Grids Review
Quiz yourself by thinking what should be in
each of the black spaces below before clicking
on it to display the answer.
Help!
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| what is the energy of the scattered xrays 1 meter from the pt. | .1% or 1/1000 of primary beam
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| what are the patient factors that affect scatter radiation | -tissue density
-size of part
-atomic # of tissue
-atomic mass of tissue
-beam restriction
*barium causes more scatter
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| what percentage of scatter makes up the useful density on a chest xray | 50%
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| what percentage of scatter makes up the useful density on a abdomen xray | 90%
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| what happens if you have too much scatter | -fogs image
-reduces contrast
-reduces visibility of detail
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| what is the purpose of an xray grid | improves contrast by reducing the amount of scatter reaching the IR
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| where is the grid located | between the patient and the IR
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| when should we use a grid | -kVp higer than 60
-part thicker than 10-12 cm
-pathology such as edema or ascites
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| how are grids constructed | alternating radio-opaque lead and radiolucent interspace(aluminum,carbon fiber,or plastic fiber)
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| what causes grid cutoff | loss of density due to absorbtion of unatenuated beam from pt
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| _________ grids cause grid cutoff on the periphery, but you can use any SID | parallel grid
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| __________ are the most commonly used grid. eliminates grid cutoff on periphery, but has limited SID(SID must be in focussing range to avoid cutoff) | focussed grids
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| cross type grid | can only be used in stationary bucky and provides no positioning lattitude
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| grid ratio | -determines grid efficiency along with amont lead in grid
-R=H/D (H=heigt of strips, D=interspace distance)
-ratio range=2:1-16:1
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| grid frequency | -number of grid lines per unit of measurement
-60-196 per inch or 152-498 per cm
-80-103 lines/in or 203-261 lines/cm is common
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| what kind of grids are used for mobile imaging | -stationary
-clip on
-grid cassettes
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| potter bucky diaphragm | -grid is held inside bucky
-recipricating or oscilating
-motion statrts before exposure and ends after and is used to blur out grid lines
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| bucky factor | -measures grid quality by analyzing pt dose with a grid vs. dose w/o a grid
-exit radiation vs radiation that actually forms image
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| when using a grid what must we do | increase mAs or kVp
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| using a grid does what to our gray scale | shortens it
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| contrast improvement ability | -measures grid quality by analyzing contrast with a grid vs. contrast w/o a grid
-increased contrast improvement ability=increased contrast
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| what are the types of grid positioning errors | -upside down
-off level
-off center
-off focus
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| causes severe peripheral cut-off and is the least common of grid errors | upside down grid
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| an angled grid or tube, causes cut-off over entire image due to oblique nature, CR cuts across or perpendicular to grid lines and is the most common of all grid errors | off level grid
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| causes cut-off of entire image with more on one side | off center(lateral decentering)grid
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| causes cut-off on periphery of image | off focus grid
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| stroboscopic effect | -in moving grids
-motion of the grid is frozen
-when using exposure time shorter than grid movement
-reciprocating mechanism may be broken
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| what are some alternate methods of reducing scatter | -in film screen, you can use a backwards cassette
-air gap technique
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| what is the rule for distance when using the air gap tecnique | for every inch of OID you must increase SID by a foot
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| focusing distance | specific range of SID's that can be used for focused grids
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