Swindell W, Morton E J, Evans P M, Lewis D G
Joint Department of Physics, Institute of Cancer Research, Sutton, Surrey, England.
Med Phys. 1991 Sep-Oct;18(5):855-66. doi: 10.1118/1.596735.
This study investigates factors associated with the imaging of a patient using a high-energy radiotherapy treatment beam. Both single-stage (e.g., solid-state detector) and two-stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter-to-primary signal-variance ratio and the differential-signal-to-noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x-ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10(-3) cGy is required to detect reliably the presence of a bone section of area 10 x 10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter-to-primary signal-variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.
本研究调查了与使用高能放射治疗束对患者进行成像相关的因素。研究考虑了单级系统（例如，固态探测器）和两级系统（例如，闪烁屏加电视）。首先，推导出一个表达式，该表达式将物体中积累深度处的剂量与物体结构、散射与原发射信号方差比以及图像中的差分信号与噪声比联系起来。其次，推导出数字化图像所需的位数。第三，研究了散射辐射对光子计数、光峰和康普顿探测器类型的影响。第四，考虑了检测过程中噪声的影响。最后，推导出X射线源尺寸、探测器孔径和图像放大倍数之间的关系。根据系统传递函数讨论了给定源尺寸和探测器孔径下的最佳放大倍数。研究表明，在2兆电子伏特的原射线能量下，要在250毫米的软组织中可靠地检测出面积为10×10毫米、厚度为4毫米的骨切片的存在，需要10^(-3) 厘戈瑞的剂量。对于此示例，还估计需要10位的数字化精度。计算表明，对于康普顿探测器，散射与原发射信号方差比从物体出射表面处的约30%的值降至距物体80厘米处的5%，从而使形成图像所需的剂量略有降低。
