• Topic ID: id_2002473
  • Version: 6.2
  • Date: Nov 4, 2019 11:02:20 AM

Running the LVShim tool (PM mode)

The objective of the LVShim tool is to fine-tune magnet homogeneity.

Prerequisites

Personnel requirements
Required persons Preliminary requirements Procedure Finalization
1 - 10 minutes -
Tools and test equipment
Item Quantity Part number Manufacturer
  1   -
LV Phantom Small Half Assembly 1 2306642 -
LV Phantom Big Half Assembly 1 2306645 -

The Gradshim process is done with a sampling diameter of 22 cm DSV and the same bandwidth as LVShim Fine, which depends on the quality (good/bad) of the fine tuning. Once the scan passes and all harmonics are within specification, the magnetic field is reanalyzed in Test mode automatically at 45 DSV for customer acceptance.

Procedure

  1. Make sure that all coils have been removed from the cradle and phantoms from the bore.
  2. Place the LVShim phantom on the patient table.

    Figure 1. Placing the LVShim phantom on the patient table

  3. Set up and align the LVShim phantom. Align the phantom so that:
    1. The phantom’s seam is oriented perpendicular to the table.
    2. The phantom’s z-axis is along the length of the cradle.
    3. The phantom is centered left to right with respect to the cradle top.
  4. Landmark on the LVShim phantom crosshairs.
  5. Press Advance to Scan.
  6. (For software version 23.x or earlier) Do a phantom centering check scan and center frequency check using manual prescan, and then click [Save Frequency].
  7. (For software version 24.x or later) Do a phantom centering check scan.
  8. On the MR Service Desktop, select the PM tab, and select PM > PM Assist > LVShim (Pre-Requisite) > Click here to start this tool.
    The LVShim tool displays.

    Figure 2. LVShim tool

  9. On the LVShim tool, select:
    • Gradient Shim shim type
    • Auto Mode radio button
    In auto mode, the tool does the following:
    • Sets up the protocol and collects image data
    • Calculates linear harmonics and compares it with the specification
    • Calculates the delta currents and the new currents based on the harmonics (dc offsets for X, Y, and Z)
    • Completes multiple iterations (maximum = five) until harmonics meets the specification
    • Shows the reanalysis of the last iteration in the final column in test mode (customer acceptance specification)
  10. Click Scan.
    After the scan is completed (with the Service Key still inserted), the tool compares all of the harmonics to specifications in the test mode.
  11. Check the results:
    • Green cell = Passed specification
    • Red cell = Failed specification
  12. When all iterations are complete and all the harmonics are within specification, click Exit in the LVShim tool.
    If any harmonics fail in Gradshim, re-run .