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ABR Medical Physics Continuing Certification (MOC) Practice Questions.
Section 1: Radiation Physics and Interactions (1–25)
1. What is the predominant interaction of 120 kV x-rays from a CT scanner with soft tissue?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: B. Above 25 keV, Compton scatter is the dominant photon interaction in soft tissue.
CT x-ray beams have higher filtration, with effective energy closer to one-half of the kV
(approximately 60 keV).
2. If a radiologic technologist uses 80 kV for an AP projection of the lumbar spine, which
interaction predominates in bone?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: C. The average energy for an 80 kV spectrum is typically 1/3 to 1/2 of maximal
energy. At approximately 40 keV, photoelectric effect dominates in bone (higher Z material).
3. What is a direct result of adding filtration to a diagnostic x-ray beam?
A. All characteristic radiation is removed
B. Image contrast is improved
C. Maximum photon energy is increased
D. Patient dose is reduced
Answer: D. Added filters reduce low-energy x-ray photons and harden the beam. Removal of
soft x-ray photons reduces patient skin dose.
4. Which of the following always increases as focal spot size increases?
A. Field of view
B. Geometric un-sharpness
C. Maximum photon energy
D. Patient dose
Answer: B. Larger focal spot size increases geometric un-sharpness when combined with
magnification.
5. The half-value layer (HVL) of an x-ray beam is defined as:
A. The thickness of material that absorbs all photons
B. The distance at which beam intensity doubles
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C. The thickness of material that reduces beam intensity by 50%
D. The time required for half of the radioactive source to decay
Answer: C. HVL is the thickness of a specified material that attenuates the beam to half its
original intensity, characterizing beam quality.
6. During imaging, the proportion of Compton scatter is increased by increasing which
parameter?
A. Exposure time
B. Focal spot size
C. kV
D. Source-to-image receptor distance
Answer: C. The proportion of Compton scattering compared to photoelectric interactions
increases with x-ray beam energy (kV, filtration).
7. The sharp increase in photoelectric absorption is related to:
A. Photon energy being just above the atomic number
B. Photon energy being just above the electron binding energy
C. Photon energy being below 10 keV
D. Photon energy being above 1 MeV
Answer: B. Photoelectric absorption is proportional to Z³/E³, with a sharp increase when
incoming photon energy is just above electron binding energy (K-edge).
8. What is the likely result when an electron vacancy in the K-shell is filled by an electron
from the L-shell?
A. Annihilation radiation
B. Characteristic x-ray
C. Bremsstrahlung
D. Compton scatter
Answer: B. Electron transition between atomic energy shells emits a characteristic x-ray
photon with energy equal to the binding energy difference.
9. A medical worker receives 30 mGy to skin on the hand from alpha particles. What is the
equivalent dose?
A. 30 mSv
B. 300 mSv
C. 600 mSv
D. 30 mGy
Answer: C. Equivalent dose = radiation weighting factor × absorbed dose. WR = 20 for alpha
particles. 30 mGy × 20 = 600 mSv.
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10. Which quantity provides a single index relating to overall stochastic risk when multiple
organs are irradiated?
A. Absorbed dose
B. Equivalent dose
C. Effective dose
D. Air kerma
Answer: C. Effective dose is a weighted sum of equivalent doses over multiple organs and
tissues, accounting for tissue radiosensitivity.
11. Which statement is true regarding effective dose?
A. It is dependent on co-morbidities
B. It is restricted to single organ doses
C. It is a weighted sum of equivalent doses over multiple organs and tissues
D. It is independent of radiation type
Answer: C. Effective dose uses tissue weighting factors (WT) for an average patient and
includes radiation weighting factors (WR).
12. Convert 20 mCi of ⁹⁹ᵐTc to MBq.
A. 0.54 MBq
B. 20 MBq
C. 37 MBq
D. 740 MBq
Answer: D. 1 mCi = 37 MBq. 20 × 37 = 740 MBq.
13. The unit of linear energy transfer (LET) is:
A. keV per μm
B. keV per mg
C. keV per g
D. keV per cm
Answer: A. LET is the average energy deposited locally per unit path length, measured in
keV/μm.
14. What is the predominant interaction of ⁹⁹ᵐTc photons (140 keV) with a sodium iodide
crystal?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: C. At 140 keV, more than 50% of interactions are photoelectric in sodium iodide
(high Z material).
ABR Medical Physics Continuing Certification (MOC) Practice Questions.
