MRI Physics Safety Exam Practice
Questions and Correct Answers
(Verified Answers) Plus Rationales 2027
Q&A | Instant Download Pdf
1. Which property of the main magnetic field primarily determines the Larmor
frequency of hydrogen nuclei?
A. Gradient slew rate
B. RF pulse bandwidth
C. Static magnetic-field strength
D. Acoustic noise level
Correct Answer: C. Static magnetic-field strength
Rationale: The Larmor angular frequency is proportional to the static magnetic
field according to ω₀ = γB₀. Therefore, increasing B₀ increases the precessional
frequency of hydrogen nuclei.
2. For hydrogen nuclei, what happens to the Larmor frequency when B₀ is
doubled?
A. It is reduced by half
B. It remains unchanged
C. It increases by a factor of four
D. It doubles
Correct Answer: D. It doubles
Rationale: Larmor frequency is directly proportional to B₀. Therefore, doubling the
static magnetic field doubles the Larmor frequency.
,3. Which quantity describes the spatial rate of change of the static magnetic
field?
A. Spatial field gradient
B. RF duty cycle
C. Specific absorption rate
D. Relaxation rate
Correct Answer: A. Spatial field gradient
Rationale: The spatial field gradient describes how rapidly the static magnetic field
changes with position. It is particularly important when assessing translational
forces on magnetic objects.
4. Why is spatial field gradient important for MR safety?
A. It determines the patient's oxygen saturation
B. It contributes to translational forces on susceptible objects
C. It determines the RF pulse frequency independently of B₀
D. It eliminates acoustic noise
Correct Answer: B. It contributes to translational forces on susceptible objects
*Rationale: Translational force on a magnetically susceptible object depends on
both the magnetic field and its spatial gradient. ACR guidance specifically
identifies spatial field gradient as an important factor in predicting forces on
ferromagnetic objects. *
5. Where around a typical cylindrical superconducting MRI system is the spatial
field gradient often particularly high?
A. Far from the scanner
B. Near the center of the bore only
C. Near the ends or faces of the magnet
D. Only inside the RF coil
Correct Answer: C. Near the ends or faces of the magnet
,*Rationale: The spatial field gradient generally increases substantially near the
ends of a cylindrical MRI scanner. This contributes to the potentially strong
translational forces experienced by susceptible objects in those regions. *
6. Which expression best represents the magnetic force relationship relevant to
a small magnetically susceptible object?
A. Force is proportional only to B₀
B. Force is proportional only to object mass
C. Force is proportional only to RF frequency
D. Force depends on magnetic susceptibility, magnetic field, and spatial field
gradient
Correct Answer: D. Force depends on magnetic susceptibility, magnetic field,
and spatial field gradient
Rationale: The force on a susceptible object is related to the interaction between
the object's induced magnetic moment and the spatial variation of the magnetic
field. Thus, both B₀ and its spatial gradient are important.
7. What is magnetic torque most directly associated with?
A. Rotation of a magnetic object toward field alignment
B. Heating caused by RF energy
C. Acoustic vibration of the gradient coils
D. Oxygen displacement during a quench
Correct Answer: A. Rotation of a magnetic object toward field alignment
Rationale: Magnetic torque acts on a magnetic dipole and tends to rotate it
toward alignment with the external magnetic field.
8. Which type of MRI field is primarily responsible for peripheral nerve
stimulation?
, A. Static B₀ field
B. RF B₁ field
C. Time-varying gradient field
D. Earth’s magnetic field
Correct Answer: C. Time-varying gradient field
Rationale: Rapidly changing gradient magnetic fields can induce electric fields in
tissue. If sufficiently large, these induced fields can stimulate peripheral nerves.
9. Which parameter is most directly associated with the rate of gradient-
induced magnetic-field change?
A. dB/dt
B. T1
C. T2*
D. B₀ homogeneity
Correct Answer: A. dB/dt
Rationale: dB/dt represents the rate of change of magnetic field with respect to
time. Rapidly changing gradients can generate induced electric fields and produce
peripheral nerve stimulation.
10. A gradient system has a maximum amplitude of 40 mT/m and a slew rate of
200 T/m/s. What does the slew rate primarily describe?
A. The maximum static field strength
B. The rate at which gradient amplitude can change
C. The RF frequency
D. The patient's SAR
Correct Answer: B. The rate at which gradient amplitude can change
Rationale: Slew rate is expressed in T/m/s and describes how rapidly the gradient
amplitude changes with time. It is an important determinant of dB/dt and
therefore of potential peripheral nerve stimulation.
