McCurdy B M, Luchka K, Pistorius S
Medical Physics Department, CancerCare Manitoba, 675 McDermot Avenue, Winnipeg R3E 0V9,
Med Phys. 2001 Jun;28(6):911-24. doi: 10.1118/1.1374244.
A two step algorithm to predict portal dose images in arbitrary detector systems has been developed recently. The current work provides a validation of this algorithm on a clinically available, amorphous silicon flat panel imager. The high-atomic number, indirect amorphous silicon detector incorporates a gadolinium oxysulfide phosphor scintillating screen to convert deposited radiation energy to optical photons which form the portal image. A water equivalent solid slab phantom and an anthropomorphic phantom were examined at beam energies of 6 and 18 MV and over a range of air gaps (approximately 20-50 cm). In the many examples presented here, portal dose images in the phosphor were predicted to within 5% in low-dose gradient regions, and to within 5 mm (isodose line shift) in high-dose gradient regions. Other basic dosimetric characteristics of the amorphous silicon detector were investigated, such as linearity with dose rate (+/- 0.5%), repeatability (+/- 2%), and response with variations in gantry rotation and source to detector distance. The latter investigation revealed a significant contribution to the image from optical photon spread in the phosphor layer of the detector. This phenomenon is generally known as "glare," and has been characterized and modeled here as a radially symmetric blurring kernel. This kernel is applied to the calculated dose images as a convolution, and is successfully demonstrated to account for the optical photon spread. This work demonstrates the flexibility and accuracy of the two step algorithm for a high-atomic number detector. The algorithm may be applied to improve performance of dosimetric treatment verification applications, such as direct image comparison, backprojected patient dose calculation, and scatter correction in megavoltage computed tomography. The algorithm allows for dosimetric applications of the new, flat panel portal imager technology in the indirect configuration, taking advantage of a greater than tenfold increase in detector sensitivity over a direct configuration.
最近开发了一种两步算法来预测任意探测器系统中的门静脉剂量图像。当前的工作在临床可用的非晶硅平板成像器上对该算法进行了验证。高原子序数的间接非晶硅探测器包含一个氧硫化钆磷光闪烁屏，用于将沉积的辐射能量转换为形成门静脉图像的光学光子。在6和18 MV的束能量以及一系列气隙（约20 - 50 cm）下，对水等效固体平板体模和拟人化体模进行了检查。在这里给出的许多示例中，磷光体中的门静脉剂量图像在低剂量梯度区域预测误差在5%以内，在高剂量梯度区域预测误差在5 mm（等剂量线偏移）以内。还研究了非晶硅探测器的其他基本剂量学特性，如与剂量率的线性关系（±0.5%）、重复性（±2%）以及机架旋转和源到探测器距离变化时的响应。后一项研究揭示了探测器磷光体层中光学光子扩散对图像有显著贡献。这种现象通常被称为“眩光”，在此已被表征并建模为径向对称的模糊核。该核作为卷积应用于计算出的剂量图像，并成功证明可解释光学光子扩散。这项工作证明了两步算法在高原子序数探测器方面的灵活性和准确性。该算法可用于提高剂量学治疗验证应用的性能，如直接图像比较、反投影患者剂量计算以及兆伏级计算机断层扫描中的散射校正。该算法允许在间接配置中对新型平板门静脉成像器技术进行剂量学应用，利用其相对于直接配置而言探测器灵敏度提高了十多倍的优势。
