Density/IR Exposure Ch. 26 Flashcards
What is a major factor in determining the density of structures?
IR exposure/Density
What is the difference between IR exposure and density?
IR exposure = digital = amount of exposure to the IR Density = film screen = degree of overall blackening
What is optical density?
F/S = measurement of the amount of light that would be transmitted thru certain densities on the film
What is subject density?
the patient
What does OD stand for?
Optical density
In digital, what is brightness?
level of brightness on the computer monitor has nothing to do with IR exposure.
What does window level do?
changes the brightness
Brightness and density are not..
interchangable
An overexposed IR has received….
too many photons and has recorded too much information.
For digital, what controls IR exposure?
mAs
The relationship between mAs and IR exposure is…
directly proportional
What is kVp in regards to IR exp/Density?
it is an influencing factor
An increase or decrease in kVp will…
increase or decrease IR exp/Density but not directly proportional
What are a graphic representation of the exposure to the IR?
Digital image histograms
If mAs controls IR exposure then what does it also control/
Density
How does kilovoltage alter the intensity of the beam reaching the IR?
- It controls the energy and therefore the strength of the electrons striking the target of the xray tube 2. It controls the average energy of the xray photons produced at the anode target.
Kilovoltage also affects the production of ______ radiation.
Scatter
Both ____ and ____ of the xray beam will vary significantly with changes to the kilovoltage.
quality and quantity
What % of increase in kilovoltage causes a doubling of exposure to the IR?
15%
The 15% rule is somewhat accurate within what range of kVp?
60-100 kVp
Which is overexposed and which is under exposed?
The first is underexposed and the third is overexposed.
High energy kVp beam gives more photons exiting the patient
What are factors that affect IR exp/density?
mAs-controlling
kVp-influencing
Anode heel effect
Distance-inversely proportional (SID)
Distance-OID
Beam restriction
Patient factors-part thickness
Contrast media
Artifacts
Pathology
Grids
F/S IR’s
Digital IRs
Focal spot size
Where is the anode heel effect the strongest?
At the cathode end of the image.
How much does the anode heel effect vary across the image? (%)
45% variation across the image
When is the anode heel effect more pronounced?
- larger exp. field
- larger IR
- smaller targer angle (an angle less than 12 degrees)
What are some projections that may use the anode heel effect to advantage?
Femur-cathode end is hip, anode end is knee
Lower leg-cathode end knee, anode end ankle
Humerus-cathode end is shoulder, anode end elbow
Forearm-cathode end elbow, anode end wrist
T-spine AP-cathode end abdomen, anode end neck
T-spine Lat- cathode end neck, anode end abdomen
L-spine-cathode end pelvis, anode end abdomen
What is the primary method of changing image contrast?
changing kilovoltage
What is the primary method of changing density/IR exposure?
change mAs
What waveform has a lower average photon energy?
single phase
Lower average photon energy results in what?
less density/IR exposure
What is the difference between large focal spot size and smaller focal spot size?
Larger utilize a greater incident electron stream than the small.
How do manufacturers account for the difference between large and small FSS?
By adjusting the actual mAs at the filament for dual focus tubes.
What does the anode heel effect alter?
The intensity of the radiation between the anode and cathode ends of the tube - and therefore the density
The variation % of the anode heel effect depends on what?
The angle of the anode
Why is the anode heel effect more pronounced when the collimator is wide open?
Because a greater portion of the peripheral beam - and a greater portion of the intensity difference - reaches the IR when the collimator is wide open.
When can the anode heel effect be used to its advantage?
By placing the portion of the object with the greatest subject density toward the cathode end of the tube.
SID alters the intensity of the beam reaching the IR according to what?
Inverse square law
Why do we use the exposure maintenance formula?
To maintain IR exposure when distance is changed. mAs must be changed when the distance is changed but mAs is directly proportional because when distance increases mAs must also increase in order to maintain exposure
What are the 3 distances diagnostic radiography uses?
40, 72, 56
If you go from 72” to 56” what should you do to mAs?
half the mAs
If you go from 56” to 40” what should you do/
half the mAs
If you go from 56” to 72” what should you do with mAs?
double it
When the air gap technique is used what is its benefit?
