Chapter 12 Flashcards
Scatter, in diagnostic x-ray is primarily due to _________ effect
Compton
What two major factors affect scatter production?
Tissue volume and kVp
Tissue volume is dependent on:
Part thickness and field size
What causes fog?
Scatter (unwanted radiation)
What two things does beam restriction do?
Limits patient dose
Reduces scatter production
True/False: Beam restriction and collimation are interchangable terms
True
What is the relationship between collimation and field size?
Inverse proportional
How does collimation affect contrast?
Less collimation = more scatter
How does collimation affect mAs?
The more collimation = the more mAs needed
Beam restricting devices
Aperture Diaphragms
Cones/Cylinders
Collimators
Flat peice of lead with a small hole in it, it attaches to the collimator
Aperture diaphragms
Long tube/cone that attaches to the collimator; most often used for dental imaging or small parts such as nasal bones and fingers
Cones/cylinders
Most used form of beam restriction
Collimator
How many lead shutters are in a collimator?
2-3 that can be adjusted using collimator controls
Automatically sets the field size depending on what exam is being completed
Positive Beam-limiting Device (PBL)
When was the grid invented?
1913
Who invented the grid?
Gustave Bucky
Drawback of using a grid
Requires more mAs
What is grid frequency?
Number of lead strips per unit length
What is grid ratio?
Ratio of the height of the lead strips and the distance between them
Two types of linear grids
Long
Short
These grids have lead lines that run parallel with the long axis of the grid
Long
These grids have lead lines that run perp to the long axis of the grid
Short
What is grid pattern?
Linear pattern of the lead lines within a grid
2 types of grid patterns
Linear
Cross-hatched
Difference between linear and cross-hatched grids
Linear - lines running in one direction
Cross hatched - lines running in two different directions, perp to each other
What type of grid removes more scatter - linear or cross hatched?
Cross-hatched
What is grid focus?
Refers to the orientation of the lead lines to one another
2 types of grid focus
Parallel
Focused
Difference between parallel and focused grids?
Parallel - lines are parallel and used primarily in fluoro and mobile x-ray
Focused - lines are angled to match beam divergence - designed to be used at specific distances
Type of grid that is stationary, same size as IR, and usually taped to secure it to IR
Wafer grid
Type of grid that is stationary, IR with a permanent grid mounted to the front surface
Grid cassette
Type of grid that is stationary, designed to hold the IR so no tape is needed - most common type
Grid cap
This type of grid moves slightly while the image is being taken
Reciprocating grid
The decrease in the number of transmitted photons that reach the IR due to grid misalignment
Grid cutoff
Grid cutoff caused by the grid being placed upside down
Upside-down focused
Most common type of grid cutoff - the IR with grid is tilted and CR is not
Off-level
What type of cutoff can occur with both focused and parallel grids?
Off-level
Grid cutoff caused when the CR is not aligned to the center of the IR
Off-center
Grid cutoff occurs when SID is outside of the focal range of the focused grid
Off-focus
Zebra stripe pattern that occurs when the grid frequency of a stationary grid used in CR imaging is similar to the laser scanning frequency of the CR reader
Moire Effect
How to eliminate the Moire effect
Use higher frequency grids or moving grids
What is the Air Gap Technique
By increasing OID, we able to limit the amount of scatter that reaches the IR
Increasing OID also increases:
Magnification
