Scatter Control Bushong Chapter 11 Unit 6 Flashcards
5 x-ray interactions with matter
Classical Coherent Scatter Compton Scatter Photoelectric Interaction Pair Production Photodisintegration
Classical Coherent Scatter
Contributes little to medical image because of low energy levels
No energy loss
No ionization
Directional change
Energies below 10 KeV
Incident x ray interacts with target atom causing it to become excited
Target atom releases excess energy as scattered x ray
Scatter x ray has same wavelength as incident but changes direction
Compton Scatter
Occurs throughout diagnostic range important in x ray imaging reduces energy ionizes atom & changes direction Incident x ray interacts with outer shell electron ejects outer shell electron from atom Incident x ray's direction is angled, deflected can be deflected in any direction
Photoelectric Interaction
X ray is absorbed
Atom is ionized
X ray interacts with inner shell electron ejecting it
Binding energy + photoelectron energy = incident x ray energy.
All of the energy is absorbed
Scatter
Scatter increases with: Increased Field size Increased kVp Increased patient thickness Grids are used for parts over 10cm kVp that is over 60 according to Carlton
Reduction of Scatter on the image
*Decreasing kVp
*Decreasing field size
*Compression
Grids**
Air Gap**
**reduces scatter on image ONLY
*reduces scatter on image and on scatter produced.
X ray energy to scatter ratio
As x ray energy increases, Compton decreases, but photoelectric interaction decreases more rapidly.
This means relative to Photoelectric, as kVp increases, Compton increases.
So when looking at percentage of interaction, raising kVp increases Compton, when compared with Photoelectric
Using Low kVp
Low kVp can not be used all the time because this would increase photoelectric absorption which increases patient dose.
To achieve proper OD we would have to increase mAs which also increases patient dose.
Low kVp techniques would increase patient dose unacceptably.
Field Size
Scatter radiation increases as field size increases.
Reducing scatter results in less OD, so technique may need to be increased.
Since field size is smaller, the increase in technique is acceptable
Density is related to mAs
Beam Restrictors
Aperture diaphragm
Cones or cylinders
Variable aperture collimators
Aperture Diaphragm
Simplest beam restrictor
Lead or lead lined metal diaphragm attached to tube.
Opening usually just smaller than IR
Mostly used in trauma, chest, and dental radiography.
Lead sheet with opening the size of the IR
Cones and Cylinders
Mostly used in specific areas (skull)
Modification of aperture diaphragm
Extended metal structure tube or cone shaped.
Most commonly a cylinder but both are typically called cones.
Must be aligned or one edge may be cut off.
Variable Aperture Collimator
Most commonly used device
shutters reduce beam to desired size
First stage shutters reduce off-focus radiation and protrude from the top of the collimator into the x ray tube housing.
Second stage shutters reduce field size. Usually made of 3mm lead.
First and Second Stage shutters on Variable Aperture Collimator
Work in pairs opposite each other independently controlled.
Can produce a square or rectangle
Light is produced from collimator housing with lamp and angled mirror
Mirror is adjusted so it coincides with x ray field
Must be precise and checked for quality control.
Paper clip test
Off Focus Radiation
Off focus radiation is when electrons interact with areas of the anode other than the focal spot.
It increases blur because it increases the size of the focal spot.
PBL
Positive Beam Limiting Devices
Found in majority of equipment
Mandated by FDA from 1974 removed in 1994.
When film is clamped in bucky a signal is sent to collimator housing to collimate to the appropriate size.
Should still collimate tighter when possible.
Only works in the bucky.
Collimators
Some collimator housings allow for changing filters
Filtration is never completely removed
Some filtration is inherent in tube, some filtration is from mirror.
X ray beam should NEVER exceed size of IR.
Patient Thickness
thicker parts scatter more radiation than thinner parts
type of tissue also effects scatter (lungs vs abdomen)
a 3 cm part vs. a 30 cm part
Average angle of deflection is increased due to multiple scattering.
Compression
Improves both spatial and contrast resolution
lowers patient dose
compression paddle, mammography compression units, compression band
Collimators
Reduce patient dose and improve contrast resolution
Filtration
Filtration is never completely removed
Some filtration is inherent in tube and mirror
Focal spot
controls spatial resolution
Scatter Radiation
controls contrast resolution
Purpose of Grids
Improve contrast
absorbs scatter radiation
Does NOT decrease production of scatter
Primary & Secondary Photons
Primary x ray beam coming from target
Secondary photons- scatter
Primary photons do one of 3 things
Primary photons do one of 3 things:
Transmit: Black image
Absorb: White image
Scatter: Grey image
Differences in absorption and transmission
create contrast on the image
Scatter increases with
increase field size
increase kVp
increased patient thickness
When are grids used?
