title: Lung motion estimation using dynamic point shifting: An innovative model based on a robust point matching algorithm.
abstract: OBJECTIVE
  Image-guided radiotherapy is an advanced 4D radiotherapy technique that has been developed in recent years. However, respiratory motion causes significant uncertainties in image-guided radiotherapy procedures. To address these issues, an innovative lung motion estimation model based on a robust point matching is proposed in this paper.
  
  
  METHODS
  An innovative robust point matching algorithm using dynamic point shifting is proposed to estimate patient-specific lung motion during free breathing from 4D computed tomography data. The correspondence of the landmark points is determined from the Euclidean distance between the landmark points and the similarity between the local images that are centered at points at the same time. To ensure that the points in the source image correspond to the points in the target image during other phases, the virtual target points are first created and shifted based on the similarity between the local image centered at the source point and the local image centered at the virtual target point. Second, the target points are shifted by the constrained inverse function mapping the target points to the virtual target points. The source point set and shifted target point set are used to estimate the transformation function between the source image and target image.
  
  
  RESULTS
  The performances of the authors' method are evaluated on two publicly available DIR-lab and POPI-model lung datasets. For computing target registration errors on 750 landmark points in six phases of the DIR-lab dataset and 37 landmark points in ten phases of the POPI-model dataset, the mean and standard deviation by the authors' method are 1.11 and 1.11 mm, but they are 2.33 and 2.32 mm without considering image intensity, and 1.17 and 1.19 mm with sliding conditions. For the two phases of maximum inhalation and maximum exhalation in the DIR-lab dataset with 300 landmark points of each case, the mean and standard deviation of target registration errors on the 3000 landmark points of ten cases by the authors' method are 1.21 and 1.04 mm. In the EMPIRE10 lung registration challenge, the authors' method ranks 24 of 39. According to the index of the maximum shear stretch, the authors' method is also efficient to describe the discontinuous motion at the lung boundaries.
  
  
  CONCLUSIONS
  By establishing the correspondence of the landmark points in the source phase and the other target phases combining shape matching and image intensity matching together, the mismatching issue in the robust point matching algorithm is adequately addressed. The target registration errors are statistically reduced by shifting the virtual target points and target points. The authors' method with consideration of sliding conditions can effectively estimate the discontinuous motion, and the estimated motion is natural. The primary limitation of the proposed method is that the temporal constraints of the trajectories of voxels are not introduced into the motion model. However, the proposed method provides satisfactory motion information, which results in precise tumor coverage by the radiation dose during radiotherapy.
authors.affiliation: College of Information Engineering, Shenzhen University, Shenzhen, Guangdong 518000, China and College of Information Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi 341000, China.
authors.forename: Jianbing
authors.initials: J
authors.lastname: Yi
authors.affiliation: College of Computer Science and Software Engineering, Shenzhen University, Shenzhen, Guangdong 518000, China.
authors.forename: Xuan
authors.initials: X
authors.lastname: Yang
authors.affiliation: National High Performance Computing Center at Shenzhen, College of Computer Science and Software Engineering, Shenzhen University, Shenzhen, Guangdong 518000, China.
authors.forename: Guoliang
authors.initials: G
authors.lastname: Chen
authors.affiliation: College of Computer Science and Software Engineering, Shenzhen University, Shenzhen, Guangdong 518000, China.
authors.forename: Yan-Ran
authors.initials: YR
authors.lastname: Li
mesh_terms: D000465:Algorithms; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000502:physiology* / Q000528:radiation effects; D008954:Models, Biological; D009068:Movement; D061089:Radiotherapy, Image-Guided; D012119:Respiration; D013997:Time Factors