title: Fast shading correction for cone beam CT in radiation therapy via sparse sampling on planning CT.
abstract: OBJECTIVE
  The image quality of cone beam computed tomography (CBCT) is limited by severe shading artifacts, hindering its quantitative applications in radiation therapy. In this work, we propose an image-domain shading correction method using planning CT (pCT) as prior information which is highly adaptive to clinical environment.
  
  
  METHODS
  We propose to perform shading correction via sparse sampling on pCT. The method starts with a coarse mapping between the first-pass CBCT images obtained from the Varian TrueBeam system and the pCT. The scatter correction method embedded in the Varian commercial software removes some image errors but the CBCT images still contain severe shading artifacts. The difference images between the mapped pCT and the CBCT are considered as shading errors, but only sparse shading samples are selected for correction using empirical constraints to avoid carrying over false information from pCT. A Fourier-Transform-based technique, referred to as local filtration, is proposed to efficiently process the sparse data for effective shading correction. The performance of the proposed method is evaluated on one anthropomorphic pelvis phantom and 17 patients, who were scheduled for radiation therapy. (The codes of the proposed method and sample data can be downloaded from https://sites.google.com/view/linxicbct) RESULTS: The proposed shading correction substantially improves the CBCT image quality on both the phantom and the patients to a level close to that of the pCT images. On the phantom, the spatial nonuniformity (SNU) difference between CBCT and pCT is reduced from 74 to 1 HU. The root of mean square difference of SNU between CBCT and pCT is reduced from 83 to 10 HU on the pelvis patients, and from 101 to 12 HU on the thorax patients. The robustness of the proposed shading correction is fully investigated with simulated registration errors between CBCT and pCT on the phantom and mis-registration on patients. The sparse sampling scheme of our method successfully avoids false structures in the corrected CBCT even when the maximum registration error is as high as 8 mm.
  
  
  CONCLUSIONS
  We develop an effective shading correction algorithm for CBCT readily implementable on clinical data as a software plug-in without modifications of current imaging hardware and protocol. The algorithm is directly applied on the output images from a commercial CBCT scanner with high computational efficiency and negligible memory burden.
authors.affiliation: Nuclear and Radiological Engineering and Medical Physics Programs, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA.
authors.forename: Linxi
authors.initials: L
authors.lastname: Shi
authors.affiliation: Landauer Medical Physics, 2 Science Road, Glenwood, IL, 60425, USA.|Department of Radiation Oncology, Cancer Treatment Centers of America - Southeastern Regional Medical Center, 600 Celebrate Life Parkway, Newnan, GA, 30265, USA.
authors.forename: Tiffany
authors.initials: T
authors.lastname: Tsui
authors.affiliation: Landauer Medical Physics, 2 Science Road, Glenwood, IL, 60425, USA.|Department of Radiation Oncology, Cancer Treatment Centers of America - Southeastern Regional Medical Center, 600 Celebrate Life Parkway, Newnan, GA, 30265, USA.
authors.forename: Jikun
authors.initials: J
authors.lastname: Wei
authors.affiliation: Nuclear and Radiological Engineering and Medical Physics Programs, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA.|Department of Modern Physics, School of Physical Sciences, University of Science and Technology of China, Hefei, China.
authors.forename: Lei
authors.initials: L
authors.lastname: Zhu
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography; D005583:Fourier Analysis; D006801:Humans; D010388:Pelvis / Q000000981:diagnostic imaging; D019047:Phantoms, Imaging
keywords: cone beam CT; radiation therapy; shading correction