金属植入物16-bit CT成像对放疗剂量分布的影响

目的：探索利用CT成像中扩展位深功能重建16?bit CT对金属植入物进行成像，通过比较扫描条件变化对金属CT值影响，分析金属CT值对计算剂量分布影响。方法将不锈钢棒和钛棒插入模体中，采用管电压为120 kV管电流为230 mA条件扫描，得到金属棒12?bit图像及16?bit图像CT值分布。分别在管电流230 mA，管电压分别为100、120、140 kV和管电压120 kV，管电流分别为180、230、280 mA下得到金属棒16?bit 图像CT值分布，并在瓦里安TPS中分别设计为单野和对穿野治疗计划和计算剂量分布。结果不锈钢棒和钛棒12?bit图像CT值均为3071 HU；16?bit图像CT值不锈钢棒比钛棒大。3种管电流下2种金属棒16?bit图像CT值均无明显变化，放疗计划剂量分布也接近。3种管电压下不锈钢棒最大CT值分别为13568、13127、12295 HU；钛棒的分别为8420、7140、6310 HU。结论12?bit图像不能区分不同高密度金属植入物，16?bit图像可得出不同金属CT值分布。金属植入物基于12?bit图像计算的剂量分布与16?bit图像的不同。管电压改变会造成金属植入物CT值明显变化，从而造成放疗剂量分布改变；管电流在一定程度内变化时，金属CT值变化较小，剂量分布差异不明显。

Effect of 16-bit computed tomography imaging of metallic implants on dose distribution in radiotherapy

Objective To reconstruct 16?bit images of metal implants using the extended function of computed tomography ( CT) imaging, and to analyze the effect of the metal CT value on calculation of dose distribution by evaluation of metal CT values in different scanning conditions. Methods A stainless steel rod and a titanium rod were inserted in a phantom. The 12?and 16?bit images and CT value distribution of metal implants were obtained by scanning the phantom using 120 kV tube voltage and 230 mA tube current. The 16?bit images and CT value distribution of metal implants were obtained by scanning the phantom using fixed tube current ( 230 mA) with varied tube voltage ( 100, 120, and 140 kV) or fixed tube voltage ( 120 kV) with varied tube current ( 180, 230, and 280 mA) . In the Varian treatment planning system, a single?field plan and a parallel?opposed field plan were designed based on the CT images. The dose distribution was calculated and compared by the paired t test. Results The CT values of the stainless steel rod and the titanium rod were both 3 071 HU in 12?bit CT images. In 16?bit CT images;however, the CT value of the stainless steel rod was significantly larger than that of the titanium rod. There were no significant differences in CT value of 16?bit image and dose distribution in radiotherapy plan between three scanning conditions with different tube currents. Under three scanning conditions with different tube voltages, the maximum CT values were 13 568, 13 127, and 12 295 HU for the stainless steel rod and 8 420, 7 140, and 6 310 HU for the titanium rod, respectively. Conclusions High?density metal implants cannot be distinguished by 12?bit images, while the distribution of metal CT value can be obtained by 16?bit images. The dose distribution of metal implants based on 12?bit images is different from that based on 16?bit images. Changes in tube voltage cause substantial changes in the CT value for metal implants, leading to changes in dose distribution in radiotherapy. Variation of tube current within a certain range causes slight changes in metal CT value and dose distribution.