铜转换板对低 Z 靶图像质量的影响。

The effect of copper conversion plates on low-Z target image quality.

机构信息

Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada.

出版信息

Med Phys. 2012 Sep;39(9):5362-71. doi: 10.1118/1.4742052.

DOI:10.1118/1.4742052

PMID:22957604

Abstract

PURPOSE

Common electronic portal imaging devices (EPIDs) contain a 1.0 mm copper conversion plate to increase detection efficiency of a therapeutic megavoltage spectrum. When used in imaging with a photon beam generated with a low atomic number (Z) target, the conversion plate attenuates a substantial proportion of photons in the diagnostic range, thereby reducing the achievable image quality. In this work, we measure directly dependence on low-Z target image quality as a function of copper plate thickness, for both planar imaging and cone beam computed tomography (CBCT).

METHODS

Monte Carlo modeling was used to quantify changes to the diagnostic spectrum and detector response for low-Z target beams generated with either 2.35 or 7.00 MeV electrons incident on a carbon target. Planar contrast-to-noise ratio (CNR) and spatial resolution measurements were made as a function of copper thickness. CNR measurements were made for CBCT imaging as a function of dose both with and without the copper plate present in the EPID.

RESULTS

The presence of copper in the EPID decreased the diagnostic photon population by up to 20% and suppressed the peak detector response (dose deposited in the scintillator) at 60 keV by a factor of 6.4. Planar CNR was increased by a factor ranging from 1.4 to 4.0, depending on the material imaged, with no copper present compared to a standard 1.0 mm thickness. Planar spatial resolution was only slightly degraded with increasing copper thickness. Increases in CBCT image CNR ranged from a factor of 1.3-2.1 with the copper plate removed.

CONCLUSIONS

It is possible to increase the proportion of photons in the diagnostic energy range (25 keV-150 keV) reaching the phosphor screen by as much as 20% when removing the copper conversion plate. This results in significant increases of planar and CBCT image CNR. Consequently, we suggest that the copper conversion plate be removed from the EPID when used for low-Z target planar or CBCT imaging.

摘要

目的

常见的电子射野影像装置（EPID）包含 1.0 毫米厚的铜转换板，以提高治疗兆伏光谱的检测效率。当使用低原子序数（Z）靶产生的光子束进行成像时，转换板会衰减诊断范围内的大量光子，从而降低可实现的图像质量。在这项工作中，我们直接测量了铜片厚度对低 Z 靶成像质量的依赖性，包括平面成像和锥形束计算机断层扫描（CBCT）。

方法

使用蒙特卡罗建模来量化由 2.35 或 7.00 MeV 电子入射到碳靶上产生的低 Z 靶束对诊断谱和探测器响应的变化。作为铜厚度的函数，进行了平面对比度噪声比（CNR）和空间分辨率测量。作为剂量的函数，在 EPID 中存在和不存在铜片的情况下，对 CBCT 成像进行了 CNR 测量。

结果

EPID 中的铜降低了多达 20%的诊断光子数，并将 60keV 处的峰值探测器响应（闪烁体中沉积的剂量）抑制了 6.4 倍。与标准 1.0 毫米厚度相比，当不存在铜时，平面 CNR 增加了 1.4 到 4.0 倍，具体取决于所成像的材料。随着铜厚度的增加，平面空间分辨率仅略有下降。当去除铜片时，CBCT 图像 CNR 的增加幅度在 1.3-2.1 倍之间。