模体大小对CBCT图像剂量计算的影响

目的：锥形束CT（CBCT）使用宽束X-射线,探测板获取的信号受散射线的影响很大。该文对扫描模体大小及散射体积对CBCT重建图像HU值及剂量计算的影响进行研究。方法：在Elekta Synergy-XVICBCT系统中对不同深度及散射体积的均匀水模和非均匀密度参考模体扫描,并测量感兴趣区域的HU值;建立考虑和不考虑散射的两组HU-物理密度曲线应用于水模及头颈部仿真模体CBCT图像进行剂量计算,与常规CT图像（FBCT）计算结果比较。结果：均匀水模CBCT图像的HU值随水模深度增加先增大后略有减少,随纵轴散射长度增加而减少,变化幅度最大均接近10%。随散射长度增加,非均匀密度参考模体CBCT图像的高密度组织的HU值减少而低密度组织HU值增加,对1.609 g/cm3致密度骨最大减少约1422 HU。均匀水模和头颈部仿真模体CBCT图像使用考虑散射的HU-物理密度修正曲线计算与FBCT图像比较结果为：点绝对剂量（cGy/MU）最大偏差小于1.5%,等剂量线偏差小于2 mm~3 mm,2%/2 mm DTA指数的通过率平均大于97%,明显优于不做散射修正的结果。结论：Elekta Synergy-XVI系统获取CBCT图像的HU值受扫描模体的几何大小及散射体积影响很大,应选择与扫描患者近似几何大小及人体组织等效的HU-密度校准模体。考虑模体大小及散射修正的头颈部模体CBCT图像用于剂量计算能满足临床要求。

The Effect of Phantom Size on Dose Calculation Accuracy of CBCT Images

Objective：Cone-beam CT（CBCT）using a wide-beam X-ray,which covers large range especially in the longitudinal direction,and scatter poses a more severe problem in the resultant signals collected by flat panel detectors.The purpose of this investigation is to assess the effect of scanning phantom size and phantom scattering volume on the CBCT image reconstruction HU values and dose calculation.Methods：We scanned a set of homogeneous water phantoms with different depths and lengths and an inhomogeneous standard density reference phantom（CIRS） embedded in different length water on Elekta Synergy-XVI respectively.Houns field Unit（HU）values of various regions of interest were measured.The HU to physical densitycurves with or without scattering correction of phantom were established separately,which were applied in an anthroporpnic phantom of head-neck for dose calculation.The dosimetric consequence of HU variation was investigated by comparing CBCT-based treatment plan to conventional CT-based（FBCT） plan for both homogeneous and inhomogeneous phantoms.Results：The HU values of water phantom in CBCT were increased with water depth increases first,and then decreased.In addition,HU value decreases with the increases in the length ofthe phantom.In which greatest magnitude of changes are both close to 10%.The effect of adding scatter material longitudinally in CIRS reference phantom in CBCT was found that HU values decreased for high-density inserts,but increased for low-density inserts.For dense-bone insert,the change in HU value was up to 1422 HU.The plans based on CBCT with scatter correction of phantom showed good agreement with those based on FBCT with less than 1.5% of MU/c Gy different or 2mm~3mm of isodose discrepancy,and for dose distribution the average pass rate with 2%/2 mm DTA index was more than 97% for head-neckphantom.And there were better results in water phantom.Large dosimetrical discrepancy occurred when CBCT was used to dose calculate without scatter correction of phantom.Conclusions