11. Scatter Control and Radiographic Grids Flashcards
what 2 things are photoelectric radiation interaction responsible for
at what energies is it more likely to occur at
absorption of photons and tissue contrast
lower energies
contrast is improved (high) with ___ kV
decreased
what are compton radiation interaction responsible for
at what energies is it more likely to occur at
scattering of photons
higher energies
scatter is more likely to degrade image quality at ___ kV
increased kV
large IR has what effect on the scattered radiation detected
more scattered radiation detected
ideally what do beam restrictors do
decrease the field size to anatomy of interest
what 4 things happen when FS is decreased
decreases unnecessary tissue exposure
scatter decreases
scale of contrast shortens
visibility of detail increases
are 3 types of beam restrictors
aperture diaphragms/cones/cylinders
collimators
ancillary devices
what do aperture diaphragms cones used for
reduces unsharpness as compared to aperture diaphragm
gives specific shape of beam
closer collimation for specific shape
what do aperture diaphragms cones introduce and what shape does it have
introduce area of unsharpness = penumbra
almost divergent shape
what do aperture diaphragms cylinders used for
what is its effectiveness at reduction compared to the cones
further reduction in unsharpness as compared to cone
what 2 components do collimators have
light localizing field light
2 sets of shutters that permits infinite number of field size
what do the 2 sets of shutters do in a collimator
what does it permit
length and width of field independently controlled
permits rectilinear collimation
which shutters of the collimator reduces the penumbra
why
bottom shutters
geometric unsharpness around periphery of image- edge unsharpness
what does collimators do to the image edge
improves sharpness of recorded image edge
what does off focus radiation occur from
areas of the x-ray tube other than focal spot areas
upper shutters of the collimator reduces what in terms of radiation reaching the IR
reduces off focus radiation reaching IR
off focus radiation produces images where
beyond exposed field of radiation
image shadows
beam restriction does what
limit the x-ray beam field size
what 2 things does limiting the x-ray beam field size do
limit patient dose
reducing the amount of scatter
beam restriction needs to consider what
the full circular x-ray field
in terms of an aperture diaphragm there is significant unsharpness due to what
owing to proximity to x-ray source
what happens inside a collimator
light comes from lightbulb and mirror angle directs where x-ray beam is going to be
why is there poor quality with wide collimation
lots of scatter
the light reflected off the mirror is used to do what
project the coverage of x-ray beam
the projection of the x-ray beam should be accurate to within +/- ___ % of the ___
+/-2% of the SID
mirror adds what kind of filtration to the system
inherent filtration
what are the 3 features of collimator housing
central ray must be marked
alignment light helps center beam with IR
some units project the AEC sensor locations in the light field
what are Positive beam limitation devices/what do they do
automatically collimates beam to size of IR
what are 2 types of ancillary devices
lead blockers/shields
lead masks attached to the collimator
what do lead blockers do
absorb scatter
what do grids do
absorb and limit the amount of scattered radiation that reaches the IR
improves the quality of radiograph
less scatter on the image has what effect on contrast
increases contrast
what do grids do in terms of the transmitted radiation
absorbs the transmitted radiation exiting the patient and therefore reduces the radiation reaching the IR
what is the grid conversion formula
mAs1/mAs2 = grid ratio1/grid ratio2
what are radiographic strips
device that has very thin lead strips with radiolucent interspaces, intended to absorb scatter radiation emitted from the patient
what is the grid frequency
measure of the number of grid lines per unit distance
what is the grid ratio equation in terms of h and D
ratio = h/D
h = height of lead strip D = interspace distance
where are grids placed
between the patient and the IR
what is the grid frequency equation
freq = 1/(t + D)
t = thickness of lead strip
increased grid ratio has what effect on scatter
more scatter removed = improved contrast
what are the 2 grid types
linear and cross-hatched
the focused grid does what to the x-ray beam and photons
matches the divergence of the x-ray beam and allwos more photons to reach the IR
what happens if the grid isnt focused
if the grid isn’t focused, may eliminate some of the radiation you were going to use
the convergent line or point of a focused grid falls within ___ ____
a focal range
what is the grid conversion factor
measure of the required mAs adjustment to keep exposure constant with and without the grid
what is the GCF equation
GCF = mAs with grid/mAs without grid
increase in GCF has what effect on the required exposure
required exposure to the IR increases
what is the limitation of stationary grids
stationary grids produce visible grid lines on the radiography
what are reciprocating grids
slightly moving the grid during the x-ray exposure will blur the grid lines which will therefore be less visible
reciprocating grid motion is controlled how
electronically
what is the grid cutoff
decrease in the number of transmitted photons that reach the IR because of grid misalignment
what are the 4 causes of grid unalignment
upside down focused grid
off level grid
off centre grid
off focus grid
what is the moire effect
artifact demonstrated when a stationary grid is used during CR imaging and the grid frequency is similar to the laser scanning frequency
higher ratio grids will increase what 3 things
scatter absorption
patient exposure
potential for grid cuttoff
