Shim Power Supply Details

1 GE Gradient/Resistive Shim (S-III Magnets)

The GE resistive shim coil set does not provide correction for x, y, or z harmonic errors, but the gradient coil does. Therefore, the resistive shim coil and gradient coil assemblies are characterized at the same time, creating a 9-coil calibration file. This file allows the magnet to be shimmed by both the gradient and resistive shim coils at the same time (one process rather than two).

1.1 Gradient Amplifiers

To set the gradient shim coil currents:

  1. At the operator workspace in the Scanning Parameters window, click Manual Prescan.

  2. In the Manual Prescan window under Gradient Shimming, set x, y, and z to the desired values, then click Done to exit Manual Prescan.

1.2 Resistive Shim Power Supply

To set the resistive shim coil currents:

At the resistive shim power supply, set the appropriate shim channel(s) to the desired current value.

note:

After setting shim channel(s) at resistive shim power supply, wait a few minutes to allow the power supply and magnetic field to stabilize before scanning.

2 GE 12-Coil, 14-Coil or 18-Coil Superconductive Magnets

2.1 Coil Set Selection

There are nine coil set choices available during S/C LVshim when the Research operation mode is used. See Table 1.

The spherical harmonics affected by the Full, Standard, Basic, No Z6, Axial Coils Only, and Transversal Coils Only sets are shown in Table 2 for S-III and S-IV, or Table 3 for S-V a. The Selected Coil sets allows you to select any combination of coils.

2.2 Setting Shim Power Supply Currents

Notice Image
  • notice
  • Equipment damage possibility. Do not attempt to perform this procedure if the helium level in the magnet is below operating level specified in the appropriate magnet manual. Low helium level may result in a quench, causing possible damage to the magnet.
  • Be sure that the vent bypass cap is removed from the shim lead extension vent line to properly cool this lead. Failure to do so will damage the Shim Lead Extension Assembly.
  • Always enter the axial currents last during the shimming procedure. This allows for proper lead cooling.
  • Always match the currents in the axial coils with the power supplies before turning on the axial switch heater. These coils couple closely with the main field and a main field quench may occur if large amounts of axial currents are suddenly dumped.
note:

Do not pulse the gradients while trying to stabilize currents in the S/C shim coils.

note:

Normally, the switch heater would stay turned on for approximately 5 minutes to allow the shim coils currents to stabilize. For S-V and S-XC_2 magnets, the shim coil inductance is higher, thereby requiring more time for the shim coils currents to stabilize. Typically 15 to 25 minutes is required for S-V and S-XC_2 magnets to stabilize.

  1. For systems with the Resistive Shim Coil option, the resistive shim power supply must be turned off during the S/C LVshim procedure.

  2. Enter the new shim current settings into the S/C shim power supply per one of the following procedures:

For GE S-II and S-III unshielded magnets, refer to Direction 15120, GE 1.5T Magnet And Cryogens Subsystem, Setup & Calibration tab, Section 9-5, Adjusting S/C Shim Coil Power Supply Currents For Magnet Shimming.

For GE S-III shielded magnets, refer to Direction 15174, GE 1.5T Magnet And Cryogens Subsystem With Add-On Shield, Setup & Calibration tab, Section 9-5, Adjusting S/C Shim Coil Power Supply Currents For Magnet Shimming.

For GE S-IV magnets, refer to Direction 15345, GE 1.5T SIV Magnet & Cryogens Subsystem, Set Up & Calibration tab, Section 11-4-3, Adjusting S/C Shim Coil Power Supply Currents For Active Shimming.

For GE S-V magnets, refer to Direction 2141548, GE 1.5T SV Active Shield Magnet & Cryogens Subsystem, Setup & Calibration tab, Section 11-4, Adjusting S/C Shim Coil Power Supply Currents For Active Shimming.

For GE Cx magnets, refer to Direction 2159496, GE 1.5T & 1.0T Cx Active Shield Magnet & Cryogens Subsystem, Setup & Calibration tab, Section 13-4, Input Shim Currents.

For GE LCC magnets, refer to Direction 2192624, GE 1.5T & 1.0T LCC Active Shield Magnet & Cryogens Subsystem, Setup & Calibration tab, Section 13-4, Input Shim Currents.

For LCC300 magnets, refer to Direction 2325852, GE LCC300 Active Shield Magnet & Cryogens Subsystem, Setup & Calibration tab, Section 9.

For GEMSO 3.0T/940 magnets, refer to Direction 2295400, GEMSO 3.0T/940 Active Shield Magnet & Cryogens Subsystem, Setup & Calibration tab, Section 12.

note:

Wait about 5 minutes after the new currents have been applied to the S/C coils to allow the switch heaters to cool before dialing the power supplies to zero in preparation for the next LVshim scan.

note:

For all magnet systems, remove the cables from the S/C shim power supply and close the RF door before performing the LVshim scan, otherwise RF noise from external sources could cause imaging artifacts that will affect the LVshim results.