MRI Artifacts, Scan Acceleration & Gradient‐Echo Techniques – Lecture Notes

Lab Session Logistics

• Groups for next week’s lab have been posted on Canvas
  • One student already requested a swap (Thursday → Wednesday).
  • Anyone able to swap in the opposite direction should see the instructor after class.
• Goal: keep daily group sizes balanced (avoid 10 on one day and 2 on another).
• If you have any other availability issues, confirm immediately so final timetable can be locked in.

Metal & Susceptibility Artifacts

• Images shown: dramatic signal voids/distortions when metal is present but MRI-safe (e.g. dental implants, cochlear implant, aneurysm clips).
• Physics
  • Metal has high magnetic susceptibility → becomes magnetised → perturbs $B_0$ → field becomes inhomogeneous.
  • Results:
  • $T_2^*$ shortening → rapid dephasing → signal loss (black voids).
  • Off-resonance frequency shifts → displaced or piled-up signal (bright/ghost lines).
• Sequence dependence
  • $T_1$ spin-echo: least affected (good image on demo).
  • $T_2$-FLAIR (inversion recovery): moderate artifact; CSF suppression fails near field distortion.
  • Gradient-echo (GRE): worst—large voids because relies on $T_2^*$.
  • Diffusion EPI: severe distortions/voids.
• Mitigation hierarchy
  1. Remove metal if clinically possible.
  2. Choose sequences with refocusing (spin-echo, fast spin-echo).
  3. Advanced metal-artifact–reduction sequences (beyond scope today).

Motion Artifacts

Respiratory & Bulk Motion

• Ghosting/streaking repeating in phase-encode direction.
  • Demonstrated as parallel replicas of anatomy.
• Counter-measures
  1. Saturation band: pre-pulse nulls anterior abdominal wall → moving tissue contributes no signal.
  2. Gating/triggering: use ECG or respiratory bellows to acquire at same chest position.
  3. Breath-hold acquisitions if short enough.
  4. Reduce scan time (topics below).

Flow-Related (Pulsatile) Motion

• Example: oval streaks from descending aorta.
• Caused by phase changes from flowing spins during phase-encoding steps.
• Fix: Cardiac gating (ECG) to acquire same phase of cardiac cycle.

Scan-Time Reduction via k-Space Manipulation

• Rectangular FOV: skip every second phase line → fewer phase steps; risk of aliasing.
• Zero-filling / zero-padding (truncate outer k-space): keeps contrast (central k-space) but blurs fine detail.
• Partial Fourier (e.g. 3⁄4 acquisition): exploit complex conjugate symmetry; some SNR/phase penalty.

Non-Cartesian & PROPELLER Sampling

• Spiral k-space: ultra-short TE; limited role for breathing but useful in spiral fMRI, MRA.
• PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction)
  • Acquire central “blade” (∼20 % of k-space) repeatedly while rotating blade angle.
  • Oversamples centre → robust to in-plane motion; corrupted blades can be discarded.
  • Great for head scans & free-breathing abdomens.

Turbo Spin Echo (Fast Spin Echo / RARE)

• Concept
  1. One $90^\circ$ excitation.
  2. Train of $180^\circ$ refocusing pulses → multiple echoes.
  3. Each echo uses different phase-encode gradient → fills several k-space lines per TR.
• Key terms
  • Echo Train Length (ETL) or Turbo Factor =\text{# echoes/TR}.
  • ETL = 8 ⇒ scan 8× faster.
• k-Space ordering
  • Linear ordering: late echoes fill centre → heavy $T_2$ weighting.
  • Centric ordering: early echoes fill centre → higher SNR, less $T_2$ blurring.
• Trade-offs
  • Longer ETL → more blurring/$T_2$ decay but faster.
  • High RF power deposition (SAR) from many 180° pulses.

Interleaved Multi-Slice Imaging

• While slice 1 recovers longitudinal $M_z$, excite slice 2, 3, …
• Can fit 20–30 slices within a single long TR.
• Avoid slice-crosstalk by
  • Leaving small gap (≈10 %)
  • Spatial interleaving (1-3-5…, then 2-4-6…)

Gradient Echo (GRE) Imaging

Basic Sequence

RF (small flip θ) → dephase gradient → rephase (opposite polarity) → echo readout

• No 180° pulse; refocusing done by gradient reversal.

