Department of Medical Biophysics, University of Toronto, Toronto, Canada.
Med Phys. 2012 Mar;39(3):1518-29. doi: 10.1118/1.3687163.
Most electronic portal imaging devices (EPIDs) developed so far use a thin Cu plate/phosphor screen to convert x-ray energies into light photons, while maintaining a high spatial resolution. This results in a low x-ray absorption and thus a low quantum efficiency (QE) of approximately 2-4% for megavoltage (MV) x-rays. A significant increase of QE is desirable for applications such as MV cone-beam computed tomography (MV-CBCT). Furthermore, the Cu plate/phosphor screen contains high atomic number (high-Z) materials, resulting in an undesirable over-response to low energy x-rays (due to photoelectric effect) as well as high energy x-rays (due to pair production) when used for dosimetric verification. Our goal is to develop a new MV x-ray detector that has a high QE and uses low-Z materials to overcome the obstacles faced by current MV x-ray imaging technologies.
A new high QE and low-Z EPID is proposed. It consists of a matrix of plastic scintillating fibers embedded in a water-equivalent medium and coupled to an optically sensitive 2D active matrix flat panel imager (AMFPI) for image readout. It differs from the previous approach that uses segmented crystalline scintillators made of higher density and higher atomic number materials to detect MV x-rays. The plastic scintillating fibers are focused toward the x-ray source to avoid image blurring due to oblique incidence of off-axis x-rays. When MV x-rays interact with the scintillating fibers in the detector, scintillation light will be produced. The light photons produced in a fiber core and emitted within the acceptance angle of the fiber will be guided toward the AMFPI by total internal reflection. A Monte Carlo simulation has been used to investigate imaging and dosimetric characteristics of the proposed detector under irradiation of MV x-rays.
Properties, such as detection efficiency, modulation transfer function, detective quantum efficiency (DQE), energy dependence of detector response, and water-equivalence of dose response have been investigated. It has been found that the zero frequency DQE of the proposed detector can be up to 37% at 6 MV. The detector, also, is water-equivalent with a relatively uniform response to different energy x-rays as compared to current EPIDs.
The results of our simulations show that, using plastic scintillating fibers, it is possible to construct a water-equivalent EPID that has a better energy response and a higher detection efficiency than current flat panel based EPIDs.
迄今为止开发的大多数电子射野影像装置（EPID）使用薄铜板/磷光屏将 X 射线能量转换为光光子，同时保持高空间分辨率。这导致低 X 射线吸收率，从而导致兆伏（MV）X 射线的量子效率（QE）约为 2-4%。对于 MV 锥形束计算机断层扫描（MV-CBCT）等应用，需要显著提高 QE。此外，铜板/磷光屏包含高原子序数（高 Z）材料，当用于剂量验证时，这会导致对低能 X 射线（由于光电效应）以及高能 X 射线（由于对产生）的过度响应。我们的目标是开发一种具有高 QE 且使用低 Z 材料的新型 MV X 射线探测器，以克服当前 MV X 射线成像技术所面临的障碍。
提出了一种新型高 QE 和低 Z EPID。它由嵌入在水等效介质中的塑料闪烁光纤矩阵组成，并与用于图像读出的光学敏感 2D 有源矩阵平板成像仪（AMFPI）耦合。它与以前使用更高密度和更高原子序数材料制成的分段结晶闪烁体来检测 MV X 射线的方法不同。塑料闪烁光纤聚焦到 X 射线源，以避免由于离轴 X 射线的斜入射而导致图像模糊。当 MV X 射线与探测器中的闪烁光纤相互作用时，将产生闪烁光。在光纤芯中产生并在光纤的接受角内发射的光光子将通过全内反射引导到 AMFPI。已经使用蒙特卡罗模拟来研究在 MV X 射线照射下提出的探测器的成像和剂量学特性。
已经研究了诸如探测效率、调制传递函数、探测量子效率（DQE）、探测器响应的能量依赖性以及剂量响应的水等效性等特性。已经发现，在 6 MV 时，所提出的探测器的零频率 DQE 可以高达 37%。与当前的 EPID 相比，该探测器还具有水等效性，并且对不同能量的 X 射线具有相对均匀的响应。
我们的模拟结果表明，使用塑料闪烁光纤，可以构建具有比当前基于平板的 EPID 更好的能量响应和更高探测效率的水等效 EPID。