Questions and Correct Answers
(Verified Answers) Plus Rationales 2027
Q&A | Instant Download Pdf
1. Which property of the main magnetic field primarily determines the Larmor
frequency of hydrogen nuclei?
A. Gradient slew rate
B. RF pulse bandwidth
C. Static magnetic-field strength
D. Acoustic noise level
Correct Answer: C. Static magnetic-field strength
Rationale: The Larmor angular frequency is proportional to the static magnetic
field according to ω₀ = γB₀. Therefore, increasing B₀ increases the precessional
frequency of hydrogen nuclei.
2. For hydrogen nuclei, what happens to the Larmor frequency when B₀ is
doubled?
A. It is reduced by half
B. It remains unchanged
C. It increases by a factor of four
D. It doubles
Correct Answer: D. It doubles
Rationale: Larmor frequency is directly proportional to B₀. Therefore, doubling the
static magnetic field doubles the Larmor frequency.
,3. Which quantity describes the spatial rate of change of the static magnetic
field?
A. Spatial field gradient
B. RF duty cycle
C. Specific absorption rate
D. Relaxation rate
Correct Answer: A. Spatial field gradient
Rationale: The spatial field gradient describes how rapidly the static magnetic field
changes with position. It is particularly important when assessing translational
forces on magnetic objects.
4. Why is spatial field gradient important for MR safety?
A. It determines the patient's oxygen saturation
B. It contributes to translational forces on susceptible objects
C. It determines the RF pulse frequency independently of B₀
D. It eliminates acoustic noise
Correct Answer: B. It contributes to translational forces on susceptible objects
*Rationale: Translational force on a magnetically susceptible object depends on
both the magnetic field and its spatial gradient. ACR guidance specifically
identifies spatial field gradient as an important factor in predicting forces on
ferromagnetic objects. *
5. Where around a typical cylindrical superconducting MRI system is the spatial
field gradient often particularly high?
A. Far from the scanner
B. Near the center of the bore only
C. Near the ends or faces of the magnet
D. Only inside the RF coil
Correct Answer: C. Near the ends or faces of the magnet
,*Rationale: The spatial field gradient generally increases substantially near the
ends of a cylindrical MRI scanner. This contributes to the potentially strong
translational forces experienced by susceptible objects in those regions. *
6. Which expression best represents the magnetic force relationship relevant to
a small magnetically susceptible object?
A. Force is proportional only to B₀
B. Force is proportional only to object mass
C. Force is proportional only to RF frequency
D. Force depends on magnetic susceptibility, magnetic field, and spatial field
gradient
Correct Answer: D. Force depends on magnetic susceptibility, magnetic field,
and spatial field gradient
Rationale: The force on a susceptible object is related to the interaction between
the object's induced magnetic moment and the spatial variation of the magnetic
field. Thus, both B₀ and its spatial gradient are important.
7. What is magnetic torque most directly associated with?
A. Rotation of a magnetic object toward field alignment
B. Heating caused by RF energy
C. Acoustic vibration of the gradient coils
D. Oxygen displacement during a quench
Correct Answer: A. Rotation of a magnetic object toward field alignment
Rationale: Magnetic torque acts on a magnetic dipole and tends to rotate it
toward alignment with the external magnetic field.
8. Which type of MRI field is primarily responsible for peripheral nerve
stimulation?
, A. Static B₀ field
B. RF B₁ field
C. Time-varying gradient field
D. Earth’s magnetic field
Correct Answer: C. Time-varying gradient field
Rationale: Rapidly changing gradient magnetic fields can induce electric fields in
tissue. If sufficiently large, these induced fields can stimulate peripheral nerves.
9. Which parameter is most directly associated with the rate of gradient-
induced magnetic-field change?
A. dB/dt
B. T1
C. T2*
D. B₀ homogeneity
Correct Answer: A. dB/dt
Rationale: dB/dt represents the rate of change of magnetic field with respect to
time. Rapidly changing gradients can generate induced electric fields and produce
peripheral nerve stimulation.
10. A gradient system has a maximum amplitude of 40 mT/m and a slew rate of
200 T/m/s. What does the slew rate primarily describe?
A. The maximum static field strength
B. The rate at which gradient amplitude can change
C. The RF frequency
D. The patient's SAR
Correct Answer: B. The rate at which gradient amplitude can change
Rationale: Slew rate is expressed in T/m/s and describes how rapidly the gradient
amplitude changes with time. It is an important determinant of dB/dt and
therefore of potential peripheral nerve stimulation.
