Improving radiochromic film's sensitivity limit with an optical common-mode rejection and a flatbed color scanner

A novel method, an optical common-mode rejection (CMR), which can greatly improve the dosimetric sensitivity limit of a radiochromic film (RCF) through use of a set of color outputs from an RGB color scanner has been developed. RCFs are known to have microscopic and macroscopic uniformity, which comes from the intrinsic thickness variations in the film's active radiochromic layer and coating. These variations in the response make the optical S/N (signal-to-noise ratio) lower, resulting in lower film sensitivity. The basic principle of the optical CMR is the compensation of RCF film nonuniform response by creating a ratio of the two signals where the factors common to both numerator and denominator cancel out. The scheme was applied to create two types of ratios using two combinations, red /green and red/blue. MD-55-2 type dosimetry films, which were exposed to100 kV X-ray beams in the range from 5 mGy to 8.1 Gy, were scanned with a flatbed color image scanner and the digital data was analyzed. The results obtained with the optical CMR and the conventional analysis method using the only red pixel values were compared. The detection limit can be greatly improved with the optical CMR scheme using the combination of red and green.