Section 1: Radiation Physics and Interactions (1–25)
1. What is the predominant interaction of 120 kV x-rays from a CT scanner with soft tissue?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: B. Above 25 keV, Compton scatter is the dominant photon interaction in soft tissue.
CT x-ray beams have higher filtration, with effective energy closer to one-half of the kV
(approximately 60 keV).
2. If a radiologic technologist uses 80 kV for an AP projection of the lumbar spine, which
interaction predominates in bone?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: C. The average energy for an 80 kV spectrum is typically 1/3 to 1/2 of maximal
energy. At approximately 40 keV, photoelectric effect dominates in bone (higher Z material).
3. What is a direct result of adding filtration to a diagnostic x-ray beam?
A. All characteristic radiation is removed
B. Image contrast is improved
C. Maximum photon energy is increased
D. Patient dose is reduced
Answer: D. Added filters reduce low-energy x-ray photons and harden the beam. Removal of
soft x-ray photons reduces patient skin dose.
4. Which of the following always increases as focal spot size increases?
A. Field of view
B. Geometric un-sharpness
C. Maximum photon energy
D. Patient dose
Answer: B. Larger focal spot size increases geometric un-sharpness when combined with
magnification.
5. The half-value layer (HVL) of an x-ray beam is defined as:
A. The thickness of material that absorbs all photons
B. The distance at which beam intensity doubles
, 2
C. The thickness of material that reduces beam intensity by 50%
D. The time required for half of the radioactive source to decay
Answer: C. HVL is the thickness of a specified material that attenuates the beam to half its
original intensity, characterizing beam quality.
6. During imaging, the proportion of Compton scatter is increased by increasing which
parameter?
A. Exposure time
B. Focal spot size
C. kV
D. Source-to-image receptor distance
Answer: C. The proportion of Compton scattering compared to photoelectric interactions
increases with x-ray beam energy (kV, filtration).
7. The sharp increase in photoelectric absorption is related to:
A. Photon energy being just above the atomic number
B. Photon energy being just above the electron binding energy
C. Photon energy being below 10 keV
D. Photon energy being above 1 MeV
Answer: B. Photoelectric absorption is proportional to Z³/E³, with a sharp increase when
incoming photon energy is just above electron binding energy (K-edge).
8. What is the likely result when an electron vacancy in the K-shell is filled by an electron
from the L-shell?
A. Annihilation radiation
B. Characteristic x-ray
C. Bremsstrahlung
D. Compton scatter
Answer: B. Electron transition between atomic energy shells emits a characteristic x-ray
photon with energy equal to the binding energy difference.
9. A medical worker receives 30 mGy to skin on the hand from alpha particles. What is the
equivalent dose?
A. 30 mSv
B. 300 mSv
C. 600 mSv
D. 30 mGy
Answer: C. Equivalent dose = radiation weighting factor × absorbed dose. WR = 20 for alpha
particles. 30 mGy × 20 = 600 mSv.
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10. Which quantity provides a single index relating to overall stochastic risk when multiple
organs are irradiated?
A. Absorbed dose
B. Equivalent dose
C. Effective dose
D. Air kerma
Answer: C. Effective dose is a weighted sum of equivalent doses over multiple organs and
tissues, accounting for tissue radiosensitivity.
11. Which statement is true regarding effective dose?
A. It is dependent on co-morbidities
B. It is restricted to single organ doses
C. It is a weighted sum of equivalent doses over multiple organs and tissues
D. It is independent of radiation type
Answer: C. Effective dose uses tissue weighting factors (WT) for an average patient and
includes radiation weighting factors (WR).
12. Convert 20 mCi of ⁹⁹ᵐTc to MBq.
A. 0.54 MBq
B. 20 MBq
C. 37 MBq
D. 740 MBq
Answer: D. 1 mCi = 37 MBq. 20 × 37 = 740 MBq.
13. The unit of linear energy transfer (LET) is:
A. keV per μm
B. keV per mg
C. keV per g
D. keV per cm
Answer: A. LET is the average energy deposited locally per unit path length, measured in
keV/μm.
14. What is the predominant interaction of ⁹⁹ᵐTc photons (140 keV) with a sodium iodide
crystal?
A. Coherent scattering
B. Compton scattering
C. Photoelectric effect
D. Pair production
Answer: C. At 140 keV, more than 50% of interactions are photoelectric in sodium iodide
(high Z material).
