LVShim for Sites with Low Level Drifting Magnets

If it is determined that the magnet being shimmed has a Low Level Drift, it is recommended that the Z2 harmonic be adjusted to a positive value to increase the time needed between re-shim.

The Low Level Drift is described as a magnet where the main frequency has a steady reduction in main field frequency of 2 to 5 Hz per day. This reduction in main field correlates to a Z2 harmonic value that will increase in a negative direction. Therefore, the Z2 harmonic should be set to a positive value.

1. Shim magnet to specification.

2. Note the value of the Z2 harmonic.

3. Adjust the Z2 current value to increase the Z2 harmonic to a positive 40 Hz.

   The ratio of current to harmonic value is +0.010 amps of current equals +7 Hz change of Z2.

   Example: If the Z2 harmonic is currently at +5 Hz, and the current for the Z2 channel is +10.000 amps, increase the current to 10.050 amps. This should result in the Z2 harmonic to increase to +40 Hz.

   note:

   It is important to note that the Z2 harmonic must be less than 50 Hz in order for IIS to pass specification.

4. Execute the LV shim scan and note the value of the Z2 harmonic. The target spec for the Z2 harmonic is 40 Hz ± 5 Hz

Return to LVShim Process.