12. MRI Physics: T1 and T2

The reasoning

1. Hydrogen protons (most abundant nucleus, large magnetic moment) align in the static field B₀ → net magnetisation vector (M) along B₀ (longitudinal).
2. Protons precess at the Larmor frequency: ω = γB₀ (42.58 MHz/T for hydrogen; ~64 MHz at 1.5 T).
3. A 90° RF pulse at the Larmor frequency tips M into the transverse plane and brings spins into phase.
4. After the pulse, relaxation produces the signal received by coils; gradients localise it (slice selection, phase and frequency encoding → k‑space → Fourier transform).