title: A fast double template convolution isocenter evaluation algorithm with subpixel accuracy.
abstract: OBJECTIVE
  To design a fast Winston Lutz (fWL) algorithm for accurate analysis of radiation isocenter from images without edge detection or center of mass calculations.
  
  
  METHODS
  An algorithm has been developed to implement the Winston Lutz test for mechanical/ radiation isocenter agreement using an electronic portal imaging device (EPID). The algorithm detects the position of the radiation shadow of a tungsten ball within a stereotactic cone. The fWL algorithm employs a double convolution to independently find the position of the sphere and cone centers. Subpixel estimation is used to achieve high accuracy. Results of the algorithm were compared to (1) a human observer with template guidance and (2) an edge detection/center of mass (edCOM) algorithm. Testing was performed with high resolution (0.05 mm/px, film) and low resolution (0.78 mm/px, EPID) image sets.
  
  
  RESULTS
  Sphere and cone center relative positions were calculated with the fWL algorithm for high resolution test images with an accuracy of 0.002 +/- 0.061 mm compared to 0.042 +/- 0.294 mm for the human observer, and 0.003 +/- 0.038 mm for the edCOM algorithm. The fWL algorithm required 0.01 s per image compared to 5 s for the edCOM algorithm and 20 s for the human observer. For lower resolution images the fWL algorithm localized the centers with an accuracy of 0.083 +/- 0.12 mm compared to 0.03 +/- 0.5514 mm for the edCOM algorithm.
  
  
  CONCLUSIONS
  A fast (subsecond) subpixel algorithm has been developed that can accurately determine the center locations of the ball and cone in Winston Lutz test images without edge detection or COM calculations.
authors.affiliation: Harvard Medical School, Massachusetts General Hospital, Boston, Massachusetts 02114, USA. winey.brian@mgh.harvard.edu
authors.forename: Brian
authors.initials: B
authors.lastname: Winey
authors.forename: Greg
authors.initials: G
authors.lastname: Sharp
authors.forename: Marc
authors.initials: M
authors.lastname: Bussière
mesh_terms: D000465:Algorithms; D055615:Electrical Equipment and Supplies; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D013997:Time Factors