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anti scatter grids

anti-scatter grids

QuestionAnswer
what controls scatter? kvp
(compton) scattered radiation incident x-ray photon changes direction with less energy following interaction within patient
what is the highest quantity of scatter generated from patient? 180 degrees
what is the highest intensity from patient? 90 degrees
the amount of scatter is equal to the same as what? primary beam
energy is how much of primary beam photon energy? .1% or 1/1000
what is the relationship between kvp and absorption? as energy increases, the percentage of scatter v. absorption increases
patient factors that can cause scatter pathology such as edema, size of part, improper collimation, atomic # & mass of tissue and barium
atomic number number of protons in the nucleus
atomic mass of tissue protons and neutrons in the nucleus
barium has water in it
when would you use a grid? for a part larger than 10 cm. if you use more than 60 kvp
scatter and useful density on chest/abdomen 50% chest 90% abdomen
what helps control scatter? appropriate collimation helps control scatter
too much scatter does what? fogs image, reduces contrast, and reduces visibility of detail/ spatial resolution
gustav bucky in 1913 developed grid that had alternating grid lines
hollis potter in 1920 developed a mechanism which moved the grid during exposure
potter/bucky diaphragm means moving grid
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
DR has no moving grid
CR has a moving grid oscillating and reciprocating
oscillating circular motion
reciprocating moving back and forth during exposure to improve image quality
the grid is placed where? between the patient and IR
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
grid cut off loss of density/ brightness due to absorption of imaging, forms pb strips
types of linear grids parallel, focused, and crisscrossed
parallel type lines are parallel
focused type majority of grids are focused. lines are angled/ tilted inwards to meet divergent of the beam @ a specific focusing difference
crisscross type most effective grid, two grids one atop another
latitude range of SID you can use
non- focused types increase positioning latitude and they are linear parallel and cross (hatch)
linear parallel pb lines run in one direction parallel to each other
cross (hatch) pb lines run in crisscross formation which improves clean up
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.
advantage of non- focused grids increases SID latitude
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)
grid ratio determines grid efficiency along with amount of pb in grid
r= h/d h= pb strip (grid) height D= interspace width
grid ratio 2:1-16:1
use of low v. high ratio kVp & amount of s/s (scatter)
grid ratio formula grid ratio = height of pb lines distance between grid lines thickness of innerspace
grid frequency grid line in cm
thicker pb strips clean up more scatter t or f? true, you want higher ratio and lower frequency.
clamp on grids are used for what? mobile imaging
grid cassettes have grid lines
we do not need moving grids because of what? we use high frequency thin pb lines
moving grids potter bucky diaphragm in table bucky. blur out grid lines and motion starts before exposure and stops afterwards.
what are the two types of movement for grids? reciprocating and oscillating
patient dose w/grid vs. patient dose w/out grid pt. dose w/grid is always higher incr. pb = increased sensitivity
k = contrast w/grid/contrast w/out grid increased contrast improvement ability = increased contrast
grid position errors upside down, off level, off center, and off focus
upside down least common, severe peripheral cut-off
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
off center or lateral decentering, cut off entire image with more on one side
off focus cut off periphery of image, SID is not properly used
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)
alternate method if there is no grid film screen technology in which you would use the cassette backwards
air gap technique increase distance between patient/part and IR (OID) incr. SID compensates and prevents magnification distortion
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
when using a grid what must be taken into account pathology, part size greater than 10-12 cm, and 60>kVp
Created by: eckoultd1972