Archambault Louis, Briere Tina Marie, Beddar Sam
Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2008 Oct;35(10):4342-51. doi: 10.1118/1.2975147.
Charge coupled devices (CCDs) are being increasingly used in radiation therapy for dosimetric purposes. However, CCDs are sensitive to stray radiation. This effect induces transient noise. Radiation-induced noise strongly alters the image and therefore limits its quantitative analysis. The purpose of this work is to characterize the radiation-induced noise and to develop filtration algorithms to restore image quality. Two models of CCD were used for measurements close to a medical linac. The structure of the transient noise was first characterized. Then, four methods of noise filtration were compared: median filtering of a time series of identical images, uniform median filtering of single images, an adaptive filter with switching mechanism, and a modified version of the adaptive switch filter. The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: The average noise cluster size was 1.2 +/- 0.6 and 3.2 +/- 2.7 pixels for the U2000 and the Luca, respectively. The median of a time series of images resulted in the best filtration and minimal image distortion. For applications where time series is impractical, the adaptive switch filter must be used to reduce image distortion. Our modified version of the switch filter can be used in order to handle nonisolated groups of noisy pixels.
电荷耦合器件（CCD）在放射治疗剂量测定中的应用越来越广泛。然而，CCD对杂散辐射敏感。这种效应会产生瞬态噪声。辐射诱导噪声会严重改变图像，从而限制其定量分析。这项工作的目的是表征辐射诱导噪声，并开发滤波算法以恢复图像质量。使用两种型号的CCD在医用直线加速器附近进行测量。首先对瞬态噪声的结构进行了表征。然后，比较了四种噪声滤波方法：相同图像时间序列的中值滤波、单幅图像的均匀中值滤波、具有切换机制的自适应滤波器以及自适应切换滤波器的改进版本。两台相机中噪声像素的强度分布相似。然而，噪声的空间分布不同：U2000和Luca相机的平均噪声簇大小分别为1.2±0.6像素和3.2±2.7像素。图像时间序列的中值滤波产生了最佳的滤波效果和最小的图像失真。对于时间序列不可行的应用，必须使用自适应切换滤波器来减少图像失真。我们改进的切换滤波器版本可用于处理非孤立的噪声像素组。
