Purpose. We propose a new detection method of gold nanoparticles (AuNP) in therapeutic megavoltage (MV) x-ray beams by means of coincidence counting of annihilation photons following pair production in gold. Methods. The proposed MV x-ray induced positron emission (MVIPE) imaging technique is studied by radiation transport computations using MCNP6 (3D) and CEPXS/ONEDANT (1D) codes for two water phantoms: a 35 cm slab and a similarly sized cylinder, both having a 5 cm AuNP filled region in the center. MVIPE is compared to the standard x-ray fluorescence computed tomography (XFCT). MVIPE adopts MV x-ray sources (Co-60, 2 MV, 6 MV, 6 MV with closed MLC and 15 MV) and relies on the detection of 511 keV photon-pairs. XFCT uses kilovoltage sources (100 kVp, 120 kVp and 150 kVp) and imaging is characterized by analysis of k _α1,2 Au characteristic lines. Three levels of AuNP concentration were studied: 0.1%, 1% and 10% by weight. Results. Annihilation photons in the MVIPE technique originate both in the AuNP and in water along the x-ray beam path with significantly larger production in the AuNP-loaded region. MVIPE signal from AuNP is linearly increasing with AuNP concentration up to 10%wt, while XFCT signal reaches saturation due to self-absorption within AuNP. The production of annihilation photons is proportional to the MV source energy. MVIPE technique using a 15 MV pencil beam and 10 wt% AuNP detects about 4.5נ10³ 511 keV-photons cm⁻² at 90 w/r to the incident beam per 10⁹ source photons cm⁻²; 500 of these come from AuNP. In contrast, the XFCT technique using 150 kVp detects only about 100 k _α1-photons cm⁻² per 10⁹ source photons cm⁻². Conclusions. In MVIPE, the number of annihilation photons produced for different MV-beam energies and AuNP concentrations is significantly greater than the k _α1 photons generated in XFCT. Coincidence counting in MVIPE allows to avoid collimation, which is a major limiting factor in XFCT. MVIPE challenges include the filtering of Compton scatter and annihilation photons originating in water.

目的。我们提出了一种新的金纳米粒子 (AuNP) 检测方法，该方法通过在金中成对产生后湮没光子的巧合计数来检测治疗性兆电压 (MV) X 射线束中的金纳米粒子 (AuNP)。方法. 提出的 MV x 射线诱导正电子发射 (MVIPE) 成像技术通过使用 MCNP6 (3D) 和 CEPXS/ONEDANT (1D) 代码对两个水体模进行辐射传输计算来研究：一个 35 厘米的平板和一个类似大小的圆柱体，两者都有中心有一个 5 cm AuNP 填充区域。MVIPE 与标准 X 射线荧光计算机断层扫描 (XFCT) 进行了比较。MVIPE 采用 MV X 射线源（Co-60、2 MV、6 MV、6 MV 封闭 MLC 和 15 MV）并依赖于 511 keV 光子对的检测。XFCT 使用千伏电压源（100 kVp、120 kVp 和 150 kVp），成像的特点是分析 k _{α 1,2} Au 特征线。研究了三个水平的 AuNP 浓度：按重量计 0.1%、1% 和 10%。结果 . MVIPE 技术中的湮没光子起源于 AuNP 和沿 X 射线束路径的水中，在 AuNP 负载区域产生显着更大的产量。来自 AuNP 的 MVIPE 信号随着 AuNP 浓度高达 10%wt 线性增加，而 XFCT 信号由于 AuNP 内的自吸收而达到饱和。湮灭光子的产生与 MV 源能量成正比。使用 15 MV 笔形光束和 10 wt% AuNP 的 MVIPE 技术在每 10 ^{9 个}源光子 cm ^-2对入射光束的 90 w/r 处检测到大约 4.5±10 ³ 511 keV-光子 cm ^-2；其中 500 个来自 AuNP。相比之下，使用 150 kVp 的 XFCT 技术仅检测到约 100 k _{α 1} -photons cm ^-2每 10 ^{9 个}源光子 cm ^-2。结论。在 MVIPE 中，不同 MV 束能量和 AuNP 浓度产生的湮没光子数量明显大于 XFCT 中产生的k _{α 1}光子。MVIPE 中的符合计数允许避免准直，这是 XFCT 中的主要限制因素。MVIPE 挑战包括过滤来自水中的康普顿散射和湮没光子。