title: Phase correlation applied to the 3D registration of CT and CBCT image volumes.
abstract: OBJECTIVE
  In this study, a 3D phase correlation algorithm was investigated to test feasibility for use in determining the anatomical changes that occur throughout a patient's radiotherapy treatment. The algorithm determines the transformations between two image volumes through analysis in the Fourier domain and has not previously been used in radiotherapy for 3D registration of CT and CBCT volumes.
  
  
  METHODS
  Various known transformations were applied to a patient's prostate CT image volume to create 12 different test cases. The mean absolute error and standard deviation were determined by evaluating the difference between the known contours and those calculated from the registration process on a point-by-point basis. Similar evaluations were performed on images with increasing levels of noise added. The improvement in structure overlap offered by the algorithm in registering clinical CBCT to CT images was evaluated using the Dice Similarity Coefficient (DSC).
  
  
  RESULTS
  A mean error of 2.35 (σ = 1.54) mm was calculated for the 12 deformations applied. When increasing levels of noise were introduced to the images, the mean errors were observed to rise up to a maximum increase of 1.77 mm. For CBCT to CT registration, maximum improvements in the DSC of 0.09 and 0.46 were observed for the bladder and rectum, respectively.
  
  
  CONCLUSIONS
  The Fourier-based 3D phase correlation registration algorithm investigated displayed promising results in CT to CT and CT to CBCT registration, offers potential in terms of efficiency and robustness to noise, and is suitable for use in radiotherapy for monitoring patient anatomy throughout treatment.
authors.affiliation: St. Luke's Radiation Oncology Network, Highfield Road, Rathgar, Dublin 6, Ireland; School of Physics, University College Dublin, Belfield, Dublin 4, Ireland. Electronic address: daniel.foley@slh.ie.
authors.forename: Daniel
authors.initials: D
authors.lastname: Foley
authors.affiliation: St. Luke's Radiation Oncology Network, Highfield Road, Rathgar, Dublin 6, Ireland; School of Physics, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: O'Brien
authors.affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: Luis
authors.initials: L
authors.lastname: León-Vintró
authors.affiliation: St. Luke's Radiation Oncology Network, Highfield Road, Rathgar, Dublin 6, Ireland.
authors.forename: Brendan
authors.initials: B
authors.lastname: McClean
authors.affiliation: St. Luke's Radiation Oncology Network, Highfield Road, Rathgar, Dublin 6, Ireland.
authors.forename: Peter
authors.initials: P
authors.lastname: McBride
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D005583:Fourier Analysis; D006801:Humans; D007089:Image Enhancement; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed / Q000379:methods*
keywords: Cone beam CT; Image registration; Phase correlation