ABSORBED DOSE IN AGBR IN DIRECT FILM FOR PHOTON ENERGIES (LESS-THAN-150KEV) - RELATION TO OPTICAL-DENSITY - THEORETICAL CALCULATION AND EXPERIMENTAL EVALUATION

In the radiological process it is necessary to develop tools so as to explore how X-rays can be used in the most effective way. Evaluation o f models to derive measures of image quality and risk-related paramete rs is one possibility of getting such a tool. Modelling the image rece ptor, an important part of the imaging chain, is then required. The ai m of this work was to find convenient and accurate ways of describing the blackening of direct dental films by X-rays.Since the beginning of the 20th century, the relation between optical density and photon int eractions in the silver bromide in X-ray films has been investigated b y many authors. The first attempts used simple quantum theories with n o consideration of underlying physical interaction processes. The theo ries were gradually made more realistic by the introduction of dosimet ric concepts and cavity theory. A review of cavity theories for calcul ating the mean absorbed dose in the AgBr grains of the film emulsion i s given in this work. The cavity theories of GREENING (15) and SPIERS- CHARLTON (37) were selected for calculating the mean absorbed dose in the AgBr grains relative to the air collision kerma (K-c,K-air) of the incident photons of Ultra-speed and Ektaspeed (intraoral) films using up-to-date values of interaction coefficients. GREENING's theory is a multi-grain theory and the results depend on the relative amounts of silver bromide and gelatine in the emulsion layer. Tn the single grain theory of SPIERS-CHARLTON, the shape and size of the silver bromide g rain are important. Calculations of absorbed dose in the silver bromid e were compared with measurements of optical densities in Ultra-speed and Ektaspeed films for a broad range (25-145 kV) of X-ray energy. The calculated absorbed dose values were appropriately averaged over the complete photon energy spectrum, which was determined experimentally u sing a Compton spectrometer. For the whole range of tube potentials us ed, the measured optical densities of the films were found to be propo rtional to the mean absorbed dose in the AgBr grains calculated accord ing to GREENING's theory. They were also found to be proportional to t he collision kerma in silver bromide (Kc,AgBr) indicating proportional ity between I(c,AgBr and the mean absorbed dose in silver bromide. Whi le GREENING's theory shows that the quotient of the mean absorbed dose in silver bromide and Kc,AgBr varies with photon energy, this is not apparent when averaged over the broad (diagnostic) X-ray energy spectr a used here. Alternatively, proportionality between Kc,AgBr and the me an absorbed dose in silver bromide can be interpreted as resulting fro m a combination of the SPIERS-CHARLTON theory, valid at low photon ene rgies (<30 keV) and GREENING's theory, which is strictly valid at ener gies above 50 keV. This study shows that the blackening of non-screen films can be related directly to the energy absorbed in the AgBr grain s of the emulsion layer and that, for the purpose of modelling the ima ging chain in intraoral radiography, film response can be represented by Kc,AgBr (at the position of the film) independent of photon energy. The importance of taking the complete X-ray energy spectrum into full account in deriving K-c,K-AgBr is clearly demonstrated, showing that the concept of effective energy must be used with care.