title: Grid patterns, spatial inter-scan variations and scanning reading repeatability in radiochromic film dosimetry.
abstract: OBJECTIVE
  When comparing different scans of the same radiochromic film, several patterns can be observed. These patterns are caused by different sources of uncertainty, which affect the repeatability of the scanner. The purpose of this work was to study these uncertainties.
  
  
  METHODS
  The variance of the scanner noise, as a function of the pixel position, was studied for different resolutions. The inter-scan variability of the scanner response was analyzed taking into account spatial discrepancies. Finally, the distance between the position of the same point in different scans was examined.
  
  
  RESULTS
  The variance of noise follows periodical patterns in both axes, causing the grid patterns. These patterns were identified for resolutions of 50, 72 and 96dpi, but not for 150dpi. Specially recognizable is the sinusoidal shape with a period of 8.5mm that is produced with 72dpi. Inter-scan variations of the response caused systematic relative dose deviations larger than 1% in 5% of the red channel images, 9% of the green and 51% of the blue. No systematic deviation larger than 1% was found after applying response corrections. The initial positioning and the speed of the scanner lamp vary between scans.
  
  
  CONCLUSIONS
  Three new sources of uncertainty, which influence radiochromic film dosimetry with flatbed scanners, have been identified and analyzed in this work: grid patterns, spatial inter-scan variations and scanning reading repeatability. A novel correction method is proposed, which mitigates spatial inter-scan variations caused by deviations in the autocalibration of the individual Charge Coupled Device detectors.
authors: Department of Medical Physics, Institute of Oncology Ljubljana, Zaloška cesta 2, Ljubljana 1000, Slovenia. Electronic address: nmendez@onko-i.si.
authors: I
authors: I
authors: Méndez
authors: Department of Medical Physics, Institute of Oncology Ljubljana, Zaloška cesta 2, Ljubljana 1000, Slovenia.
authors: Ž
authors: Ž
authors: Šljivić
authors: Department of Medical Physics, Institute of Oncology Ljubljana, Zaloška cesta 2, Ljubljana 1000, Slovenia.
authors: R
authors: R
authors: Hudej
authors: Department of Medical Physics, Institute of Oncology Ljubljana, Zaloška cesta 2, Ljubljana 1000, Slovenia.
authors: A
authors: A
authors: Jenko
authors: Department of Medical Physics, Institute of Oncology Ljubljana, Zaloška cesta 2, Ljubljana 1000, Slovenia.
authors: B
authors: B
authors: Casar
mesh_terms: D002138:Calibration; D003196:Computer Graphics; D004307:Dose-Response Relationship, Radiation; D004566:Electrodes; D005373:Film Dosimetry / Q000295:instrumentation* / Q000379:methods*; D010315:Particle Accelerators; D010363:Pattern Recognition, Automated; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted; D015203:Reproducibility of Results; D035501:Uncertainty
keywords: Film dosimetry; Flatbed scanners; Radiochromic