MRI MRI Pulse Sequences 2 — Questions and Answers
Question 1: Inversion recovery sequences null a specific tissue when the readout pulse is applied at time TI equal to:
- 0.693 × T1 of the tissue (T1 × ln2) (Correct answer)
- 2 × T1 of the tissue
- T1/2 of the tissue
- 0.5 × TE
Correct answer: 0.693 × T1 of the tissue (T1 × ln2)
The longitudinal magnetization of a tissue crosses zero at TI = T1 × ln(2) ≈ 0.693 × T1; applying the readout at this moment nulls that tissue's signal.
Question 2: The b-value in diffusion-weighted imaging controls the degree of diffusion weighting. Increasing the b-value:
- Increases overall image SNR
- Increases diffusion contrast but decreases SNR (Correct answer)
- Decreases T2 contribution
- Has no effect on image quality
Correct answer: Increases diffusion contrast but decreases SNR
Higher b-values provide greater diffusion contrast by increasing the dephasing of mobile water protons, but at the cost of reduced SNR due to greater signal attenuation from all spins.
Question 3: SSFP (steady-state free precession) sequences such as TrueFISP/FIESTA produce contrast that is proportional to which tissue property ratio?
- T1/T2
- T2/T1 (Correct answer)
- TE/T2*
- TR/T1
Correct answer: T2/T1
SSFP signal is proportional to T2/T1 ratio; tissues with high T2/T1 (like blood and fluids) appear very bright, making SSFP ideal for cardiac cine and vascular imaging.
Question 4: Which sequence parameter directly controls the slice thickness in a 2D spin-echo acquisition?
- Bandwidth of the readout gradient
- Bandwidth of the RF pulse divided by the slice-select gradient amplitude (Correct answer)
- TE duration
- TR duration
Correct answer: Bandwidth of the RF pulse divided by the slice-select gradient amplitude
Slice thickness = RF pulse bandwidth ÷ slice-select gradient amplitude; thicker slices can be produced by widening the RF bandwidth or reducing the gradient strength.
Question 5: What is the primary advantage of parallel imaging techniques (e.g., GRAPPA, SENSE) in MRI?
- They increase SNR at the same scan time
- They reduce scan time or allow higher spatial resolution by using multiple coil elements to under-sample k-space (Correct answer)
- They eliminate the need for gradient coils
- They improve T1 contrast
Correct answer: They reduce scan time or allow higher spatial resolution by using multiple coil elements to under-sample k-space
Parallel imaging exploits spatial sensitivity differences among phased-array coil elements to reconstruct fully sampled images from under-sampled k-space, reducing scan time by the acceleration factor (R).
Question 6: MR spectroscopy (MRS) of the brain measures metabolite concentrations. Which metabolite, reduced in neuronal loss and gliosis, is considered a marker of neuronal integrity?
- Choline (Cho)
- Creatine (Cr)
- N-acetylaspartate (NAA) (Correct answer)
- Lactate (Lac)
Correct answer: N-acetylaspartate (NAA)
NAA (N-acetylaspartate) is located predominantly in neurons; its reduction indicates neuronal loss or dysfunction, making it a key spectroscopic marker of neuronal integrity.
MRI MRI Pulse Sequences 2
Inversion recovery sequences null a specific tissue when the readout pulse is applied at time TI equal to: