Image Characteristics Flashcards
What is radiographic density?
Darkening of the film due to interactions with x-radiation, the overall darkening of the film is radiographic density
What is the formula for optical density?
Log of initial density divided by the transmitted intensity
What is base plus fog?
Inherent density that manufacturers add to the base of a film to enhance viewing characteristics
What are the two subject factors that affect intensity?
Subject density and subject thickness. If both of these factors increase the initial intensity is lowered and the optical density increases
What are the three exposure factors that affect intensity?
Tube current, higher current increases optical density
Exposure time, more photons increases optical density
Tube voltage, increases the number of electrons that hit the target which in turn increases optical density
Out of the three exposure factors, which one does not change intensity?
Exposure time
What is the characteristic curve?
Plot of exposure on the x-axis vs. radiographic density on the y-axis.
Increasing exposure makes the radiograph lighter or darker?
Darker
What is film latitude?
The range of useful exposures
What is radiographic contrast?
The difference in densities between two adjacent areas
How does gray scale relate to contrast?
Narrower gray scale means higher contrast. The less shades of gray the higher the contrast
Which dentists prefer high contrast and why? Low contrast?
General dentists prefer high contrast or narrow grayscale because it is easier to see caries
Periodontists prefer low contrast because its better for seeing soft tissue lesions
What is subject contrast?
Inherent properties of the person being radiographed
What happens to image contrast with increasing tube voltage?
It decreases, increases the grayscale
How can you compensate for higher tube voltage?
Decrease exposure time or the tube current
What is film contrast?
Capacity of film to display small differences in subject contrast. Greater contrast means more small differences will be highlighted
Is low or high contrast film best for seeing subtle structures? Small differences in densities like pulp canals?
Low and high respectively
How does scattered radiation affect density and contrast?
Increases density but lowers contrast
How can scattered radiation be counteracted?
Lowering voltage, focus the beam more narrowly, or use a grid (extra-oral only)
How will using an aluminum filter affect mean beam energy>
Increases it, same affect as increasing voltage while lowering contrast
How does collimation affect scatter?
Reduces the exit area meaning less tissue is irradiated
What are two causes of radiographic noise?
Mottle and artifacts
Mottle is due to variation in the film’s structure and artifacts and due to improper handling and processing
What is radiographic sharpness?
Ability to define and edge or two adjacent areas of different densities
How does sharpness affect contrast?
Decreases it
What can affect sharpness?
Film characteristics such as double emulsion, motion of the patient, and geometric blurring
What speed film is typically used in dentistry?
F speed
What is resolution?
Ability to distinguish small adjacent structures
How does focal spot size affect sharpness and resolution? What size focal spot is optimal?
Smaller focal spot increases sharpness and resolution. Less than 1.0mm is optimal
How do we minimize the size of the effective focal spot?
Tilting the anode at 10-20 degrees
How does using a long collimator affect image sharpness and resolution?
Allows only the parallel central rays to reach the object. Has a similar affect to using a small focal spot
