Week 6 - Scatter Control And Grids Flashcards
T/F
Scatter radiation is helpful in some ways, but not dianostically
True
What are 3 ways to control scatter?
Beam restriction, kvp selection, grids
How does collimation effect scatter?
Beam restriction leads to fewer primary photons created
How does kvp selection affect scatter?
Increased kvp increases the amount of Compton interactions created within the body
How do grids affect scatter?
Decrease scatter that is created after X-rays interact with the patient
What shape is the primary beam without collimation?
Circular
What is the most common beam restrictor used?
Collimator
What does off focus radiation mean?
Photons that are created outside the target (off the focal spot)
What are the top lead shutters of the collimator for?
Decreasing off focus radiation
What are the bottom lead shutters of the collimator used for?
Decreasing penumbra
Photons that were not produced at the focal spot
Off focus radiation
A geometric unsharpness around the periphery of the image, also known as edge unsharpness
Penumbra
Result of X-rays photons being created in all areas of the focal spot rather than at just a single point
Penumbra
Automatic collimation to size of IR
Positive beam limitation (PBL) device
T/F
mAs needs to be increased when collimation is increased
True
Increased collimation
Reduced field size
Decreased collimation
Increased field size
Flat sheet of metal (diaphragm) with opening in the center (aperture)
Aperture Diaphragm
What is a disadvantage to an aperture diaphragm?
Fixed field size
An aperture diaphragm that has an extended flange attached to it. The flange can vary in length and can be shaped as either a cone or cylinder
Beam restrictors: cone and cylinder
What does a higher kvp result in?
Decreased attenuation, increased transmission
What does a lower kvp result in?
Increased attenuation, decreased transmission
When increasing kvp, what should be accompanied to result in decreased scatter?
Decrease in mAs, less photons are needed to create an acceptable image
How do you identify if a low kvp was used?
Image will have a high contrast
(Greater variation of xray intensities exiting the patient)
(Less shades of grey)*
How do you identify if an image was taken with a high kvp?
Lower image contrast
(Less variation in xray intensities exiting the patient)
(More shades of grey)*
What are compression devices used for?
Applying compression during radiographic procedures (mammography)
Xray photon passes through matter without interaction
Transmission
What creates image fog?
Scatter exit radiation
What is a drawback of using a grid?
Increased mAs = increased patient dose
What anatomical part is a grid usually used for?
A part greater than 10cm thick
What kvp is a grid usually used for?
Greater than 70 kvp
Expresses the number of lead lines per unit length in inches, cm or both
Can range in value from 25-80 lines/cm
Grid frequency
(A typical grid frequency might be 40 lines/cm or 100 lines/in
Defined as ratio of height of lead strips to the distance between them
Grid ratio
h/D
Grid ratio
(Range 4:1 - 16:1)
What are the two types of grid patterns based on the direction of lead strips?
Linear
Crossed grid or cross hatched
Strips run only in one direction, more commonly used because it allows for tube angulation
Linear
Strips run at a right angle to each other, no angulation can be used
Crossed grids
What are the two types of grid focus based on the orientation of the lead lines
Parallel (non-focused)
Focused
Lead lines and interspace are parallel to each other, primarily used in fluoroscopy and mobile imaging, best used at long SID
Parallel (non-focused)
Central grid strips parallel, with increasing angulation as the move away from the centre, xray tube must be located along convergence line to match divergence
Focused
Recommended range of SIDs that can be used with a focused grid
Focal range
The range in which you can position the grid without producing grid cutoff
Positioning latitude
*less error = more positioning latitude
*higher grid ratio = more lead, less positioning latitude, more error
Grid permanently mounted on the front surface of the IR
Grid cassette
IR slides behind grid
Grid cap
Matches size of cassette; taped to IR to prevent IR from slipping
Wafer or slip on grid
Can cause grid lines or blur if moved during exposure
Stationary
Grid moves slightly back and forth in lateral direction over the IR during entire exposure
*motor driven movement
Reciprocating
Strips parallel to long axis of grid
*most commonly used
Long dimension gird
Strips perpendicular to short axis of grid
Short dimension grid
Electromagnet pulls grid then releases it during exposure
Oscillating grid
mAs with grid/mAs without grid
Grid conversion factor
A decrease in the number of transmitted photons reaching the IR due to misalignment of grid with divergence beam
Grid cut-off
When a focused grid is placed upside down on the IR
Upside down focused
Xray beam is angled across the lead strips
(Grid error)
Off level
Central day is not aligned side to side with focused grid; divergence doesn’t match angle of lead strips
Off center
Occurs when the SID is outside the recommended focal range
Off focus
Occurs when the laser scanning lines are parallel to the grid lines
Moire effect
What focal range does a typical grid have?
One that includes an SID of 40”(100cm) or 72”(180cm)
How much air gap is equivalent to a 8:1 grid?
15cm
