Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada.
Med Phys. 2010 Jan;37(1):249-57. doi: 10.1118/1.3273032.
Megavoltage computed tomography (MVCT) and megavoltage cone beam computed tomography (MVCBCT) can be used for visualizing anatomical structures prior to radiation therapy treatments to assist in patient setup and target localization. These systems are less susceptible to metal artifacts and provide better CT number linearity than conventional CT scanners. However, their contrast is limited by the properties of the megavoltage photons and the low detective quantum efficiency (DQE) of flat panel detector systems currently available. By using higher DQE, thick, segmented cadmium tungstate detectors, the authors can improve the low contrast detectability of a MVCT system. This in turn would permit greater soft tissue visualization for a given radiation dose, allowing MVCT to be used in more clinical situations.
This article describes the evaluation of our prototype system that uses thick, segmented detectors. In order to create images using a dose that would be acceptable for day to day patient imaging, the authors evaluated their system using the low intensity bremsstrahlung component of a 6 MeV electron beam. The system was evaluated for its uniformity, high contrast resolution, low contrast detectability, signal to noise ratio, contrast to noise ratio, and CT number linearity.
The prototype system was found to have a high contrast spatial resolution of about 5 line pairs per cm, and to be able to visualize a 15 mm 1.5% contrast target with 2 cGy of radiation dose delivered. SNR2 vs radiation dose and mean pixel value vs electron density curves were linear.
This prototype system shows a large improvement in low contrast detectability over current MVCBCT systems.
兆伏级计算机断层扫描（MVCT）和兆伏级锥形束计算机断层扫描（MVCBCT）可用于在放射治疗前可视化解剖结构，以帮助患者定位和目标定位。与传统 CT 扫描仪相比，这些系统不易受到金属伪影的影响，并且 CT 数线性度更好。然而，它们的对比度受到兆伏级光子的特性和当前可用的平板探测器系统的低探测量子效率（DQE）的限制。通过使用具有更高 DQE 的厚、分段的碲化镉钨探测器，作者可以提高 MVCT 系统的低对比度检测能力。这反过来又允许在给定剂量下进行更大的软组织可视化，从而使 MVCT 能够在更多的临床情况下使用。
本文介绍了作者对使用厚、分段探测器的原型系统的评估。为了使用可接受的日常患者成像剂量创建图像，作者使用 6 MeV 电子束的低强度韧致辐射成分来评估他们的系统。该系统的均匀性、高对比度分辨率、低对比度检测能力、信噪比、对比噪声比和 CT 数线性度进行了评估。
原型系统被发现具有约 5 线对/厘米的高对比度空间分辨率，并且能够以 2 cGy 的辐射剂量可视化 15 毫米 1.5%对比度的目标。SNR2 与辐射剂量和平均像素值与电子密度曲线呈线性关系。
与当前的 MVCBCT 系统相比，该原型系统在低对比度检测能力方面有了很大的提高。