Patient parts over 10 cm
kVp that is over 60 according to Carlton
Grid Construction
Series of radiopaque strips that alternate with radiolucent interspace strips.
Grid radiopaque strips
Lead is most commonly used for radiopaque material
Grid radiolucent strips
Plastic fiber and aluminum are most commonly used for radiolucent material
Grid Ratio
Ratio of height of lead strips to distance between the strips.
Higher ratio grids absorb more scatter than lower ratio grids
Grid Ratio formula
h= height
D= interspace material
Grid ratio=h/D
Grid Frequency
Number of lead strips per inch or centimeter
Range from 60-200lines per inch
What does using grid ratio and frequency determine?
Ratio and frequency determine the total lead content of the grid.
Does higher lead content grids absorb more scatter?
Higher lead content grids will absorb more scatter than lower lead content grids.
Grid Patterns
Linear: grid strips running in only one direction.
“Crossed”: criss-cross or cross hatched- Grid strips running in 2 directions perpendicular to each other.
What was Bucky’s original grid design?
Crossed
Are linear grids less likely to produce grid lines.
Yes, linear grids are less likely to produce grid lines.
Linear Grids
Allow the radiographer to angle along the grid lines but not against.
Which way do linear grid lines run?
Linear grid lines run along the long axis of the grid, however, some run along the short axis.
Which way will linear grids cut offs produce?
Linear grids will produce cutoff along their lateral edges.
Crossed grids
more likely to produce grid cutoff as the angulation in either direction (crosswise or lengthwise) could produce cutoff
Grid Cleanup
Grid cleanup on a 6:1 crossed grid is comparable to a 12:1 linear grid
A 6:1 crossed grid is composed of (2) 6:1 linear grids.
Grid Types
Parallel or Focused
Parallel Grids
all grid lines and interspace material run parallel to each other
These lines are perpendicular to the IR
Focused Grids
Focused grids the center lines are parallel and the rest of the grid lines angle inward at an increasing angle as they move outward from the center of the grid.
How are focused grids used?
Focused grids must be used at a particular SID to meet the angle of divergence of the x ray beam.
Where is the focal range of the focused grid?
Focal range of the grid should be labeled on the inside of the grid itself.
How do parallel grids work best?
Parallel grids work better at long SIDS
What happens with parallel grids at shorter SIDs?
At shorter SIDs grid cutoff will be apparent at the lateral edges of the grid.
At what distance will the x ray beam of parallel grids be more perpendicular to the IR?
At longer distances, the x ray beam will be more perpendicular to the IR, and converge with the parallel grid better.
Grid Uses
Grids can be stationary or portable
Portable Grids
Simply snap on the cassette or the cassette slides into the grid.
Gridded Cassettes
have a grid built into them
the cassette must be reloaded with the film between exposures.
Moving Grids
Can be mounted in the upright bucky or inside the x ray table.
Which way do grid lines run in a moving grid?
Grid lines in a moving grid run lengthwise.
Movement of moving grids
Reciprocating or Oscillating
Reciprocating moving grids
a motor drives the grid side to side blurring grid lines
Oscillating moving grids
an electromagnet pulls the grid to one side, it is released during exposure, moving in a circular pattern
Grid Conversion Factor (GCF)
GCF=mAs with Grid/mAs without Grid
How does GCF increase?
GCF increases with increasing kVp
How much mAs more or less do I need if I decide to use or remove my grid?
Contrast Improvement Ability (K)
K=Contrast Improvement Ability
Contrast Improvement Ability
The principal function of a grid is to improve contrast by removing scatter
K is how much the contrast is improved by employing the grid
Contrast Improvement Ability formula
K-Image contrast without grid/Image contrast with grid.
This is a measure of performance, it does not matter the grid frequency or ration, just the contrast on the finished image
K of 1 would mean no change * there has to be change.
Does a higher K mean better or worse contrast?
Higher K would mean better contrast improvement or clean up.
Off Level
a result of improper tube or grid positioning
Primary beam is angled against the lead strips.
Is Off Level possible with focused or parallel grids?
Off level is possible with both focused and parallel grids.
Occurs with any angulation on a crossed grid
What is the ONLY problem with parallel grids?
Off level which is a result of improper tube or grid position is the ONLY problem with parallel grids.
Where does grid cutoff occur with off level?
Grid cutoff occurs across the entire image.
Off Center
Happens when the x ray beam is not centered to the central axis of the grid.
