Radiographic contrast is the density difference between neighbouring 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).
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 radiographs are considered 'high-contrast' whereby density differences albeit greater, overall possess fewer in density steps (lesser shades of grey).
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 examing the lung fields, where subtle changes in density are pertinent to a diagnostic image.
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 will decrease the contrast of the radiograph, however, the use of close collimation and grids are two factors that can be applied to counterbalance this undesirable effect.
- 1. John Lampignano, Leslie E. Kendrick. Bontrager's Textbook of Radiographic Positioning and Related Anatomy. (2017) ISBN: 9780323399661
- 2. Ching, William, Robinson, John, McEntee, Mark. Patient‐based radiographic exposure factor selection: a systematic review. (2014) Journal of Medical Radiation Sciences. 61 (3): 176. doi:10.1002/jmrs.66 - Pubmed
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