- The photon beam that exits the tissue is no longer the random, filtered, polychromatic beam that entered the tissue.
- It has been altered by the interactions with the tissue that have selectively attenuated the incoming photons.
- These exiting photons form an invisible dataset that interacts with the image receptor (film or digital).
- Once the image is processed, the resultant image must be interpreted.
- Our knowledge of image characteristics helps us to interpret the image.
- Density is defined as the darkening of the film due to interactions with x-radiation (or light, in the case of screen-film combinations)
- Overall darkening of the film = radiographic density; depending on the number of photons striking the film.
- Optical Density = Lg(Io/It). Io is the intensity of light from the light box and It is the intensity of the light that has been transmitted through the film.
- Additionally, there is inherent density that can be seen in an unexposed film. This is called base plus fog. Manufacturers add a tint to the base of the film to enhance the viewing characteristics.
- the plot of the range of useful optical densities as a function of the log of the exposure
- determines the range of useful exposures for a given receptor, corresponding to the toe-shoulder segment of the curve
Exposure factors that affect radiographic density
- mA. By increasing the milliamperage, the number of photons produced is increased.
- Time. Similarly, an increase in exposure time will increase the number of photon produced.
- kVp. An increase in the peak kilovoltage will increase the mean energy of the photons and therefore, as a side effect, increase the number of photons that reach the image receptor.
- Subject Thickness. The thicker the subject, the greater the attenuation.
- Subject Density. The greater the density, the greater the attenuation.
- the images of thick and/or dense structures appear lighter.
- The difference in densities between two adjacent areas. The great the difference, the higher the contrast.
- A radiograph demonstrating both black and white areas is considered high contrast. This is also called a narrow gray scale. Showing caries.
- Similarly, radiographs with a wide gray scale exhibit a large range of gray tones. Good for soft tissue.
- Interwined with density. All factors affecting one affect the other. Contrast suffers when density is non-optimal.
- High contrast is not always the best.
- Inherent properties of subject being radiographed - different types and number of interactions occur in different tissues
- Oral cavity contains a variety of tissues with different subject contrasts - distinguishable structures.
- Many (but not all) common oral/dental lesions contrast well with the surrounding normal tissues.