T1- and T2-weighted imaging
T1-weighted imaging emphasizes longitudinal recovery differences, while T2-weighted imaging emphasizes transverse decay differences. TR and TE are central to classic spin-echo weighting, but modern sequences add refocusing trains, variable flip angles, acceleration and reconstruction choices that change the practical result.
FLAIR and inversion recovery
FLAIR uses inversion recovery timing to suppress fluid signal while preserving T2-sensitive information. Inversion time is selected so the target tissue crosses near zero longitudinal magnetization at the excitation point. The same nulling principle appears in other inversion-recovery techniques.
STIR versus Dixon fat suppression
STIR uses inversion recovery to suppress fat based on its T1 behavior and is relatively robust to some field inhomogeneity, but it is not chemically selective. Dixon methods exploit the phase relationship between fat and water to separate their signals and can produce water-only and fat-only images. Each approach has different tradeoffs and applications.
DWI and ADC
Diffusion-weighted imaging sensitizes signal to microscopic water motion using diffusion gradients. ADC maps help separate diffusion-related signal behavior from T2 shine-through effects. Diffusion concepts extend into DTI, where directional information is modeled to characterize anisotropic diffusion.
Gradient echo and susceptibility
Gradient-echo families can be fast and flexible but do not use a 180-degree refocusing pulse in the same way as spin echo, making them more sensitive to field inhomogeneity and susceptibility effects. That sensitivity can be useful or problematic depending on the imaging goal.