How it works

Magnetic resonance imaging

A superconducting magnet, three gradient coils and an RF chain that turn hydrogen nuclei into a spatially encoded radio signal, then into images.

Course: MRI Service — Level 1

Interactive. Hover to open the magnet; stop the compressor and watch the helium level. Open full screen ↗

Principle

A superconducting coil held in liquid helium at about 4.2 K produces a strong, permanent static field (B0, typically 1.5 T or 3 T) that aligns the spins of hydrogen nuclei and makes them precess at the Larmor frequency, roughly 42.58 MHz per tesla (about 64 MHz at 1.5 T, 128 MHz at 3 T). An RF transmit pulse at that frequency tips the magnetisation; as it relaxes, it induces a weak signal in the receive coils. Three gradient coils (X, Y, Z) superimpose small, rapidly switched field slopes on B0 so that frequency and phase of the signal depend on position; the reconstructor then turns this frequency/phase-encoded data (k-space) into images with a Fourier transform. For the engineer the machine is four systems sharing one room: the magnet and its cryogenics, the gradient chain, the RF chain inside a Faraday cage, and the computing chain. The field is always on, even with the building power off.

The chain, block by block

Block	What it does	How it fails
Magnet and cryostat (B0)	Superconducting coil in liquid helium inside a vacuum-insulated cryostat with thermal shields; provides the static field and, with passive and active shims, its homogeneity.	Homogeneity drift (geometric distortion, poor fat suppression) often has an external cause such as new steel near the magnet. Loss of vacuum or cooling shows as rising helium boil-off and, in the worst case, a quench.
Cryocooler: cold head, helium compressor, adsorber	Cold head re-condenses evaporating helium so a healthy modern magnet is near zero boil-off; compressor in the technical room supplies it with pressurised helium gas; adsorber removes oil from the circuit.	Falling helium level is a symptom, not consumption: compressor stopped (power, water cooling, overheating), contaminated adsorber, worn cold head. Signs: changed cold-head rhythm, frost or condensation on the head or lines, compressor pressures off nominal.
Gradient amplifiers	High-power switching amplifiers, one per axis, that drive the gradient coils with fast, precise current waveforms.	Over-temperature and current-limit errors, scans aborting on demanding sequences, one-axis faults giving distortion or ghosting along that axis.
Gradient coil and its cooling	Water-cooled X/Y/Z coils cast into a cylinder inside the bore; eddy-current compensation corrects for currents induced in surrounding metal.	Overheating from poor water flow or a failing chiller is one of the commonest causes of aborted exams. Changed acoustic noise (new rattle or buzz) points to loose covers or mounts. Eddy-current calibration drift causes distortion and artefacts.
RF transmitter and body coil	RF amplifier drives the body coil built into the bore to excite the spins; SAR monitoring limits RF power to the patient.	No signal or uniformly low signal over the whole volume, transmit/reflected-power errors, SAR limit trips. Relatively rare compared with coil and cage faults.
Receive coils	Surface and array coils pick up the MR signal; on many current platforms they contain preamplifiers and even digitisation, so they are active electronic devices, not passive loops.	Local signal drop-out in one element, uneven brightness, raised noise in one region, coil not recognised. Connectors, latches, cables and drops cause most failures; physical inspection matters as much as the screen.
Faraday cage (RF room shield)	Continuous shield in walls, floor and ceiling, door finger-stock contacts, shielded window, waveguides and filtered penetrations keep external RF out.	External RF leaking in gives a 'zipper' line across the image at a fixed frequency position. Typical causes: door contacts worn or door not latched, an electronic device carried into the room, a cable run through the wall bypassing the filter panel, damage during building works.
Spectrometer / receiver and reconstructor	Generates sequence timing and RF waveforms, digitises received data (if not done in the coil), reconstructs images and stores them.	Reconstruction errors, hangs, full disks, failing fans or drives; the system may scan but return no or corrupted images.
Host computer, network and PACS	Operator console, database, archive and DICOM transfer to PACS and worklist.	Full local archive, DICOM node changes, network or licence issues stopping work even though the hardware is healthy.

Block

What it does

How it fails

Magnet and cryostat (B0)

Superconducting coil in liquid helium inside a vacuum-insulated cryostat with thermal shields; provides the static field and, with passive and active shims, its homogeneity.

Homogeneity drift (geometric distortion, poor fat suppression) often has an external cause such as new steel near the magnet. Loss of vacuum or cooling shows as rising helium boil-off and, in the worst case, a quench.

