Object ID: 00000018WIA3085CE20GYZ
Topic ID: id_13106395 Version: 3.2
Date: Jun 19, 2020 1:26:01 AM

B0 Drift Calibration

Prerequisites

Table 1. Personnel requirements
Required persons	Preliminary requirements	Procedure	Finalization
1	-	4 to 12 hours	-

Table 2. Tools and test equipment
Item	Quantity	Effectivity	Part number	Manufacturer
Body TLT Phantom and Sphere	1	-	2360037	-
Service Filler Panel	2	Curved table	5344577	-

Table 3. Required conditions
Condition	Reference	Effectivity
All other applicable calibrations must be complete.	-	-

About this task

Unwanted changes in the main magnetic field strength can occur when the spatially uniform field component is altered for a short term, during the process of scanning, or when the spatially uniform field component is permanently altered. This is known as short-term B0 drift. All MR systems that use passive shimming have B0 drift. However, the MR750w system has a much higher drift (up to 150 Hz per hour) because it uses a 3.0T magnet and is passive shim only. Other systems, such as MR750 and MR450w, have lower B0 drift (less than 30 Hz per hour).

With short-term B0 drift, the main magnetic field returns to its original value after scanning stops. Given sufficient non-active time, short-term B0 drift is reversible. However, too much short-term drift can have an adverse impact on image quality. It can cause an image intensity shift during fMRI, TRICKS, and VIBRANT MPH, and banding artifacts in SSPS-based sequences, such as FIESTA and COSMIC. Short-term B0 drift shall not exceed 30 Hz per 10 minutes. This requirement is for the 3TLC magnet design and the G3 magnet design, regardless of temperature variants.

The B0 Drift tool characterizes the B0 drift of a given magnet by monitoring the frequency of the system during a high power scan. The tool’s compensation factor is applied to any scan that affects B0. The B0 Drift tool is part of system calibration and is the last calibration executed during the installation process.

B0 drift is directly affected by the amount of shim steel in the magnet. This calibration should be effective for the life of the system.

Procedure

(For flat table) Place the TLT phantom and sphere on the flat table. Position the phantom in the center of the cradle and center it on the PA coil. To achieve the best test measurement, the phantom must be located in the center (long direction) of the cradle.

Note:
The MR table in the figure is a flat (GEM) table.

(For curved table) Place the TLT phantom directly on the cradle.

Figure 1. TLT Phantom and Sphere on Flat Table
Start the B0 Drift tool:
1. (For non-proprietary service tools) From the Common Service Desktop, select Calibration > B0 Drift Calibration. Select Click here to start this tool.
The B0 Drift window appears.

Figure 2. B0 Drift Window with X, Y, and Z Axes Highlighted Non-Proprietary
All three axes are processed together. However, you can select X, Y, and Z axes individually, depending on the progress of the installation. To select more than one axis, hold the Shift key while clicking the axes to select.
(For non-proprietary mode) Select Calibrate Only.
Click Scan.

Note:
Images are generated during B0 drift calibration. Image quality (IQ) is not relevant. Ignore any image quality issues, or any error message related to IQ.
B0 drift calibration requires total time of 12 hours for non-proprietary mode. Wait 30 minutes before you leave the site to confirm that the B0 drift scan is working properly.
When B0 drift calibration completes, a screen displays the data for each axis.

Figure 3. Data from B0 Calibration
Select Exit to save results and exit the B0 Drift tool.

To troubleshoot calibration failures, use the procedure in B0 Drift Functional Check.

Finalization

Remove service equipment, phantom, and head coils.

(For curved table) Remove service filler panels from each cradle top panel.