Ortega P G, Torres-Espallardo I, Cerutti F, Ferrari A, Gillam J E, Lacasta C, Llosá G, Oliver J F, Sala P R, Solevi P, Rafecas M
CERN (European Organization for Nuclear Research), Route de Meyrin 385, 1217 Meyrin, Switzerland.
Phys Med Biol. 2015 Mar 7;60(5):1845-63. doi: 10.1088/0031-9155/60/5/1845. Epub 2015 Feb 6.
Compton Cameras emerged as an alternative for real-time dose monitoring techniques for Particle Therapy (PT), based on the detection of prompt-gammas. As a consequence of the Compton scattering process, the gamma origin point can be restricted onto the surface of a cone (Compton cone). Through image reconstruction techniques, the distribution of the gamma emitters can be estimated, using cone-surfaces backprojections of the Compton cones through the image space, along with more sophisticated statistical methods to improve the image quality. To calculate the Compton cone required for image reconstruction, either two interactions, the last being photoelectric absorption, or three scatter interactions are needed. Because of the high energy of the photons in PT the first option might not be adequate, as the photon is not absorbed in general. However, the second option is less efficient. That is the reason to resort to spectral reconstructions, where the incoming γ energy is considered as a variable in the reconstruction inverse problem. Jointly with prompt gamma, secondary neutrons and scattered photons, not strongly correlated with the dose map, can also reach the imaging detector and produce false events. These events deteriorate the image quality. Also, high intensity beams can produce particle accumulation in the camera, which lead to an increase of random coincidences, meaning events which gather measurements from different incoming particles. The noise scenario is expected to be different if double or triple events are used, and consequently, the reconstructed images can be affected differently by spurious data. The aim of the present work is to study the effect of false events in the reconstructed image, evaluating their impact in the determination of the beam particle ranges. A simulation study that includes misidentified events (neutrons and random coincidences) in the final image of a Compton Telescope for PT monitoring is presented. The complete chain of detection, from the beam particle entering a phantom to the event classification, is simulated using FLUKA. The range determination is later estimated from the reconstructed image obtained from a two and three-event algorithm based on Maximum Likelihood Expectation Maximization. The neutron background and random coincidences due to a therapeutic-like time structure are analyzed for mono-energetic proton beams. The time structure of the beam is included in the simulations, which will affect the rate of particles entering the detector.
康普顿相机作为粒子治疗（PT）实时剂量监测技术的一种替代方案而出现，它基于对瞬发伽马射线的探测。由于康普顿散射过程，伽马射线的起源点可以被限制在一个圆锥体（康普顿圆锥）的表面上。通过图像重建技术，可以估计伽马射线发射源的分布，方法是将康普顿圆锥通过图像空间进行圆锥表面反投影，同时使用更复杂的统计方法来提高图像质量。为了计算图像重建所需的康普顿圆锥，要么需要两次相互作用，最后一次是光电吸收，要么需要三次散射相互作用。由于PT中光子能量很高，第一种选择可能不适用，因为光子一般不会被吸收。然而，第二种选择效率较低。这就是采用光谱重建的原因，在重建反问题中将入射γ能量视为一个变量。与瞬发伽马射线一起，与剂量图相关性不强的次级中子和散射光子也会到达成像探测器并产生虚假事件。这些事件会降低图像质量。此外，高强度束流会在相机中产生粒子积累，这会导致随机符合事件增加，即从不同入射粒子收集测量值的事件。如果使用双事件或三事件，噪声情况预计会有所不同，因此，重建图像会受到虚假数据的不同影响。本工作的目的是研究虚假事件在重建图像中的影响，评估它们对束流粒子射程确定的影响。本文展示了一项模拟研究，该研究包括在用于PT监测的康普顿望远镜的最终图像中存在误识别事件（中子和随机符合事件）的情况。使用FLUKA模拟了从束流粒子进入体模到事件分类的完整探测链。随后根据基于最大似然期望最大化的双事件和三事件算法获得的重建图像来估计射程。针对单能质子束，分析了类似治疗时间结构下的中子本底和随机符合事件。束流的时间结构包含在模拟中，这将影响进入探测器的粒子速率。
