Department of Radiology, UC Davis Medical Center, University of California, Davis, 4860 Y Street, Suite 3100, Sacramento, California 95817, USA.
Med Phys. 2010 Jan;37(1):40-8. doi: 10.1118/1.3264616.
Dosimetry in computed tomography (CT) is increasingly based on Monte Carlo studies that define the dose in the patient (in mGy) as a function of air kerma (free in air) at isocenter (mGy). The accuracy of Monte Carlo studies depends in part on the accuracy of the characterization of the bow tie filter for a given CT scanner model. A simple method for characterizing the bow tie filter attenuation profile in CT scanners would therefore be very useful. The theory behind such a method is proposed.
A measurement protocol is discussed mathematically and demonstrated using computer simulation. The proposed method requires the placement of a radiation monitor at the periphery of the CT field, and the time domain signal (kerma rate versus time) is measured with good temporal resolution (-200 Hz or better) and with all other objects (e.g., patient couch) retracted from the field of view. Knowledge of the source to isocenter distance (or alternately, the isocenter to probe distance) is required. The stationary detector records the kerma rate versus time signal as the gantry rotates through several revolutions. From this temporal data, signal processing techniques are used to extract in-phase peaks, as well as out-of-phase kerma rate levels. From these data, the distance from isocenter to the probe can be determined (or, alternatively, the source to isocenter distance), and the angle-dependent bow tie filter attenuation can be computed. By measuring the angle-dependent bow tie filter attenuation at several kVp settings, the bow tie composition versus fan angle can be computed using basis decomposition techniques.
The simulations illustrated that with 2% added noise in the kerma rate versus time signal, the attenuation properties of a hypothetical two component (aluminum and polymethyl methacrylate) bow tie filter could be determined (r2 > 0.99). Although the computed basis material thicknesses were not exactly equal to the actual thicknesses, their combined attenuation factors matched that of the actual filter across kVp's to within an average of 0.057%.
It is concluded that the proposed method may provide a simple noninvasive approach to characterizing the performance of bow tie filters in CT systems; however, experimental validation is necessary.
计算机断层扫描（CT）中的剂量学越来越依赖于蒙特卡罗研究，该研究将患者（以毫戈瑞 mGy 为单位）中的剂量定义为等中心处的空气比释动能（自由空气中）的函数（以毫戈瑞 mGy 为单位）。蒙特卡罗研究的准确性部分取决于给定 CT 扫描仪模型的蝶形滤波器特性的准确性。因此，一种简单的方法来描述 CT 扫描仪中的蝶形滤波器衰减特性将非常有用。提出了这种方法的理论基础。
讨论了一种测量协议，并用计算机模拟进行了演示。该方法需要将辐射监测器放置在 CT 场的外围，并用良好的时间分辨率（-200 Hz 或更好）和所有其他物体（例如患者床）从视场中缩回，测量时域信号（比释动能率与时间的关系）。需要知道源到等中心的距离（或者，交替地，等中心到探头的距离）。固定探测器在旋转架旋转几转时记录比释动能率与时间的信号。从这个时间数据中，使用信号处理技术提取同相峰值以及异相比释动能率水平。从这些数据中，可以确定等中心到探头的距离（或者，交替地，源到等中心的距离），并计算角度相关的蝶形滤波器衰减。通过在几个 kVp 设置下测量角度相关的蝶形滤波器衰减，可以使用基分解技术计算蝶形滤波器组成与扇形角的关系。
模拟结果表明，在比释动能率与时间信号中添加 2%的噪声，就可以确定（r2 > 0.99）一个假设的双组分（铝和聚甲基丙烯酸甲酯）蝶形滤波器的衰减特性。尽管计算出的基材料厚度并不完全等于实际厚度，但它们的组合衰减因子在 kVp 范围内与实际滤波器的衰减因子匹配，平均误差为 0.057%。
可以得出结论，所提出的方法可能为 CT 系统中蝶形滤波器的性能提供一种简单的非侵入式的描述方法；但是，需要进行实验验证。
