title: Technical Note: Statistical dependences between channels in radiochromic film readings. Implications in multichannel dosimetry.
abstract: OBJECTIVE
  This note studies the statistical relationships between color channels in radiochromic film readings with flatbed scanners. The same relationships are studied for noise. Finally, their implications for multichannel film dosimetry are discussed.
  
  
  METHODS
  Radiochromic films exposed to wedged fields of 6 MV energy were read in a flatbed scanner. The joint histograms of pairs of color channels were used to obtain the joint and conditional probability density functions between channels. Then, the conditional expectations and variances of one channel given another channel were obtained. Noise was extracted from film readings by means of a multiresolution analysis. Two different dose ranges were analyzed, the first one ranging from 112 to 473 cGy and the second one from 52 to 1290 cGy.
  
  
  RESULTS
  For the smallest dose range, the conditional expectations of one channel given another channel can be approximated by linear functions, while the conditional variances are fairly constant. The slopes of the linear relationships between channels can be used to simplify the expression that estimates the dose by means of the multichannel method. The slopes of the linear relationships between each channel and the red one can also be interpreted as weights in the final contribution to dose estimation. However, for the largest dose range, the conditional expectations of one channel given another channel are no longer linear functions. Finally, noises in different channels were found to correlate weakly.
  
  
  CONCLUSIONS
  Signals present in different channels of radiochromic film readings show a strong statistical dependence. By contrast, noise correlates weakly between channels. For the smallest dose range analyzed, the linear behavior between the conditional expectation of one channel given another channel can be used to simplify calculations in multichannel film dosimetry.
authors.affiliation: Hospital Clínico Universitario Virgen de la Arrixaca, Ctra. Madrid-Cartagena, El Palmar, Murcia 30120, Spain.
authors.forename: Antonio
authors.initials: A
authors.lastname: González-López
authors.affiliation: Servicio de Protección Radiológica y Física Médica Hospital Universitari Sant Joan de Reus, Av. del Dr. Josep Laporte, 2, Reus, Tarragona 43204, Spain.
authors.forename: Juan Antonio
authors.initials: JA
authors.lastname: Vera-Sánchez
authors.affiliation: Hospital IMED Elche, Max Planck No. 3, Elche, Alicante 03203, Spain.
authors.forename: Carmen
authors.initials: C
authors.lastname: Ruiz-Morales
mesh_terms: D016477:Artifacts; D005373:Film Dosimetry / Q000295:instrumentation* / Q000379:methods*; D016014:Linear Models; D011336:Probability; D012815:Signal Processing, Computer-Assisted