caused by radiation-induced RI changes. For in-plane rayline errors, streaks in an irradiated region (>7 Gy) were as high as 49% for unfiltered data, 14% for AM, and 6% for ISG. For out-of-plane rayline errors, overestimations of dose in a low-dose region (∼50 cGy) were as high as 13 Gy for unfiltered data, 10 Gy for AM, and 3.1 Gy for ISG. The ISG routine also addressed unrelated artifacts that previously needed to be manually removed in sinogram space. However, the ISG routine blurred reconstructions, causing losses in spatial resolution of ∼5 mm in the plane of the fan-beam and ∼8 mm perpendicular to the fan-beam.
This paper reveals a new category of imaging artifacts that can affect the optical CT readout of polyacrylamide gel dosimeters. Investigative scans show that radiation-induced RI changes can cause significant rayline errors when rays confront a prolonged dose gradient that runs perpendicular to their trajectory. In fan-beam optical CT, these errors manifested in two ways: (1) distinct streaking artifacts caused by in-plane rayline bending and (2) severe overestimations of opacity caused by rays bending out of the fan-beam plane and missing the detector array. Although the ISG filtering routine mitigated these errors better than an adaptive-mean filtering routine, it caused unacceptable losses in spatial resolution.
本研究旨在展示在光学计算机断层扫描（CT）过程中，由于聚丙烯酰胺凝胶中辐射诱导的折射率（RI）变化，聚合物凝胶剂量计可能出现的成像伪影。
用6 MV光子的3×3 cm²方形射束照射1 L圆柱形聚丙烯酰胺凝胶剂量计。使用一台原型扇束光学CT扫描仪对剂量计进行成像。进行研究性光学CT扫描以检查两种类型的射线弯曲：（i）在扇束平面内的弯曲和（ii）偏离扇束平面的弯曲。为了解决结构化误差，设计了一种迭代Savitzky - Golay（ISG）滤波程序，用于在正弦图空间中对二维投影进行滤波。作为比较，二维投影也使用自适应均值（AM）滤波器进行滤波。
在光学CT投影中最明显观察到平面内射线弯曲，即扇束的射线遇到持续的剂量梯度，该梯度垂直于其轨迹但在扇束平面内。这些误差由于较高强度射线向剂量计更不透明区域的折射，在图像重建中导致明显的条纹伪影。在剂量计的切片中观察到平面外射线弯曲，这些切片具有垂直于扇束平面的剂量梯度。由于当光线从探测器阵列弯曲时扫描仪感知到的不透明度高于实际值，这些误差在图像重建中导致剂量的广泛、严重高估。对于由辐射诱导的RI变化引起的平面内和平面外射线误差，ISG滤波程序的性能优于AM滤波。对于平面内射线误差，在照射区域（>7 Gy）中，未滤波数据的条纹高达49%，AM滤波为14%，ISG滤波为6%。对于平面外射线误差，在低剂量区域（约50 cGy）中，未滤波数据的剂量高估高达13 Gy，AM滤波为10 Gy，ISG滤波为3.1 Gy。ISG程序还解决了以前需要在正弦图空间中手动去除的无关伪影。然而，ISG程序使重建图像模糊，导致在扇束平面内的空间分辨率损失约5 mm，垂直于扇束方向的空间分辨率损失约8 mm。
本文揭示了一类新的成像伪影，其可影响聚丙烯酰胺凝胶剂量计的光学CT读数。研究性扫描表明，当射线遇到与它们的轨迹垂直的长时间剂量梯度时，辐射诱导的RI变化可导致显著的射线误差。在扇束光学CT中，这些误差以两种方式表现出来：（1）由平面内射线弯曲引起的明显条纹伪影，以及（2）由射线弯曲出扇束平面并错过探测器阵列导致的不透明度严重高估。尽管ISG滤波程序比自适应均值滤波程序能更好地减轻这些误差，但它导致了不可接受的空间分辨率损失。
