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66 Cards in this Set
- Front
- Back
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Name 2 purposes of the grid
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-improves radiographic contrast in image
-absorbs scatter radiation before it reaches IR |
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What 3 things will occur w/ the primary photons that originate at the target
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pass through the body unaffected; be absorbed by body; or interact & change direction
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absorption of photons occurs as the result of
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photoelectric interaction
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primary radiation that interacts & changes direction is known as
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scatter radiation
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True/False: the exposure from scatter radiation to the IR has no diagnostic value
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True
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the % of compton interactions increases w/
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increased kVp
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As scatter increases and contrast is further impaired as kVP ____
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increases
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the amount of scatter radiation increases w/
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patient thickness and larger field sizes
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the amout of scatter radiation decreases w/
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atomic number of the tissue
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what is a grid designed to do
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absorb unwanted scatter radiation
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when should a grid be used
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body part thickness exceeds 10 cm and the kVp is above 60
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basic grids are constructed of
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radiopaque lead strips separated by radiolucent interpace material
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the interspace material is made up of
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aluminum
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grid construction involves the selection of
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materials, grid ration and grid frequency
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primary disadvantage of grid use is
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grid lines on film
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it can improve contrast and is defined as the ratio of the height of the lead strips to the distance b/w strips
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grid ratio
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grid ratio formula
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h/D
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increasing the distance b/w the lead strips would result in a
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decrease in grid ratio
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____ ____ ratios allow less scatter radiation to pass through their interspace material to reach the IR
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higher grid
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realigned lead strips to run in one direction and allowed grid to move during exposure
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Potter-Bucky diaphragm
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more efficient in removing scatter is a
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higher grid ratio
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typical grid ratio range is
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5:1 to 16:1
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is defined as the number of grid lines per inch or cm
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grid frequency
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grid frequency range is
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60-200 lines/inches or
25-80 lines/cm |
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grids with higher grid frequencies have _____ lead strips
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thinner
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recommended for digital imaging systems and have a very high frequency grid range is
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103-200 lines/in
41-80 lines/cm |
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most important in determining the grid's efficiency at cleaning of scatter
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grid's lead content
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as the lead content of a grid increases, the ability of the grid to remove scatter & improve contrast _____
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increases
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high ratio, low frequency grids tend to have
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highest lead content
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2 types of grid patterns
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criss-cross or cross-hatched
and linear |
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linear grids allow the technologist to angle the tube only ...
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"along" the long axis of the grid
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angulation across the long axis would result in
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grid cut-off
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a problem of the primary beam angled into the lead, the lead will absorb an undesirable amount of primay radiation is known as
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grid cut-off
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grids that are more commonly used b/c they allow for tube angulation
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linear
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2 linear grid types
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parallel and focused
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are made w/ the lead and interspace strips running parallel to one another
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parallel grids
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are designed so that the central grid strips are parallel & as the strips move away from the central axis they b/c more & more inclined
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focused grids
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for the grid to be properly focused, the x-ray tube must be located along the
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convergence line
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has horizontal & vertical lead strips, primary beam perpendicular to grid, & grid must remain flat
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criss-cross or cross-hatched
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2 movement mechanisms are
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reciprocating and oscillating
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a motor drives the grid back and forth during the exposure for a total distance of no more than 2-3 cm
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reciprocating grid
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an electromagnet pulls the grid to one side and then releases it during exposure
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oscillating grid
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grid conversion factor (GCF) formula is
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mAs with the grid
____________________ mAs without the grid |
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the more efficient a grid is at absorbing scatter, the _____ exposure will be received by the IR
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less
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the better the grid cleans up scatter, the _____ will the dose to the patient to
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greater
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grid-conversion factors increas with _____ ____ ____ & ____ ____
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higher grid ratios and increasing kVp
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ICRU defines 2 criteria for measuring a grid's performance
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selectivity and contrast improvement ability
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describes a grids ability to allow primary radiation to reach IR & absorb a greater % of scatter
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selectivity
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selectivity formula is
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% primary radiation
--------------------- % scatter radiation |
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a grid with a higher lead content would have a ______ selectivity
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greater
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is dependent on the amount of scatter produced, which is controlled by the kVp and volume of irradiated tissue
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contrast improvement factor or the "K" factor
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as the amount of scatter radiation increases, the ____ will be the contrast and the ____the contrast improvement factor
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lower, lower
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contrast improvement formula
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K= Rad contrast w/ grid
---------------------- Rad contrast w/o grid |
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the higher the K factor, the _____ the contrast improvement
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greater
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proper tube/grid alignment is essential to prevent the undesirable absorption of primary radiation known as
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grid cut-off
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error occurs when the tube is angled across the long axis of the grid strips
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off-level
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to prevent this error the x-ray tube must be centered along the central axis of a focused grid
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off-center
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when a grid is used at a distance other than that specified as the focal range it results in
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off-focus error
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severe peripheral grid cut-off will occur
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upside-down
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a grid error that occurs with digital IR systems when the grid lines are captured and scanned parallel to the scan lines in the imaging plate readers
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the Moire Effect
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Moire Effect can be prevented by
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high frequency grids
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the most important way to improve image quality is to
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decrease the amount of scatter initially created-best done by restricting primary beam & collimation
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an alternative to the use of a grid is
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air-gap technique
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moving the patient away from the IR, the amount of scatter reaching the IR will be reduced
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air-gap technique
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primary disadvantage of the air-gap technique is the
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loss of sharpness that results from increased OID
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10 inch air gap has the same degree of clean-up of scatter as a _____ grid for a 10 cm body part.
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15:1 grid
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