ABR Medical Physics Certification
Examination Practice Questions And
Correct Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
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1. The half-life of a radioactive isotope is the time required for:
A. Its activity to double
B. Half of its nuclei to decay
C. All of its nuclei to decay
D. Its energy to reduce by half
Rationale: The half-life is defined as the time in which 50% of the
radioactive nuclei undergo decay.
2. Which interaction predominates for diagnostic X-rays in soft tissue
(20–150 keV)?
A. Pair production
, B. Photoelectric effect
C. Compton scattering
D. Coherent scattering
Rationale: In soft tissue and diagnostic energies, the photoelectric effect
dominates at lower energies, especially in high atomic number materials.
3. In linear accelerators, the target used to generate X-rays is typically
made of:
A. Aluminum
B. Tungsten
C. Copper
D. Lead
Rationale: Tungsten has a high atomic number and melting point, making
it efficient for bremsstrahlung production in X-ray targets.
4. The unit of absorbed dose in the SI system is:
A. Rad
B. Gray (Gy)
C. Rem
D. Sievert
Rationale: Gray (Gy) is the SI unit for absorbed dose, defined as 1 J/kg.
5. Which of the following factors primarily affects beam hardening in CT
imaging?
A. Detector efficiency
, B. Slice thickness
C. X-ray filtration
D. Pitch
Rationale: Beam hardening occurs when lower energy photons are
absorbed preferentially, which is directly affected by filtration.
6. The buildup region in megavoltage photon beams refers to:
A. The depth at maximum electron density
B. The depth at which dose rises to its maximum
C. The surface dose only
D. The dose beyond the maximum
Rationale: High-energy photons have a dose that increases with depth
until the maximum (dmax) due to electron scatter buildup.
7. Which dosimeter is most appropriate for measuring skin dose in
interventional radiology?
A. Ionization chamber
B. TLD (thermoluminescent dosimeter)
C. Film badge
D. Geiger-Müller counter
Rationale: TLDs are small, sensitive, and can accurately measure surface
doses where direct ion chamber measurement is impractical.
8. The primary purpose of a beam collimator in radiation therapy is to:
A. Increase dose
, B. Reduce photon energy
C. Shape and limit the beam to the target
D. Measure tissue attenuation
Rationale: Collimators restrict the beam to conform to the tumor shape,
minimizing dose to surrounding tissue.
9. Compton scattering probability:
A. Increases with atomic number
B. Decreases with photon energy
C. Is independent of electron density
D. Only occurs in metals
Rationale: Compton scattering is primarily dependent on electron density
and decreases as photon energy increases.
10. In radiation protection, the effective dose accounts for:
A. Only absorbed dose
B. Only equivalent dose
C. Both tissue weighting and radiation type
D. Only high-energy photons
Rationale: Effective dose considers radiation type and the sensitivity of
different tissues to provide a risk-based measure.
11. Which MRI sequence is most sensitive to water content?
A. T1-weighted
B. Proton density
Examination Practice Questions And
Correct Answers (Verified Answers) Plus
Rationales 2026 Q&A | Instant
Download Pdf
1. The half-life of a radioactive isotope is the time required for:
A. Its activity to double
B. Half of its nuclei to decay
C. All of its nuclei to decay
D. Its energy to reduce by half
Rationale: The half-life is defined as the time in which 50% of the
radioactive nuclei undergo decay.
2. Which interaction predominates for diagnostic X-rays in soft tissue
(20–150 keV)?
A. Pair production
, B. Photoelectric effect
C. Compton scattering
D. Coherent scattering
Rationale: In soft tissue and diagnostic energies, the photoelectric effect
dominates at lower energies, especially in high atomic number materials.
3. In linear accelerators, the target used to generate X-rays is typically
made of:
A. Aluminum
B. Tungsten
C. Copper
D. Lead
Rationale: Tungsten has a high atomic number and melting point, making
it efficient for bremsstrahlung production in X-ray targets.
4. The unit of absorbed dose in the SI system is:
A. Rad
B. Gray (Gy)
C. Rem
D. Sievert
Rationale: Gray (Gy) is the SI unit for absorbed dose, defined as 1 J/kg.
5. Which of the following factors primarily affects beam hardening in CT
imaging?
A. Detector efficiency
, B. Slice thickness
C. X-ray filtration
D. Pitch
Rationale: Beam hardening occurs when lower energy photons are
absorbed preferentially, which is directly affected by filtration.
6. The buildup region in megavoltage photon beams refers to:
A. The depth at maximum electron density
B. The depth at which dose rises to its maximum
C. The surface dose only
D. The dose beyond the maximum
Rationale: High-energy photons have a dose that increases with depth
until the maximum (dmax) due to electron scatter buildup.
7. Which dosimeter is most appropriate for measuring skin dose in
interventional radiology?
A. Ionization chamber
B. TLD (thermoluminescent dosimeter)
C. Film badge
D. Geiger-Müller counter
Rationale: TLDs are small, sensitive, and can accurately measure surface
doses where direct ion chamber measurement is impractical.
8. The primary purpose of a beam collimator in radiation therapy is to:
A. Increase dose
, B. Reduce photon energy
C. Shape and limit the beam to the target
D. Measure tissue attenuation
Rationale: Collimators restrict the beam to conform to the tumor shape,
minimizing dose to surrounding tissue.
9. Compton scattering probability:
A. Increases with atomic number
B. Decreases with photon energy
C. Is independent of electron density
D. Only occurs in metals
Rationale: Compton scattering is primarily dependent on electron density
and decreases as photon energy increases.
10. In radiation protection, the effective dose accounts for:
A. Only absorbed dose
B. Only equivalent dose
C. Both tissue weighting and radiation type
D. Only high-energy photons
Rationale: Effective dose considers radiation type and the sensitivity of
different tissues to provide a risk-based measure.
11. Which MRI sequence is most sensitive to water content?
A. T1-weighted
B. Proton density
