Radiographic contrast

Radiographic contrast is the density difference between neighboring regions on a plain radiograph. High radiographic contrast is observed in radiographs where density differences are notably distinguished (black to white). Low radiographic contrast is seen on radiographic images where adjacent regions have a low-density difference (black to grey). 

Contrast scale

As radiographs have varying regions of density, one cannot simply make assumptions based on a small region of interest. It is due to this that the radiographic contrast of an entire image is referred to as 'long-scale' or 'short-scale.' 

Short-scale contrast

Short-scale radiographs are considered 'high-contrast' whereby density differences albeit greater, overall possess fewer in density steps (lesser shades of grey).

Long-scale contrast

Long-scale radiographs are considered 'lower-contrast' whereby density differences are less noticeable however possess many more shades of grey. Long-scale radiographs are preferred while examining the lung fields, where subtle changes in density are pertinent to a diagnostic image.

Contrast control

Kilovoltage

Radiographic contrast is dependent on the technical factors of the radiographs taken. The kilovoltage (kV) during the radiographic examination will determine the primary beams' energy; higher energy effects increased penetrating power. A primary beam with greater kV results in an overall rise in penetration through all tissues (decrease in attenuation differences), therefore resulting in a lower contrast radiograph. Hence the high kV technique of the chest x-ray is employed to present a more uniformly dense image to better appreciate the lung markings.

A 15% increase in kV will essentially correlate to an increase in density similar to double the mAs ².

Scatter radiation 

Scatter radiation travels in all directions and will decrease the contrast of the radiograph ⁶. Factors that contribute to scatter radiation are increasing volume of tissue, tube kilovoltage, the density of matter, and field size ³. Ways to reduce scatter include close collimation, grids, or air gap technique ³. However, the use of grids will increase the dose to the patient because it attenuates the primary beam as well as the scattered radiation ⁴. Meanwhile, the air gap technique can increase the geometric unsharpness (decrease the blur) of the image due to increase in patient-film distance, reduced field of view, and increase in patient dose ⁵.