引导螺旋断层放疗用兆伏CT图像质量定量分析

Quantitative evaluation of image quality of megavoltage computed tomography for guiding helical tomotherapy

目的:定量分析螺旋断层放疗系统3种兆伏CT扫描模式获取的2套模体图像质量,探索和验证螺旋断层放疗系统兆伏CT图像的半自动质控方法。方法:分别使用螺旋断层放疗系统上螺距为4 mm、8 mm、12 mm/圈的Fine、Normal和Coarse模式,获取CatPhan504和Cheese模体的兆伏CT图像。基于Pylinac、Matlab和Eclipse软件自动分析对比3种模式下图像的空间分辨率、图像噪声和低对比度分辨率。结果:对CatPhan504空间分辨率模块的分析显示,Coarse模式的前3个线对(间距分别为0.500 cm、0.250 cm和0.167 cm)清晰但从线对4(间距为 0.125 cm )开始发生了明显混叠,而Normal和Fine的前4个线对均可清晰观测。Coarse、Normal和Fine模式下Cheese模体的总体不均匀度分别为0.157 7、0.136 8和0.122 9。3种成像模式所得Cheese图像的低对比度分辨率相近,且随着构造材料与局部背景CT平均差值的增大,插件的可观测性呈近乎线性增长。结论: 3种成像模式在3个图像质量评估指标上并未呈现一致优劣性。临床上螺旋断层放疗系统成像模式的选择可基于图像观测的需求,参考本研究的评估结果,从而做出决策,相关图像质量评估工作也可参考本研究的方法,定期进行相关质控测试。

Objective: To quantitatively analyze image quality of two sets of phantom (CatPhan504 and Cheese) Megavoltage computed tomography (MVCT) images acquired by Helical Tomotherapy with three scanning modes (Fine, Normal and Coarse), and to explore and validate a semi-automatic quality assurance procedure for MVCT images of Helical Tomotherapy. Methods: On Helical Tomotherapy, CatPan504 and Cheese phantoms were scanned with three pitch levels (Fine, Normal, Coarse: 4 mm, 8 mm, 12 mm/circle) respectively. Pylinac, Matlab and Eclipse were used to calculate and compare spatial resolution, noise level and low contrast resolution of images obtained under three scanning modes respectively. The spatial resolution can be evaluated by the blurring of line-pair CT value in the images of CatPhan504’s CTP528 module. The noise level can be evaluated by the integral non-uniformity in the images of Cheese’s uniformity module. the low contrast resolution can be evaluated by contrast-to-noise ratio of both phantoms’ plug-in module, or visibility of the region of interest (Supra-Slice) in the images of CatPhan504’s CTP515 module. Results: Analyses on CatPhan504’s line pair module(CTP528 module ) showed that the first three line pairs(the gap size are 0.500 cm, 0.250 cm and 0.167 cm respectively ) could be clearly observed but blurring began to occur from the fourth line pair(the gap size is 0.125 cm) under Coarse mode. Meanwhile, the first four line pairs were all observable under the Normal and Fine modes. Integral non-integrity index(the value negatively correlated with the noise level) were 0.155 7, 0.136 8 and 0.122 9 for Coarse, Normal and Fine modes respectively. None of the Supra -Slice in CatPhan504’s CTP515 module could be observed under three imaging modes. Low contrast contrast-to-noise ratio of Cheese phantom was similar under three modes and the insert visibility exhibited nearly linear growth with the increasing difference between CT average value of the insert material and background. Conclusion: Superiority and inferiority of three image modes in terms of the three image quality index was not consistent. Evaluation results above could provide reference for more rational decision on scanning modes selection of helical tomotherapy, which was based on image visualization demands in clinical practice. The proposed method could also provide guidance for similar image quality assessment and periodic quality assurance.