Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298, USA.
Med Phys. 2011 May;38(5):2508-14. doi: 10.1118/1.3582944.
To investigate the cause of a bow-tie wobble artifact (BWA) discovered on Varian OBI CBCT images and to develop practical correction strategies.
The dependence of the BWA on phantom geometry, phantom position, specific system, and reconstruction algorithm was investigated. Simulations were conducted to study the dependence of the BWA on scatter and beam hardening corrections. Geometric calibration was performed to rule out other gantry-angle dependent mechanical non-idealities as BWA causes. Air scans were acquired with ball-bearing markers to study the motions of the x-ray head assembly as functions of gantry angle. Based on measurements, we developed hypothesis regarding the BWA cause. Simulations were performed to validate our hypothesis. Two correction strategies were implemented: a measurement-based method, which acquires gantry-dependent normalization projections (NPs); and a model-based method that involves numerically shifting the single-angle NP to compensate for the previously-measured bow-tie-filter (BTF) motion.
The BWA has a diameter of approximately 15 cm, is centered at the isocenter, and is reproducible independent of phantom, position, system, reconstruction, and standard corrections, but only when the BTF is used. Measurements identified a 2D sinusoidal gantry-angle-dependent motion of the x-ray head assembly, and it was the BTF motion (>3 mm amplitude projected onto the detector) resulting an intensity mismatch between the all-angle CBCT projections and a single-angle NP that caused the BWA. Both correction strategies were demonstrated effective.
A geometric mismatch between the BTF modulation patterns on CBCT projections and on the NP causes the BWA. The BTF wobble requires additional degrees of freedom in CBCT geometric calibration to characterize.
研究在瓦里安 OBI CBCT 图像中发现的蝴蝶结摆动伪影（BWA）的原因，并开发实用的校正策略。
研究了 BWA 对体模几何形状、体模位置、特定系统和重建算法的依赖性。进行了模拟研究以研究 BWA 对散射和束硬化校正的依赖性。进行了几何校准以排除其他与机架角度相关的机械非理想性作为 BWA 原因。进行了空气扫描，使用滚珠轴承标记来研究 X 射线头组件的运动作为机架角度的函数。根据测量结果，我们提出了关于 BWA 原因的假设。进行了模拟以验证我们的假设。实施了两种校正策略：一种是基于测量的方法，它获取与机架相关的归一化投影（NPs）；另一种是基于模型的方法，涉及数值地移动单角度 NP 以补偿先前测量的蝴蝶结滤波器（BTF）运动。
BWA 的直径约为 15 厘米，位于等中心，并且与体模、位置、系统、重建和标准校正无关，但是仅在使用 BTF 时才具有可重复性。测量确定了 X 射线头组件的二维正弦机架角度相关运动，并且是 BTF 运动（投影到探测器上的幅度大于 3 毫米）导致所有角度 CBCT 投影和单角度 NP 之间的强度不匹配，从而导致 BWA。两种校正策略都证明是有效的。
在 CBCT 投影和 NP 上的 BTF 调制模式之间的几何不匹配导致了 BWA。BTF 摆动需要在 CBCT 几何校准中增加自由度来进行特征描述。
