Quantitative optical densitometry with scanning-laser film digitizers

A new process for eliminating two types of artifacts inherent in commercial ly available transmission scanning-laser film digitizers is presented. The first kind of artifact results in nonreproducible interference-pattern fluc tuations as large as 7%. The second kind results in spreading of transmitte d light from low-to-high optical density (OD) in regions with rapidly varyi ng ODs, producing errors as large as 50%. These OD artifacts cause the loss of precision for films with low-OD regions (first type) and the loss of ac curacy for films with regions of high-OD near high-OD gradients (second typ e).:Test radiochromic films, produced by uniform exposure:to a 6 MV photon beam and a high dose rate Ir-192 brachytherapy source, along with test radi ographic films were used to characterize the artifacts of a commercially av ailable scanning-laser film digitizer. The interference-pattern artifact wa s eliminated by digitizing the films on a masked diffusing ground-glass sca nning bed. The light-transmission artifact was eliminated through discrete- fast-Fourier-transform (DFFT) de convolution of transmission profiles with measured digitizer line-spread functions. Obtaining precise: OD distributio ns after the DFFT deconvolution required prior removal of the interference- pattern artifact and application of a low-pass Wiener noise filter. Light-t ransmission artifacts are particularly significant for applications requiri ng measurement of high-gradient OD distributions, such as brachytherapy or conformal photon-beam film dosimetry and quantitation of two-dimensional el ectrophoresis gels. Errors as large as 15%-35% occur in OD distributions re presentative of these applications. The data collection and correction:proc ess developed in this study successfully removes these artifacts. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00308-9].