rad phys questions Flashcards
to double the density of a film by using mAs what would you do?
increase the mAs by 100%
of the choices below which is true about a reciprocating grid?
a. requires a 10:1 ratio
b. it requires 103 lines per inch
c. it is thicker than a stationary grid
d. it requires a faster exposure time
e. it is less expensive than a stationary grid
c. it is thicker than a stationary grid
when converting from a grid with low ratio to one with high ratio which of the following would be the best to do to maintain a good film quality?
a. increase kV
b. increase mAs
c. decrease kV
d. decrease mAs
b. increase mAs
which of the following is recommended for spinal radiography in the usual chiro office?
a focused grid
what effect does using a high kV have?
produces more scatter
more Compton compared to photoelectric effect
more penetration= less absorption
more graying from film fog
more x-rays pass through without interaction
what is the preferred grid focus range for a spinal radiography?
40” to 72”
how do you control scatter from a high kV?
use the optimal kV technique
what is the relationship of beam size and scatter?
the larger the beam the more scatter
what do you do to control the scatter from a large size beam?
collimate to film or part size
describe optimal kV
the lowest possible mAs with acceptable contrast on the film
how many lines per inch are recommended for a stationary grid?
100-103
diminished penetration at the periphery of a grid is called?
grid cut off
if a film is said to be burned out, what would it look like?
too dark
which choice below is the main reason why a “fixed kV technique is preferred?
a. contrast is consistent
b. less stress on the tube
c. patient exposure is reduced
d. faster exposure time can be used
a. contrast is consistent
scattered or primary rays that leave the patient and reach the grid is called?
remnant radiation
what are some common grid ratios and there max kV?
8: 1- 90 kV
10: 1- 100 kV
12: 1- 110 kV
16: 1
which grid ratios are recommended for spinal use?
10:1 or 12:1
grid frequency is the # of lead strips per in also called?
lines per inch LPl
what are the 3 types of stationary grids?
crossed- not practical for spinal
parallel
focused
with parallel strips there is an attenuation of the periphery of the beam due to an angle of diverging rays, what does this create?
uneven exposure with sides being lighter
which type of stationary grid has graduated angles to match the angle of divergence?
focused- the periphery of the beam is not attenuated
what is grid radius?
the angle of the strips that match the divergence extending from the grid to the focal spot
which type of grid will produce the least amount of grid lines?
moving grid because moving the grid during exposure can avoid the lines all together
when are grid lines more noticeable?
with lower frequencies and parallel grids
when are grid least noticeable?
with a 10:1 focused grid and 103 lines
what is another name for a moving grid?
reciprocating grid or Potter Bucky diaphragm
exceeding the optimal kV while still maintaining good density will yield what type of film?
long scale contrast
which of the following would increase patient exposure?
a. using high kV
b. using 10:1 stationary grid
c. using high speed screen
d. using air gap technique
b. using 10:1 stationary screen
about how much increase in mAs is required when going from a non grid technique to using a technique with a 12:1 grid?
12 times
what are some negatives to using a moving grid?
more expensive
can fail
thicker so increase OFD
can cause motion of film leading to blurring
can be too slow for fast exposures causing grid lines
to get through any grid what is needed?
more exposure
when should grids normally be used?
when 70+ kVp is used or part is over 10-15cm thick
what hold a grid in place?
grid cabinet
true or false, the patient is more stable when they are in contact with the cabinet?
true
which films need to use air gap technique?
lateral, flexion, extension and oblique cervical films
if a part of separated from the cabinet some scatter will miss the film, what is this called?
air gap technique
what is the term for attenuation of part of the beam by the grid?
grid cut off
what is the main effect of using a grid with a patient?
reduces x-ray exposure to patient
the higher the kVp the more ______ radiation is produced?
compton
what is an advantage of a high kVp technique?
lowered patient radiation exposure
if using high frequency with a chart made for single phase what do you do with the mAs?
cut the mAs in half
what is a technique chart?
estimators that indicate what settings to use for what situation
what is a mini phantom for measuring a unit’s output?
penetrometer
which setting device fits in your pocket and is “super tech” that allows for more specific design
slide rule calculator
it is more difficult to see what on films that are too light, too dark or to grainy? distortion or detail?
detail
if no unsharpness is seen there is _____ detail?
good recorded
penumbra =
unsharpness
umbra=
sharpness
with unsharpness the edges of a structure will be?
unclear or blurry
what are the 3 basic causes of unsharpness?
geometric
screen
motion
which cause of unsharpness relates to distance between the focal spot and object and film?
geometric unsharpness
does INcrease in OFD increase or decrease unsharpness?
increases
does DEcrease in SID increase or decrease unsharpness?
increases
decrease in OFD increases_____
clarity or sharpness
a small focal spot ________ sharpness
increase
to decrease geometric unsharpness….
use short OFD
long SID
small focal spot
single screen has _____ clarity than no screen and double screen has _____ clarity than single screen
less, less
high speed screens have ___ clarity than slow speed screens
less; increase in size of crystals increases unsharpness
to reduce screen unsharpness and maintain low patient exposure…
use 400 speed rare earth screens
maintain good film/ screen contact
change screens if warped
what is the most common cause of image blur?
patient motion
magnification distortion causes _______ _______
geometric unsharpness
shape distortion is also known as?
true distortion
distortion of a shape occurs from?
angulation of a ray, film or object
what are the 3 things that happen when x-rays enter a body?
- total absorption- don’t reach film
- partial absorption and deflection- some rays reach film but aren’t desirable
- passing through patient with no interaction- image producers
when rays actually reach a film it is called
remnant radiation
to overcome the binding energy of an inner electron, the incoming photon’s energy is totally absorbed this is called
photoelectric effect
dense tissue like bone _____ rays
absorbs
less dense tissue like muscle and fat _____ as much
don’t absorb
the contrast between structures of different densities is called?
differential absorption
when an incoming photon strikes an outer shell electron displacing it its called?
compton scatter/effect
the angle of photo deflection determines how much energy is lost, the greater the angle the _______ energy is lost
more
when photons are scattered toward the tube it is called?
back scatter
film fog=
graying of a film
what are 2 causes of film fog?
secondary rays or scatter that reaches the film
when a weak incoming photon doesn’t displace but vibrates an electron or atom its called
classical or coherent scatter
classical scatter ___ energy to the atom which gives off x-ray in order to return to original balance
adds
at 70 kVp what % of scatter is classical?
3-5%