What is off center grid cut off called?
Off Center grid cutoff is called lateral decentering.
How is the central ray supposed to line up with the focused grids?
The central ray is supposed to line up with the perpendicular central portion of the grid.
This problem occurs with focused grids ONLY.
How will grid cutoff be produced with Off Center?
Will produce grid cutoff across entire image.
The greater the lateral movement, the greater the grid cutoff.
Off Focus
Occurs when using a grid at a distance other than that specified.
What does Off Focus grid cutoff cause?
Causes a mismatch in angle of divergence between the grid and the beam.
How will grid cutoff appear with Off Focus?
Cutoff will appear at lateral edges of grid.
Occurs with focused grid ONLY.
Upside-Down Grid
Occurs when back of grid faces tube.
X rays pass through in center of grid where strips are perpendicular, but not along lateral edges where strips are angled more severely.
Where does Upside-Down grid cutoff appear?
SEVERE grid cutoff appears at lateral edges.
Occurs with focused grid.
Many grids today are manufactured so that this is impossible.
Air Gap
IR is placed 10-15cm away from patient.
Portion of the scattered x rays diverge away from the image receptor, improving contrast.
Loss of detail will occur due to increased OID
Do grids increase Patient Dose?
Grids considerably raise patient dose, twice as much.
How much do grids increase patient dose?
15% more dose with a moving grid as compared with a stationary grid.
With a low kVp, a low ratio grid should be used, with a high kVp, a high ratio grid should be used.
Compton Scatter
Produces noise
Reduces image contrast
Reduces Contrast Resolution
Grids Remove
Noise
Improve image contrast
Spatial Resolution is determined by?
Focal spot size and
Other sources of blur
Contrast Resolution is determined by?
Scatter Radiation and
other sources of noise
As scatter increases
Image looses contrast and appears gray (dull)
Increase in Photoelectric Absorption
Increases patient dose
occurs with low kVp
Increase in field size
Results in poor image contrast because
it increases scatter
Thicker body parts result in:
More scatter radiation
x rays undergo multiple scattering and the
average angle of scatter in remnant beam is therefor greater
What is contrast?
The degree of difference in OD between areas of radiographic image.
The ability to distinguish soft tissues on image
Most remnant x rays are?
Most remnant s rays are scatter radiation
Collimation
Reduces patient dose
improves contrast resolution
What does off focus radiation increase?
Off focus radiation increases blur
Grids
The greater the atomic number of tissue the less scatter.
Less scatter in bone vs. soft tissue
Higher Grid Ratios
Allow less scatter through to the IR
The Higher grid ratio straighter the scatter photons have to be to pass through to the IR
Higher grid ratios require greater accuracy in positioning
More prone to grid errors.
Grid Frequency
Number of grid lines per inch or cm
Frequency range is 60-200 lines/inch
Most common frequency 85-103 lines/inch or 33-41 lines/cm.
Lead content of grids
Lead content is higher in grids w/higher grid ratios and lower grid frequencies.
As lead content increases the ability to remove scatter increases.
Higher lead content improves contrast
Higher Grid Ratio
The higher the grid ratio the more critical for proper grid alignment
Lower Grid Ratio
The lower the grid ratio the greater the latitude in tube alignment
Parallel Grids
Do not coincide with beam divergence
Grid cut off occurs along the lateral edges
work best at long SIDs because
Beam is straighter as distance increases
Grid Clean up
The better the clean up the greater the patient dose.
Grid Conversion Factor(GCF)=mAs with grid/ mAs without grid
Formula needed to know how much to increase your mAs if you can not use a grid.
Grid Errors
Off Level
Off Center
Off Focus
Upside Down
Off Level Grid Error
occurs when the tube is angled across the long axis of the grid strips
Occurs with focused grids.
ONLY positioning error with parallel grids
Errors occur across the entire image.
Off Center Grid Error
Tube must be centered along central axis of focused grid to prevent this error.
Occurs with focused grids
Error occurs across entire image
The greater degree of off-centering the greater grid cut off.
Off Focus Grid Error
Results from grid use at a distance other than the specified focal range on the grid
Occurs with focused grids
Errors occur along the peripheral edges of the image.
High grid ratio requires greater positioning accuracy to prevent grid cut off.
Upside Down Grid Error
Severe peripheral grid cut off occurs when the grid is placed on the IR upside down.
Air Gap Technique
Alternate technique to the use of a grid.
Places the patient part at a greater OID creating an air gap between patient and IR
Amount of scatter reaching IR is reduced
Same amount of scatter is produced but less scatter reaches the IR
