Unit 2 Radiographic Quality Factors II: Contrast Flashcards
Image Contrast
The differences between adjacent areas of IR exposure on the image
- Differences in IR exposure
- Capabilities of processing and monitor
Dynamic Range
Range of brightness of the display monitor
Window Width
Post-processing function that allows the user to produce changes in displayed shades of gray
Window level vs. Window width
-Window Level: changes the level of brightness (center shade of gray) displayed on the viewing monitor
-Window Width: changes the displayed CONTRAST (the number of shades of gray displayed) on the viewing monitor
Physical Contrast vs Visible Contrast
-Physical Contrast: Recorded on the Image Receptor
-Visible Contrast: Displayed on the Viewing Monitor
Long Scale Contrast
• Differences between adjacent shades of gray is small
• Many Shades of Gray
• More diagnostic information
Short Scale Contrast
• Differences between adjacent shades of gray is large
• Fewer Shades of Gray
• Less diagnostic information
High contrast vs Low contrast
-High Contrast: extremely bright and dark areas
-Low contrast: many shades of gray, very little difference between light and dark areas
What type of contrast is most desirable?
Long scale contrast because there is more diagnostic information
What is Total image contrast? What 2 sub-types of contrast make it up?
-A result of the total amount of contrast from both the anatomical part and image processing.
-Subject contrast and Image contrast
Subject Contrast
Differences in intensity of the beam after it has been attenuated by the tissue, Differential Absorption
What is the controlling factor for subject contrast?
kVp (absorption of the x-ray beam)
Subject contrast: High kVp vs Low kVp
-High kVp = more uniform penetration of the body = Long Scale Contrast
-Low kVp = more photons absorbed = Short Scale Contrast
Subject Contrast and Differential Attenuation:
-High subject contrast occurs when the differences in absorption properties of adjacent tissues of body parts is High
-Low subject contrast occurs when the differences in absorption properties of adjacent tissues of body parts is Low
How does tissue atomic number affect subject contrast?
-High Atomic Number Increases absorption (Photoelectric Interactions)
-High subject contrast occurs when the differences in adjacent atomic numbers of tissues in High
Atomic numbers for Soft tissue, Barium sulfate, and Iodine
Soft tissue: 7.3
Barium Sulfate: 56
lodine: 53
Subject Contrast and Tissue Density
-High tissue density Increases absorption
-Increased differences in absorption between adjacent tissue means Increased subject contrast
T or F: As contrast decreases, scatter radiation increases
True
kVp is the Controlling Factor for Subject Contrast, but it also controls
_________ __________ in the tissue and the relationships between _________ and __________
-Differential Absorption
-Photoelectric and Compton
As kVp increases, PE(photoelectric) ____________, Compton ____________, and Contrast ___________
Decreases; Increases; Decreases
Look Up Table (LUT)
Stored data used to map incoming exposure data with a gray shade on the monitor
Image contrast vs Subject contrast
Image contrast: BY the computer
Subject contrast: TO the computer
Subject Contrast influencing factors
OID
Grid use
Beam restriction
Contrast media
Patient factors
Scatter
-OID Reduces scatter = Increases subiect contrast
-Beam Restriction Reduces scatter = Increases subject contrast
-Patient Factors Part thickness, atomic #, tissue density all affect Photoelectric (increased PE = increased contrast)
-Grid Use Reduces scatter = Increases subject contrast
