Parsons David, Robar James L
Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia B3H 1V7, Canada.
Department of Radiation Oncology and Department of Physics and Atmospheric Science, Dalhousie University, 5820 University Avenue, Halifax, Nova Scotia B3H 1V7, Canada.
Med Phys. 2015 Sep;42(9):5258-69. doi: 10.1118/1.4928474.
The focus of this work was to investigate the improvements in image quality and dose reduction for volume-of-interest (VOI) kilovoltage-cone beam CT (CBCT) using dynamic collimation.
A prototype iris aperture was used to track a VOI during a CBCT acquisition. The current aperture design is capable of 1D translation as a function of gantry angle and dynamic adjustment of the iris radius. The aperture occupies the location of the bow-tie filter on a Varian On-Board Imager system. CBCT and planar image quality were investigated as a function of aperture radius, while maintaining the same dose to the VOI, for a 20 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Corresponding scatter-to-primary ratios (SPR) were determined at the detector plane with Monte Carlo simulation using EGSnrc. Dose distributions for various sizes VOI were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full-field distributions.
SPR was reduced by a factor of 8.4 when decreasing iris diameter from 21.2 to 2.4 cm (at isocenter). Depending upon VOI location and size, dose was reduced to 16%-90% of the full-field value along the central axis plane and down to 4% along the axis of rotation, while maintaining the same dose to the VOI compared to full-field techniques. When maintaining constant dose to the VOI, this change in iris diameter corresponds to a factor increase of approximately 1.6 in image contrast and a factor decrease in image noise of approximately 1.2. This results in a measured gain in contrast-to-noise ratio by a factor of approximately 2.0.
The presented VOI technique offers improved image quality for image-guided radiotherapy while sparing the surrounding volume of unnecessary dose compared to full-field techniques.
本研究旨在探讨使用动态准直技术对感兴趣区（VOI）千伏级锥形束CT（CBCT）图像质量的改善及剂量降低情况。
在CBCT采集过程中，使用一个原型虹膜孔径来跟踪VOI。当前的孔径设计能够根据机架角度进行一维平移，并动态调整虹膜半径。该孔径占据了Varian机载成像系统上蝴蝶结滤波器的位置。对于一个直径20 cm的圆柱形水模体，其中心位于等中心处且插入了一个直径9 mm的骨插件，在保持对VOI剂量相同的情况下，研究了CBCT和平面图像质量与孔径半径的关系。使用EGSnrc通过蒙特卡罗模拟在探测器平面确定相应的散射与原发射线比率（SPR）。使用动态BEAMnrc库和DOSXYZnrc对各种大小VOI的剂量分布进行建模。将所得的VOI剂量分布与全场分布进行比较。
当虹膜直径从21.2 cm减小到2.4 cm（在等中心处）时，SPR降低了8.4倍。根据VOI的位置和大小，沿中心轴平面剂量降低至全场值的16% - 90%，沿旋转轴降低至4%，同时与全场技术相比，保持对VOI的剂量相同。当保持对VOI的剂量恒定时，虹膜直径的这种变化对应于图像对比度增加约1.6倍，图像噪声降低约1.2倍。这导致测量的对比度噪声比提高了约2.0倍。
与全场技术相比，所提出的VOI技术在图像引导放射治疗中提供了更高的图像质量，同时减少了周围不必要剂量的体积。
