EXAM #3 Flashcards
What is attenuation?
Reduction in the number or intensity of x-rays as a result of absorption and scattering
Differences in attenuation cause subject contrast
Can x-rays be transmitted without any interaction?
yes
What can x-rays interact with?
- entire atom
- orbital electron
- nucleus of an atom
X-rays interact with the nucleus of an atom at _____ _____ energies
very high energies
What are the five interactions between x-rays and matter
- ) Coherent scattering
- ) Photoelectric (PE) absorption (2° radiation)
- ) Compton scattering
- ) Pair production
- ) Photodisintegration
http: //youtu.be/4p47RBPiOCo
What is secondary radiation?
Radiation produced using the patient’s atoms
What is off-focus radiation?
Radiation produced off the focal spot of the x-ray tube
What is photoelectric absorption?
- More photoelectric absorption at high/low kVp?
- Is photoelectric absorption ionizing?
- The incident electron interacts with and _____ the _____ shell electron
- The incident electron’s energy is divided into…?
What gives us those white areas in our image
- More absorption at a low kVp
- Photoelectric absorption is ionizing
- The incident electron interacts with and ejects the inner shell electron
- The incident electron’s energy is divided into breaking the binding energy and giving off kinetic energy to the ejected electron
When is an ion pair formed, and in what interaction?
- What is the product called?
- What happens to the affected atom?
An ion pair is formed when an electron is ejected from the atom in photoelectric absorption
- It becomes known as the [photoelectron]
- Remaining atom has a vacancy in its inner electron shell
When is photoelectric absorption most likely to occur?
Most likely to occur when x-ray photon has just slightly more energy than Eb of a K or L-shell electron
[*] Photoelectrons are related to what interaction?
Photoelectric absorption
Photoelectron energy can be determined with what equation ?
Ei = Eb + Eke
Ei = energy of the incident photon
Eb = binding energy of the electron
Eke = kinetic energy of the photoelectron
[*] Recoil electrons are related to what interaction?
Compton scatter
What are the three characteristics of a photoelectron?
• Kinetic energy (Eke)
• Mass
• Reabsorbs quickly
- Within 1-2mm of tissue
Three steps to characteristic cascade?
What interaction?
- Vacancy filled by an outer shell electron
- Electron undergoes change in energy level
• Emits characteristic photon
– (secondary radiation)
• Photoelectric
What is secondary radiation?
• When do characteristic photons emit?
Radiation that originates from irradiated material outside of x-ray tube (from the patient’s atoms)
• Characteristic photons emitted from atoms of patient after PE absorption interaction has occurred
_____ of resulting x-ray is dependent on the atomic (Z#) of the material.
Low Z# in tissue = _____ energy secondary radiation
Higher Z# with contrast agents = _____ energy secondary radiation
Energy of resulting x-ray is dependent on the atomic (Z#) of the material.
Low Z# in tissue = low energy secondary radiation
Higher Z# with contrast agents = higher energy secondary radiation
Photoelectric Absorption Condition #1
Incident photon energy (Ei) must be _____ than or _____ to binding energy (Eb) of inner-shell electron
Incident photon energy (Ei) must be greater than or equal to binding energy (Eb) of inner-shell electron
Photoelectric Absorption Condition #2
PE absorption interaction is more likely to occur if:
• Incident photon energy (Ei) and inner-shell electron binding energy (Eb) are _____ to each other
[*] As photon energy increases, chance of PE interaction _____ _____
– _____ _____ relationship (1/E^3)
PE absorption interaction is more likely to occur if:
• Incident photon energy (Ei) and inner-shell electron binding energy (Eb) are close to each other
[*] As photon energy increases, chance of PE interaction decreases dramatically
– Inverse cubed relationship (1/E^3)
Example: 1 / 2^3 = 8
If you double your energy, the probability of absorption goes down to 1/8
Photoelectric Absorption Condition #3
• Increased Z# has a dramatic impact on the amount of PE absorption, with a _____ _____ relationship
• Double Z#
Increase chance of PE absorption interaction by a factor of __.
Kerry: Denser things, things that have higher binding energies are most likely to absorb more
PP: PE absorption interaction is more likely to occur in elements with a higher Z#, and therefore higher binding energy (Eb) of inner-shell electrons
• Increased Z# has a dramatic impact on the amount of PE absorption
– Direct cubed relationship (Z^3)
• Double Z#
Increase chance of PE absorption interaction by a factor of 8.
PE absorption is most influenced by:
A. Subject volume
B. Subject thickness
C. Subject mass density
D. Subject atomic #
D. Subject atomic #
because atomic number is cubed and mass density is proportional
What is Coherent scatter
- Are electrons ejected?
- Does ionization take place?
- High/low energy photons?
- What are the two types and how are they produced?
- What percentage of x-ray interactions in patients? Does this affect our image?
Electrons are excited and vibrate at photon frequency (momentarily bound to orbital electron which excites the atom and is reemitted)
- No electrons are ejected
- No ionization takes place
- Involves low energy photons (below 10 keV)
- Two types with same result:
- Thompson (single outer-shell electron interaction)
- Rayleigh (all electrons of the atom interaction)
• less than 5% Slightly
What is Compton scatter?
- What is formed because of Compton scatter?
- Where is the incident photon’s kinetic energy transferred to?
- Does ionization take place?
- Increasing kVp _____ the likelihood of Compton scatter
- The amount of Compton scatter is _____ proportional to tissue density
- What does Compton scatter create?
Incident photon (Ei) interacts with outer-shell, loosely bound electron and ejects it
- Ion pair is formed
- Incident photon transfers some of its kinetic energy to the recoil (Compton) electron and continues on in a different direction (never leaves patient, and has no affect on image quality)
- Ionization takes place in Compton scatter
- Increasing kVp decreases the likelihood of Compton scatter
- The amount of Compton scatter is directly proportional to tissue density
- Compton scatter degrades image quality with a blanket of exposure called noise
What is the equation for Compton scatter?
Ei = Es + Eb + Eke
Incident photon energy (Ei) is distributed between recoil electron (Eke) and scattered photon (Es)
Energy transferred to recoil electron (Eke) affects angle and energy of scattered photon (Es)