Unit 2: Interaction with Matter Flashcards
What are the five types of interactions with matter?
coherent scatter
compton scatter
photoelectric effect
pair production
photodisintegration
Coherent scatter can also be called:
Thompson scatter or Unmodified scatter
describe Coherent scatter:
Interaction between the incident x-ray and a target atom that becomes excited. The atom releases this excess energy as a scattered x-ray photon that changes in direction
no energy transfer and no ionization occur
What scatter is of little importance to diagnostic radiology?
coherent scatter
describe Compton scatter:
interaction between incident x-rays and outer-shell electrons and ejects electron and ionizes atom
photon changes direction with a reduction in its energy
What is the ejected electron called?
Compton/recoil electron
In the energy transfer of Compton scatter, what retains most of the energy?
the scattered x-ray
define absorbed dose:
amount of energy that ionizing radiation gives to a given mass of matter (radiation absorbed by an object)
What are the three types of Compton scatter?
back
side
forward
describe back scatter:
photon scattered 180° perp. to beam, strikes the IR/pt and travels back toward the tube
What causes back scatter?
photon strikes an outer shell electron with a head on collision
Back scatter can contribute to:
additional patient dose
describe side scatter:
photon scatters straight off to the side
What causes side scatter?
photon strikes an outer shell electron on its outer edge
What is side scatter the main source of?
occupational dose in radiography and fluoro
describe forward scatter:
photon travels in a forward direction and strikes the IR
provides no diagnostic information (no anatomy represented)
What causes forward scatter?
photon grazes an orbital electron by hitting side of atom and changes in a different forward direction
barely gives up any of its energy
What is the type of scatter that reduces image contrast/noise?
forward
List the factors that affect production of Compton scatter radiation:
field size (collimation)
body part thickness
body part density
body part electron density
photon energy
?kVp
What is the relationship between Compton scatter and field size?
larger field size = more Compton scatter (more noise and lower image contrast)
what is the most effective way of reducing pt dose?
collimation
What is the relationship between Compton scatter and body part thickness?
thicker part = more Compton scatter
compress thicker body parts
What is the relationship between Compton scatter and body part density?
greater the density = greater the Compton scatter production
T/F: atomic number of tissue does not effect the likelihood of Compton scatter.
true
Why do more denser parts increase scatter?
atoms are packed more tightly, meaning more electrons leading to increase in scatter
The greater amount of water present in tissue will produce a _________ amount of Compton.
greater
What is the relationship between Compton scatter and kVp?
increasing kVp will increase Compton scatter
Why do we use grids?
higher body parts require higher kVp settings, which will produce more scatter
What do grids do?
clean up forward Compton scatter
list the ways to reduce Compton scatter lowering image contrast:
lowest possible kVp
grids
smaller field size
air gap technique
compression
describe the Photoelectric effect:
x-rays undergo ionizing interactions with inner-shell electrons that become totally absorbed by the K-shell electron
pt dose and absorption
Photoelectric interaction produce secondary radiation through what?
cascading effect
What must happen in order for the photoelectric interaction to happen?
incident x-ray energy must be equal to or slightly greater than the electron binding energy
What are the factors that affect photoelectric effect?
kVp
body part thickness
body part density
atomic number tissue
What is the relationship between the photoelectric effect and kVp?
increasing kVp -> increases photoelectric interaction due to increase in quantity
increasing kVp will:
decrease diff. absorption
What is the relationship between the photoelectric effect and body part thickness and density?
thicker and denser body parts -> increases chances of photoelectric interactions
What is the relationship between the photoelectric effect and the atomic number of tissue?
atomic number increases -> photoelectric effect increases greatly
increasing x-ray beam energy will result in:
overall increase all interactions
fewer avg. Compton interactions
much fewer avg. photoelectric interactions
more transmission through pt
describe pair production:
photon escapes influence w/ orbital electrons and interacts w/ the nucleus
what happens to the photon during pair production?
it disappears and is replaced with two electrons (positron and negatron)
pair production requires a photon energy that is:
greater than 1.02 MeV
describe photodisintegration:
photon escapes interaction w orbital electrons and becomes absorbed by nucleus
What happens to the nucleus during photodisintegration?
nucleus enters an excited state and emits a nucleon or other nuclear fragment
photodisintegration requires a photon energy that is:
10 MeV or greater
