Bowman Wesley A, Robar James L, Sattarivand Mike
Department of Medical Physics, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada.
Department of Radiation Oncology, Nova Scotia Cancer Centre, Halifax, Nova Scotia, B3H 2Y9, Canada.
Med Phys. 2017 Mar;44(3):823-831. doi: 10.1002/mp.12093. Epub 2017 Feb 21.
Stereoscopic x-ray image guided radiotherapy for lung tumors is often hindered by bone overlap and limited soft-tissue contrast. This study aims to evaluate the feasibility of dual-energy imaging techniques and to optimize parameters of the ExacTrac stereoscopic imaging system to enhance soft-tissue imaging for application to lung stereotactic body radiation therapy.
Simulated spectra and a physical lung phantom were used to optimize filter material, thickness, tube potentials, and weighting factors to obtain bone subtracted dual-energy images. Spektr simulations were used to identify material in the atomic number range (3-83) based on a metric defined to separate spectra of high and low-energies. Both energies used the same filter due to time constraints of imaging in the presence of respiratory motion. The lung phantom contained bone, soft tissue, and tumor mimicking materials, and it was imaged with a filter thickness in the range of (0-0.7) mm and a kVp range of (60-80) for low energy and (120,140) for high energy. Optimal dual-energy weighting factors were obtained when the bone to soft-tissue contrast-to-noise ratio (CNR) was minimized. Optimal filter thickness and tube potential were achieved by maximizing tumor-to-background CNR. Using the optimized parameters, dual-energy images of an anthropomorphic Rando phantom with a spherical tumor mimicking material inserted in his lung were acquired and evaluated for bone subtraction and tumor contrast. Imaging dose was measured using the dual-energy technique with and without beam filtration and matched to that of a clinical conventional single energy technique.
Tin was the material of choice for beam filtering providing the best energy separation, non-toxicity, and non-reactiveness. The best soft-tissue-weighted image in the lung phantom was obtained using 0.2 mm tin and (140, 60) kVp pair. Dual-energy images of the Rando phantom with the tin filter had noticeable improvement in bone elimination, tumor contrast, and noise content when compared to dual-energy imaging with no filtration. The surface dose was 0.52 mGy per each stereoscopic view for both clinical single energy technique and the dual-energy technique in both cases of with and without the tin filter.
Dual-energy soft-tissue imaging is feasible without additional imaging dose using the ExacTrac stereoscopic imaging system with optimized acquisition parameters and no beam filtration. Addition of a single tin filter for both the high and low energies has noticeable improvements on dual-energy imaging with optimized parameters. Clinical implementation of a dual-energy technique on ExacTrac stereoscopic imaging could improve lung tumor visibility.
立体X射线图像引导的肺癌放射治疗常因骨骼重叠和软组织对比度有限而受到阻碍。本研究旨在评估双能成像技术的可行性，并优化ExacTrac立体成像系统的参数，以增强软组织成像，应用于肺部立体定向体部放射治疗。
使用模拟光谱和物理肺部模型来优化滤过材料、厚度、管电压和加权因子，以获得去骨双能图像。基于定义的用于分离高能和低能光谱的指标，使用Spektr模拟来识别原子序数范围（3-83）内的材料。由于在呼吸运动存在的情况下成像的时间限制，两种能量使用相同的滤过器。肺部模型包含骨骼、软组织和模拟肿瘤的材料，并在低能（0-0.7）mm的滤过器厚度和（60-80）kVp范围内、高能（120,140）kVp范围内进行成像。当骨与软组织的对比噪声比（CNR）最小时，获得最佳双能加权因子。通过最大化肿瘤与背景的CNR来实现最佳滤过器厚度和管电压。使用优化参数，获取了在肺部插入球形模拟肿瘤材料的拟人化Rando模型的双能图像，并评估了去骨效果和肿瘤对比度。使用有无束流滤过的双能技术测量成像剂量，并使其与临床传统单能技术的剂量相匹配。
锡是束流滤过的首选材料，可提供最佳的能量分离、无毒和无反应性。在肺部模型中，使用0.2mm锡和（140,60）kVp对可获得最佳的软组织加权图像。与无滤过的双能成像相比，使用锡滤过器的Rando模型的双能图像在骨消除、肿瘤对比度和噪声含量方面有显著改善。在有和没有锡滤过器的情况下，临床单能技术和双能技术的每次立体视图表面剂量均为0.52mGy。
使用优化采集参数且无束流滤过的ExacTrac立体成像系统，双能软组织成像在不增加成像剂量的情况下是可行的。为高能和低能添加单个锡滤过器对优化参数的双能成像有显著改善。在ExacTrac立体成像上临床实施双能技术可提高肺部肿瘤的可视性。
