MRI Pulse Sequences Flashcards

A technologist adds a -90° 'flip-back' pulse at the end of each echo train in a fast spin echo sequence. What is the main benefit?

Answer: It returns residual transverse magnetization to the longitudinal axis, allowing bright fluid with a shorter TR

Driven equilibrium (DRIVE/FRFSE/RESTORE) flips leftover transverse magnetization back to +Z so fluid stays bright on T2 images with a reduced TR.

In a fast spin echo sequence, the effective TE is determined by which factor?

Answer: The echo acquired when the central lines of k-space are filled

Image contrast comes mostly from the center of k-space, so the echo that fills the central lines sets the effective TE.

For a spoiled gradient echo sequence, the flip angle that gives maximum signal for a tissue with a given T1 and TR is called the:

Answer: Ernst angle

The Ernst angle, where cos θ = e^(-TR/T1), produces the highest signal for a given TR and T1.

A post-contrast T1 fat-suppressed image is requested. Why is STIR a poor choice for fat suppression here?

Answer: STIR also nulls gadolinium-enhancing tissue, since gadolinium shortens T1 toward that of fat

STIR nulls signal by T1 rather than frequency, so enhancing tissue with T1 shortened toward fat's may be suppressed too.

Time-of-flight MRA makes flowing blood bright mainly because:

Answer: Fresh unsaturated spins enter the slice while stationary tissue is saturated by repeated short-TR pulses

TOF uses flow-related enhancement: inflowing fully magnetized blood gives strong signal compared with saturated stationary tissue.

On a phase contrast flow study, velocities above the selected VENC will cause:

Answer: Aliasing, where high velocities are shown as flow in the opposite direction

Phase shifts beyond ±180° wrap around, so velocities exceeding the VENC are encoded as flow in the opposite direction.

Which sequence uses both magnitude and phase data from a high-resolution 3D gradient echo acquisition to highlight hemosiderin, deoxyhemoglobin, and calcification?

Answer: Susceptibility-weighted imaging (SWI)

SWI combines magnitude and filtered phase images to greatly increase sensitivity to susceptibility differences such as blood products and calcium.