MRI Knowledge 3 — Questions and Answers
Question 1: What is the Larmor frequency in MRI?
- The frequency at which gradient coils switch polarity
- The resonant frequency at which protons precess in a given magnetic field (Correct answer)
- The rate of T1 relaxation for water
- The bandwidth of the RF receiver coil
Correct answer: The resonant frequency at which protons precess in a given magnetic field
The Larmor frequency (ω₀ = γB₀) is the precise resonant frequency at which protons absorb and emit RF energy in the applied field.
Question 2: Which MRI sequence is most sensitive for detecting acute subarachnoid hemorrhage?
- T2-weighted FSE
- FLAIR (Fluid Attenuated Inversion Recovery) (Correct answer)
- Gradient Echo (GRE)
- Diffusion-Weighted Imaging (DWI)
Correct answer: FLAIR (Fluid Attenuated Inversion Recovery)
FLAIR suppresses CSF signal, making subarachnoid blood appear hyperintense against a dark CSF background.
Question 3: What is the effect of increasing the number of signal averages (NSA/NEX) on MRI image quality?
- SNR decreases proportionally
- SNR improves proportionally to the square root of NSA (Correct answer)
- Spatial resolution increases
- Scan time decreases
Correct answer: SNR improves proportionally to the square root of NSA
Averaging multiple acquisitions improves SNR by a factor of √NSA, though it linearly increases scan time.
Question 4: Which MRI technique is used to assess brain perfusion by tracking the first pass of a gadolinium bolus?
- Arterial Spin Labeling (ASL)
- Dynamic Susceptibility Contrast (DSC) perfusion (Correct answer)
- Diffusion Tensor Imaging (DTI)
- Magnetic Resonance Spectroscopy (MRS)
Correct answer: Dynamic Susceptibility Contrast (DSC) perfusion
DSC perfusion MRI uses rapid T2*-weighted sequences to measure signal drop as gadolinium transits through the brain capillaries.
Question 5: A patient with a cochlear implant is scheduled for MRI. What is the primary concern?
- Heating of the cochlear fluid
- Device displacement or demagnetization by the static field and RF heating (Correct answer)
- Acoustic noise damage
- Claustrophobia from the device signal
Correct answer: Device displacement or demagnetization by the static field and RF heating
Cochlear implants contain magnets that can torque, displace, or demagnetize in the MRI field, posing significant safety risks.
Question 6: What does 'isotropic voxel' mean in MRI acquisition?
- Voxel with equal field strength in all dimensions
- Voxel with equal dimensions in all three spatial directions (Correct answer)
- Voxel that produces equal signal in T1 and T2 sequences
- Voxel aligned with the main magnetic field
Correct answer: Voxel with equal dimensions in all three spatial directions
An isotropic voxel has identical length, width, and height, allowing high-quality multiplanar reformats without resolution loss.
Question 7: Which MRI contrast agent class is associated with the highest risk of nephrogenic systemic fibrosis (NSF)?
- Macrocyclic ionic agents
- Linear non-ionic agents (Correct answer)
- Blood pool agents
- Manganese-based agents
Correct answer: Linear non-ionic agents
Linear non-ionic (and some linear ionic) gadolinium agents release free Gd³⁺ more readily and carry the highest NSF risk in patients with severe renal impairment.
MRI Knowledge 3
What is the Larmor frequency in MRI?