MRI Knowledge 5 — Questions and Answers
Question 1: What is the Specific Absorption Rate (SAR) a measure of in MRI?
- Rate of image data acquisition in k-space
- Rate at which radiofrequency energy is absorbed by the patient's tissue per unit mass (Correct answer)
- Speed of gradient switching in Tesla per meter per second
- Contrast agent uptake rate in tissue
Correct answer: Rate at which radiofrequency energy is absorbed by the patient's tissue per unit mass
SAR (W/kg) quantifies RF energy deposition in tissue and is a primary safety limit to prevent excessive tissue heating during MRI.
Question 2: Which MRI sequence is best suited for evaluating articular cartilage because it provides excellent contrast between cartilage, fluid, and bone?
- T1-weighted SE
- Proton density fat-saturated (PD FS) (Correct answer)
- FLAIR
- Inversion recovery (STIR)
Correct answer: Proton density fat-saturated (PD FS)
PD fat-saturated sequences highlight cartilage defects as intermediate-signal cartilage contrasts sharply against bright joint fluid and suppressed fat.
Question 3: What causes the 'zipper' artifact that appears as a line of noise across the MRI image?
- Patient motion during phase encoding
- External RF interference entering the scan room through a breach in shielding (Correct answer)
- Eddy currents induced by gradient switching
- Incomplete fat suppression
Correct answer: External RF interference entering the scan room through a breach in shielding
A zipper artifact is caused by external RF noise at a specific frequency leaking into the Faraday-shielded room, appearing as a bright line at that frequency position.
Question 4: In MRI, what does the term 'bandwidth' typically refer to in clinical practice?
- The range of flip angles used in a sequence
- The range of frequencies sampled during readout, affecting SNR and chemical shift (Correct answer)
- The number of phase-encode steps acquired
- The RF pulse duration in milliseconds
Correct answer: The range of frequencies sampled during readout, affecting SNR and chemical shift
Receiver bandwidth is the range of frequencies digitized during readout; wider bandwidth reduces chemical shift and distortion but decreases SNR.
Question 5: What is the primary advantage of a 3T MRI scanner compared to a 1.5T scanner?
- Shorter T1 relaxation times for all tissues
- Higher SNR that can be traded for faster scans or higher spatial resolution (Correct answer)
- Elimination of susceptibility artifacts
- Lower SAR deposition in patients
Correct answer: Higher SNR that can be traded for faster scans or higher spatial resolution
Field strength doubling theoretically doubles SNR, which can be reinvested in thinner slices, smaller voxels, or reduced scan time.
Question 6: Which MRI technique uses magnetically labeled water protons as an endogenous tracer to measure cerebral blood flow without contrast injection?
- Dynamic Contrast Enhanced (DCE) MRI
- Arterial Spin Labeling (ASL) (Correct answer)
- Dynamic Susceptibility Contrast (DSC) MRI
- Blood Oxygen Level Dependent (BOLD) fMRI
Correct answer: Arterial Spin Labeling (ASL)
ASL uses RF inversion of arterial water protons proximal to the imaging plane as a flow tracer, enabling non-invasive perfusion measurement.
Question 7: What is the primary reason gradient coils produce loud acoustic noise during MRI scanning?
- RF pulses vibrate the RF coil housing
- Lorentz forces cause the gradient coil windings to physically vibrate as current rapidly switches (Correct answer)
- Cryogen boiling creates mechanical vibration in the magnet bore
- The patient table motor produces noise as it moves into position
Correct answer: Lorentz forces cause the gradient coil windings to physically vibrate as current rapidly switches
Rapidly switching electrical current in gradient coils within the strong static field produces Lorentz forces that physically deflect the coils, generating loud knocking sounds.
MRI Knowledge 5
What is the Specific Absorption Rate (SAR) a measure of in MRI?