Interactions Flashcards
Photoelectric (PE) Absorption
Incident photon energy is completely absorbed by an inner shell electron
-occurs when x-ray photon has just slightly more energy than the Eb (binding energy of the electron) of a k or l-shell electron
- x-ray photon knocks the inner shell electron of an atom out of orbit causing the atom to ionize
- *Patient tissues DO absorb radiation during this interaction**
What type of electron goes along with PE absorption?
Photo electron
How can the photoelectron energy be determined?
Ei = Eb + Eke
- Ei: energy of incident photon
- Eb: binding energy of the electron
- Eke: Kinetic energy of the photoelectron
Characteristics of Photoelectron
- Kinetic Energy
- Mass
- Reabsorbs quickly
Characteristic Cascade
- Vacancy filled by an outer shell electron
- electron undergoes change in energy level
- emits characteristic photon
Secondary Radiation
-when a characteristic photon are emitted from atoms of the patient after PE absorption interaction has occurred
radiation that originates from irradiated material outside of xray tube
PE Absorption is most influenced by what? A. Subject volume B. Subject thickness C. Subject mass density D. Subject atomic #
D. Subject atomic #
As you increase kVp (increasing photon energy), what happens to the probability of absorption?
Decreases by 1/8 (inverse cubed relationship)
Low atomic number materials and soft tissue result in what?
-photoelectrons maintaining the majority of the incident photon’s energy.
What interactions are both energy and atomic number dependent?
Photoelectric Interactons
When kVp decreases the, what happens to the probability of interactions (PE and Compton)?
The probability of Interactions of PE or Compton decrease
Attenuation
The absorption or scatter of a beam as it travels through a target material
Coherent Scatter
Involves low energy photons (below 10 keV)
-Can also be called unodified scatter, classical scatter, thompson (single outer-shell e), or rayleigh (all electrons of the atom)
Transmission of an x-ray
When x-rays pass through a patient instead of being absorbed
Compton Scatter
Incident photon (Ei) interacts w. outer shell electron and knocks it out of orbit -incoming xray does not lose all of its energy; it becomes a scattered xray photon and continues in a different direction with a lower amount of energy
In what interaction is the energy of the incident photon shared between the scattered photon and the KE of the orbiting electron that was knocked out of its orbit?
Compton Scatter
interactions that happen at low xray energies such as in mammography are called what?
Unmodified (coherent) scattering
-Instead of the atom losing an electron, the electron or atom vibrates and redirects incident photons
Which Interaction creates the vast majority of scatter in the range of xray energies used in diagnostic imaging?
Compton Scatter
What 3 variables affect the quality of an xray beam?
Part thickness, atomic number, and tissue density
if a tissue made out of a substance with a very low atomic number and fewer inner shell electrons make up the tissue, then the chance of photoelectric absorption interactions increases or decreases?
Decreases
Which atom would have an increased chance of photoelectric absorption occuring, calcium or oxygen and why?
Calcium, because it has a bigger atomic number, and there are more electrons orbiting around the nucleus for an incident photon to interact with than in an oxygen atom
Tissue density affects how tightly bound atoms are in any given tissue.
True or False?
True
Which of the 4 major body tissues has the greatest effective atomic number A. Bone B. Muscle C .Fat D. Air
A. Bone
What is the effective atomic number for Bone
14 (calcium)
What is the predominant interaction taking place between bone and the x-ray beam?
Photoelectric Absorption
Muscle is not the most difficult tissue for an x-ray beam to penetrate
True or False?
False…Muscle is the most difficult tissue for an x-ray beam to penetrate
Less photoelectric absorption and more Compton scatter happens in what body tissue?
Muscle
What two factors make fat easier for the x-ray beam to penetrate?
Both its atomic number and density being less than the other body tissues
Why doesn’t air attenuate more of the primary beam than muscle or fat?
Because air’s tissue density is much, much lower than muscle or fat.
-Therefore, air doesn’t absorb any of the x-ray beam at all.
Thicker parts cause more beam scatter and greater secondary radiation.
True or False?
True
increased kVp exposure produces more scatter and secondary photons in the patient that tend to travel in What direction?
a forward direction, toward the image receptor.
The Compton interaction, which occurs between what three things?
The signal, x-ray beam, and the patients tissue
Noise
The source of noise is anything that prevents information from reaching an image receptor.
Sources of Noise?
- Reduced signal strength from thicker parts
- incorrect use of an anti-scatter grid and too few x-ray photons making it through the patient, table or grid
- Scatter is also an example of noise
Pair Production
- When a super energetic photon enters an atom and interacts with the nucleus–nucleus can’t hold all of the energy, so it is given off as a negatron and positron
- Nucleus becomes unstable - — Happens in therapy and PET scans
Increasing kVp does what to the total attenuation?
Decreases total attenuation
When kVp is increased extremely, what happens regarding PE Absorption and Compton Scatter?
PE Absorption decreases
Compton Scatter increases
Photodisintegration
Happens with extremely high energy photon
- Absorption of photon by nucleus
- excited nucleus releases alpha particle
- *Not significant in imaging
As kVp increases,the # of photons transmitted without interactions increases or decreases?
Increases
As kVp increases and the # of photons increases, what does this do to the PE and Compton interactions?
Decreases the probability of PE absorption and Compton interactions
Within the attenuated beam… As kVp increases
PE absorption decreases
Compton increases
- Increases percentage of scatter
- Decreases percentage of absorption
Differential Absorption
The difference in what is transmitted through different body parts
-result in image contrast
Mass Density
Relates to how closely atoms are located within a material
As Mass density increases, the space between atoms ___?
decreases
X-ray transmission is reduced as what increases?
Mass Density
Mass Density and X-ray attenuation coefficients are inversely proportional
True or False?
False, they are directly proportional–if you double the mass density, the the x-ray attenuation coefficients doubles
