Scatter Correction For Cone Beam CT Images

Accurate radiotherapy techniques tried to find a balance between killing the tumor cells and protecting surrounding normal tissues at maximum using advanced equipment and technologies. During the treatment course, the target or organ shape changes severely so that the adjustment of treatment plan is needed. Dose calculationand verification based on CBCT images has the advantages of less time consuming, low cost and lower concomitant dose. However, the usage of CBCT images for dose calculation was hampered by image artifacts mainly caused by scatter radiation, which leads to instable HU values and mis-information. To achieve the clinical practical purpose of dose calculation based on CBCT images in Adaptive Radiotherapy, the scatter correction of CBCT images was investigated in this paper.The module modeling of CBCT system method was developed. EGSnrc program was adopted to simulate the geometry construction of x-ray tube-phantom-detector full system. The method could differentiate the primary and scatter radiation and separate the scatter distribution from phantom and detector. The system model was verified by Beamdp program analysis, dose measurement of water phantom and scatter extraction of simulated CBCT images of three phantoms. The dose error between the calculation and simulation was within2%.On the basis of scatter kernels calculated using Pencil Beam model, the Scatter Kernel Deconvolution method was adopted to correct the scatter in the CBCT images. The key techniques involved were further conducted, including Signal Conversion modeling, Scatter Kernel calculation and scatter distribution modeling. The CBCT images of Water phantom and anthropomorphic head phantom were adopted to verify the method. The results showed that images quality was obviously improved, the overall Scatter-to-Primary Ratios was reduced an order of magnitude.Finally, dose calculation was investigated based on CBCT images after scatter correction. The HU-ED calibration model was established using the standard phantom. The dose calculation for CATphan phantom and nasopharyngeal carcinoma patient were carried out based on calibration model. Compared with dose distribution calculated based on CT-ED calibration curve, the results showed that the precision of dose calculation based on scatter corrected CBCT images had been greatly improved. The pass rates of gamma analysis were95.3%and97.7%respectively, meeting the clinical requirement. The study of dose calculation based on scatter corrected CBCT images would provide technical support for Adaptive Radiotherapy.