Physics Flashcards

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

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

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

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

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

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

What is the Ernst angle in MRI?

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

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

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

Answer: Magnetic susceptibility

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).

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

Answer: Greater diffusion weighting and lower overall signal

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

The apparent diffusion coefficient (ADC) map is calculated to eliminate which confounding effect in DWI?

Answer: T2 shine-through

ADC maps remove T2 shine-through by computing the diffusion coefficient from multiple b-values, isolating true diffusion restriction from long-T2 tissue signal.

What is the purpose of crusher (spoiler) gradients in a spin echo sequence?

Answer: To dephase residual transverse magnetization before the next RF pulse

Crusher gradients surround the 180° refocusing pulse to eliminate stimulated echoes from imperfect flip angles that could contaminate image quality.