Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
47 Cards in this Set
- Front
- Back
|
what is the purpose of a grid
|
improves radiographic contrast, absorbs larger angle scatter
|
|
|
what are the three ways photons get absorbed
|
pass through unattenuated, completely absorbed, change direction after interaction with patient
|
|
|
how does scatter effect the image
|
creates fog (gray image), contrast decreases, visibility of detail decreases
|
|
|
how does scatter increase
|
kV increases, tissue volume undergoing exposure increases
|
|
|
when do you use a grid
|
high kVs 65ish or higher, large field size, thick part
|
|
|
how is a grid constructed
|
radiopaque lead strips, radiolucent interspace material, encased in Al/plastic protection
|
|
|
what will grid ratio indicate
|
the degree of efficiency
|
|
|
what will changing the height of the lead strips and the length of the space between do to a grid
|
alter the effectiveness
|
|
|
what is the equation for grid ratio
|
h/d
|
|
|
the ____ the grid ratio, the ____ effective the grid
|
higher, more
|
|
|
what is grid frequency
|
the number of grid lines/ square inch or centimeter
|
|
|
grids with higher grid frequency have (thinner or thicker) strips
|
thinner
|
|
|
higher grid ratios have (more or less) lead
|
more measured in grams/square cm (g/cm squared)
|
|
|
as lead content increases
|
scatter decreases, contrast increases, image quality increases, visibility of detail increases
|
|
|
where does a parallel grid absorb most of the primary beam
|
at the edges
|
|
|
how do you make less cut off of a parallel grid
|
increase SID
|
|
|
grid lines are angled to match the primary beam, focused for particular distance ranges, which type of grid
|
focused
|
|
|
if lead strips of a focused grid were extended up in the site beyond the grid surface, at some point they would intersect each other or combine together
|
convergence line
|
|
|
the range of acceptable distance for a particular grid
|
focal range
|
|
|
most common used grid
|
linear focused grid
|
|
|
grids remove scatter or add density to an image so what must be increased to compensate for loss of exposure to IR
|
mAs (patient dose will rise)
|
|
|
when a grid is used do these go up or down,,, image quality
|
up
|
|
|
scatter or fog
|
down
|
|
|
contrast
|
up
|
|
|
visibility of detail
|
up
|
|
|
patient dose
|
up
|
|
|
when is the grid conversion factor used
|
when one needs to convert a technique, from grid to no grid, or from one grid type to another
|
|
|
as kV goes ____, GFC goes____
|
up, up
|
|
|
as grid radio goes _____, GFC goes ____
|
up, up
|
|
|
what is the grid conversion equation, no grid to grid
|
mas1/mas2=GFC1/GFC2
|
|
|
GFC= mAs with a grid/mAs without a grid
|
grid conversion equation
|
|
|
ICRU
|
international commission of radiologic units and measurements
|
|
|
the icru evaluates a grids performance by examining a grids selectivity
|
ability to allow primary radiation through to the ir while preventing scatter photons through
|
|
|
contrast improvement ability is defined by the
|
k factor
|
|
|
what does contrast improvement ability compare
|
radiographic contrast of an image taken with a grid to one without a grid (higher k factor greater the contrast)
|
|
|
focused grid must have proper ____ between grid and cr
|
alignment
|
|
|
higher the grid ratios the greater the effect/visibility of
|
errors
|
|
|
grid errors occur two ways (off level)
|
grid is tilted so not perpendicular to cr, grid is straight but tube is angled into the grid lines
|
|
|
cr must be centered along central aspect of grid to prevent lateral decentering
|
off center
|
|
|
tube must be within focal range or beam divergence will not match the angle of the lead strip
|
off focus
|
|
|
tube side is always facing the tube
|
upside down
|
|
|
wide latitude is ___ to use, narrow latitude is ____ challenging
|
easier, more
|
|
|
grid lines are captured and scanned parallel to the scan lines in the imaging plate reader
|
moire effect
|
|
|
how do you minimize the moire effect
|
scan plates in the transverse axis, user stationary grid with high frequency
|
|
|
physical space between the patient and the image receptor
|
air gap
|
|
|
what is an advantage of air gap
|
scatter emitted from the patient will miss the IR
|
|
|
what is a disadvantage of air gap
|
image anatomy will be magnified so shape distortion occurs, technique up, increase SID
|
