1
CNT Written Exam Actual Exam 2026/2027
– Complete Exam-Style Questions with
Detailed Rationales | 100% Verified | Pass
Guaranteed – A+ Graded
[SECTION 1: Radiation Physics & Instrumentation — Questions 1-22]
Q1: Which interaction between a gamma photon and a Sodium Iodide (NaI) crystal is primarily
responsible for the photopeak in a pulse height spectrum?
A. Compton scattering
B. Pair production
C. Photoelectric effect
D. Coherent scatter
Correct Answer: C
Rationale: The photoelectric effect results in the complete absorption of the photon's energy by
the crystal, which is then converted into light photons proportional to the incident energy. This
total energy deposition creates the photopeak, which corresponds to the specific energy of the
radionuclide (e.g., 140 keV for Tc-99m). Compton scattering (A) results in partial energy
deposition and creates the Compton plateau and edge. Pair production (D) only occurs at
energies > 1.02 MeV.
Q2: What is the primary function of the photomultiplier tube (PMT) in a gamma camera?
A. To generate high voltage for the camera system.
B. To filter scattered radiation from the primary beam.
C. To convert light photons from the crystal into an electrical pulse and amplify it.
D. To collimate the incoming photons before they strike the crystal.
Correct Answer: C
,2
Rationale: The PMT takes the faint light signal produced by the scintillation crystal and converts
it into an electrical signal via the photocathode. This signal is then amplified through a series of
dynodes to a level that can be processed by the electronics. Options A, B, and D describe the
functions of the high-voltage power supply, energy discrimination circuits, and collimator,
respectively.
Q3: Which type of collimator is best suited for imaging small organs (like the thyroid) with high
spatial resolution?
A. High-energy parallel-hole
B. Diverging
C. Pinhole
D. Converging
Correct Answer: C
Rationale: The pinhole collimator provides magnification and excellent spatial resolution for
small structures that can fit within the field of view of the aperture. Parallel-hole (A) provides no
magnification. Diverging (B) provides minification. Converging (D) provides magnification but
is typically used for larger organs or specific tomographic setups; pinhole is the gold standard for
thyroid.
Q4: When using a Low Energy All Purpose (LEAP) parallel-hole collimator with Tc-99m (140
keV), what is the expected trade-off compared to a Low Energy High Resolution (LEHR)
collimator?
A. Lower spatial resolution but higher sensitivity.
B. Higher spatial resolution but lower sensitivity.
C. Higher septal penetration.
D. Lower sensitivity and lower resolution.
Correct Answer: A
Rationale: LEAP collimators have larger holes and/or thinner septa than LEHR collimators,
allowing more photons to pass through (higher sensitivity) at the cost of spatial resolution.
,3
LEHR collimators sacrifice counts (sensitivity) to achieve sharper images. Option C is incorrect
because septal penetration is primarily related to the energy of the isotope relative to the septal
design, though LEAP may have slightly thinner septa, the primary trade-off is resolution vs
sensitivity.
Q5: What is the purpose of the Pulse Height Analyzer (PHA)?
A. To regulate the high voltage supplied to the PMT.
B. To convert analog signals to digital signals for storage.
C. To accept only electrical pulses corresponding to the photopeak energy and reject scatter.
D. To position the X and Y coordinates of the event.
Correct Answer: C
Rationale: The PHA (or energy window) discriminates between valid photons that have
deposited full energy in the crystal (photopeak) and those that have lost energy via scatter
(Compton scatter). By setting a window (e.g., 20% around 140 keV), the system rejects lower
energy pulses that would degrade image contrast. Positioning (D) is handled by the resistance
matrix or delay lines.
Q6: Energy resolution is defined as the ability of the detector to distinguish between photons of
different energies. How is it quantitatively expressed?
A. Full width at half minimum (FWHM)
B. Full width at tenth maximum (FWTM)
C. Full width at half maximum (FWHM) as a percentage of the photopeak energy.
D. The total counts under the photopeak.
Correct Answer: C
Rationale: Energy resolution is calculated by taking the width of the photopeak at half its
maximum height (FWHM) and dividing it by the peak energy, expressed as a percentage (e.g.,
10% at 140 keV). A lower percentage indicates better resolution. Option A is a non-standard
term. Option B is used sometimes but FWHM is the standard.
, 4
Q7: Which of the following factors has the greatest effect on spatial resolution in a nuclear
medicine image?
A. Count density
B. Matrix size
C. Collimator hole diameter
D. Number of PMTs
Correct Answer: C
Rationale: The collimator is the primary determinant of spatial resolution; smaller holes provide
better resolution but lower sensitivity. While count density (A) and matrix size (B) affect the
digital resolution or noise, the physical limiting factor for the system's ability to resolve detail is
the collimator design.
Q8: What is the primary advantage of using a thicker NaI(Tl) crystal (e.g., 1 inch vs 3/8 inch)?
A. Increased sensitivity (detection efficiency) for higher energy isotopes.
B. Improved intrinsic spatial resolution.
C. Reduced septal penetration.
D. Better energy resolution.
Correct Answer: A
Rationale: Thicker crystals increase the probability that a photon will interact within the crystal,
especially for higher energy isotopes (e.g., I-131, F-18). This increases sensitivity. However,
thicker crystals can degrade intrinsic resolution because light spreads more before reaching the
PMTs. Options B and D are generally negatively correlated with crystal thickness.
Q9: What is "dead time" in a gamma camera system?
