Density Flashcards
For selection of an optimal radiographic technique, three groups of factors must be considered
- image quality factors ex density, contrast
- exposure technique factors ex kvp, mAs, SID
- patient factors ex pathology
Visibility is achieved with the balance of
- radiographic density
- radiographic contrast
Radiographic density
- the amount of overall BLACKNESS produced on the image after processing
- must achieve balance
- must have sufficient density (allows us to visualize the structures, excess or insufficient density results in poor visualization of anatomy)
What are the factors that alter radiographic density of images (aka prime exposure factors)
- kVp
- exposure time
- mA
- SID
What does it mean when an image is said to be too dark?
- has a high OD
- caused by overexposure
- more information
What does it mean when an image is said to be too light?
- has a low OD
- caused by underexposure
- less information
OD can be controlled by two major factors
- mAs
- SID
- direct relationship between mAs and OID
MAs and exposure
- mAs is primary factor used for density
- determines the number of x-rays produced (quantity)
- direct relationship
- double the mAs=double the exposure
- provided nothing else changes
- also generally doubles the density
Reciprocity law
- the density produced on a radiograph remains the same for any combination of mA and exposure times as long as the value of the mAs is the same
- as long as the resultant mAs is the same, regardless of the mA and time combination used, the OD produced on the image will be the same
- failure of law occurs with: extremely short exposures and extremely long exposures
When do we use the reciprocity law?
- motion
- focal spot size
- breathing technique
MAs and density
- for us to notice a visible change in radiographic density
- mAs needs to be changed by at least 25-30%
- what if the mAs is changed by 10%?
- the change will not be visible to us
- the general rule of thumb for mAs changes is to make adjustments in doubles or halves
What happens when mAs is decreased?
Radiographic density is decreased
What happens when mAs is increased?
Radiographic density is increased
True or false: mAs controls density; directly proportional
TRUE!!
What role does kVp play in optical density?
- affects the intensity of the beam
- controls energy (strength) with which the electrons hit the target (anode)
- major player in quality (some effect on x-ray quantity)
- primary controller of x-ray beam quality and controls RADIOGRAPHIC CONTRAST
- what is contrast? Shades of grey on the image
Contrast
- function: to make anatomy more visible
- the difference in OD between the adjacent structures, or the variation of OD on an image
- without it, would not be able to discern structural borders
- is a result of the x-ray beam being attenuated differently as it passes through the tissues of the body
Kvp and density
- has an effect on density because it ALTERS PENETRABILITY OF THE BEAM (this also affects the amount of radiation that reaches the IR)
- if the kVp is decreased, less radiation reaches the IR, resulting in: decreased OD (image will be lighter)
- if the kVp is increased, more radiation reaches the IR, resulting in: increased OD
15% rule
- increasing the kVp by 15% will have the equivalent effect on radiographic density, as doubling the mAs
- this rule can be applied for maintaining or adjusting density
KVp vs mAs
- so if both affect radiographic density, how do I decide what to adjust?
- mAs THE SIMPLEST ADJUSTMENT TO MAKE, as it has a direct relationship with density
- when do we adjust kVp?
- when we want to change the contrast of the image
- when we want to control motion (as kVp is increased, mAs is decreased, meaning shorter exposure times can be achieved)
The density of a radiographic image can be defined as?
-the amount of overall blackness produced on the image after processing
What type of property is density?
- geometric
- photographic
What range of OD can be detected by human vision?
-0.25-2.50 OD
What are prime factors?
- kVp
- exposure time
- mA
- SID
If you had to choose between an image with too much or too little density you would choose?
- an image with too much density
How is visibility achieved?
-with the balance of radiographic density and radiographic contrast
What type of relationship does mA and time have with radiographic density?
-directly proportional
If all other factors were left unchanged, and the exposure time was decreased from 0.1 sec to 0.05s, how will the radiographic density on the image be affected?
-the density has been halved
KVp primarily controls
Contrast
True or false: The number of electrons in the thermionic emission cloud is controlled by kVp
FALSE
X-rays with more penetrating power have a ____ wavelength than weaker x-rays
Shorter!
When kVp is increased
- the # of x-rays transmitted is increased
- compton scattering is increased
What is the relationship between dose and distance?
- if we increase one we have to decrease the other
- inversely proportional
Does focal spot size influence the density of an image?
No!
What is another name for the density maintenance formula?
Direct square law
What is the relationship between distance and density maintenance?
⬆️ distance, ⬇️ density
⬇️ SID (distance), ⬆️ density
How can OID affect radiographic density?
⬆️ OID,results in ⬇️radiograophic density
A ______pathology will increase the radiopacity of tissue
Constructive!
A _____ pathology will increase the radiolucency of tissue
Destructive
When you ____ the kVp, you ____ the number of interactions
-increase, decrease
Why is there a difference in intensity of the x-ray beam when the SID is changed? How does this affect radiographic density?
-with increased SID, get a change in intensity. Same # of photons cover a larger area, so they become less intense
