MRI Knowledge 5

Question 1

What is the Specific Absorption Rate (SAR) a measure of in MRI?

Accepted Answer

Rate at which radiofrequency energy is absorbed by the patient's tissue per unit mass

Answer

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?

Accepted Answer

Proton density fat-saturated (PD FS)

Answer

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?

Accepted Answer

External RF interference entering the scan room through a breach in shielding

Answer

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?

Accepted Answer

The range of frequencies sampled during readout, affecting SNR and chemical shift

Answer

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?

Accepted Answer

Higher SNR that can be traded for faster scans or higher spatial resolution

Answer

Field strength doubling theoretically doubles SNR, which can be reinvested in thinner slices, smaller voxels, or reduced scan time.

MRI - Magnetic Resonance Imaging Practice Test

MRI - Magnetic Resonance Imaging Practice Test

MRI Knowledge 5