Caracterização de filmes radiográficos de uso odontológico a partir de sensitometria e espectrometria por fluorescência de raios X

Abstract

This dissertation aims to show a method to raise characteristic curves of dental radiographic films of two different brands, Carestream Dental and Contrast, traded as films of E class, through the intensity quantification of the characteristic radiation produced by the metallic silver fixed after the development process and the determination of different sensitometric parameters from the raised curves. The characteristic curve (also termed as sensitometric curve or H-D curve) correlates the degree of blackening of a region of film, called optical density, with the exposure applied. The measurement instrument the optical density is the densitometer and the reading process is called sensitometry. The optical density is measured by the attenuation of the intensity of a light beam produced by a source installed on the densitometer. The greater blackening of the film, the greater the attenuation of the beam intensity. However, densitometers have a limitation in the reading process when the degree of blackening is sufficient to attenuate the light beam completely. In this situation, the densitometer saturates at optical density values lower than the maximum predicted for radiographic films. On the other hand, the X-ray fluorescence spectrometry has no such limitation once it does not correspond to the radiation transmission but to the characteristic radiation output from the radiation interaction with the environment. For radiographic films, the intensity of the detected metallic silver fluorescence is proportional to its mass fixed after development process. Greater amount of metallic silver, greater is the blackening and greater is the fluorescence count. Even after the densitometer saturation, the spectrometer continues to produce fluorescence counts proportional to different amounts of silver in the film. Based on this fact, the tracing method of the characteristic curve has been drawn correlating optical density data, obtained by sensitometry, with silver fluorescence counting upto the densitometer saturation point. From the correlation, it was produced a transfer function for each set of films, converting fluorescence count in optical density. From the experimental and adjusted data from the films, it has been estimated the values of the expression arguments which define the characteristic curves. Traced the curves, other parameters linked to radiographic films could be estimated, such as film speed, average gradients, contrast, density and mass of grains. In addition, it was inferred the photon fluence and exposure rate applied to the film. The method has proved effective in compliance the established objectives and reveals an important potential to some specific applications such as dosimetry using dosimetric films, digitization of radiographic films and imaging techniques.