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Grids

Grids Review

QuestionAnswer
what is the energy of the scattered xrays 1 meter from the pt. .1% or 1/1000 of primary beam
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
what percentage of scatter makes up the useful density on a chest xray 50%
what percentage of scatter makes up the useful density on a abdomen xray 90%
what happens if you have too much scatter -fogs image -reduces contrast -reduces visibility of detail
what is the purpose of an xray grid improves contrast by reducing the amount of scatter reaching the IR
where is the grid located between the patient and the IR
when should we use a grid -kVp higer than 60 -part thicker than 10-12 cm -pathology such as edema or ascites
how are grids constructed alternating radio-opaque lead and radiolucent interspace(aluminum,carbon fiber,or plastic fiber)
what causes grid cutoff loss of density due to absorbtion of unatenuated beam from pt
_________ grids cause grid cutoff on the periphery, but you can use any SID parallel grid
__________ 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
cross type grid can only be used in stationary bucky and provides no positioning lattitude
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
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
what kind of grids are used for mobile imaging -stationary -clip on -grid cassettes
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
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
when using a grid what must we do increase mAs or kVp
using a grid does what to our gray scale shortens it
contrast improvement ability -measures grid quality by analyzing contrast with a grid vs. contrast w/o a grid -increased contrast improvement ability=increased contrast
what are the types of grid positioning errors -upside down -off level -off center -off focus
causes severe peripheral cut-off and is the least common of grid errors upside down grid
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
causes cut-off of entire image with more on one side off center(lateral decentering)grid
causes cut-off on periphery of image off focus grid
stroboscopic effect -in moving grids -motion of the grid is frozen -when using exposure time shorter than grid movement -reciprocating mechanism may be broken
what are some alternate methods of reducing scatter -in film screen, you can use a backwards cassette -air gap technique
what is the rule for distance when using the air gap tecnique for every inch of OID you must increase SID by a foot
focusing distance specific range of SID's that can be used for focused grids
Created by: bigad1982