title: A method for volumetric imaging in radiotherapy using single x-ray projection.
abstract: OBJECTIVE
  It is an intriguing problem to generate an instantaneous volumetric image based on the corresponding x-ray projection. The purpose of this study is to develop a new method to achieve this goal via a sparse learning approach.
  
  
  METHODS
  To extract motion information hidden in projection images, the authors partitioned a projection image into small rectangular patches. The authors utilized a sparse learning method to automatically select patches that have a high correlation with principal component analysis (PCA) coefficients of a lung motion model. A model that maps the patch intensity to the PCA coefficients was built along with the patch selection process. Based on this model, a measured projection can be used to predict the PCA coefficients, which are then further used to generate a motion vector field and hence a volumetric image. The authors have also proposed an intensity baseline correction method based on the partitioned projection, in which the first and the second moments of pixel intensities at a patch in a simulated projection image are matched with those in a measured one via a linear transformation. The proposed method has been validated in both simulated data and real phantom data.
  
  
  RESULTS
  The algorithm is able to identify patches that contain relevant motion information such as the diaphragm region. It is found that an intensity baseline correction step is important to remove the systematic error in the motion prediction. For the simulation case, the sparse learning model reduced the prediction error for the first PCA coefficient to 5%, compared to the 10% error when sparse learning was not used, and the 95th percentile error for the predicted motion vector was reduced from 2.40 to 0.92 mm. In the phantom case with a regular tumor motion, the predicted tumor trajectory was successfully reconstructed with a 0.82 mm error for tumor center localization compared to a 1.66 mm error without using the sparse learning method. When the tumor motion was driven by a real patient breathing signal with irregular periods and amplitudes, the average tumor center error was 0.6 mm. The algorithm robustness with respect to sparsity level, patch size, and presence or absence of diaphragm, as well as computation time, has also been studied.
  
  
  CONCLUSIONS
  The authors have developed a new method that automatically identifies motion information from an x-ray projection, based on which a volumetric image is generated.
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235 and Department of Biomedical Engineering, Southern Medical University, Guangzhou, Guangdong 510515, China.
authors.forename: Yuan
authors.initials: Y
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235.
authors.forename: Hao
authors.initials: H
authors.lastname: Yan
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235.
authors.forename: Luo
authors.initials: L
authors.lastname: Ouyang
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235.
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.affiliation: Department of Biomedical Engineering, Southern Medical University, Guangzhou, Guangdong 510515, China.
authors.forename: Linghong
authors.initials: L
authors.lastname: Zhou
authors.affiliation: Center for Advanced Radiotherapy Technologies and Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, California 92037.
authors.forename: Laura
authors.initials: L
authors.lastname: Cervino
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235.
authors.forename: Steve B
authors.initials: SB
authors.lastname: Jiang
authors.affiliation: Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235.
authors.forename: Xun
authors.initials: X
authors.lastname: Jia
mesh_terms: D003964:Diaphragm / Q000000981:diagnostic imaging; D056973:Four-Dimensional Computed Tomography / Q000295:instrumentation / Q000379:methods; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D000069550:Machine Learning; D008954:Models, Biological; D009038:Motion; D019047:Phantoms, Imaging; D025341:Principal Component Analysis; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D012119:Respiration; D013997:Time Factors; D014965:X-Rays