grids
composed of lead and intermaterial space
lead absorbs the scattered photons
both the perp ones and the ones that travel at angle cuz the hit the intermat and get bounced in
use grids for
increasing contrast and reduced noise
reduce scatter by
collimating,compression,keep kvp low,wearing protection,anti scatter grids
anti scattter grid
placed between the patient and ir
used for
big patients,high tissue areas like lumbar spine and abdomen
parralel grid lines
dead straight and have more felxibility and aligment
focussed grid lines
at an angle towards centre,needs precision
parra grid lines
primary beam gets absorbed and theres geometric cut off
focussed grid lines
angled so divergent beams arent absorbed = better image
mistakes
mas must be increased,primary beam shouldnt be absorbed,correct side facing tube,central beam to central grid,perp to short axis of grid
sid same as grid apparent focus
sid is wrong
random beams escape making bad image
central beam not perp
gradient effect and geomtric cutoff
oscillating grids
moving slats to blur them
stationary grids
cast slats onto receptor
advantages
imrpove iq
thicker lead slab
more scatter removed
improve effiecency
disadvantages
increase oid
unit not transportable
expensive
grid like software
create grid like image wiht no grid
post processing features like manipulation and chnage histograms
doesnt increase mas
better at chest then abdomen
increase contrast
grid ratio
relationship between hieght of lead and distance between them
affect how much scatter is absorbed and how precise positing needs to be
the higher the ratio the proportion of scatter removed
grid effeciency
ability to remove scattered radiation with minimal loss of useful primary beam
effected by
grid ratio
thickness
type of grid
nature of interspacematerial
effect on exposure factors
increase mas
reduction in exposure at image receptor
reduction in number of photons making image
reduce signal
underexposed image
absorbtion reduces signal
put photons back
mas=quantaty and duration of photons
kvp=energy of electrons between cathode and anode
grids are classified as
exposue factors
grid factor
amount of additional exposure needed
changes as kv changes
more foward scatter
greater mas
diagnostic levels 3,4 fold increase
