MRI Scanner

Overview

A magnetic resonance imaging (MRI) scanner makes cross-sectional images of the body without ionising radiation. It works by immersing the patient in an intense, very uniform magnetic field, nudging the hydrogen nuclei in their tissue with radio waves, and listening to the faint radio signal those nuclei give off as they relax. Because the signal depends on how hydrogen is bound in water and fat, MRI gives soft-tissue contrast that X-ray CT cannot match, which is why it dominates neurological, musculoskeletal, and many oncological exams.

The machine is built around four cooperating systems: a Superconducting Magnet that supplies the static field, a Gradient Coil Assembly set that encodes position, an RF System that excites and receives the signal, and a Reconstruction System chain that turns raw data into images. A Patient Table, Gantry Housing, Cooling System loop, Helium Compressor, Operator Console, and RF Shield Room Interface support them.

The superconducting magnet

Image quality scales with field strength and uniformity, so clinical scanners use a Superconducting Magnet running at 1.5 or 3 tesla — tens of thousands of times the Earth's field. Such a field can only be held economically by a superconductor, so the NbTi Superconducting Coils are wound from niobium-titanium filaments embedded in copper and cooled below their critical temperature. They sit in a Cryostat / Vacuum Vessel: a Liquid Helium Vessel holding around 1,500 litres of liquid helium at roughly 4 kelvin, wrapped in Thermal Radiation Shields inside an evacuated Outer Vacuum Vessel so almost no heat reaches the cold mass. Once energised the current flows in a closed persistent loop with effectively no resistance, so the field stays on for years without a power supply — the magnet is never really "off".

Field uniformity matters as much as strength; even parts-per-million variation blurs the image. The main coils are trimmed by Superconducting Shim Coils wound on the same Coil Former, and further corrected by passive shims and the gradients. A Cold Head / Cryocooler cryocooler re-condenses helium that boils off so the cryogen is not consumed, and if the windings ever lose superconductivity — a quench — the stored energy dumps into the helium, which flash-boils and vents safely outdoors through the Quench Pipe.

Gradient coils: encoding position

A uniform field alone tells you nothing about where a signal came from. The Gradient Coil Assembly assembly solves this by superimposing small, controllable field gradients along three axes. The X Gradient Coil and Y Gradient Coil coils are saddle-shaped windings producing transverse gradients, while the Z Gradient Coil coil is a Maxwell pair producing an axial gradient. Switching these on and off makes a proton's resonant frequency and phase depend on its position, which is how a slice is selected and an image is spatially encoded.

The gradients must switch in well under a millisecond and carry hundreds of amps, so they are driven by a Gradient Amplifier — a three-channel switching amplifier whose per-axis Gradient Power Stages use IGBT and MOSFET H-bridges across large DC Link Capacitor banks, sequenced by the Gradient Control Board. This rapid switching against the main field is what produces MRI's characteristic loud knocking, and it dumps heat, so the Coolant Water Lines run coolant directly through the potted coil.

The RF system

The RF System both excites the protons and detects their reply. A RF Power Amplifier drives the RF Body / Transmit Coil — a birdcage transmit coil built into the bore with non-magnetic tuning capacitors — to send a precise radio pulse at the proton resonance frequency (63.9 MHz at 1.5 T). That pulse tips the nuclear magnetisation; as it relaxes, the nuclei re-emit an extremely weak radio signal.

Because the returning signal is so faint, it is captured close to the body by a Receive Coil Array, a phased array of Receive Coil Elements each with its own low-noise preamplifier, which also enables parallel imaging to speed up scans. A T/R Switch built from PIN diodes isolates the sensitive receivers from the kilowatt transmit pulse so the preamplifiers survive.

Reconstruction, console, and siting

Received signals are digitised and demodulated by the Spectrometer Board in the Reconstruction System chain, which also synthesises the transmit waveforms. The raw data fills a frequency-domain matrix known as k-space; the Image Reconstruction Engine, accelerated by a GPU Accelerator Module, runs a Fourier transform and modern reconstruction to produce images, which the Host Computer and the Operator Console present to the operator. An Intercom Control Panel keeps two-way contact with the patient in the bore.

Mechanically, a non-magnetic Patient Table driven by a Table Drive Motor glides the patient into the Bore Liner Tube inside the Gantry Housing. A Cooling System chiller and the helium Helium Compressor reject heat from the gradients, RF amplifier, and cold head. Finally, because the scanner both emits and listens at radio frequencies, it must sit in a shielded cabin: the RF Shield Room Interface carries power and signals through a Penetration Panel fitted with RF Line Filters so outside radio noise cannot corrupt the image and the scanner's own pulses cannot escape.