MRI Knowledge 3

Question 1

What is the Larmor frequency in MRI?

Accepted Answer

The resonant frequency at which protons precess in a given magnetic field

Answer

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?

Accepted Answer

FLAIR (Fluid Attenuated Inversion Recovery)

Answer

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?

Accepted Answer

SNR improves proportionally to the square root of NSA

Answer

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?

Accepted Answer

Dynamic Susceptibility Contrast (DSC) perfusion

Answer

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?

Accepted Answer

Device displacement or demagnetization by the static field and RF heating

Answer

Cochlear implants contain magnets that can torque, displace, or demagnetize in the MRI field, posing significant safety risks.

MRI - Magnetic Resonance Imaging Practice Test