title: Role of "the frame cycle time" in portal dose imaging using an aS500-II EPID.
abstract: BACKGROUND
  This paper evaluates the role of an acquisition parameter, the frame cycle time "FCT", in the performance of an aS500-II EPID.
  
  
  METHODS
  The work presented rests on the study of the Varian EPID aS500-II and the image acquisition system 3 (IAS3). We are interested in integrated acquisition using asynchronous mode. For better understanding the image acquisition operation, we investigated the influence of the "frame cycle time" on the speed of acquisition, the pixel value of the averaged gray-scale frame and the noise, using 6 and 15MV X-ray beams and dose rates of 1-6Gy/min on 2100 C/D Linacs.
  
  
  RESULTS
  In the integrated mode not synchronized to beam pulses, only one parameter the frame cycle time "FCT" influences the pixel value. The pixel value of the averaged gray-scale frame is proportional to this parameter. When the FCT <55ms (speed of acquisition V(f/s)>18 frames/s), the speed of acquisition becomes unstable and leads to a fluctuation of the portal dose response. A timing instability and saturation are detected when the dose per frame exceeds 1.53MU/frame. Rules were deduced to avoid saturation and to optimize this dosimetric mode.
  
  
  CONCLUSIONS
  The choice of the acquisition parameter is essential for the accurate portal dose imaging.
authors.affiliation: Tenon Hospital, Radiation Physics Department, CancerEst, GHU EST, 75970 Paris Cedex 20, France. zeinakattar1@hotmail.com
authors.forename: Zeina
authors.initials: Z
authors.lastname: Al Kattar Elbalaa
authors.forename: Jean Noel
authors.initials: JN
authors.lastname: Foulquier
authors.forename: Alexandre
authors.initials: A
authors.lastname: Orthuon
authors.forename: Hanna
authors.initials: H
authors.lastname: Elbalaa
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Touboul
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*; D014963:X-Ray Intensifying Screens