QUIZ #4 {QUICK STUDY EDITION} Flashcards
3 main factors influencing production of scatter + rules for using a grid
(from most significant to least significant)
Patient or part thickness >13cm (more chance for scatter)
(large) field size
(extremely high) kVp
Scatter affects density and contrast in what way
Scatter INCREASES density
Scatter DECREASES contrast
15% rule to change contrast, and/or density or to maintained density
Slight lengthening of gray scale with maintained density (slightly denser due to penetration)
Types of beam restriction?
Which is best and why?
- Aperture Diaphragms
- Cones/Cylinders
- Collimators
- Grids
Cylinders are best
DECREASE penumbra/blur, field size, & scatter
INCREASE contrast/detail/spatial resolution
What is PBL?
Sensors in the Bucky that know the size of the IR and collimates to correct size (auto-collimation)
Off-level grid error…
tube or grid is angled along so that the beam crosses the parallel grid lines, decreases density
Off-center grid error…
when central ray is not centered correctly, increases cut off
Calculation for grid ratio
h/D
height/Depth
THE BEST INDICATOR OF HOW WELL A GRID WORKS, HIGHER RATIO = HIGHER GCF
How does grid ratio help us, what does it tell us?
Increasing grid ratios INCREASES the amount of scatter that is absorbed
What is bucky factor or grid conversion factor? be able to use it in a calculation
A Grid Conversion Factor formula will assist in selecting the appropriate mAs, this will provide the correct exposure to the IR and prevent noise.
(How the mAs without a grid compares to mAs with a grid)
What is grid frequency?
Number of grid lines per inch/centimeter
The higher the grid frequency, the LESS obvious the grid lines on an image
41-80 L/cm
What special distance do we need to know with a focused grid?
Focal distance
Upside-down grid error…
Seen with focused grid that is placed tube side down. Center will be exposed, with ~all being absorbed towards periphery.
Upside-down grid error…
Seen with focused grid that is placed tube side down. Center will be exposed, with ~all being absorbed towards periphery.
what is air gap?
how do we use it to our advantage?
Alternative to grid, used in magnification exams. OID is increased, causing air gap between patient and receptor.
Causes the amount of scatter that reaches the film to decrease.
Disadvantage is loss of sharpness from Increases OID (more magnification)
Used for lateral C-spines.
what is air gap?
how do we use it to our advantage?
Alternative to grid, used in magnification exams. OID is increased, causing air gap between patient and receptor.
Causes the amount of scatter that reaches the film to decrease.
Disadvantage is loss of sharpness from Increases OID (more magnification)
Used for lateral C-spines.
What is air gap?
How do we use it to our advantage?
Alternative to grid, used in magnification exams. OID is increased, causing air gap between patient and receptor.
Causes the amount of scatter that reaches the film to decrease.
Disadvantage is loss of sharpness from Increases OID (more magnification)
Used for lateral C-spines.
What is the fulcrum?
Object plane?
Slice thickness?
How do we get thinner slices?
Tissue of interest should be positioned at the _____?
Fulcrum is the pivot point or point where the imaginary line connecting tube + receptor at start and end positions cross.
Object plane is the area surrounding the pivot point and tissue in this area is imaged ”in focus”
Tomographic angle describes the amount of tube movement and the slice thickness.
Greater angles = more thinness
Tissue of interest should be positioned at the FULCRUM
What is the fulcrum?
Object plane?
Slice thickness?
How do we get thinner slices?
Tissue of interest should be positioned at the _____?
Fulcrum is the pivot point or point where the imaginary line connecting tube + receptor at start and end positions cross.
Object plain is the area surrounding the pivot point and tissue in this area is imaged ”in focus”
Tomographic angle describes the amount of tube movement and the slice thickness.
Greater angles = more thinness
Tissue of interest should be positioned at the FULCRUM
Exposure times in tomo are _____ that of regular x-ray
Exposure times in tomo are GREATER than that of regular x-ray
Know the types of tube movement and which produces the thinnest slices.
The greater the angle (linear arc) the thinner the slice
10°-40° range
What are the 3 Prime Factors?
- kVp (primarily quality, but affects quantity. How penetrating the beam is. Higher kVp with denser tissue/material)
- mAs
- SID
What controls quantity and quality in the image?
QUANTITY = mAs
QUALITY = kVp
What primarily controls density and contrast in an image?
mAs
What are the 3 main factors effecting technique selection?
- Thickness
- Composition
- Pathology
For denser parts, we increase kVp/mAs
For thicker parts, we increase kVp/mAs
For denser parts, we increase kVp
For thicker parts, we increase mAs
Describe detail and distortion and what controls each
DETAIL = seeing small objects, increased SID and collimation
DISTORTION = misrepresentation of objects, increased SOD
What is the most common technique chart?
Fixed kVp
What are variable kVp charts?
SHORT/LONG scale contrast?
HIGHER/LOWER patient dose?
NARROW/WIDE exposure latitude?
ADVANTAGES?
DISADVANTAGES?
A fixed mAs is used and kVp is varied to accommodate differences in thickness (historic, not used much anymore)
SHORT scale contrast
HIGHER patient dose
NARROW exposure latitude
ADVANTAGES:
• Allows for small changes in exposure for small changes in part thickness
• Higher contrast
DISADVANTAGES:
• More patient dose
• Higher repeat rate
• Increased Tube wear
What are fixed kVp chart s?
SHORT/LONG scale contrast?
HIGHER/LOWER patient dose?
NARROW/WIDE exposure latitude?
_____ or _____ the mAs for every __cm of part thickness
ADVANTAGES?
DISADVANTAGES?
kVp is selected to provide proper penetration of the part, mAs is varied to provide adequate density.
LONG scale contrast
LOWER patient dose
WIDER exposure latitude
Double or half the mAs for every 5cm of part thickness
ADVANTAGES:
• Reduction in patient dose
• Provides more information within the image
• Increased consistency in density and contrast
• Lengthens exposure latitude
• Reduces x-ray tube wear
• Allows for shorter exposure times, thus reduces patient motion
• Easier to memorize
DISADVANTAGES:
• Lower overall contrast when compared to variable kVp systems
• Perceived as less pleasing to the eye
• Higher optimal kVp results in more scatter
Images have more gray
What are AEC charts?
- Appropriate kVp must be selected for the part.
- Proper cell configurations must be selected.
- Density control selections are also available, 2,1,0, -1,-2. These adjust the set point of radiation intensity needed to activate the system. (generally a decrease in mAs)
