Optical density is a measure of the degree of radiographic film darkening, and is related to the proportion of incident x-ray photons that are transmitted through the tissue and strike the film 1.
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Usage
Optical density is used to describe the level of film exposure in film-screen radiography. In contrast, ‘exposure index’ is typically used to describe the level of exposure in computed radiography and digital radiography systems 2.
Measurement
Optical density is equal to the logarithm of the ratio of incident light intensity and transmitted light intensity (figure 1).
Clinical relevance
High optical density values indicate higher levels of x-ray photon transmission through the tissue, which corresponds to a higher degree of film darkening. Dense structures such as bone attenuate a relatively high proportion of incident X-ray photons and therefore produce lower optical density values than less dense structures such as muscle and fat.
Dose response curves can be plotted for different radiographic systems to describe detector response over a range of incident radiation exposures. In the case of film-screen radiography, plotting optical density values at different relative levels of radiation exposure generates a sigmoid curve shape 3. This is significant as it demonstrates film emulsion has a non-linear response to dose (figure 2):
adequate image contrast is achieved using doses that correspond to the linear region of the curve
at the low optical density extremity of the curve, low dose low contrast images are produced, which are not clinically useful
at the high optical density extremity of the curve, high dose low contrast images are produced, which are not clinically useful either
The dynamic range is the ratio of the maximum and minimum doses that can be used to generate a clinically useful radiographic image 2. Film-screen radiography has a narrower dynamic range than other modalities such as computed radiography and digital radiography 3.