Characteristics

• Signal governed by $T<em>2^*$, not $T</em>2$.
• Advantages
  • Very short TE & TR (e.g. TE 5 ms, TR 15 ms).
  • Low SAR (small flip angles, no 180°).
  • Enables rapid 3-D imaging, dynamic studies, low power in implants.
• Disadvantages
  • Highly sensitive to field inhomogeneity → metal, air/tissue interfaces.

Flip-Angle Dependence & Ernst Angle

• Optimal flip (max SNR) $\theta<em>E=\arccos\big(e^{-TR/T</em>1}\big)$.
• Example graphs:
  • TR = 15 ms → $\theta<em>E\approx 10^\circ$ (given brain $T</em>1\approx 1\,\text{s}$).
  • TR = 500 ms → $\theta_E\approx 45^\circ$.
• Contrast now governed by TR + flip angle + TE trio.

Steady-State Free Precession (SSFP) Family

Concept

• With ultra-short TR, residual transverse magnetisation persists into next RF pulse → chain of stimulated echoes.
• After several pulses a steady state is reached (RF energy in = energy lost).

Two Main Flavours

1. Unbalanced / Spoiled / Incoherent SSFP

   • Apply strong spoiler gradient to destroy residual transverse M.
   • Sequence names:
     • Siemens: FLASH / SPGR
     • GE: SPGR
     • Philips: FFE
   • Weighting: can sample
     • FID immediately → $T_1$-weighted
     • Stimulated echo later → $T_2$-weighted

2. Balanced SSFP (bSSFP, TrueFISP, FIESTA, b-FFE)

   • Re-phase all gradients each TR (area = 0) → preserve transverse M.
   • Signal $\propto \tfrac{T<em>2}{T</em>1}$ ⇒ very bright fluids, blood.
   • High SNR, excellent blood/myocardium contrast (cardiac cine gold-standard).
   • Susceptible to off-resonance → dark banding; careful shimming or frequency offsets needed.

Echo-Planar Imaging (EPI)

• Single small flip RF → long oscillating readout gradient; tiny “blips” in phase encode produce zig-zag through k-space.
• Whole 2-D image in one shot (≲100 ms) or a few shots.
• Spin-echo EPI exists but longer.
• Artifacts
  • Geometric distortion along phase direction.
  • $T_2^*$ blurring (long echo train).
• Remedies
  • Parallel imaging (GRAPPA/SENSE) reduces phase lines.
  • Lower field strength.
  • Smaller matrix/FOV.
  • Maximum gradient slew rates.
• Key applications
  • BOLD fMRI, diffusion-weighted imaging (DWI/DTI), dynamic perfusion, rapid breath-hold body scans.

Practical Trade-Off Matrix

Numerical / Formula Summary

• Echo Train Length speed-up: $t<em>{scan}^{\text{TSE}} \approx \frac{N</em>{phase}}{\text{ETL}} \times TR$
• Ernst angle (max SNR GRE): $\theta<em>E = \arccos\big(e^{-TR/T</em>1}\big)$
• SAR ↓ with flip angle: $SAR \propto \theta^2$ (approx., no 180° pulses in GRE).
• Balanced condition for bSSFP gradients: $\sum<em>{TR} G</em>x = \sum<em>{TR} G</em>y = \sum<em>{TR} G</em>z = 0$.

Links to Upcoming Lectures

• Gradient-Echo EPI underpins:
  • $BOLD$ functional MRI (Week 6)
  • Diffusion Tensor Imaging (Week 7)
  • Dynamic susceptibility contrast perfusion (Week 8)
• Balanced SSFP forms basis of advanced cardiac cine and phase-contrast flow quantification.
• Turbo Spin Echo variants feed into 3-D $T_2$ mapping and fast musculoskeletal protocols.

Take-Home Messages

• Artifact control (metal, motion) begins with sequence choice.
• Scan acceleration combines k-space tricks, multi-echo trains, slice interleaving, and ultra-fast readouts.
• Gradient Echo family provides versatility: low SAR, rapid 3-D, special contrasts—but mind $T_2^*$ pitfalls.
• Steady-state and balanced designs exploit residual transverse M to boost SNR/contrast.
• Echo-Planar Imaging sacrifices geometric fidelity for raw speed—indispensable for fMRI & diffusion.