MRI Physics 4

Question 1

Which phenomenon causes the chemical shift artifact in MRI, and in which direction does it occur?

Accepted Answer

Resonance frequency difference between fat and water; frequency-encode direction

Answer

Fat protons resonate at ~3.5 ppm lower frequency than water, causing spatial misregistration along the frequency-encoding direction.

Question 2

In MRI, the term 'flip angle' refers to:

Accepted Answer

The angle by which the net magnetization is rotated from B0 by the RF pulse

Answer

The flip angle is the degree of rotation of the net magnetization vector away from the longitudinal axis produced by an RF pulse.

Question 3

What is the Ernst angle in MRI?

Accepted Answer

The flip angle that maximizes signal for a given TR and T1

Answer

The Ernst angle (cos θ = e^(−TR/T1)) is the optimal flip angle for steady-state gradient echo imaging that maximizes SNR efficiency.

Question 4

Susceptibility-weighted imaging (SWI) exploits differences in which magnetic property between tissues?

Accepted Answer

Magnetic susceptibility

Answer

SWI uses the phase and magnitude information from a long-TE GRE sequence to detect subtle differences in magnetic susceptibility (e.g., deoxyhemoglobin, iron, calcium).

Question 5

In diffusion-weighted imaging (DWI), a high b-value results in:

Accepted Answer

Greater diffusion weighting and lower overall signal

Answer

Higher b-values apply stronger diffusion-sensitizing gradients, so freely diffusing water loses more signal, increasing lesion conspicuity for restricted diffusion.

MRI - Magnetic Resonance Imaging Practice Test

MRI - Magnetic Resonance Imaging Practice Test

MRI Physics 4