2) Interactions of Radiation and Tissue
1.
What is the approximate binding energy for the innermost electrons of Tungsten?
A) 69.5 keV
B) -69.5 keV
C) 140 keV
D) -140 keV
Answer:
B) -69.5 keV
2.
Specific ionization is directly proportional to which of the following?
A) Energy
B) Electrons
C) Charge
D) Velocity
Answer:
C) Charge
3.
Which photon interaction is dominant at the photon energies used in mammography?
A) Photoelectric absorption
B) Coherent Scatter
C) Compton Scatter
D) Bremsstrahlung
Answer:
A) Photoelectric absorption
4.
Positron annihilation results in the production of what?
A) two gamma rays
B) electron-electron pair
C) electron-positron pair
D) positron-positron pair
Answer:
A) two gamma rays
5.
Which is the dominant photon interaction in a chest x-ray?
A) Photoelectric absorption
B) Coherent Scatter
C) Compton Scatter
D) Bremsstrahlung
Answer:
C) Compton Scatter
6.
What are the units of linear attenuation coefficient?
A) kev/cm
B) 0.693/μ
C) line pairs/mm
D) 1/cm
Answer:
D) 1/cm
7.
The probability of Compton scatter:
A) Increases with increasing atomic number
B) Remains constant with increasing photon energy
C) Decreases with increasing photon energy
D) Increases with increasing electron binding energy
Answer:
C) Decreases with increasing photon energy
8.
Which of the following is NOT associated with charged particles?
A) Radiative losses
B) Scattered photons
C) Orbital electron energy transitions resulting in infrared heat emission
D) Ejection of a K-shell electron
Answer:
B) Scattered photons
9.
A sharp increase in photoelectric absorption occurs at:
A) Photon energies just below the K shell binding energy
B) Photon energies just above the K shell binding energy
C) Electron energies just below the K shell binding energy
D) Electron energies just above the K shell binding energy
Answer:
B) Photon energies just above the K shell binding energy
10.
Biological damage is primarily caused by:
A) Interaction of radiation directly with the nucleus
B) The ionization of water molecules
C) Interaction of radiation with carbon atoms
D) Heat generated during interaction of radiation with H2O atoms.
Answer:
B) The ionization of water molecules
11.
Beam hardening refers to the removal of:
A) low energy x-rays from a monoenergetic beam
B) low energy x-rays from a polyenergetic beam
C) high energy x-rays from a monoenergetic beam
D) high energy x-rays from a polyenergetic beam
Answer:
B) low energy x-rays from a polyenergetic beam
12.
The linear attenuation coefficient describes the:
A) Number of x-rays transmitted through an absorber
B) Number of x-rays attenuated in an absorber
C) Fraction of x-rays transmitted per unit length
D) Fraction of x-rays attenuated per unit length
Answer:
D) Fraction of x-rays attenuated per unit length
13.
The probability of photoelectric absorption in an absorber is proportional to:
A) ZE
B) Z3 E-3
C) ZE3
D) Z3E3
Answer:
B) Z3 E-3
14.
The charged particle interaction that occurs most frequently in a tungsten target x-ray tube is:
A) Excitation
B) Ionization
C) Photoelectric absorption
D) Bremsstrahlung
Answer:
A) Excitation
15.
Bremsstrahlung interactions result in:
A) Characteristic x-rays
B) Ionization
C) Polyenergetic x-rays
D) Heat
Answer:
C) Polyenergetic x-rays
16.
Which photon interaction delivers the least dose to the patient?
A) Photoelectric absorption
B) Coherent Scatter
C) Compton Scatter
D) Bremsstahlung
Answer:
B) Coherent Scatter
Coherent scattering occurs when an incoming photon excites the entire atom in an absorbing medium, causing the electrons to oscillate. The excited atom then de-excites by emitting a photon of the same energy, but in a different direction than the original photon. The interaction does not result in ionization, and there is no net transfer of energy to the absorbing medium (no absorbed dose). It appears macroscopically that the incident photon has merely changed direction. Coherent scatter interactions are more likely to occur at the lower energies used in diagnostic radiology. They are most likely to occur with low energy photons such as those used in mammography. Coherent scatter events account for about 10-15% of total photon interactions at x-ray energies below 30 keV, and less than 5% of the interactions at x-ray energies above 70 keV. Coherent scattering is also called classical or Rayleigh scattering. Coherent scatter photons that reach the image receptor result in a loss of contrast in the recorded image.
Figure 12. Coherent scatter occurs when an incoming photon transfers its energy to an atom in an absorbing material. The atom re-emits a photon of the same energy in a different direction.
Figure 13. Compton scatter occurs when an incoming photon transfers some of its energy to a loosely bound orbital or free electron. The scattered photon changes direction and has lower energy than the incident photon. The recoil electron departs in a billiard ball fashion.
Figure 14. Photoelectric absorption occurs when an incoming photon transfers all its energy to an inner shell electron, ejecting it from the atom. Characteristic x-rays are subsequently emitted when outer shell electrons move inward to fill the electron void. These characteristic x-rays are locally absorbed.
Photoelectric absorption is also the primary contributor to patient dose.