Hilaire Estelle, Sarrut David, Peyrin Françoise, Maxim Voichiţa
Université de Lyon, CREATIS, CNRS UMR5220, Inserm U1044, INSA-Lyon, Université Lyon 1, France.
Phys Med Biol. 2016 Apr 21;61(8):3127-46. doi: 10.1088/0031-9155/61/8/3127. Epub 2016 Mar 23.
In proton therapy, the prompt-γ (PG) radiation produced by the interactions between protons and matter is related to the range of the beam in the patient. Tomographic Compton imaging is currently studied to establish a PG image and verify the treatment. However the quality of the reconstructed images depends on a number of factors such as the volume attenuation, the spatial and energy resolutions of the detectors, incomplete absorptions of high energy photons and noise from other particles reaching the camera. The impact of all these factors was not assessed in details. In this paper we investigate the influence of the PG energy spectrum on the reconstructed images. To this aim, we describe the process from the Monte Carlo simulation of the proton irradiation, through the Compton imaging of the PG distribution, up to the image reconstruction with a statistical MLEM method. We identify specific PG energy windows that are more relevant to detect discrepancies with the treatment plan. We find that for the simulated Compton device, the incomplete absorption of the photons with energy above about 2 MeV prevents the observation of the PG distributions at specific energies. It also leads to blurred images and smooths the distal slope of the 1D PG profiles obtained as projections on the central beam axis. We show that a selection of the events produced by γ photons having deposited almost all their energy in the camera allows to largely improve the images, a result that emphasizes the importance of the choice of the detector. However, this initial-energy-based selection is not accessible in practice. We then propose a method to estimate the range of the PG profile both for specific deposited-energy windows and for the full spectrum emission. The method relies on two parameters. We use a learning approach for their estimation and we show that it allows to detect few millimeter shifts of the PG profiles.
在质子治疗中，质子与物质相互作用产生的瞬发γ（PG）辐射与束流在患者体内的射程相关。目前正在研究断层康普顿成像以建立PG图像并验证治疗效果。然而，重建图像的质量取决于许多因素，如体积衰减、探测器的空间和能量分辨率、高能光子的不完全吸收以及到达相机的其他粒子产生的噪声。所有这些因素的影响尚未得到详细评估。在本文中，我们研究了PG能谱对重建图像的影响。为此，我们描述了从质子辐照的蒙特卡罗模拟，到PG分布的康普顿成像，再到使用统计最大似然期望最大化（MLEM）方法进行图像重建的过程。我们确定了与检测治疗计划差异更相关的特定PG能量窗口。我们发现，对于模拟的康普顿装置，能量高于约2 MeV的光子的不完全吸收阻碍了对特定能量下PG分布的观察。这也导致图像模糊，并使在中心束轴上作为投影获得的一维PG轮廓的远端斜率变得平滑。我们表明，选择几乎将所有能量沉积在相机中的γ光子产生的事件可以在很大程度上改善图像，这一结果强调了探测器选择的重要性。然而，这种基于初始能量的选择在实际中是无法实现的。然后，我们提出了一种方法来估计特定沉积能量窗口和全谱发射情况下PG轮廓的射程。该方法依赖于两个参数。我们使用一种学习方法来估计它们，并且我们表明它能够检测到PG轮廓几毫米的偏移。
