MRI (Magnetic Resonance Imaging)

Uses strong magnetic fields and radio waves to image soft tissue without ionizing radiation. Superconducting magnet cooled by liquid helium produces the main field; gradient coils encode spatial information; RF coils transmit and receive.

Physics

1. Static main field (1.5 or 3 T clinical) aligns hydrogen nuclei.
2. RF pulse tips nuclei away from alignment.
3. As nuclei return to alignment, they emit RF that receiver coils detect.
4. Gradient fields encode spatial position by varying the main field.
5. Fourier transform of the signal reconstructs the image.

Pulse sequences (T1, T2, FLAIR, DWI, EPI) are different timing patterns of RF and gradients that emphasize different tissue properties.

History

• 1973 — Paul Lauterbur publishes first MRI image (a water-filled test tube).
• 1977 — first human MRI image.
• 1980s — commercial clinical MRI deployed.
• 1984 — first 1.5 T clinical system.
• 2002 — clinical 3 T deployed broadly.
• 2017 — first FDA-approved 7 T clinical MRI (research tool since 1999).

Key specs buyers evaluate

• Field strength — 0.3, 1.5, 3, 7 T
• Bore size — 60 cm (standard), 70 cm (wide-bore)
• Gradient strength + slew rate
• RF channels — parallel-imaging ceiling
• Coil platform — TIM, HD, dStream
• Cryogen — helium refill vs sealed

Systems

• Siemens Symphony (legacy 1.5 T), Avanto, Aera (1.5 T wide-bore), Skyra, Prisma (3 T)
• GE Signa HDxt / Voyager / Pioneer
• Philips Achieva / Ingenia 1.5T / Ingenia 3T

Service reality

Unlike most imaging equipment, MRI has unique cost drivers: helium refills, cold head replacements, quench risk, RF amplifier aging, gradient cooling loops. Cost of ownership is dominated by cryogen + service contract, not capital.

Regulatory

ACR MRI accreditation. MRI safety program (FDA + Joint Commission). Zone I-IV access control, 5-gauss line management, ferromagnetic screening.