Relaxation, Contrast Mechanisms, and Pulse Sequence Timing
Core concepts
Contrast is engineered by timing. After excitation, M_xy decays with T₂ (and T₂*), while M_z recovers with T₁. The TR (repetition time) governs T₁ weighting; the TE (echo time) governs T₂ weighting. Short TR + short TE → T₁-weighted; long TR + long TE → T₂-weighted; long TR + short TE → proton-density. Inversion recovery adds TI to null a tissue (e.g., STIR nulls fat; FLAIR nulls CSF).
Signal equations and traps
Spin-echo signal ∝ ρ·(1−e^(−TR/T₁))·e^(−TE/T₂). Gradient-echo adds a flip-angle term: sinα·(1−e^(−TR/T₁))/(1−cosα·e^(−TR/T₁)), and uses T₂* decay. Common traps: assuming a long TE increases T₁ contrast (it does not); forgetting that a 180° refocusing pulse removes static dephasing but not T₂ decay; misapplying the Ernst angle. The Ernst angle α_E = arccos(e^(−TR/T₁)) maximizes GRE signal for a given TR.
Worked example
For T₁ = 1000 ms, TR = 500 ms: e^(−0.5) ≈ 0.607, so α_E = arccos(0.607) ≈ 52.6°. Using 90° here wastes signal and adds SAR.
Practical notes
Fat is bright on T₁ and T₂ but suppressed by fat saturation; fluid is dark on T₁ and bright on T₂. Understanding these patterns resolves most contrast questions.