Abstract
Radiochromic film with a dye incorporated into the radiation sensitive layer [Gafchromic EBT2, Ashland, Inc.] may be digitized by a color transparency scanner, digitally processed, and calibrated so that a digital image in units of radiation absorbed dose is obtained. A transformation from raw scanner values to dose values was developed based upon a principal component analysis of the optical densities of the red, green and blue channels of the color image of a dose of 0.942 Gy delivered by a Sr-90/Y-90 disk-shaped source. In the order of increasing eigenvalue, the three eigenimages of the principal component analysis contained, by visual inspection, 1) mainly noise; 2) mainly a pattern of irregular streaks; and 3) most of the expected dose information along with some of the same background streaking that predominated in the second eigenimage. The combination of the second and third eigenimages that minimized the background streaking was converted into a transformation of the red, green and blue channels’ optical densities and applied to films with a range of doses from 0 to 63.7 Gy. The curve of dose vs. processed optical density was fit by a two-phase association curve. This processing was applied to a film exposed from its edge by a different Y-90 source in a configuration that was modeled by Monte Carlo simulation. The depth-dose curves of the measurement and simulation agree closely, suggesting that this approach is a valid method of processing EBT2 radiochromic film into maps of radiation absorbed dose.
Highlights
Self-developing radiochromic film, which darkens without external chemical processing when exposed to ionizing radiation, is a convenient means of measuring radiation absorbed dose in a variety of experimental and clinical applications
A radiochromic film that incorporates a yellow dye in its sensitive layer [Gafchromic EBT2, Ashland, Inc.] is commercially available
The “eigenimages”, which reflect the projections of each pixel’s feature vector onto each of the basis functions, separate different components in the underlying images, that need not be the case in general. It is a noteworthy characteristic of principal component analysis that the basis functions are dependent upon the data, in contrast to a transformation such as the Fourier transform or a wavelet transform in which the basis functions are independent of the data
Summary
Self-developing radiochromic film, which darkens without external chemical processing when exposed to ionizing radiation, is a convenient means of measuring radiation absorbed dose in a variety of experimental and clinical applications. The color of the dye is designed to be sufficiently different from that of the dose information in an exposed film that the image of the dye may be used to correct for subtle variations in the thickness and measurement of the sensitive layer of the film [1]-[3]. Principal component analysis [4]-[6], called the Hotelling transformation or the discrete Karhunen-Loève transformation, is a method of analyzing multidimensional data It is described in an accessible manner in many modern textbooks on digital image processing [7] and is summarized in the Appendix. It is a noteworthy characteristic of principal component analysis that the basis functions are dependent upon the data, in contrast to a transformation such as the Fourier transform or a wavelet transform in which the basis functions are independent of the data
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