Density Part II Flashcards
1
Q
Latent image
A
- occurs immediately after exposure, but before processing
- invisible
- this happens to all images, regardless of how it is acquired (film, digital DR, CR)
2
Q
Image formation
A
- how do we get it? Result of differential absorption of the x-ray beam
- differential absorption is: process where some of the beam is ABSORBED and some is TRANSMITTED
- it is called differential absorption because the beam is not absorbed to the same degree over the entire area exposed
3
Q
Differential absorption
A
- creates an image that represents the structures of anatomy (results from the variation between absorption and transmission
- when an image is created this way, several processes are required
- occurs in beam ATTENUATION
4
Q
Beam attenuation
A
- when the primary x-ray beam passes through tissue and loses some of its energy (this reduction is known as attenuation)
- attenuation (loss of energy) consists of:
- absorption
- scattering
- photon transmission
5
Q
Absorption
A
- interaction called photoelectric effect
- complete absorption of x-ray photon
- inner shell electron ejected (by x-ray photon)
- photoelectron
- outershell electron drops to fill the spot
- characteristic radiation emitted
- low energy (usually remains in the body)
- atom is ionized
6
Q
Probability of PE interactions increases if:
A
- energy levels are CLOSER together
- binding energy of inner shell electron and energy of incoming x-ray photon
- the closer they are, the higher the odds of PE occurring
- complete absorption of the x-ray photon
- what other interactions are classified as absorption? Pair production and photo disintegration
7
Q
Scattering
A
- what interaction would this be? Compton
- incoming x-ray photon is not absorbed
- ejects outer shell electron
- compton electron, secondary electron
- x-ray photon loses energy, changes direction
- can go onto interact with IR
- can leave body
8
Q
Scattering cont
A
- what else would be in this category? Coherent
- not significant
- occurs with very low energy x-rays
- can interact with IR but very minimally
- In diagnostic ranges, very little contribution from coherent scatter
9
Q
Scattered photons
A
Why are they an issue?
-if they reach the IR they degrade image, contribute nothing useful to the image, AKA image fog
10
Q
KVp and image fog
A
- when kVp is increased there are: fewer interactions in total, increased amount of x-rays transmitted, increased compton scattering
- scattered radiation creates unwanted exposure on the image called image fog
- does not offer any useful information
11
Q
Scatter and image fog
A
- when kVp is INCREASED the amount of fog is INCREASED
- how is density affected? Increased fog will increase overall image density
- how is contrast affected? Increased fog leads to a decrease in contrast
12
Q
Transmission
A
- defined as: the x-ray photon passes through to the image receptor
- direct transmission
- indirect transmission
13
Q
Interactions and kVp
A
⬆️ kVp ⬇️interactions
14
Q
Exit radiation
A
= transmitted and scattered radiation
=(direct and indirect transmission)
-amount of absorption and transmission are varied (differential absorption)
15
Q
On the image
A
- WHITE (low radiographic density, areas of absorption, photoelectric interactions)
- BLACK (transmission, high radiographic density)
- GREY (mix of both absorption and transmission)
16
Q
All about compromise
A
⬆️ kVp ⬇️ patient dose
17
Q
Exponential absorption
A
- as x-rays pass through tissue, there is no fixed range to describe how far they go
- attenuated exponentially
- for every increment of thickness, the x-rays are decreased in #, by a certain percentage
- never reaches 0
18
Q
Distance and density
A
SID
- source to image distance aka FFD (focus to film distance)
- intensity changes with distance
- inverse square law
19
Q
SID
A
- as the distance between the focal spot and the IR changes, the intensity of the radiation will change also
- you’ll notice as the distance is increased, the light covers a larger area
- what happens to the # of photons as the area is increased? Same # of photons covering a larger area, so less density
20
Q
SID cont
A
- What happens to the image as the area is increased? Because it is receiving less radiation, there will be less radiographic density
- what if we have a very short SID? The reverse is true, more photons in a smaller area -> increased intensity, more radiographic density
21
Q
Density maintenance formula
A
- derived from the inverse square law
- when do we use it? Portables (especially chests), to fit larger parts to a receptor, patient condition
22
Q
OID
A
- object to image distance
- increased OID allows less scatter to reach the IR (coming off in all directions and may just miss the IR)
- how would less scatter affect radiographic density? It would decrease it
- increased OID, results in decreased radiographic density
23
Q
Body habitus
A
- sthenic (average, muscular)
- hyposthenic (thin, slim, healthy)
- hypersthenic (large)
- aesthenic (small and frail)
24
Q
Thickness
A
- calipers
- with increased thickness, how would we adjust our exposure factors? MUST INCREASE EXPOSURE FACTORS
- can change mAs or kVp
- in most situations would choose to adjust mAs
25
General rule of technical factors
- chest (high kVp)
- soft tissue (low kVp, high mAs)
- bony extremities (low kVp)
26
Pathology
Radiographic technique needs to be adjusted in cases with known pathology
- destructive (increases radiolucency of tissue)
- constructive (increases radiopacity of tissue)
27
Filtration
Three types
- added
- inherent
- compensating
28
Inherent filtration
- glass or metal envelope
- collimator usually provides some additional filtration
- tech has no control over this
- built into the unit
29
Added filtration
- can be adjusted or selected
- aluminum is the material most commonly used
- technique charts are based on units using the lowest allowable added filtration
- when you add more, you must adjust your technique to compensate
- usually increase filtration for areas with high subject contrast
- can decrease dose
30
Compensating filters
- sole purpose is to balance the intensity of the x-ray beam to deliver a more UNIFORM exposure to the IR
- can be aluminum and attached by collimator
- can be placed on the patient, to even out density
31
Filtration
- Why do we use it? Increased x-ray beam quality, increased penetrability (due to losing lower energy photons)
- what is the result on our image? More scatter, decreased image contrast, compensating -> more uniform density
32
Grids
- device placed between patient and IR
- purpose: absorb scatter exiting the patient
- why do we use it? Improve image quality, high ratio grid can absorb 80-90% of scatter
33
Grids and density
- how will a grid affect radiographic density? It will decrease the amount of density on the image
- so then: mAs must be adjusted when adding, removing, or changing a grid
- this is to maintain appropriate density
34
Beam restriction
- collimation (adjusted by tech)
- what does this affect? The amount of tissue being irradiated, the amount of scatter reaching the IR
- what does this mean for density? Less scatter reaching the IR results in decreased radiographic density
35
Generators and density
- exposure factors that we use are developed based on the type of generator in the room
- generator factors:
- if the output is more efficient, LOWER technique settings
- if the output is less efficient, HIGHER technique settings