It increases OID to prevent scatter radiation from reaching the IR.
How does using the air gap technique reduce scatter?
Scatter that would normally strike the IR will miss the receptor, causing a decrease in density/IR exposure
When is the air gap technique used? What projection?
Lateral c-spine
Which image is using the anode heel effect? How can you tell the difference?
The pic on the right is using the effect. I can tell because the pic on the left is darker at the top and right, whereas the right picture is more uniform
The exposure maintenance formula is used with this concept…
40/56/72 rule of thumb
What does increasing collimation do to scatter on an image?
reduces it…increasing collimation is tighter collimation
What is a good and maybe the only example of when effects are most common for beam restriction reducing scatter?
Portable CXR
Lg body part w/out grid and higher kVp
What happens to density/IR exposure when filters are used?
it decreases
Why does beam restriction reduce the amount of scatter and density/IR exposure?
Because it reduces the total number of photons available
When is a technical factor compensation for changes in density/IR exposure required?
- large anatomical part
- high kVp
- low grid efficiency
- non grid examinations (port CXR)
When dealing with patient factors and density/IR exposure, what does attenuation depend on?
the part thickness & tissue type
What does increased attenuation result in?
less IR exposure/density
When would tissue ave atomic # & density changes happen with pt factors?
with contrast, artifacts and pathology
What type of relationship is there between tissue thickness/type and density/IR exposure?
an inverse relationship
What would a radiolucent contrast media such as air do to density/IR exposure?
It would increase it
What can happen if we have part thickness and severe tube angles together?
You could have a 60% IR exposure difference from one part end to another
If you are dealing with a cast, why would you do any adjustment to your technique?
Because of the additional thickness of the cast…not because of the material of the cast.
What would emphysema do to IR exp/density?
increase it
What would pneumonia do to IR exp/density?
reduce it
Do patient artifacts increase or decrease IR exp/density?
decrease it
The use of grids does what to scatter?
reduces it
If you have a grid with more lead in it what should you do with your technique?
increase mAs
The amount of change to mAs when using a grid can be calculated by using what?
the grid conversion factor or GCF
Which have an impact on density/IR exposure? Film screen systems or digital systems?
both
How does film/screen systems have an effect on IR exposure/density?
Intensifying screen phosphors convert xray photons to the light photons that will expose the silver halides in film. When the silver halide crystals in the film emulsion form latent image centers, they establish the physical foundation for the black metallic silver that is film density
What numbers are the most useful parameter for digital imaging systems?
EI numbers
What are the most useful numbers for parameters for film screen combinations?
RS numbers
Compensatioins for changes in relative speed can be made by adjusting…
mAs
What is the relationship between mAs and RS?
directly proportional
in F/S IRs what does speed refer to?
the screen phosphor layer & emulsion silver crystal layer
What effects F/S density most when processing the film?
the developer temp
What will happen if the developer temperature increases?
density will increase
What is the ideal temp for developer?
95 degrees
What is the most common relative speed?
400
What does the phosphor layer do to xrays?
turns them to light
Digital has greater ___ ___ ___ than F/S
margin of error
Because of linear response effects and histogram, digital areas of high/low exposures are…
rescaled
Determining over and underexposure is easier with digital than F/S. T or F?
False
Deficient or excessive exp in digital can result in what/
Quantum moddle or degraded image quality
The linear response gives the digital system increased…
exposure latitude over F/S systems
What is increase exposure latitude?
areas that receive very little radiation can be enhanced by the computer and vice versa
What makes the image in a histogram?
pixels
What is window width?
digital processing that produces changes in the range of density/brightness, which can be used to control contrast
What is window leveling?
The digital processing that produces changes in density/brightness
Compare window width and window leveling
Window level: change brightness/density
Window width: change the range of brightness/density which can in turn control contrast
For visible changes to IR exp/density what is the least % that has to be done to see a change? (mAs)
30% but it is easier to double or half mAs instead of calculating 30%
How much change in kVp needs to be done to see a visible change in IR exp/density?
15% (15/50 rule)
What is the most common generator type?
high frequency because it creates more exposure
What determines FSS?
the filament size & then selected at the panel for part size
Small changes in filtration do or do not have a big change in IR exp/density?
Do not
