Research On Contour Propagation Methods For Lung Tumor Delineation In 4D-CT Images

In order to reduce the redundancy of internal gross tumor volume,it is useful to precisely delineate gross tumor volume(GTV)on all phase images of four-dimensional computed tomography(4D-CT).However,this procedure is tedious and time-consuming for physicians,meanwhile the intraobserver variation and interobserver variation may cause troubles for radiotherapy plan.At present,contour propagation methods are usually used to delineate GTV on all phase images semi-automatically,but accuracy and time efficiency leave much to be improved.Contour propagation can be done in two ways:volume registration or contour registration.It is required by contour registration to accurately convert three-dimensional tumor contours into planar ones,but not required by volume registration,therefore volume registration is usually used for contour propagation.However,on the one hand,the intensity is nearly homogeneous inside the tumor,which is disadvantageous for volume registration,and on the other hand,the tumor contour can provide a wealth of boundary information,which is advantageous for contour registration.In order to use contour registration,a method of converting three-dimensional tumor contours into planar ones is proposed:each three-dimensional tumor contour is fitted by a B-spline curve;a set of line segments can be defined by every two adjacent B-spline curves in superior-inferior direction;exterior-point method is used to define a set of line segments between the top or bottom contour and the exterior point;the intersections of an axial plane and all the line segments are fitted by a B-spline curve to obtain the planar tumor contour.Experiment results show that contour registration using the proposed method is significantly superior to volume registration on the accuracy of contour propagation.Since contour usually deforms quite differently from the tissues outside the boundary,the deformation field near the contour is usually discontinuous.Therefore when the moving image is aligned with the fixed image as far as possible,the moving contour in the moving image may be far from alignment with the reference contour in the fixed image.To address the problem,contour propagation method using B-spline curve(BSC)is proposed:a tumor contour is modeled by a B-spline curve;the shape and position of the B-spline curve are adjusted by moving its control vertexes,thus modeling the deformation of the moving contour.BSC method can treat the moving contour as a deformable entity,and aim to maximize the intensity-distribution similarity between the moving contour and the reference contour.Therefore BSC method can avoid the discontinuous deformation field near the contour to be smoothed by its surrounding tissues so that to improve the accuracy of contour propagation,and minimize the optimization objective so that to greatly speed up contour propagation.Experiment results show that BSC method is effective in improving the accuracy and speed of contour propagation.All the phases in 4D-CT should be treated as an organic whole.However,lots of contour propagation methods joint only two phases for contour propagation.To address the problem,contour propagation method using B-spline surface(BSS)is proposed:the trajectory surface swept by a contour is modeled by a B-spline surface;the shape and position of the B-spline surface are adjusted by moving its control vertexes,thus modeling the deformation of the moving contours on all the phases.BSS method can treat the trajectory surface as a deformable entity,and aim to maximize the intensity-distribution similarity between the moving contours on all the phases and the reference contour.Therefore it can joint all the phases as an organic whole for contour propagation so that to overcome the drawbacks of jointing only two phases.Experiment results show that BSS method is superior to BSC method on the accuracy of contour propagation,and is inferior to BSC method on the speed of contour propagation.