• Discovery MR750w and SIGNA™ Architect T 3.0T System Service Methods
  • 5690002-2EN Revision 4
  • Object ID: 00000018WIA305EAE20GYZ
  • Topic ID: id_13107521 Version: 4.0
  • Date: Feb 21, 2021 9:03:39 PM

LVShim Procedures

Prerequisites

Table 1. Personnel requirements
Required personsPreliminary requirementsProcedureFinalization
Not Applicable30 minutes10 minutes
Table 2. Safety
Table 3. Required conditions
ConditionReferenceEffectivity

Confirm that passive shimming is complete and is within specification.

--

Perform Setting up grafidy hardware.

--

Remove all coils from the cradle and phantoms from the bore.

--

About this task

LVShim - Gradshim

The objective is to perform fine-tuning of the homogeneity of the magnet by using gradients only. The Gradshim is performed on 22 cm DSV. Test mode is performed in 5 volumes for peak to peak and rms values. The interface accordingly shows these values and their specs. Once the scan passes and all harmonics are within specification, the magnetic field is reanalyzed in Test mode.

The Gradshim process includes LVShim - Test. The objective of this test is to analyze completed scans at 45 DSV without calculating currents.

Note:

For DV systems 22.x and earlier, 1.5T LVShim uses green phantoms and 3.0T LVShim uses pink phantoms.

For DV systems 23.x and later, 1.5T LVShim uses green phantoms and 3.0T LVShim uses either green or pink phantoms.

Note:

During LVShim procedures, the system performs a SNR check to verify system integrity. If the SNR check fails, troubleshoot the SNR issue (in particular, check the I and Q drive cable connections) before continuing with LVShim.

Topic ID: id_SL4463850-1144893

Preparing for LVShim Procedure

Procedure

  1. Set up and align the LVShim phantom. Align the phantom so that:

    • The phantom’s seam is oriented perpendicular to the table.

    • The phantom’s z-axis is along the length of the cradle.

    • The phantom is centered left to right with respect to the cradle top.

    Do not use foam service filler panels for LVShim procedures.

    For flat (GEM) tables, use the curved cradle insert.

    Figure 1. Placing LVShim Phantom on Patient Table
  2. Landmark on the LVShim phantom crosshairs.
  3. Press the Advance to Scan button.
  4. Perform a phantom centering check scan and center frequency check using manual prescan, and then click Save Frequency.
  5. Perform a phantom centering check scan.
  6. Start the LVShim tool from the Common Service Desktop, on the Calibration tab.

    Click Calibration > LVShim, and then click Click here to start this tool.

Topic ID: id_SL4464188-1144893

Gradshim LVShim Procedure

About this task

By default, Gradshim is performed in Auto mode, although Manual mode is also available. Both modes are described below.

Topic ID: id_SL4464189-1144893

Auto Mode

About this task

In Auto mode, gradient currents are automatically calculated after you click Scan to fine tune the homogeneity of the magnet. In this mode, the tool stops after the X, Y, and Z harmonics are within specification. In Test mode, the image from the last scan is automatically reanalyzed.

Procedure

  1. Note:

    During LVShim procedures, the system performs a SNR check to verify system integrity. If the SNR check fails, troubleshoot the SNR issue (in particular, check the I and Q drive cable connections) before continuing with LVShim.

    Verify that the Auto Mode radio button is selected. If it is not, select it.
  2. Select Gradient Shim, and then click Scan.
    Figure 2. Gradient Shim – Auto Mode

    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

    • Completes multiple iterations (maximum of 5) until harmonics meets the specification

    • Shows the reanalysis of the last iteration in the final column in Test mode (customer acceptance specification)

Topic ID: id_SL4464199-1144893

Manual Mode

Procedure

  1. Prepare the phantom for the LVShim procedure as described above.
  2. Select Gradient Shim and Manual Mode, then click Scan.
    Figure 3. Gradient Shim – Manual Mode

    After the scan is complete, the tool calculates harmonics and compares X, Y, and Z to specifications.

    • Green cell = Passed specification

    • Red cell = Failed specification

    Figure 4. Gradient Harmonics and Gradient Bits

    The tool calculates the X, Y, and Z gradient delta currents (for gradient bits only) and displays the new currents based on the harmonics in another window.

    Figure 5. Original Delta and New Currents (Examples)
  3. To accept or decline the new currents:
    1. If X/Y/Z harmonics data cells are green, click Cancel to decline the new currents. Ignore cells with the specification NA.
    2. If any harmonics data cells are red, click Dial In Currents to accept the new currents. The new currents will load in a new column in the LVShim user interface as shown in Figure 4. Repeat Step 2.
  4. For more details about the scan (cal file name, center frequency, etc.), select More Actions > More Info.
    Figure 6. More Information on Scan
  5. When all iterations are complete and all harmonics are within specification, click Exit on the LVShim window.

    If any harmonics fail in Gradshim, re-run Grafidy 3 Eddy Current Calibration.

    If this also fails, run passive shim using the instructions in the applicable Magnet and Cryogens Subsystem Manual.

Topic ID: id_14979067

Finalization

Procedure

  1. Remove any phantoms and/or tools used.
  2. Perform a Doing a check scan before turning the unit over to the customer.