Medical Physics Unit, McGill University Health Center, 1650 Avenue Cedar, Montreal, Quebec H3G 1A4, Canada.
Med Phys. 2010 Jan;37(1):124-31. doi: 10.1118/1.3267040.
Recently, several authors have shown contrast improvements in megavoltage portal imaging and cone-beam computed tomography using low atomic number (Z) targets. This work compliments previous studies by investigating the effects of varying different beam production parameters including target atomic number, target thickness, and incident electron energy on spatial resolution.
Target materials of beryllium, aluminum, and tungsten were investigated over a range of thicknesses between 10% and 100% of the continuous slowing down approximation range of electrons. Incident electron kinetic energies of 4.5 and 7.0 MeV were used, in conjunction with custom targets installed above the carousel of a modern radiotherapy linear accelerator. Monte Carlo simulations of the accelerator were constructed and compared to the experimental results.
The results showed that thinner targets, as well higher incident electron energies, generally produce more favorable modulation transfer function (MTF) curves. Due to an MTF dependence of the detector system on the photon energy, the experimental results showed that low-Z targets produced superior MTF curves. Simulations showed 14.5% and 21.5% increases in f50 for the 7.0 and 4.5 MeV targets (A1; 60% R% CSDA), respectively, when moved from the carousel to the location of the clinical target. f50 values for the custom targets were compared to the clinical 6 MV beam and were found to be between 10.4% lower (4.5 MeV/W) and 15.5% higher (7.0 MeV/Be).
Integration of low-Z external targets into the treatment head of a medical linear was achieved with only minor modifications. It was shown that reasonably high resolution images on par or better than those acquired with the clinical 6 MV beam can be achieved using external low-Z targets.
最近，几位作者已经表明，在兆伏级门控成像和锥形束计算机断层扫描中使用低原子序数（Z）靶可以提高对比度。这项工作通过研究不同束流产生参数（包括靶材原子序数、靶材厚度和入射电子能量）对空间分辨率的影响，补充了之前的研究。
研究了铍、铝和钨三种靶材，靶材厚度在连续慢化近似范围内的 10%到 100%之间变化。使用了 4.5 和 7.0 MeV 的入射电子动能，并结合安装在现代放射治疗直线加速器旋转架上方的定制靶材进行实验。构建了加速器的蒙特卡罗模拟，并将模拟结果与实验结果进行了比较。
结果表明，较薄的靶材和更高的入射电子能量通常会产生更有利的调制传递函数（MTF）曲线。由于探测器系统对光子能量的 MTF 依赖性，实验结果表明低 Z 靶材产生了更优的 MTF 曲线。模拟结果表明，当从旋转架移动到临床靶区位置时，7.0 和 4.5 MeV 靶材（A1；60%R%CSDA）的 f50 值分别增加了 14.5%和 21.5%。定制靶材的 f50 值与临床 6 MV 束进行了比较，发现分别比临床 6 MV 束低 10.4%（4.5 MeV/W）和高 15.5%（7.0 MeV/Be）。
仅通过微小的修改，就可以将低 Z 外部靶材集成到医用直线加速器的治疗头中。研究表明，使用外部低 Z 靶材可以获得与临床 6 MV 束相当或更好的高分辨率图像。
