Fast, Parallel, and Advanced Acquisition Techniques
Core concepts
Fast imaging trades time for SNR, resolution, or artifacts. Fast spin echo (FSE/TSE) uses multiple 180° refocusing pulses to fill multiple k-space lines per TR; echo train length (ETL) shortens scan time but blurs and adds T₂ weighting. EPI acquires many lines after one excitation, enabling fMRI and diffusion but prone to distortion and ghosting. Gradient echo sequences (SPGR, FLASH) use short TR and low flip angles for 3D imaging.
Parallel imaging
Techniques such as SENSE and GRAPPA use coil sensitivity to skip phase-encode lines, reducing scan time by the acceleration factor R at an SNR cost of roughly √R (geometry-dependent).
Traps
- Assuming acceleration is free; g-factor noise amplification grows with R.
- Confusing ETL with number of excitations.
- Forgetting EPI's long readout causes susceptibility distortion.
Rule of thumb
Scan time reduction by factor R costs at least √R SNR; beyond R ≈ 2–3, artifacts dominate for most arrays.