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The Atom & Interactions - Physics I Unit 6 - Coggle Diagram
The Atom & Interactions - Physics I Unit 6
kVp
Increasing kVp
Smaller differences in shades of gray
Low subject contrast
Homogeneous Remnant Beam
Decrease Image Contrast
Increases scatter % radiation
Decreases the S/N Ratio
Fogy
Low mAs
Quantum Noise
Decreased patient dose
Decreased Attenuation
More Exposure to IR
Increases the total number of photons transmitted without an interaction increase
Fewer Interactions
Higher % of Compton Scatter interactions
Lower % of PE interactions
Decreasing kVp
More attenuated
Less transmission
Increased image contrast
Larger differences in shades of gray
Black & White
High Contrast
Higher S/N Ratio
High Subject Contrast
More Detail
Short Gray Scale/Narrow Window
Decreased % Scatter
More Total Interactions
High % of PE interactions
Lower % of Scatter interactions
Quality
Power
Energy
Penetration
Compton Scatter
Intermediate-diagnostic range
Higher than Photoelectric (PE)
An incident photon interacts with a loosely bound outer-shell electron
The incident photon uses SOME of its kinetic energy to eject an electron from an atom
The atom is now ionized (it’s missing an electron, there’s an atomic shell vacancy)
The ejected electron acts as a free electron that can fill an atomic shell vacancy created by other ionizing interactions
The incident photon continues on its way with less energy and in a different direction as a scattered photon
A single X-ray photon can have multiple Compton scatter interactions
The remaining photon energy is slightly above the binding energy of an electron shell
A PE interaction can occur, and the photon will be absorbed
Coherent Scatter
Very low photon energies
Below 10 keV
The incident photon does NOT lose or transfer energy
The scattered photon changes direction slighlty
The energy & wavelength of the scattered x-ray is equal to the wavelength of the incident x-ray
Does not contribute to useful information
No Clinical Value, diagnostically
Thompson - Involves a single outer shell electron
Rayleigh - Involves all the electrons of an atom
Not an important x-ray interaction
Interacts with the whole atom rather than the electrons or nucleus
Photon interacts with matter (the patient)
Causes an electron (in the patient's body) to vibrate at the same frequency as the incident x-ray photon.
The atom stabilizes itself by releasing a secondary (scattered) x-ray photon with the same energy and wavelength as the incident photon
Photoelectric Effect (PE)
Intermediate-diagnostic range
lower energy than Compton
Complete absorption of the x-ray photon
The incident (incoming) x-ray photon interacts with an inner shell electron in an atom
All the incident photon is transferred to the electron in the atom
The atom is missing an electron and is now "ionized"
An electron (now with higher energy) is ejected from the atom
Characteristic Cascade
An outer shell electron drops down to fill the hole in the inner shell producing secondary/characteristic radiation
The Atom
Binding Energy (BE)
The Energy Required to remove an electron from its shell
K-Shell has the highest binding energy
Electron Total Energy
Electron total energy = 0 - BE
The further the electron from the nucleus the higher the total energy
Examples
Closest to Nucleus
K-Shell = -69 electron energy
Farthest from Nucleus
M-Shell = -3 electron energy
Outer shell electrons have less binding energy than inner shell electrons
The closer the electron is to the nucleus the higher the binding energy
Scatter
Some energy of the photon is transferred to the matter. The photon still exists, but it has less energy
Absorption
All the energy of the photon is transferred to the matter and the photon no longer exists
Photon Energy
Low energy photons
Tend to interact with the whole atom.
Intermediate energy photon
Typically interact with orbital electrons
Also capable of interacting with the nucleus
Very high energy photons
Not normally associated with diagnostic radiology
Pair production and photodisintegration occur at much higher energy levels
Pair Production
Very high photon energies
Minimum photon energy of 1.02 MeV
Megaelectron Volt (1 Million Volts)
Does not occur in the diagnostic radiology range
Occurs when an incident photon travels close to the forcefield of the nucleus
The incident photon loses all of its energy in the interaction
The “lost” energy results in “pair” of electrons
Positron
Positively charged electron resulting from pair production
Extremely volatile
Has some characteristics of a proton, combines with a negative electron instantaneously
Both particles (positron & electron) disappear
Their energy becomes 2 photons moving in opposite directions (energy of 0.51 MeV each
1 more item...
Negatron
Negatively charged electron resulting from pair production
Common in nature - quickly absorbed by nearby atoms
Immediately absorbed
Utilized in PET scanning
More significant in radiation therapy
Photodisintegration
Extremely high photon energies
Strikes the nucleus
Photon can escape interaction with electrons and the nuclear field
The excited (higher energy) nucleus releases an alpha particle (nuclear fragment)
Alpha particles are particles consisting of two protons and two neutrons tightly bound together
Absorbs all the photon energy
As atomic number is increased, the binding energies move closer to the photon
energy and PE is more likely to occur.
