加速器附加锥形束CT图像质量评价

Evaluation of Varian kV X-ray cone beam CT image

目的评价瓦里安23EX加速器附加kV X线锥形束CT的图像质量。方法应用安装在直线加速器上的锥形束CT系统,重复扫描Catphan 504体模,用图像分析软件评价获取图像的质量;并用电离室测量扫描体模时等中心点的辐射剂量;并扫描5例头颈部肿瘤患者,进行临床评价。结果图像均一性较好,大部分测量点的CT值偏差在±3 HU以内,图像边缘均一性略差,最大偏差值16 HU;空间分辨率达0．63 mm;体模锥形束CT图像的CT值具有较好的准确性和稳定性,实测值与标称值的最大差值为22 HU;空间线性距离的标称值和实测值的最大差别为0．4%,几何精度较高;低对比度分辨率为1．0%的对比度为4 mm,0．5%的对比度为5 mm;扫描体模时等中心焦点的辐射剂量为（2．97±0．19）cGy。5例头颈部肿瘤患者的锥形束CT图像结果显示,鼻咽部肿瘤和脑膜瘤显示清晰,颅内高分化胶质瘤显示清晰度欠佳。结论瓦里安23EX加速器附加kV X线锥形束CT图像均匀性好,有较准确而稳定的CT值,几何精度高,辐射剂量低,但低对比度分辨率欠佳。锥形束CT可以实现肿瘤和软组织成像,是图像引导放疗的有效工具。

Objective To evaluate the image quality of kV X-ray cone beam CT installed on Varian 23EX linear accelerators. Methods This test was done by scanning the phantom and 5 patients with head and neck carcinoma using a 2.5 mm slice reconstruction by cone beam CT, with the area and line profile measured for the phantom. Dose measurements were done with a 10 cm long thimble ionisation chamber placed at the isocentre. Results A variation of less than 3 HU was seen across the images in the centre of the phantom and higher variation of 16 HU at the edges in the uniformity modular of phantom. The highest variation of 22 HU was seen between the images of cone beam CT and the standard value of phantom. This indicated an accurate and stationary correlation betwwen the images of cone beam CT and the real electronic density of target substance. An excellent geometric accuracy was observed with a spatial linearity of pixel size verification less than 0.4% and consistent in the 8th set corresponding to a gap size of 0.63 mm seen in the high resolution modular. It was possible to detect the circle with a diameter of 4 mm at the 1% contrast level and the circle with a diameter in 5 mm at the 0.5% contrast level, and a mean dose （2.97 ±0.19） cGy was measured at the isocenter. Limpid lesion images were available for patients with nasopharyngeal carcinoma and meningothelioma,but unavailable for patients with well differentiated glioma. Conclusions The images of cone beam CT dose show a good performance in scan uniformity, electronic density, geometric accuracy, spatial resolution, low irradiation dose and limited low contrast resolution. Tumor and soft images are available by cone beam CT scan. Cone beam CT is effective for image guided radiotherapy.