Cryocooler: cold head, helium compressor, adsorber

Cold head re-condenses evaporating helium so a healthy modern magnet is near zero boil-off; compressor in the technical room supplies it with pressurised helium gas; adsorber removes oil from the circuit.

Falling helium level is a symptom, not consumption: compressor stopped (power, water cooling, overheating), contaminated adsorber, worn cold head. Signs: changed cold-head rhythm, frost or condensation on the head or lines, compressor pressures off nominal.

Gradient amplifiers

High-power switching amplifiers, one per axis, that drive the gradient coils with fast, precise current waveforms.

Over-temperature and current-limit errors, scans aborting on demanding sequences, one-axis faults giving distortion or ghosting along that axis.

Gradient coil and its cooling

Water-cooled X/Y/Z coils cast into a cylinder inside the bore; eddy-current compensation corrects for currents induced in surrounding metal.

Overheating from poor water flow or a failing chiller is one of the commonest causes of aborted exams. Changed acoustic noise (new rattle or buzz) points to loose covers or mounts. Eddy-current calibration drift causes distortion and artefacts.

RF transmitter and body coil

RF amplifier drives the body coil built into the bore to excite the spins; SAR monitoring limits RF power to the patient.

No signal or uniformly low signal over the whole volume, transmit/reflected-power errors, SAR limit trips. Relatively rare compared with coil and cage faults.

Receive coils

Surface and array coils pick up the MR signal; on many current platforms they contain preamplifiers and even digitisation, so they are active electronic devices, not passive loops.

Local signal drop-out in one element, uneven brightness, raised noise in one region, coil not recognised. Connectors, latches, cables and drops cause most failures; physical inspection matters as much as the screen.

Faraday cage (RF room shield)

Continuous shield in walls, floor and ceiling, door finger-stock contacts, shielded window, waveguides and filtered penetrations keep external RF out.

External RF leaking in gives a 'zipper' line across the image at a fixed frequency position. Typical causes: door contacts worn or door not latched, an electronic device carried into the room, a cable run through the wall bypassing the filter panel, damage during building works.

Spectrometer / receiver and reconstructor

Generates sequence timing and RF waveforms, digitises received data (if not done in the coil), reconstructs images and stores them.

Reconstruction errors, hangs, full disks, failing fans or drives; the system may scan but return no or corrupted images.

Host computer, network and PACS

Operator console, database, archive and DICOM transfer to PACS and worklist.

Full local archive, DICOM node changes, network or licence issues stopping work even though the hardware is healthy.

Symptom → where to look first

Symptom	Suspect first
Helium level steadily falling	Cryocooler first: compressor running and cooled? water flow and temperature, adsorber hours, cold-head sound and frost. Do not just top up; find why helium stopped re-condensing.
Bright line or 'zipper' across images at a fixed position	Faraday cage first: door closed and finger-stock contacts intact, any electronic device in the room, cables bypassing the filter panel. Then a noisy component inside the room.
Local signal loss or uneven brightness in one area	Receive coil: wrong or damaged element, connector or cable. Swap to another coil of the same type; if the problem moves with the coil, it is the coil.
Uniform loss of signal on all coils	Transmit chain: RF amplifier, body coil, transmit/receive switching; also check centre frequency and RF calibration (prescan) results.
Exams abort with gradient over-temperature or current errors	Cooling first: chiller, water flow and temperature to the gradient coil and amplifiers, technical-room temperature. Only then the amplifier itself.
Geometric distortion or poor fat suppression appearing over time	B0 homogeneity: new ferromagnetic objects or building changes near the magnet, then shim. Also eddy-current compensation for gradient-related distortion.
New knocking, rattling or buzzing noise during scans	Mechanical: loose covers, panels, coil mounts or objects in the bore; a change in noise character is a symptom even though MRI is normally loud.
System scans but images do not arrive or reconstruction hangs	Reconstructor and host: disk space, archive, fans, network and DICOM configuration before suspecting acquisition hardware.
Cold head cycling sound changed or stopped	Cryocooler: compressor fault, power, water cooling, helium circuit pressure; treat as urgent because boil-off starts as soon as cooling stops.

Symptom

Suspect first

Helium level steadily falling

Cryocooler first: compressor running and cooled? water flow and temperature, adsorber hours, cold-head sound and frost. Do not just top up; find why helium stopped re-condensing.

Bright line or 'zipper' across images at a fixed position

Faraday cage first: door closed and finger-stock contacts intact, any electronic device in the room, cables bypassing the filter panel. Then a noisy component inside the room.

