LVShim Calibration File Generation Details

When this option is set to yes, the gradient shim coil perturb currents are automatically set by the software. When the option is set to no, the gradient shim coil perturb currents must be manually set.

To manually set the gradient shim coil currents:

1. At the operator workspace, select Manual Prescan.

2. Set x, y, and z to the desired values, then select Files, then Save Shim File. Select Done to save the file.

3. Select Done to exit Manual Prescan.

When this option is set to yes, the suggested perturb currents for the coil set being calibrated are displayed, allowing you to get a hard copy of the currents. This option allows you to change the perturb currents to be used for the coil set being calibrated. When selected, the software prompts for the new currents, one at a time. Illustration 5-2A shows a typical suggested perturb currents display (shown in Genesis format; the Lx display is different).

Figure 2. SUGGESTED PERTURB CURRENTS (OXFORD RESISTIVE SHIM COIL)

There are two types of storage: Accumulate and Replace. Accumulate is used when you wish to average multiple scans for a given coil type/perturbation (such as taking three Z1 baseline scans to average the data). Replace is used when you wish to throw out the old data and replace them with data from a different scan. If you choose Replace mode, all previous data collected for the coil are lost.

1. Perform a Phantom Setup, if you haven’t already done so.

2. At the operator workspace, select the Scan icon in the desktop control panel and set up LVshim scan as follows:

   1. In RX Manager, click New Series.

   2. At the operator workspace, prepare the system for LVshim scanusing the "Service Protocols" procedure located on the service methods CD-ROM, or, for the alternate proprietary procedure, see below.

   3. In Additional Parameters, click User CVs Screen.

   4. In the User Control Variables window enter the following:

      • No. of Scan Planes: 6

      • Bandwidth: 2000

   5. Click Accept and then Save Series.

   6. In RX Manager, click Prepare to Scan.

3. Click Manual Prescan. While in the Center Freq Coarse (CFC) mode, verify the frequency peak is centered in the display. Adjust the center frequency peak as necessary. After locating the best signal, open the [Frequency] menu at the top of Manual Prescan window and select Save Frequency to save the new center frequency.

4. Click Scan TR (R1/R2). Verify that R1 and R2 are not over ranged. Adjust R1 and R2 for approximately 50%.

5. Click Done to exit manual prescan.

6. Click Scan.

   note:

   A Resistive Shim Calibration file can be created by using one baseline scan and one perturbation scan for each coil in a coil set. However, if time allows, three scans should be taken for each baseline and perturbation to insure accuracy of the calibration matrix (by averaging the data).

7. If more than one scan is desired, when the first scan is finished, press Scan again to collect a second set of scan data. Then, when the second scan is finished, press Scan again to collect a third set of scan data.

8. When the baseline scan(s) is finished, in RX Manager, copy and paste the present series, then select View Edit. Change the Series Description to Z1 0.1, then click Save Series, followed by Prepare to Scan. Perturb the Z1 coil baseline current by 0.1 amps as described in Suggested Perturb Currents and Series Descriptions, then repeat steps 3 through 8 to collect the Z1 perturbation scan(s). After the Z1 perturbation scan(s) have been collected, set the Z1 coil current back to its baseline value.

9. To collect scan data for all of the coils in a coil set before processing, follow these steps:

   1. In RX Manager, copy and paste the present series, then select View Edit.

   2. Change the Series Description to Z2 baseline, then click Save Series, followed by Prepare to Scan.

   3. Perform steps 3 through 9 to collect baseline and perturbation scans for the Z2 coil (use Z2 0.5 as the Series Description name for the Z2 perturbation scans).

   4. Repeat steps a through c for the remaining coils in the coil set.

   5. When scan data for all coils have been collected, perform Shim Calibration Processing (Silent Mode = Yes).

10. To collect and process scan data for each coil, one at a time

    1. Perform Shim Calibration Processing on the Z1 coil scans (baseline and perturbation) (Silent Mode = No).

    2. When the processing is done, in RX Manager, copy and paste the present series, then select View Edit. Change the Series Description to Z2 baseline, then click Save Series, followed by Prepare to Scan.

    3. Perform steps 3 through 9 to collect baseline and perturbation scans for the Z2 coil.

    4. Repeat steps a through c for the remaining coils in the coil set.

Existing current values are displayed for all the coils in the coil set being shimmed. This option allows you to change the existing values. When the gradient current values are changed, the new values are automatically sent to the gradient amplifiers by the software.

1. In the Service Browser, select the Calibration tab, then in the menu options on the left side of the browser, click LVShim. On the right side, click the link Click here to start this tool. See Figure 1.

2. For TwinSpeed, highlight the GradMode and click on OK. Continue as shown in Table 6.

The coil characterization status is located in the area under the menu title line. It consists of the following information for each of the coils in the coil set:

• Coil name

• Number of Baseline scans for the coil (designated with a B)

• Number of Perturbation scans for the coil (designated with a C)

The following is an example of the coil characterization status for a typical coil (with three baseline scans and three perturbation scans):

AX_1: ( B 3, C 3 )

The coil characterization status area also shows whether a common baseline has been processed for the coil set. A common baseline is a baseline scan used during characterization of all the coils in a coil set. A common baseline is not necessary if a new baseline scan is performed before each coil perturbation.

This is the name of the coil to be characterized by the selected image data set. If the selected image data set is being used as the baseline scan for all the coils, use the name common baseline.