MRI MRI Pulse Sequences 1 — Questions and Answers
Question 1: In a spin-echo sequence, which combination of TR and TE produces a T2-weighted image?
- Short TR, short TE
- Short TR, long TE
- Long TR, short TE
- Long TR, long TE (Correct answer)
Correct answer: Long TR, long TE
T2-weighting requires long TR (to minimize T1 contrast by allowing full recovery) and long TE (to allow T2 differences between tissues to accumulate).
Question 2: Fast spin-echo (FSE/TSE) sequences acquire multiple lines of k-space per TR using which approach?
- A single 90° pulse followed by multiple 180° refocusing pulses (echo train) (Correct answer)
- Multiple 90° pulses with a single 180° refocusing pulse
- Rapidly toggling the frequency-encoding gradient
- Using a series of partial flip-angle pulses
Correct answer: A single 90° pulse followed by multiple 180° refocusing pulses (echo train)
FSE uses an echo train (RARE principle): after a 90° excitation, a series of 180° pulses generates multiple spin echoes, each phase-encoded differently, filling multiple k-space lines per TR.
Question 3: Which gradient-echo sequence uses a very short TR and a spoiler gradient to eliminate residual transverse magnetization before the next RF pulse?
- Steady-state free precession (SSFP/TrueFISP)
- Spoiled gradient echo (SPGR/FLASH) (Correct answer)
- FIESTA
- GRASS
Correct answer: Spoiled gradient echo (SPGR/FLASH)
Spoiled GRE sequences (SPGR, FLASH) use RF or gradient spoiling to destroy residual transverse magnetization between pulses, producing T1-weighted images with high SNR efficiency.
Question 4: What does the acronym STIR stand for, and what tissue does it primarily null?
- Short-T1 Inversion Recovery — nulls fat signal (Correct answer)
- Steady-state T1 Inversion Recovery — nulls water signal
- Short-TE Inversion Recovery — nulls fat signal
- Standard Tissue Inversion Recovery — nulls all tissue equally
Correct answer: Short-T1 Inversion Recovery — nulls fat signal
STIR (Short-T1 Inversion Recovery) applies an inversion pulse with a TI timed to null fat signal (TI ≈ 150 ms at 1.5T), providing robust fat suppression independent of B0 homogeneity.
Question 5: Diffusion-weighted imaging (DWI) measures the random Brownian motion of water molecules. Restricted diffusion (e.g., in acute stroke) appears as:
- Low signal on DWI, high ADC
- High signal on DWI, low ADC (Correct answer)
- High signal on DWI, high ADC
- Low signal on DWI, low ADC
Correct answer: High signal on DWI, low ADC
In acute ischemia, cytotoxic edema restricts water movement; restricted diffusion shows high signal on DWI (b=1000) and low signal on ADC map (low diffusivity).
Question 6: Which MRI sequence is used to visualize flowing blood as bright signal ('bright blood') without contrast injection by exploiting the inflow of unsaturated spins into the imaging slice?
- Phase contrast MRA
- Contrast-enhanced MRA
- Time-of-flight (TOF) MRA (Correct answer)
- VIBE
Correct answer: Time-of-flight (TOF) MRA
TOF MRA uses gradient echo with short TR to saturate stationary tissue; fresh unsaturated blood flowing into the slice appears bright due to the inflow enhancement effect.
MRI MRI Pulse Sequences 1
In a spin-echo sequence, which combination of TR and TE produces a T2-weighted image?