Enhanced Radiographic Contrast: Key Factors for Optimal Imaging

1. Radiographic Contrast

2. Contrast • The range of density variation among the light and dark areas on a radiographic image. • A difference in density on adjacent anatomic structures.

3. Radiographic Contrast • The overall contrast of a radiograph • Dependent on two factors. • Subject contrast • Film contrast

4. Subject Contrast • The contrast in the aerial image. • The ratio of the number photons in two or more equal zones of the aerial image. • Aerial Image = The image that contains the information derived from the patient.

5. Aerial Image • Also contaminated by unwanted noise which impairs the quality of information that is displayed on the imaging system. • Noise factors • Quantum mottle • Scattered radiation • Fogging

6. Determining Factors of Subject Contrast • Patient Size • Density • Attenuating characteristics of the subject.

7. Determining Factors of Subject Contrast • Patient Size (thickness) • With increased total thickness of tissue traversed by the x-ray beam, there are more atoms available for interactions (absorption and scatter). • Greater attenuation means that less radiation is transmitted through the patient; less exit radiation is available for interaction with the imaging system.

8. Determining Factors of Subject Contrast • Part density • An increase in density (grams/cm3) increases x-ray attenuation per cm of tissue. • With higher density, more electrons are present per cm3 of tissue to engage in Compton’s scatter.

9. Determining Factors of Subject Contrast • Attenuating characteristics of the subject. • Photoelectric absorption increases dramatically with increasing atomic number. • Attenuation is higher per cm of bone than per soft tissue.

10. Film Contrast • Films vary in inherent contrast, depending on their emulsion characteristics. • Films are designed for long, medium, and short contrast. • Film development also affects film contrast.

11. Film Contrast • Intensifying screens convert 98% or more of the radiologic image to light. • The aerial image is changed almost completely to a light image, which is then recorded by the film. • This process enhances the contrast because screen type film has more inherent contrast for the light emitted by the screens than for x-rays directly.

12. Film and Subject contrast relationship • Radiopacity is the degree of attenuation in a tissue. • Radiolucency is the degree of transmission in a tissue. • The greater the tissue attenuation, less radiation transmitted through the patient.

13. Film and Subject contrast relationship • Various soft tissues are represented as shades of gray, depending on their differential absorption/attenuation. • Fat capsule around the kidneys. • Fat around the psoas muscles. • Both appear as dark gray or nearly black lines. • This ultimately produces subject contrast which improves the image quality.

14. Controlling factors • Kilovoltage (kVp) • Determines the penetrability of the remnant beam. • As the penetrability increases, the scale of contrast on the radiographic image lengthens thus giving the appearance of an increase in the number of gray shades.

15. Controlling factors • Likewise, as the penetrability of the remnant beam decreases, the scale of contrast shortens thus giving the image a more black and white appearance.

16. Influencing Factors • Milliampere seconds (mAs) • Grids • Beam Limiting Devices • Intensification Factor • Patient Considerations • Contrast Media

17. References Bushong, S. Radiologic Science for Technologists, Physics, Biology, and Protection, 6th Edition, Mosby, 1997. Bushberg, et al. The Essential Physics of Medical Imaging, Williams & Wilkins, 1994. Carlton et al. Principles of Radiographic Imaging an Art and a Science, 3rd Edition, Delmar, 2001. Selman, J. The Fundamentals of X-Ray and Radium Physics, 8th Edition, Charles Thomas, 1994.