Radiographic Contrast

Contrast is the difference in density or the degree of grayness between areas of the radiographic image.⁷ The radiographic contrast depends on the following three factors:

1. Subject Contrast: it refers to the difference in the intensity transmitted through the different parts of an object. For example, in an intraoral radiograph, the enamel will attenuate x-rays more than dentin. Subject contrast is affected by the following factors:

   • Thickness difference: when two different thicknesses of the same material attenuate an x-ray beam, the thicker part will attenuate more x-rays than the thinner part. ^1,4,5,12

   • Density difference: this is also known as the mass per unit volume. It is the most critical factor contributing to subject contrast. A higher density material will attenuate more x-rays than a lower density material.^5,6,10,11

   • Atomic number difference: A higher atomic number material will attenuate more x-rays than a lower atomic number material.^2,6,7

   • Radiation quality or kVp: this has a significant effect on subject contrast. A lower kVp will make the x-ray beam less penetrating. It will result in a more substantial difference in attenuation between the different parts of the subject, leading to higher contrast. A higher kVp will make the x-ray beam more penetrating. It will also result in less difference in attenuation between the different parts of the subject, leading to lower contrast.^1,3,4,12

2. Receptor Contrast: It refers to a receptor's ability to adequately show the information that the photons transmitted through the subject. In conventional radiography, the contrast depends on the size of the grains, the development time, the concentration and temperature of the developing solution, and overall film density.^2,4,7,12As there is limited usage of conventional film, we will not discuss the details of these factors. In digital imaging, contrast depends on the bit-depth of the receptor. Bit-depth refers to the number of possible grey values stored in an image. The higher the bit-depth, the more gray values it can show. A 1-bit image can only show two colors, black and white. 1-bit can only store one of two values, 0 (white) and 1 (black). An 8-bit image can store 256 possible gray values, while a 12-bit image can display 4096 gray values.^1,4,9

3. Factors that Affect Radiographic Contrast: Scattered radiation will increase the image noise and decrease the radiographic image's contrast. Radiographic collimation can reduce the amount of scattered radiation and improve the radiographic contrast.^1,4,7,8