title: A heuristic approach to edge detection in on-line portal imaging.
abstract: OBJECTIVE
  Portal field edge detection is an essential component of several postprocessing techniques used in on-line portal imaging, including field shape verification, selective contrast enhancement, and treatment setup error detection. Currently edge detection of successive fractions in a multifraction portal image series involves the repetitive application of the same algorithm. As the number of changes in the field is small compared to the total number of fractions, standard edge detection algorithms essentially recalculate the same field shape numerous times. A heuristic approach to portal edge detection has been developed that takes advantage of the relatively few changes in the portal field shape throughout a fractionation series.
  
  
  METHODS
  The routine applies a standard edge detection routine to calculate an initial field edge and saves the edge information. Subsequent fractions are processed by applying an edge detection operator over a small region about each point of the previously defined contour, to determine any shifts in the field shape in the new image. Failure of this edge check indicates that a significant change in the field edge has occurred, and the original edge detection routine is applied to the image. Otherwise the modified edge contour is used to define the new edge.
  
  
  RESULTS
  Two hundred and eighty-one portal images collected from an electronic portal imaging device were processed by the edge detection routine. The algorithm accurately calculated each portal field edge, as well as reducing processing time in subsequent fractions of an individual portal field by a factor of up to 14.
  
  
  CONCLUSIONS
  The heuristic edge detection routine is an accurate and fast method for calculating portal field edges and determining field edge setup errors.
authors.affiliation: Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA 19111, USA.
authors.forename: K P
authors.initials: KP
authors.lastname: McGee
authors.forename: T E
authors.initials: TE
authors.lastname: Schultheiss
authors.forename: E E
authors.initials: EE
authors.lastname: Martin
mesh_terms: D001943:Breast Neoplasms / Q000532:radiotherapy; D005260:Female; D006801:Humans; D008297:Male; D008953:Models, Anatomic; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011785:Quality Assurance, Health Care; D018787:Radiation Oncology / Q000379:methods* / Q000592:standards; D011881:Radiotherapy, Computer-Assisted / Q000379:methods* / Q000592:standards; D015203:Reproducibility of Results