chapter 11 xray physics Flashcards
compton predominates at higher what?
kVp settings
Compton is associated with that kind of e-?
scatter
what are 3 things that can be used to control scatter?
patient positioning, exposure technique, collimation
when there is less tissue to penetrate how does that affect the scatter?
it lessens it
how does higher kVp/lower mA technique affect scatter?
it will increase scatter
collimation does what?
reduces the field size to area of interest, reduces scatter, reduces pt. dose,
as the pt, becomes thicker what controlling mechanism of scatter if important?
collimation
why would you want to place pt. in recumbent position instead of a standing?
when they are thicker
More than___of the exiting (remnant) beam may be scatter
½
purpose of the grid is to do what?
reduce scatter “clean up”
what does scatter have an effect on?
it will reduce image contrast
what is the trade off when using a grid?
the increased exposure to the pt. and the improved dx. quality of image
what % can a grid reduce scatter by?
90-80
what is the grid ratio?
“height” of Pb strip divided by distance between the Pb strip
what does the height of the Pb strip refer too in recumbent xray technique?
the horizontal measurement from front to back
grid frequency =
lines/inch
grid selectivity is also known as
sigma
sigma =
1^0 radiation t’mitted/scatter t’mitted
are higher frequency grids generally less or more efficient than lower frequency grids of the same ratio?
they are less efficient
the greater the lead in a grid, what happens to the selectivity?
it becomes greater
what is the difference of effectiveness when using higher ratio grids vs lower ratio grids?
the effectiveness increases with higher ratio
As SID decreases what margins of the grid are cut off?
lateral
what are focused grids used for?
to reduce the grid cut off
in a focused grid the grid strip are accommodating what?
accommodate the increasingly divergent angle of photons further away from central ray.
focal range is defined as?
the limited range of SID that can be used without grid cutoff
grids that are used for general radiography are what kind of grids?
focus
what is the relationship between grid ratio and focal range?
an inverse relationship meaning that as grid ratio increases the focal range will decrease
will the SID need to decrease or increase as you increase the grid ration?
it will need to decrease
what are crossed grids ?
sandwhiching two parallel grids together so their grid strips are perpendicular
what do cross grids reduce?
scatter photons
how do cross grids negatively affect the image in an general radiograpy?
grid cutoff will occur in the superior to inferior angulation of the xray beam
potter bucky diaphragm is known as the
moving grid
is a moving grid a new technology or old?
old
what is the purpose of the moving grid?
to blur the grid strips on the xray image
the disadvantages of the moving grid?
- freezing of image may happen if exposure time is too short
- moving parts may require service
- motion of grid might show up on the image
- require 15% more exposure
what may appear on the image screen if grid is not perpendicular to the central ray?
grid lines on the image
grid cutoff on image will move towards what?
the displaced central ray
when SID in not in the focal range where will the grid cutoffs be?
at the sides
what are some common grid errors?
- grid not perpendicular
- central ray not in the center
- SID not in focal range
- grid upside down
Air gap technique
space between the image receptor and object imaged cause scatter photon to diverge and not hit the image receptor.
what are the drawbacks of air gap?
image magnification/distortion
Higher exposure is also necessary (about 10% increase per cm of air gap)
What projection includes a natural “air gap”?
Looking top down on the pt. the image receptor and cervical spine will have a air gap –> creating scatter photons that do not end up hitting the image receptor
