X, Y and Z gradient coil assemblies for MRI scanners · Gradient amplitude and slew rate, sensitivity and inductance against amplifier current and voltage, bore diameter, linearity and nerve stimulation, shielding, force balancing, water cooling and acoustic noise
An MRI gradient coil is the set of three coils, for the x, y and z axes, mounted inside the magnet bore that superimpose a controlled, linearly varying field on the static field so the scanner can select slices and encode position. Each axis is driven by its own gradient power amplifier. Ask three things first: what bore, field of view and body region the coil must serve, what peak amplitude and slew rate it must deliver with the available amplifier current and voltage, and how it will be shielded, cooled and kept quiet within nerve-stimulation limits.
Gradient coil assemblies (primary, shielding and shim layers with cooling and epoxy structure) for whole-body and head-only MRI systems, and their interface to gradient power amplifiers. Complete MRI scanners, RF coils, superconducting magnets and the amplifiers themselves are outside this page except where compared.
01
A Siemens Healthineers white paper describes gradient amplitude (mT/m) as set by coil current, G = ηI, where the coil sensitivity η is in the range of 0.05 mT/m per ampere, and slew rate (T/m/s) as set by amplifier voltage, SR = ηU/L, where L is coil inductance; coil resistance affects slew rate by only about 5%. Doubling winding density doubles amplitude and halves slew rate, leaving the product Gmax · SRmax unchanged; that product is proportional to amplifier power through the coil efficiency ε = η²/L. It cites whole-body amplifiers of 100 to 1,200 A and 500 to 2,250 V, gives 80 mT/m and 200 T/m/s as the highest whole-body 3 T figures at the time, and notes that efficiency falls with the fifth power of the coil's inner diameter, so a 70 cm patient bore (76 cm coil) needs about twice the amplifier power of a 60 cm bore (66 cm coil) for the same performance. [1]
The paper's example: a 45 mT/m gradient at 200 T/m/s reaches full amplitude in 225 µs, and it advises checking that maximum amplitude and slew rate can be used together in one pulse. GE HealthCare reports its head-only SIGNA MAGNUS system at Gmax 300 mT/m and SRmax 750 T/m/s from a 930 A, 2,150 V (2 MVA) amplifier, against 1 to 2 MVA per axis for 40 to 80 mT/m in conventional 60 to 70 cm bores. [1] [2]
Editorial calculation: using SR = G / Trise, 300 mT/m at 750 T/m/s gives a rise time of 0.3 / 750 s = 400 µs, and 930 A × 2,150 V ≈ 2.0 MVA, matching GE's stated amplifier rating; ask the supplier for η (mT/m/A) and L for each axis so you can check the claimed amplitude and slew rate against your amplifier. [2] [1]
Give the supplier the magnet warm bore, patient bore and RF body coil thickness, the field of view and body region, target Gmax and SRmax per axis, the amplifier's peak current and voltage, duty cycle and sequences to be run (diffusion, EPI), shim requirements, and limits on stray field, noise and weight.
02
The white paper describes a state-of-the-art coil as actively shielded: primary x, y and z coils produce the gradient, outer shielding coils reduce the stray field and the eddy currents it induces in the magnet, both layers are water cooled, the body is filled with epoxy for stiffness, and passive shim pockets take iron plates during installation. Lower linearity raises efficiency and lowers peripheral nerve stimulation (PNS) but shrinks the usable field of view; lower shielding raises efficiency but increases eddy-current artefacts; force compensation reduces vibration at some cost in efficiency. Heat rises with the square of current, so large bores have lower long-term performance at the same peak rating. Hollow water-cooled conductors cool static high currents well but Siemens says “hollow wires are mostly used for the z direction only” because they cannot follow the tight x and y fingerprint patterns and generate more eddy-current heat at high slew rates. [1]
GE notes that PNS limits whole-body systems to a usable slew rate of about 100 to 120 T/m/s, while its head-only coil has PNS thresholds four to five times higher, allowing the full 750 T/m/s for typical brain EPI; it also notes the separate concern of cardiac stimulation addressed in IEC 60601-2-33. [2]
A Frontiers in Physics review explains that Lorentz forces on switched conductors in the static field make coils vibrate and radiate sound; it reports echo-planar imaging at 110 to 120 dB against the NIOSH 85 dB limit, simulated a 20 mm ceramic bore lining reducing average SPL by 10.9 dB (about 30 dB with inner and outer layers), and modelled a head coil with hollow copper conductors carrying 16 °C water at 0.72 to 1.74 L/min, where a 6 mm solid third axis performed nearly as well as all-hollow cooling. [3]
The U.S. FDA's guidance on significant-risk criteria for magnetic resonance diagnostic devices (issued June 20, 2014) sets the regulatory lines a gradient coil's slew rate and noise are judged against in clinical investigations: any rate of change of gradient fields (dB/dt) "sufficient to produce severe discomfort or painful nerve stimulation", a peak unweighted sound pressure level above 140 dB, or an A-weighted rms sound pressure level above 99 dBA with hearing protection in place makes the operating condition significant risk. The same guidance sets static-field thresholds of 8 T for adults, children and infants older than 1 month and 4 T for neonates. [4] Editorial calculation: the 110–120 dB echo-planar levels in the review sit 20–30 dB below FDA's 140 dB peak line, but are unweighted measurements, so ask the supplier for A-weighted rms levels to compare with the 99 dBA criterion. [3] [4]
Ask for linearity over the field of view, shielding factor and eddy-current data, PNS thresholds or limits per axis, cooling water flow, temperature and pressure drop, conductor type per axis, force balancing, acoustic data, and the coil's mass and dimensions.
