Most of projection radiography is currently performed using digital receptors such as computed radiography which makes use of photostimulable phosphors (e.g., BaFBr). The data acquired by a digital radiographic system can be characterized by two key numbers: (1) the average radiation intensity incident on the imaging receptor, expressed as an air kerma (mGy) or exposure (mR) where 1 mR ~ 10 mGy; (2) the dynamic range of the radiation incident on the imaging receptor, which may be taken as the ratio of the maximum radiation intensity of clinical interest to the corresponding minimum intensity. For example, in a chest x-ray, if the maximum lung intensity corresponds an air kerma of 50 mGy, and the lowest intensity in the mediastinum is 0.5 mGy, the dynamic range would be 100 (i.e., 50 mGy/0.5 mGy).

The CR system used to generate most of the radiographs in this teaching file generates two numbers that relate to the characteristics of the pattern of radiation that is incident on the imaging plate: (1) an S number that is an approximate measure of the average radiation intensity, and where an S value of 200 corresponds to an average air kerma incident on the plate of 10 mGy (i.e., 1 mR exposure). If the radiation intensity incident on the plate increases by a factor of two, the S number would be halved and vice versa; (2) an L number that is a measure of the range of intensities incident on the plate (dynamic range). If the spread of radiation values in the image ranges from 3 mGy to 30 mGy, the dynamic range is 10 (30 divided by 3), the L number is the log of the dynamic range, or 1.0. If the L value was 2, this would mean that the maximum intensity in the image divided by the minimum intensity would be approximately 100 (i.e., log 100 is 2).