Robar J, Parsons D, Berman A, MacDonald A
Dalhousie University, Halifax, Nova Scotia.
Nova Scotia Cancer Centre, Halifax, Nova Scotia.
Med Phys. 2012 Jun;39(6Part24):3913. doi: 10.1118/1.4735971.
This study demonstrates feasibility and advantages of volume of interest (VOI) cone beam CT (CBCT) imaging performed with an x-ray beam generated from 2.35 MeV electrons incident on a carbon linear accelerator target.
The electron beam energy was reduced to 2.35 MeV in a Varian 21EX linear accelerator containing a 7.6 mm thick carbon x-ray target. Arbitrary imaging volumes were defined in the planning system to produce dynamic MLC sequences capable of tracking off-axis VOIs in phantoms. To reduce truncation artefacts, missing data in projection images were completed using a priori DRR information from the planning CT set. The feasibility of the approach was shown through imaging of an anthropomorphic phantom and the head-and-neck section of a lamb. TLD800 and EBT2 radiochromic film measurements were used to compare the VOI dose distributions with those for full-field techniques. CNR was measured for VOIs ranging from 4 to 15 cm diameter.
The 2.35 MV/Carbon beam provides favorable CNR characteristics, although marked boundary and cupping artefacts arise due to truncation of projection data. These artefacts are largely eliminated using the DRR filling technique. Imaging dose was reduced by 5-10% and 75% inside and outside of the VOI, respectively, compared to full-field imaging for a cranial VOI. For the 2.35 MV/Carbon beam, CNR was shown to be approximately invariant with VOI dimension for bone and lung objects. This indicates that the advantage of the VOI approach with the low-Z target beam is substantial imaging dose reduction, not improvement of image quality.
VOI CBCT using a 2.35 MV/Carbon beam is a feasible technique whereby a chosen imaging volume can be defined in the planning system and tracked during acquisition. The novel x-ray beam affords good CNR characteristics while imaging dose is localized to the chosen VOI. Funding for this project has been received from Varian Medical, Incorporated.
本研究展示了利用2.35 MeV电子束入射到碳线性加速器靶上产生的X射线束进行感兴趣体积（VOI）锥形束CT（CBCT）成像的可行性和优势。
在配备7.6 mm厚碳X射线靶的Varian 21EX线性加速器中，将电子束能量降至2.35 MeV。在计划系统中定义任意成像体积，以生成能够跟踪体模中离轴VOI的动态MLC序列。为减少截断伪影，利用计划CT数据集的先验DRR信息填充投影图像中的缺失数据。通过对人体模型和羔羊头颈部的成像展示了该方法的可行性。使用TLD800和EBT2放射变色胶片测量来比较VOI剂量分布与全场技术的剂量分布。测量了直径为4至15 cm的VOI的CNR。
2.35 MV/碳束提供了良好的CNR特性，尽管由于投影数据截断会出现明显的边界和杯状伪影。使用DRR填充技术可基本消除这些伪影。与颅骨VOI的全场成像相比，VOI内部和外部的成像剂量分别降低了5 - 10%和75%。对于2.35 MV/碳束，对于骨骼和肺部物体，CNR显示出与VOI尺寸大致无关。这表明低Z靶束的VOI方法的优势在于大幅降低成像剂量，而非提高图像质量。
使用2.35 MV/碳束的VOI CBCT是一种可行的技术，可在计划系统中定义选定的成像体积并在采集过程中进行跟踪。这种新型X射线束具有良好的CNR特性，而成像剂量局限于选定的VOI。本项目已获得Varian Medical公司的资助。
