MRI 3 Axis Gradient Coils

MRI 3-Axes Gradient Coils

    • Active shielding
    • B0 eddy current compensation
    • Maximum Sample Volume
    • Low Noise and Vibration
    • High Continuous Gradients

Advances in hardware for magnetic resonance imaging (MRI) are needed to improve image quality, ease of use and functionality in high-field MRI research using small objects. Doty’s MRI gradient coils fill this need.

Low-amplitude B0 eddies are induced in the magnet radiation shields primarily from minute variations in coil diameters along the axis or from axial registration errors between the gradient and shield coils. Our use of alumina ceramic for both the gradient and shield formers allows higher precision to be maintained, and low-amplitude eddy current to be minimal. Ceramic coil forms, together with heavy Golay windings dramatically reduce vibration and noise, even at the highest fields. Any remaining B0 eddy is compensated by a time-dependent correction applied to a B0 shim coil. Another advantage of the alumina coil form is its very high thermal conductivity, which helps equilibrate hot spots. The cooling requirements are then satisfied with relatively minor constraints on the winding geometry.

Higher-order eddies are minimized by active shielding. Our coil designs often acheive a factor of 2 better shielding of the transverse gradients than alternative designs.

There is a strong benefit from gradient coil construction with an alumina ceramic coilform and multilayer windings. We significantly reduce acoustic noise, vibration and recovery time compared to gradient coils from other microscopy MR vendors.

Parameter

Model
26-40

Units

Cooling method

Water

Diameter di for 4% local deviation

14

mm

Length zi for 4% local deviation

17
mm

Diameter di for 10% local deviation

18
mm

Length zi for 10% local deviation

22

mm

Nearest Gradient Null point

15.4

mm

Outside diameter, do

39.6

mm

Coil half-length, h1

36.1

mm

RF shield diameter,dS

26

mm

Clear bore, di

23.6

mm

Max inductance, L

37

μH

Max DC resistance, RE

1.4

Ω

Min gradient gain, α

48

mT/Am

Max shielding error at 1.5 d0

0.4

%

Min slew rate, GS = αV/L, at 1 V

1,189

T/m/s

Continuous current, IRMS

11

A

Continuous gradient, GC

53

G/cm

Peak Voltage

120

V

Approx. EPI Acoustic Noise, 7 T

70

dBa

Rise time to Gfor 100 V

4.6

μs

Total mass

0.4

kg

In the table above, the performance for the X and Y axes are each as indicated above, and the Z axis is typically better in slew rate, continuous gradient, and volume of linear region.

Local deviation (or differential linearity) is defined as the rms deviation from the mean gradient over the specified diameter, di, and length, zi, of the cylindrical sample region.  The half-length h1 is the distance from the center to the closer of the two external end surfaces.  Eddy currents from the internal RF shield are negligible.  The gradient slew rateGS is the instantaneous rate of change in gradient when a 1 V step is applied.  The continuous current ratings are true continuous ratings for a single axis with no time limit and adequate cooling. Derate the current 30% when all three axes are driven simultaneously.

 

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