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. The longest-lived single piece of capital equipment in most departments — magnets routinely outlast the host computers, gradient amplifiers, and clinical software they shipped with.

Physics

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

Pulse sequences — T1, T2, FLAIR, DWI, EPI, gradient-echo, MR angiography, MR spectroscopy — are different timing patterns of RF and gradients that emphasize different tissue properties.

History

1973 — Paul Lauterbur publishes the first MRI image (a water-filled test tube).
1977 — first human MRI image.
1980s — commercial clinical MRI deployed.
1984 — first 1.5T clinical system.
2002 — clinical 3T deployed broadly.
2017 — first FDA-approved 7T clinical MRI (research tool since 1999).
2020s — sealed-bore / zero-boil-off magnets reach broad commercial deployment (BlueSeal on Philips Ambition, equivalents on Siemens / GE); deep-learning reconstruction (Deep Resolve, AIR Recon DL, Compressed SENSE) reshapes throughput economics.

Key specs buyers evaluate

Field strength — 0.55T (low-field re-emergence), 1.5T (workhorse), 3T (premium), 5T / 7T (research / specialty).
Bore size — 60 cm (legacy), 70 cm (wide-bore, modern standard).
Gradient strength and slew rate — define advanced-application capability.
RF channels — parallel-imaging ceiling.
Coil platform — TIM 4G (Siemens), dStream / dS coils (Philips), AIR Coils (GE), BioMatrix (Siemens).
Cryogen posture — conventional helium refill, low-helium, or sealed / zero-boil-off.
Acceleration license tier — Compressed SENSE (Philips), Deep Resolve (Siemens), AIR Recon DL (GE).

Systems

Siemens — MAGNETOM Symphony (legacy 1.5T), Avanto, Aera (1.5T wide-bore), Skyra / Prisma / Vida (3T), Free.Max (0.55T)
GE — Signa HDxt / Voyager / Pioneer / Premier
Philips — Achieva / Ingenia 1.5T / Ingenia 3T / Ingenia Elition
Canon — Vantage Galan / Orian
Hitachi (now Fujifilm) — Echelon, Oasis, Airis

Clinical applications

Routine Brain MRI
Breast MRI
Stroke Thrombectomy (workup)
Cardiac MRI, prostate multiparametric MRI, MR enterography, MSK, MR spectroscopy, fMRI, DTI

Service and refurb reality

Cost of ownership dominated by cryogen + service contract, not capital. Helium refill costs are non-trivial and track market volatility.
Cold-head service interval typically every 3 years; warm rooms / 24-7 scanning push that to 2.
Quench risk drives PM discipline; a quench is an expensive day plus potential magnet damage.
Coil portfolio is the system. A bare magnet without proper coil inventory is a gutted scanner — coils carry meaningful standalone value.
Acceleration license tier is the modern refurb price-determining variable. A 3T without Compressed SENSE / Deep Resolve runs at older platform speeds regardless of chassis age.
Host-computer obsolescence drives most retirements — the magnet outlasts the OS. See MAGNETOM Symphony Field Guide and Ingenia Field Guide.

Regulatory and safety

ACR MRI accreditation.
MRI safety program (FDA + Joint Commission frameworks).
Zone I–IV access control, 5-gauss-line management, ferromagnetic screening.
Cryogen safety — quench-vent stack patency, oxygen-deficiency monitoring.
Implant compatibility screening — pacemakers, ferromagnetic foreign bodies, cochlear implants.