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Photoelectric effect

The photoelectric effect is the most important interaction with reference to producing a radiographic image

The complete absorption of the x-ray results in reduced exposure of the film under that tissue

The probability of a photoelectric interaction is directly proportional to the atomic number of the tissues being exposed

Different body tissues such as fat, bones, and lungs have different atomic number, which  results in different amount of x-rays being absorbed when these tissues are exposed

If it not were for this different absorption of x-rays between tissues there would not be enough radiographic contrast, the image would have an uniform opacity (color)

Tissues with a higher atomic number absorb more x-rays with a lower atomic number

Scatter

A photon interacts with an object and the direction of the photon is changed, but the object does not absorb the photon

Scattering is not useful in the production of a radiographic image, in fact, it is desirable

Scattered x-rays may strike the x-ray film and degrade image contrast or they may strike the radiographer and increase personnel exposure