IGRT锥形束CT图像的CT值与物理密度关系的研究

Relationships between cone beam CT value and physical density in image guided radiation therapy

目的 通过比较标准物理密度参考模体在IGRT锥形束CT（CBCT）和常规扇形束CT（FBCT）扫描图像的CT值，讨论影响CBCT中密度与CT值对应关系的主要因素。方法 将一种可拆分的带有人体组织等效密度插件的参考模体在IGRT-CBCT和FBCT中选择多档能量、不同视野（FOV）进行扫描，对得到的各密度插件CT值进行比较。结果 IGRT-CBCT获取图像的密度与CT值对应关系受密度体插件的空间位置、模体大小、扫描孔径范围及能量影响明显大于FBCT。在CBCT中，FOV、模体的大小对CT值影响很大。能量越高，CT值受空间位置、FOV大小及不同大小模体的影响越小，能量越高密度对比度越差。结论 由于目前厂家未对CBCT的CT值进行校正，CBCT的CT值不是真正意义上的HU值，不能用于治疗计划系统中的剂量计算。

Objective To evaluate the main factors affecting the relationship between physical density and CT value in cone-beam computed tomography（CBCT） for imaging guided radiation therapy（IGRT） by comparing the CT value in the image from cone-beam scanner and from fan-beam （FBCT） scanner of a reference phantom. Methods A taking-park reference phantom with a set of tissue equivalent inserts was scanned at different energies different fields of view（FOV） for IGRT-CBCT and FBCT. The CT value of evely insert was measured and compared. Results The position of inserts in phantom, the size of phantom, the FOV of scanner and different energies had more effect on the relationships between physical density and the CT value from IGRT-CBCT than those from the normal FBCT. The higher the energy was, the less effect of the position of inserts in phantom, the size of phantom and the FOV of scanner on CT value, and the poorer density contrast was observed. Conclusion At present, the CT value of IGRT-CBCT is not in the true HU value since the manufacturer has not corrected its number. Therefore, we are not able to use the CT value of CBVT for dose calculation in TPS.