03
Geometries described in the sources.
| Geometry | Characteristics | Points to compare |
|---|---|---|
| Whole-body cylindrical coil | 60–70 cm patient bore; 40–80 mT/m at 1–2 MVA per axis. [2] [1] | Bore, linearity, long-term duty. |
| Head-only or insert coil | 200–300 mT/m at 1–3 MVA per axis; higher PNS thresholds. [2] | Bore taper, FOV, acoustic level. |
| Hollow-conductor axes | Water flows inside the conductor. [1] [3] | Which axes, flow and pressure. |
| Thin-wire axes with cooling layers | Tighter curvature, lower eddy heat. [1] | Cooling layer coupling. |
04
Layers named in the Siemens white paper and the review.
| Layer | Role | Fix in the order |
|---|---|---|
| Primary x, y, z coils | Create the gradient field. [1] | Sensitivity, inductance, conductor. |
| Shielding coils | Cancel stray field and eddy currents. [1] | Shielding factor. |
| Cooling layers or hollow conductors | Remove ohmic heat. [1] [3] | Water flow and temperature. |
| Epoxy body | Stiffness and stability. [1] [3] | Material and potting quality. |
| Shim pockets and high-order shim coils | Improve B0 homogeneity. [1] | Pocket count, shim channels. |
| Acoustic lining | Reduces noise in the bore. [3] | Material, thickness, added mass. |
05
The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Rated current | Sets amplitude via G = ηI; amplifiers 100–1,200 A; 930 A in one system. [1] [2] | Current for Gmax per axis. |
| Inductance | Sets slew rate per volt, SR = ηU/L. [1] | L per axis. |
| Resistance | About 5% effect on slew rate; sets heat ∝ I². [1] | R per axis and heat load. |
| Gradient strength | 45–80 mT/m whole-body; 300 mT/m head-only. [1] [2] | Gmax per axis. |
| Slew rate | 200 T/m/s whole-body; 750 T/m/s head-only. [1] [2] | SRmax and PNS-limited value. |
| Linearity | Deviation from ideal gradient, in %. [1] | Linearity over FOV. |
| Operating temperature | Not stated; heat and cooling define it. [3] | Temperature limits. |
| Insulation class | Not stated in the reviewed sources. | Insulation system and voltage rating. |
| Cooling method | Water cooling; hollow or thin-wire conductors. [1] [3] | Flow, pressure, inlet temperature. |
| Weight | Not stated; a ceramic lining adds 20.4–89.5 kg in one model. [3] | Coil mass. |
| Dimensions (L×W×H) | Inner diameter = patient bore + 2 × RF coil thickness. [1] | Inner and outer diameter, length. |
| Operating humidity | Not stated in the reviewed sources. | Environmental limits. |
| IP rating | Not stated in the reviewed sources. | Enclosure requirements. |
| Acoustic noise | Significant risk above 140 dB peak unweighted or 99 dBA rms with hearing protection. [4] EPI measured at 110–120 dB. [3] | Peak and A-weighted SPL per sequence. |
06
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Geometry: [warm bore; patient bore; RF coil thickness; length] Performance: [Gmax; SRmax; linearity over FOV] Amplifier: [peak current; peak voltage] Shielding: [shielding factor; eddy currents] Cooling: [water flow; temperature; pressure] Safety and noise: [PNS limits; acoustic targets] Supply: [quantity; testing; documentation]
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Amplitude follows current (G = ηI); slew rate follows voltage divided by inductance (SR = ηU/L). [1]
Efficiency falls with the fifth power of diameter, and head coils have higher nerve-stimulation thresholds. [1] [2]
The review reports echo-planar imaging at 110–120 dB, above the NIOSH 85 dB limit. [3]
Thick hollow wires cannot follow tight x and y patterns and generate more eddy heat at high slew rates. [1]
FDA's 2014 guidance treats a peak unweighted level above 140 dB, or an A-weighted rms level above 99 dBA with hearing protection, as significant risk, along with dB/dt high enough to cause painful nerve stimulation. [4]
Sources checked 2026-10-12. The Siemens Healthineers white paper and GE HealthCare article describe each maker's view of gradient design; the review summarises noise and cooling research; the FDA guidance gives the U.S. significant-risk criteria for gradient dB/dt and sound level. Values for your coil must come from the supplier offered.
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