2D and 3D Optical Flow Based Interpolation of the 4DCT Image Sequences in the External Beam Radiotherapy

Although in the external beam radiotherapy tumor motionis a crucial andchallenging issue due to respiration motion, temporal changes in anatomy duringimaging cause considerable problems. Moreover, the Four Dimensional ComputedTomography (4DCT) imaging has been proposed to track these changes at thedifferent breathing phases. Also at real time tumor tracking, the accuracy of motiontracking models that are necessary can be increased by constructing virtual imagesdue to obtaining additional motion data. In this study, the 4DCT data set of five real patients who have had lungcancer were provided by DIR-lab site in addition to deformable image registrationalgorithms presented in MATLAB software and DIRART software respectively tocalculate 2D and 3D vector felids between two respiratory volumes. Moreover, the2D and 3D displacement vector were calculated by optical flow based on Horn-Schunck method, these vector fields were used to generate an interpolated image at thedesired time by 2D and 3D interpolation methods. Although 2D interpolation methodsincluded nearest, cubic, linear, and B-spline, the 3D interpolation method was based onthe 3D spatial interpolation. In this study, the reconstructed image at the desired timeby two methods was compared with real image at the same time. Considering RootsMean Square Error (RMSE) between actual and interpolated imageis used to measurethe accuracy of interpolated images. Also the accuracy of our reconstruction imagesdepends on the accuracy of displacement field. All of the methods are able to generate images at the desired time withless RMSE and high correlation coefficient. While the 2D interpolation methodsthat include nearest, cubic, linear, and B-spline were able to generate an imagewith less errors, the performance of the 2D interpolation method is less efficientthan other methods. The behavior and capability of the algorithmsare demonstrated bysynthetic image examples. Furthermore, to compare 2D and 3D optical flow basedinterpolation methods, the RMSE quantitative measures are calculated. Results indicatethat both 2D and 3D interpolation presented methods are outperformed significantly,and the patient is kept away from re-scanning for getting new images.