Magnetic Resonance: Physics, Sequences, and Safety
Core concepts
Magnetic resonance questions reward a clear model of spin behavior in a magnetic field. Understand B0 field strength and homogeneity, precession and the Larmor equation (frequency proportional to field strength times the gyromagnetic ratio), and the RF excitation that tips magnetization into the transverse plane. Know T1 (longitudinal recovery) and T2 (transverse decay) relaxation, plus T2* dephasing from field inhomogeneity. Be able to map TR and TE onto image weighting: short TR and short TE gives T1 weighting; long TR and long TE gives T2 weighting; long TR and short TE gives proton density. Study gradient echo versus spin echo, inversion recovery and fat suppression, and how parallel imaging and higher field strength trade signal for artifact and SAR.
Traps to expect
Examiners love the difference between T2 and T2*, between fat suppression techniques, and between SAR limits and static field limits. A frequent distractor suggests that increasing TE improves signal-to-noise; it actually increases T2 weighting while reducing SNR. Another trap confuses the safety zones and the projectile risk of ferromagnetic objects with RF heating of implants.
Worked example
A fluid-bright image is requested. Fluid has long T1 and long T2. To make fluid bright, use a long TR (removes T1 contrast) and a long TE (allows T2 decay differences to dominate), giving a T2-weighted spin-echo sequence. If the same fluid appears dark on a short-TR, short-TE image, that is expected T1 weighting, not a scanner fault. For a suspected hemorrhage or metal artifact, consider gradient echo and T2* effects.