García-Vicente F, Delgado J M, Peraza C
Hospital Universitario La Princesa, Departamento de Radiofisica, Madrid, Spain. fgarcia/
Med Phys. 1998 Feb;25(2):202-7. doi: 10.1118/1.598182.
One of the most important parameters in the characterization of a detector is its spatial convolution kernel. This kernel contains all of the information about the influence that the detector size has on the measured beam profile. In this paper we present an experimental method for the determination of the spatial convolution kernel for commonly used detectors that are employed in the x-ray profile measurement: film + densitometer, diode, and ionization minichamber. Our work is based on first assuming a step function pattern on a photographic film is known and is a perfect step function. The kernel of the densitometer system was then derived from the deconvolution of the scanned profile to the step function. Next a film was exposed to a penumbra area of an x-ray beam from a linac. The film was scanned using the same densitometer. The "real profile" that emerges from a linear accelerator was derived by the deconvolution of the scanned profile using the now known kernel of the film densitometer. Under the same irradiation condition the x-ray profile was measured with other detectors and with this information we obtained the convolution kernels for these detectors by solving numerically their basic convolution integrals. The results show that the Gaussian convolution kernel is the most consistent with the measurements. The best numerical values for the FWHM of the kernels are 1.1 mm, 2.2 mm, and 5.4 mm for densitometer, diode, and minichamber, respectively.
探测器特性描述中最重要的参数之一是其空间卷积核。该卷积核包含了探测器尺寸对测量光束轮廓影响的所有信息。在本文中，我们提出了一种实验方法，用于确定X射线轮廓测量中常用探测器（胶片+密度计、二极管和电离小型室）的空间卷积核。我们的工作首先基于假设已知摄影胶片上的阶跃函数图案且它是一个完美的阶跃函数。然后通过将扫描轮廓与阶跃函数进行去卷积来推导密度计系统的卷积核。接下来，将一张胶片暴露于直线加速器X射线束的半影区域。使用同一台密度计对胶片进行扫描。通过使用现在已知的胶片密度计卷积核对扫描轮廓进行去卷积，得出直线加速器产生的“真实轮廓”。在相同的照射条件下，用其他探测器测量X射线轮廓，并利用这些信息通过数值求解它们的基本卷积积分来获得这些探测器的卷积核。结果表明，高斯卷积核与测量结果最为一致。密度计、二极管和小型室的卷积核半高宽的最佳数值分别为1.1毫米、2.2毫米和5.4毫米。