A. The time required for the PMT to cool down after use.
B. The time between patient injections.
C. The period during which the detector is processing an event and cannot record another.
CNT Written Exam Actual Exam 2026/2027
– Complete Exam-Style Questions with
Detailed Rationales | 100% Verified | Pass
Guaranteed – A+ Graded
[SECTION 1: Radiation Physics & Instrumentation — Questions 1-22]
Q1: Which interaction between a gamma photon and a Sodium Iodide (NaI) crystal is primarily
responsible for the photopeak in a pulse height spectrum?
A. Compton scattering
B. Pair production
C. Photoelectric effect
D. Coherent scatter
Correct Answer: C
Rationale: The photoelectric effect results in the complete absorption of the photon's energy by
the crystal, which is then converted into light photons proportional to the incident energy. This
total energy deposition creates the photopeak, which corresponds to the specific energy of the
radionuclide (e.g., 140 keV for Tc-99m). Compton scattering (A) results in partial energy
deposition and creates the Compton plateau and edge. Pair production (D) only occurs at
energies > 1.02 MeV.
Q2: What is the primary function of the photomultiplier tube (PMT) in a gamma camera?
A. To generate high voltage for the camera system.
B. To filter scattered radiation from the primary beam.
C. To convert light photons from the crystal into an electrical pulse and amplify it.
D. To collimate the incoming photons before they strike the crystal.
Correct Answer: C
,2
Rationale: The PMT takes the faint light signal produced by the scintillation crystal and converts
it into an electrical signal via the photocathode. This signal is then amplified through a series of
dynodes to a level that can be processed by the electronics. Options A, B, and D describe the
functions of the high-voltage power supply, energy discrimination circuits, and collimator,
respectively.
Q3: Which type of collimator is best suited for imaging small organs (like the thyroid) with high
spatial resolution?
A. High-energy parallel-hole
B. Diverging
C. Pinhole
D. Converging
Correct Answer: C
Rationale: The pinhole collimator provides magnification and excellent spatial resolution for
small structures that can fit within the field of view of the aperture. Parallel-hole (A) provides no
magnification. Diverging (B) provides minification. Converging (D) provides magnification but
is typically used for larger organs or specific tomographic setups; pinhole is the gold standard for
thyroid.
Q4: When using a Low Energy All Purpose (LEAP) parallel-hole collimator with Tc-99m (140
keV), what is the expected trade-off compared to a Low Energy High Resolution (LEHR)
collimator?
A. Lower spatial resolution but higher sensitivity.
B. Higher spatial resolution but lower sensitivity.
C. Higher septal penetration.
D. Lower sensitivity and lower resolution.
Correct Answer: A
Rationale: LEAP collimators have larger holes and/or thinner septa than LEHR collimators,
allowing more photons to pass through (higher sensitivity) at the cost of spatial resolution.
,3
LEHR collimators sacrifice counts (sensitivity) to achieve sharper images. Option C is incorrect
because septal penetration is primarily related to the energy of the isotope relative to the septal
design, though LEAP may have slightly thinner septa, the primary trade-off is resolution vs
sensitivity.
Q5: What is the purpose of the Pulse Height Analyzer (PHA)?
A. To regulate the high voltage supplied to the PMT.
B. To convert analog signals to digital signals for storage.
C. To accept only electrical pulses corresponding to the photopeak energy and reject scatter.
D. To position the X and Y coordinates of the event.
Correct Answer: C
Rationale: The PHA (or energy window) discriminates between valid photons that have
deposited full energy in the crystal (photopeak) and those that have lost energy via scatter
(Compton scatter). By setting a window (e.g., 20% around 140 keV), the system rejects lower
energy pulses that would degrade image contrast. Positioning (D) is handled by the resistance
matrix or delay lines.
Q6: Energy resolution is defined as the ability of the detector to distinguish between photons of
different energies. How is it quantitatively expressed?
A. Full width at half minimum (FWHM)
B. Full width at tenth maximum (FWTM)
C. Full width at half maximum (FWHM) as a percentage of the photopeak energy.
D. The total counts under the photopeak.
Correct Answer: C
Rationale: Energy resolution is calculated by taking the width of the photopeak at half its
maximum height (FWHM) and dividing it by the peak energy, expressed as a percentage (e.g.,
10% at 140 keV). A lower percentage indicates better resolution. Option A is a non-standard
term. Option B is used sometimes but FWHM is the standard.
, 4
Q7: Which of the following factors has the greatest effect on spatial resolution in a nuclear
medicine image?
A. Count density
B. Matrix size
C. Collimator hole diameter
D. Number of PMTs
Correct Answer: C
Rationale: The collimator is the primary determinant of spatial resolution; smaller holes provide
better resolution but lower sensitivity. While count density (A) and matrix size (B) affect the
digital resolution or noise, the physical limiting factor for the system's ability to resolve detail is
the collimator design.
Q8: What is the primary advantage of using a thicker NaI(Tl) crystal (e.g., 1 inch vs 3/8 inch)?
A. Increased sensitivity (detection efficiency) for higher energy isotopes.
B. Improved intrinsic spatial resolution.
C. Reduced septal penetration.
D. Better energy resolution.
Correct Answer: A
Rationale: Thicker crystals increase the probability that a photon will interact within the crystal,
especially for higher energy isotopes (e.g., I-131, F-18). This increases sensitivity. However,
thicker crystals can degrade intrinsic resolution because light spreads more before reaching the
PMTs. Options B and D are generally negatively correlated with crystal thickness.
Q9: What is "dead time" in a gamma camera system?
A. The time required for the PMT to cool down after use.
B. The time between patient injections.
C. The period during which the detector is processing an event and cannot record another.
