Control Of Scatter Radiation Flashcards
Define contrast
Arises from areas of light, dark, and shades of gray in the image
Define Contrast resolution
Ability to image adjacent similar tissues as different
Scatter puts what in the image
A density in the film that is not related to patient anatomy and makes the image less visible=noise
Scatter is produced when and destroys what
Produced during Compton interaction
High kV ranges
Scatter destroys contrast
Factors affecting scatter PRODUCTION
Volume of material irradiated
Atomic number of material being irradiated
KV
Ways to control scatter
- kVp selection
- Beam restriction
- Compression
- Grid use
What is the best way to reduce scatter?
Beam restriction
Aperture diaphragm
Flat sheet of metal with a hole in the center Attached to tube head Simple, cheap, easy to use High penumbra High off focus radiation No light field
Comes/cylinders
Extended metal structure attached to tube head or collimator
Cheap, easy to use
Less penumbra and off focus than diaphragm
Collimator
Lead shutters at right angles to one another regulate field size
More complex, more expensive
Infinite field sizes, provides light source
PBL
Positive beam limitation devices
Automatic collimator a were required from 1974-1993
First stage shutters
Decrease off focus radiation
Fixed, don’t move
2nd stage shutters
Control field size and decrease penumbra
Lead strips and lead masks
Attach to collimator head used in oncology in size of tumor
Compression
Used for bigger patients to decrease patient thickness
Grid use
Doesn’t affect scatter production
Absorbs scatter after it’s been produced
What is the purpose of a grid?
Improves contrast of image by absorbing scatter
Use when: body part is greater than 10 cm thick
kV is greater than 60 in film and CR
kV is greater or equal to 90 in digital
What is a grid made out of?
Lead strips with Al or plastic fiber interspace
What is the Al interspace for?
More durable Easier for manufacture Nonhygroscopic Acts as a grids grid Absorbs more low energy primary also
Grid ratio
Height/distance
Higher grid ratio absorbs more scatter
Grid frequency
Number of grid lines/in
Range from 60-110 lines per inch
High frequency grids show less distinct grid lines
Linear parallel grid
Strips in one direction, run parallel to each other.
Some grid cut off bc not canted along divergence of beam
Better for long SID
What is grid cutoff?
Absorption of primary beam
Linear focused grid
Central strips run parallel as strips move farther away from center they become more and more inclined
Less cut off
SID must be within focal range
Convergence line
Imaginary line in space where strips would meet on linear focused grid
Grid radius
Distance from face of grid to convergence line
Cross cross or cross hatched grid
Lead strips in 2 directions at right angles of each other
More efficient than linear
Centering is crucial
Cannot angle tube against grid
Stationary grid uses
Portable procedures
Most uprise procedures
Grid cassette
Moving grid uses
Table top procedures
Usually focused grids
Blurs grid lines
Single stroke mechanism
Spring, requires manual resetting for each exposure
Reciprocating mechanism
Motor, back and forth
Most common
Oscillating mechanism
Electromagnet circular motion
Vibrating
Stroboscopic effect
Grid lines occurring at very short exposure times. With moving grids
What grid to use up to 80 kV?
5:1 & 6:1
What grid to use up to 100 kV?
8:1 & 10:1
What grid to use over 100 kV?
12:1 & 16:1
Grid conversion factor
Bucky factor
Tells how much to increase mAs with grid
B= mAs with grid/mAs without grid
B= incident radiation/transmitted radiation
Selectivity of grid
Ability of grid to absorb scatter and not primary radiation
% primary radiation transmitted/% scatter radiation transmitted
Want a higher selectivity
Contrast improvement factor
K factor Usually 1.5-3.5 If K=1, then no improvement is seen K=radio graphic contrast with grid/without grid Want higher number Average gradient
Off level
Tube angled across long axis of grid
Grid cutoff on entire image: underexposed
Off center
Tube not centered along central axis of focused grid
Beam divergence my aligned
Grid cut off on entire image
Off focus
Tube not within focal range o focused grid (SID)
Grid cut off along periphery
Upside down
Tube side of focused grid against film
Line down the middle of image
Severe peripheral grid cut off
Off center and off focus
Grid cut off on one side
Air gap method
Use greater OID
Increase mAs 10% for every inch of air gap
What is the grid conversion equation?
mAs2= mAs1 x GCF2/GCF1
Lead content
Measured in g/cm^2
High lead content means high ratio with low frequency
High lead content means higher absorption of scatter
Lead strips become thinner as you add more
