anti-scatter grids
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| what controls scatter? | kvp
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| (compton) scattered radiation | incident x-ray photon changes direction with less energy following interaction within patient
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| what is the highest quantity of scatter generated from patient? | 180 degrees
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| what is the highest intensity from patient? | 90 degrees
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| the amount of scatter is equal to the same as what? | primary beam
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| energy is how much of primary beam photon energy? | .1% or 1/1000
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| what is the relationship between kvp and absorption? | as energy increases, the percentage of scatter v. absorption increases
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| patient factors that can cause scatter | pathology such as edema, size of part, improper collimation, atomic # & mass of tissue and barium
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| atomic number | number of protons in the nucleus
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| atomic mass of tissue | protons and neutrons in the nucleus
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| barium | has water in it
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| when would you use a grid? | for a part larger than 10 cm. if you use more than 60 kvp
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| scatter and useful density on chest/abdomen | 50% chest
90% abdomen
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| what helps control scatter? | appropriate collimation helps control scatter
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| too much scatter does what? | fogs image, reduces contrast, and reduces visibility of detail/ spatial resolution
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| gustav bucky | in 1913 developed grid that had alternating grid lines
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| hollis potter | in 1920 developed a mechanism which moved the grid during exposure
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| potter/bucky diaphragm means | moving grid
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| what is the purpose of a moving grid? | to blur out lines and clean up scatter. improves contrast by reducing the amount of scatter reaching the IR
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| DR | has no moving grid
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| CR | has a moving grid oscillating and reciprocating
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| oscillating | circular motion
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| reciprocating | moving back and forth during exposure to improve image quality
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| the grid is placed where? | between the patient and IR
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| construction of grid | we have carbon fiber thin pb which has alternating radio-opaque pb which absorbs scatter and radiolucent interspace which absorbs x-ray beam light for latent image, plastic for low kvp
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| grid cut off | loss of density/ brightness due to absorption of imaging, forms pb strips
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| types of linear grids | parallel, focused, and crisscrossed
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| parallel type | lines are parallel
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| focused type | majority of grids are focused. lines are angled/ tilted inwards to meet divergent of the beam @ a specific focusing difference
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| crisscross type | most effective grid, two grids one atop another
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| latitude | range of SID you can use
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| non- focused types | increase positioning latitude and they are linear parallel and cross (hatch)
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| linear parallel | pb lines run in one direction parallel to each other
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| cross (hatch) | pb lines run in crisscross formation which improves clean up
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| construction of non- focused grids | problem is cut off (loss of density due to absorption of exit beam by pb strips) seen in periphery of image.
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| advantage of non- focused grids | increases SID latitude
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| focused parallel or cross grids | manufactured so peripheral grid lines are tilted to meet beam divergence to eliminate peripheral grid cut off. less latitude/ focusing range (grid radius)
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| grid ratio | determines grid efficiency along with amount of pb in grid
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| r= h/d | h= pb strip (grid) height D= interspace width
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| grid ratio | 2:1-16:1
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| use of low v. high ratio | kVp & amount of s/s (scatter)
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| grid ratio formula | grid ratio = height of pb lines
distance between grid lines thickness of innerspace
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| grid frequency | grid line in cm
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| thicker pb strips clean up more scatter t or f? | true, you want higher ratio and lower frequency.
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| clamp on grids are used for what? | mobile imaging
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| grid cassettes have | grid lines
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| we do not need moving grids because of what? | we use high frequency thin pb lines
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| moving grids | potter bucky diaphragm in table bucky. blur out grid lines and motion starts before exposure and stops afterwards.
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| what are the two types of movement for grids? | reciprocating and oscillating
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| patient dose w/grid vs. patient dose w/out grid | pt. dose w/grid is always higher
incr. pb = increased sensitivity
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| k = | contrast w/grid/contrast w/out grid
increased contrast improvement ability = increased contrast
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| grid position errors | upside down, off level, off center, and off focus
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| upside down | least common, severe peripheral cut-off
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| off level | more common (to light) angled grid or tube, cut-off entire image due to oblique nature, CR cuts across or perpendicular to grid lines
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| off center or | lateral decentering, cut off entire image with more on one side
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| off focus | cut off periphery of image, SID is not properly used
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| stroboscopic effect | captures grid lines when grid is not moving. is seen when reciprocating mechanism is broken and when using short exp time (shorter than movement of grid)
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| alternate method if there is no grid | film screen technology in which you would use the cassette backwards
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| air gap technique | increase distance between patient/part and IR (OID) incr. SID compensates and prevents magnification distortion
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| to prevent OID you would increase what? what is the rule? | SID. for every 1" OID, increase 1' SID. EX. 6" OID and 6' (72") SID
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| when using a grid what must be taken into account | pathology, part size greater than 10-12 cm, and 60>kVp
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