Objective:To compare and study the improvement of different iterative reconstruction(IR)algorithms of the tomotherapy(TOMO)Radixact system on the image quality of megavoltage computed tomography(MVCT)imaging system,and the influence of that on the precision of automatic registration,and to explore the reconstruction algorithm that is suitable for clinical application.Methods:Using the MVCT imaging system to respectively scan the Tomo-Phantom HE phantom and the Catphan 604 phantom,and to analyze three groups of images were generated by three kinds of reconstruction algorithms,which included the Standard(STD)algorithm,IR General(IR-G)algorithm and IR Soft Tissue(IR-ST)algorithm,in MVCT image.The noise index(NI),uniformity index(UI)of image,modulation transfer function(MTF),low contrast visibility(LCV)index and contrast-to-noise ratio(CNR)of three groups of images were calculated respectively.The Lucy phantom was used to test the accuracy of automatic registration algorithm.The registration data of four dimensions,included left-right(X-axis),head-foot(Y-axis),vertical(Z-axis)and free rotation(Roll)around the Y-axis,were used to conduct verification analysis for the influences of them on the precision of automatic registration.Results:The NI values of IR-G and IR-ST reconstruction algorithms were respectively 39.58±0.10 and 14.62±0.26,which were better than 39.58±0.10 of STD algorithm,and the UI values of them were respectively 19.87±0.83 and 15.84±2.51,which were better than 24.51±1.81 of STD algorithm,and LCV values of them were respectively 2.50±0.03 and 1.74±0.11,which were better than 3.67±0.04 of STD algorithm.All of them appeared significant increase,but the resolution with high contrast of MTF were respectively 0.23 and 0.21,which were lower than 0.32 of STD.The overall image quality of the IR algorithms was superior to that of the STD algorithm.In the accuracy test of the automatic registration algorithm,the registration precisions of IR-G and IR-ST algorithms on Y-axis were respectively(0.360±0.142)mm and(0.245±0.050)mm,which were significantly higher than 0.145±0.136 of STD algorithm,and the differences of them were significant(Z=6.0,15.0,P<0.05).The differences of registration precisions of other directions were not significant(P>0.05).Conclusion:The IR algorithm has advantages in terms of noise,uniformity and resolution with low-contrast,however,it shows reduction on resolution with high-contrast.This reduction of resolution with high-contrast do not lead to the decrease of the precision of automatic registration.

Objective:To quantitatively analyze the differences in image quality and generated synthetic-computed tomography(sCT)between cone beam CT(CBCT)and iteration CBCT(iCBCT),which based on filtered back projection(FBP)and iterative reconstruction(IR)techniques of Varian Ethos adaption system.Methods:Using the special Catphan604 phantom of the test of CT image quality,five times of scanning series of CBCT and iCBCT were respectively performed based on thoracic and abdominal scanning protocols,so as to obtain four sets of images,and each set of image included the CBCT image,the composited sCT and iCBCT images based on CBCT,the composited sCT bases on iCBCT.The noise power spectrum(NPS),image uniformity(UI),modulation transfer function(MTF),low-contrast visibility(LCV)and contrast-to-noise ratio(CNR)of them were analyzed.Additionally,a retrospective analysis was conducted on images of six patients who underwent clinical radiotherapy,and the images of them were obtained by using thoracic and abdominal scanning protocols.The sCT images,which were generated by deformable registrations on the basis of CBCT and iCBCT,were analyzed,and they were compared with located CT images,and their mean absolute error(MAE)and root mean square error(RMSE)were further quantitatively analyzed.Results:Compared to CBCT,iCBCT significantly reduced NPS(thoracic images reduced by 0.08,abdominal images reduced by 0.06).The UI values of thoracic images and abdominal images of iCBCT were respectively 0.37 and 0.28,and those of CBCT were respectively 0.65 and 0.39.The LCV values of thoracic images and abdominal images of iCBCT were respectively 0.67 and 0.56,and those of CBCT were respectively 0.93 and 0.90.The MTF50％were 0.28 lp/mm except abdominal image of iCBCT was 0.29 lp/mm.The resolution rate of high contrast between iCBCT and CBCT was basic consistency.The differences of similarity analysis of the generated sCT and located CT bases on CBCT and iCBCT was no significant(P＞0.05).Conclusion:The image quality of iCBCT is superior to conventional CBCT.The accuracy of CT value of generated sCT bases on CBCT and iCBCT can meet requirement,and both of them can be used in the dose calculation of adaptive radiotherapy.