MRI Magnetic Field: Superconducting Magnets, 1.5–3 Tesla, Proton Alignment, and Signal Generation

MRI scanners use a superconducting electromagnet (niobium-titanium coils cooled to 4 K with liquid helium) to generate a stable, homogeneous magnetic field.

Field Strength

• Clinical standard: 1.5 T (15,000 Gauss) and 3 T (30,000 Gauss).
• Earth's field: ~0.5 Gauss (0.00005 T) — MRI is 30,000–60,000× stronger.
• Research/Ultra-high field: 7 T, 10.5 T, 11.7 T (human).

Physics of Signal Generation

1. Equilibrium Magnetization (M₀): Protons (¹H nuclei) align parallel/anti-parallel to B₀; net longitudinal magnetization.
2. RF Excitation (B₁): 90° pulse tips M₀ into transverse plane.
3. Free Induction Decay (FID): Transverse magnetization precesses at Larmor frequency (ω₀ = γB₀), inducing signal in receiver coil.
4. Relaxation: T1 (longitudinal recovery), T2 (transverse decay) → tissue contrast.
5. Spatial Encoding: Gradient coils (slice select, phase encode, frequency encode) localize signal.

Without B₀, no net magnetization → no signal → no image. Field homogeneity (shimming) is critical for spectral fat suppression and diffusion imaging.