Full Field Distortion

Prerequisites

A Full Field Distortion (FFD) phantom is scanned at the maximum FOV in the sagittal and coronal planes at the center of the scanner bore. The resultant images display a matrix of 50 mm x 50 mm squares from which the field linearity is measured. Body scans are used for all scan sequences.

note:

For TwinSpeed, the test should be repeated separately for each Gradient mode.

1 Sagittal Scan

Procedure

  1. Remove the Quad head coil from the cradle.
  2. Place the FFD phantom on the positioner assembly and center it on the table with plates oriented vertically (black alignment line on top) and the alignment block on the right side of the phantom (as you face the magnet). The side of the phantom with the alignment lines should face up. Landmark (axial) on the middle vertical tube of the center plate.
  3. At the keypad on the front magnet enclosure, press LANDMARK and MOVE TO SCAN. See Figure 1 .

    Figure 1. Sagittal FFD Phantom Positioning

  4. At the operator workspace, prepare the system for a Full Field Distortion (sagittal) scan using the Service Protocols procedure located on the service methods CD-ROM, or for the alternate proprietary procedure, see below. This alternate proprietary procedure is available for GE use, and to sites with a valid Advanced Service Package Limited License.
    1. Click on New PtEnter

      ID: geservice

      Name: ffd

      Weight (lb): 111

      Set Patient Protocols to Service

    2. Select the scan protocol by clicking on Other, select the Full Field Distortion protocol and series 1 from the menu then click [Accept] to load the Full Field Distortion (Sagittal) protocol.
    3. For TwinSpeed, select the Gradmode.
    4. Save Series, then Prepare to Scan.
  5. Click on Autoview, just below the Autoview image display screen. Your images will be automatically displayed.
  6. Click on Scan (The system runs Auto Prescan first.)
  7. For analysis, see Section 4- FFD Image Analysis.

2 Coronal Scan

Procedure

  1. Remove the Quad head coil from the cradle.
    note:

    Equipment damage possibility. Completely remove the Quad Head Coil from the cradle before performing any body scans. Failure to do so may damage the head coil T/R network.

  2. Place the FFD phantom (with the alignment block facing down) on the holder for a coronal scan, and align with the alignment beams as shown in Figure 2. Landmark (axial) on the center of the phantom.

    Figure 2. Coronal FFD Phantom Positioning

  3. At the keypad on the front magnet enclosure, press LANDMARK and MOVE TO SCAN.
  4. At the operator workspace, prepare the system for a Full Field Distortion (coronal localizer) scan using the Service Protocols procedure on the Service Methods CD-ROM, or, for the alternate proprietary procedure, see below. This alternate proprietary procedure is available for GE use, and to sites with a valid Advanced Service Package Limited License.
    1. Click on New PtEnter.

      ID: geservice

      Name: ffd

      Weight (lb.): 111

      Set Patient Protocols to Service

    2. Select the scan protocol by clicking on Other, select the Full Field Distortion protocol and series 2 from the menu then click [Accept] to load the Full Field Distortion (Coronal Localizer) protocol.
    3. For TwinSpeed, select the GradMode.
    4. Save Series, then Prepare to Scan.
      note:

      This localizer scan is required to determine the A/P location for scanning the center plate of the FFD phantom.

  5. Click on Autoview, just below the Autoview image display screen. Your images will be automatically displayed.
  6. Click on Scan. (The system runs Auto Prescan first.)
  7. Using Autoviewer, display the image of the localizer scan (see Figure 3).

    Figure 3. Axial Localizer Display

  8. Select Report Cursor in the Autoview window. Place the cursor on the center bright spot of the axial localizer display. The cursor position should lie within the center plate.
  9. Check for skewing of the phantom in relation to the horizontal plane. If skewing is present, reposition the phantom and rescan.
  10. Record the A/P location (e.g., A14 mm) with the cursor centered on the center plate. This location will be used when you perform the coronal FFD scan.
  11. At the operator Workspace, prepare the system for a Full Field Distortion (coronal) scan using the Service Protocols procedure located on the Service Methods CD-ROM.
  12. Click on Scan (The system runs Auto Prescan first.)

    You are now ready to analyze the image.

3 FFD Image Analysis

Procedure

  1. To run the Image Quality Analysis – NEMA tool from the Service Desktop, click Utility, Image Quality - Nema, and Click on 'Click here to start this tool'. Figure 4.

    Figure 4. Starting the Image Quality - NEMA Tool from the Service Browser

  2. For TwinSpeed, highlight the same GradMode that was used with the sagittal and coronal scans, then click OK.
  3. A NEMA Image Quality window will appear on the desktop. Type 5 for Full Field Distortion Check. Type 5Enter for Full Field Distortion Check. See Figure 5.

    Figure 5. NEMA Image Quality Analysis Menu

  4. Select the sagittal exam, series, and image numbers for the image to be analyzed.
  5. Analysis then begins. The final values are displayed on the screen. (See Figure 6.)

    Figure 6. FFD Analysis Report Screen

  6. Record the Lengths, Percent Distortions, and Superior to Inferior Coil Symmetry in the Data Sheet. For TwinSpeed, identify the GradMode used.
  7. Repeat steps Step 3 through Step 6 for the coronal image analysis.
  8. Store all completed data sheets for future reference.
  9. When the analysis is finished, type 6 and press Enter to exit the NEMA Image Quality and close the window.

Finalization

No finalization steps.