Local signal loss or uneven brightness in one area

Receive coil: wrong or damaged element, connector or cable. Swap to another coil of the same type; if the problem moves with the coil, it is the coil.

Uniform loss of signal on all coils

Transmit chain: RF amplifier, body coil, transmit/receive switching; also check centre frequency and RF calibration (prescan) results.

Exams abort with gradient over-temperature or current errors

Cooling first: chiller, water flow and temperature to the gradient coil and amplifiers, technical-room temperature. Only then the amplifier itself.

Geometric distortion or poor fat suppression appearing over time

B0 homogeneity: new ferromagnetic objects or building changes near the magnet, then shim. Also eddy-current compensation for gradient-related distortion.

New knocking, rattling or buzzing noise during scans

Mechanical: loose covers, panels, coil mounts or objects in the bore; a change in noise character is a symptom even though MRI is normally loud.

System scans but images do not arrive or reconstruction hangs

Reconstructor and host: disk space, archive, fans, network and DICOM configuration before suspecting acquisition hardware.

Cold head cycling sound changed or stopped

Cryocooler: compressor fault, power, water cooling, helium circuit pressure; treat as urgent because boil-off starts as soon as cooling stops.

Safety

Site and infrastructure

Electrical supply: dedicated feed with correct voltage and phases measured under load, protective earth, and the site's emergency power-off; the cryocooler compressor should be on a supply that keeps running or restarts reliably, because a stopped compressor starts helium boil-off.
Chilled water: capacity, supply temperature, flow and pressure to the gradient/electronics cooling and the helium compressor, as specified by the manufacturer; check for leaks, filter condition and alarm function.
Room climate: temperature and humidity in the magnet room and technical room within manufacturer limits, year-round, including summer peaks.
RF shielding integrity: door contacts, window, filter panel and waveguides intact; no unauthorised penetrations.
Quench (vent) pipe: continuous path to the outside, supports intact, outlet unobstructed and protected, discharge area away from people and air intakes.
Oxygen monitor in the magnet room working and within its check interval; room ventilation and emergency exhaust functioning.
Magnetic field zoning: controlled-access zones and the 0.5 mT (5 gauss) line marked; no new ferromagnetic structures or moving steel (lifts, vehicles, cabinets) close to the magnet.
Floor and vibration: floor loading rated for the magnet; no new sources of vibration nearby.

Preventive maintenance basics

Record helium level, cryostat pressure, compressor pressures, cold-head and adsorber hours, technical-room temperature at every visit and keep a trend; a single reading says little, the slope says everything.
Inspect the cold head and lines: listen to the cycling sound, look for frost or condensation, check compressor water flow and temperatures.
Check gradient and electronics cooling: flow, pressure, temperature, filters, leaks, chiller alarms.
Test every receive coil: recognised by the system, run a test scan per coil, inspect connectors, latches, cables and housings.
Inspect the RF door contacts and shield penetrations; check for zipper artefacts on a test scan.
Run the manufacturer's periodic image-quality phantom test (SNR, uniformity, geometry, ghosting) and compare with baseline.
Review error and magnet logs; check disk space, archive and PACS transfer.
Verify safety items: oxygen monitor, quench pipe condition, 0.5 mT line marking, emergency buttons accessible and guarded, signage.

Safety

The magnetic field is always on. Before entering Zone IV remove every ferromagnetic object from your pockets and body, every time; bring in only tools and test equipment verified as MR-safe or MR-conditional.
Any ferromagnetic object (oxygen cylinder, ladder, trolley, tool) becomes a projectile near the bore. Never let unchecked equipment through the door, including from contractors.
Persons with pacemakers, neurostimulators or other active implants must not cross the 0.5 mT line without screening; the engineer is responsible for anyone they bring.
Never operate the magnet emergency rundown (quench) button except to free a trapped person in a life-threatening situation; never open cryostat valves, vacuum ports or disconnect pressurised helium lines without the manufacturer's procedure and training.
In a quench (loud venting, white cloud, falling oxygen) evacuate everyone immediately and do not re-enter until oxygen level is confirmed normal; cold helium gas displaces oxygen and can cause asphyxiation and cold burns. The door may be hard to open due to room over-pressure.
Gradient and RF amplifiers carry hazardous voltages and stored energy; apply lockout and wait for discharge before opening cabinets.

Manufacturers

Siemens Healthineers · GE HealthCare · Philips · Canon Medical Systems · United Imaging · Fujifilm (former Hitachi MRI) · Neusoft Medical · Esaote (low-field / dedicated)