Dual drive quadrature tool

Swapping the RF transmit or receive cables does not cause hardware damage, but it does create a situation known as reverse quadrature. When this occurs, the combined image has a low signal. To determine if this is the problem, generate the three images using the Dual Drive Quadrature Calibration tool.

The illustration below shows the results of a system that has proper quadrature and the proper signal strength from both channel 1 and channel 2.

Values are approximate and may vary if system gain was not previously executed. The ratio between the combined scan and the single channel scan is approximately 1.41 : 1. If the value for the combined image is 1400, the value for the single channel scans should be approximately 1000.

1. Place the body TLT sphere and loader on the table. If you are using a flat (GEM) table, use the foam holder to contain the phantom.
2. Landmark the phantom and drive it into the magnet bore.
3. Click New Patient to set up for a scan.
4. Under Protocol, select Service. Then select DDquadrature.

   

5. Set up and scan the combined image. DO NOT perform Auto Prescan. Perform Manual Prescan and set the values to:

   • TG = 150
   • R1 = 11
   • R2 = 14
6. After scanning the combined image, removed the RF input cable from RF amplifier channel 2 by disconnecting J21. Leave channel 1 (J14) connected. Repeat the same scan WITHOUT changing any of the values for TG, R1, or R2. This linear scan uses only channel 1.
7. When the image for channel 1 is complete, reconnect J21 (channel 2 input) and disconnect J14 (channel 1 input). Repeat the same scan WITHOUT changing any of the values for TG, R1, or R2. This linear scan uses only channel 2.
8. Now there are three images. Review them. Figure 13 shows the results for a system that has proper quadrature and the proper signal strength from both channel 1 and channel 2. Use Table 7 to troubleshoot your images.

   Table 7. Troubleshooting Scan Images

   Scan result	Problem	Resolution
   If the combined image has a weaker signal than either image from the single channels:	The system could be transmitting in reverse quadrature.	Somewhere in the RF transmit chain, the channel 1 and channel 2 cables are swapped. Locate the swapped cables and connect them correctly. Cables between DTRSW and the body coil must always be in the correct locations, or the body receive signal will be poor. Swapping channel 1 and channel 2 cables somewhere else in the transmit chain will not restore the body receive signal quality to normal.
   If the signal for one of the channel images is lower than the other channel:	There could be low or no RF signal from one of the channels.	Check the RF output using: RF Amp Power Check tool or Power Measurement Kit (see Body and Head Maximum Power Setup and Calibration).

9. If all the cables and the RF outputs are correct and the source cannot be determined by visual inspection of the images, proceed to Determining Effects of PA Coil.

The presence of the PA coil in the bore affects the results of DD Quadrature Calibration. Both the magnitude and the phase are changed when the PA coil is in the magnet bore (compared to being out of the magnet bore). SNR aborted by DD Quadrature Calibration should not change.

To check the interaction of the PA coil, scan with the PA coil and cradle out of the bore as described below.

1. Landmark the phantom as usual. Mark the phantom location on the body coil with tape.
2. Press the emergency release on the cradle and pull the cradle back onto the table. Manually position the phantom on the panel or pad so it is horizontally and vertically centered in the location on the bridge.

   

3. Apply tape to push in on the cradle sensor, which will permit scanning with the cradle unlatched from the LPCA.

   

4. In the DD Quadrature Calibration Tool, verify that only the center location is selected, and then run the calibration tool, Dual drive quadrature tool.
5. Compare the results of DD Quadrature Calibration when the PA coil is in the bore with the results when the PA coil is out of the bore.

   In a good working system the difference in attenuation and phase results between the PA coil in bore and out of bore are small. We have typically observed a -0.5 dB (or -5 count) decrease in the magnitude and a -5 decrease in phase when comparing results with the PA in bore to results with PA removed from bore.

6. If DD Quadrature Calibration fails only when the GEM PA coil is in the bore due to low SNR, replace the GEM PA coil. Otherwise, perform the next troubleshooting action.

This section is designed to help the FE locate the largest contributor of phase shift in the transmit chain. This is done by swapping cables and hardware between the DTX1 exciter and the RF body coil, and checking for the largest change in phase after every swap. A change in polarity after a cable swap indicates that you have reached one of the cables contributing a majority of the phase shift. Ideally, each channel 1 and 2 transmit chain section should contribute almost no phase shift. In a normal system, swapping channel 1 and 2 cables in any location results in almost no change in phase polarity.

1. Using a normal system configuration, perform quadrature calibration in Test mode and record the initial residual phase value and polarity as result A.
2. In Figure 17, The circled numbers show where to swap cables. The numbers progress from the most-likely to the least-likely problem area.

   For each action, swap all cable connections identified with swap symbol <—>. For example, for action 1, swap the channel 1 and channel 2 cables (swap J9 and J11 on the PGR cabinet, and swap J22 and J15 on the PEN cabinet).

   In each action (except action 5) each end of the cable is swapped so that the entire section of cable is relocated to the other transmit line. Action 5, however, only swaps the cable ends connecting to the DTRSW.

   

3. Starting with action 1, swap both ends of cables as shown to relocate the entire cable onto the other transmit line. (For action 5, swap cables only at the DTRSW.)

   For action 2, try the cable ends at the PGR and DTRSW first. If the problem continues, try the cable ends at the pen panel.

4. After swapping the cables, repeat quadrature calibration in Test mode for the given action, and record the new initial residual phase value and polarity as result B.
5. Compare the values recorded as result A and result B:
   1. If the polarities are the same, neither of the cables are the cause of phase failure. Return the cables to the normal positions and repeat the steps above with the next numbered action, until you find the problem or all actions are done.
   2. If the polarities are different and this is action 5, stop and replace the DTRSW because it is the source of most of the additional phase.
   3. If the polarities are different and result A is positive, then most of the additional phase is in the channel 2 transmit cable that was swapped. Inspect the cable and confirm that it is the correct cable for the system. Reseat the cable.
   4. If the polarities are different and result A is negative, then most of the additional phase is in the channel 1 transmit cable that was swapped. Inspect the cable and confirm that it is the correct cable for the system. Reseat the cable.
   5. If the situation remains unchanged, the only items left are the DTX1 exciter and the body coil. Contact support before continuing.

Cable swapping can also be utilized to isolate magnitude issues with dual drive systems. The magnitude change between channels should not vary more than 5 counts when the RF transmit cables are swapped. The following process tests the entire transmit chain from the output of the exciter to the output of the DTRSW. The only cables that are not be tested are the cables that run between the DTRSW and the body coil.

1. Using a normal system configuration, perform quadrature calibration in Test mode and record the initial residual magnitude value and polarity as result A.
2. Swap cables between exciter outputs at J12 and J13 at the exciter, and swap cables between J2 and J3 at the DTRSW.

   

3. After swapping the cables, repeat quadrature calibration in Test mode for the given action, and record the new initial residual magnitude value and polarity as result B.
4. Compare the results. If there is a 5 count difference between the two magnitude values, the transmit chain is contributing to the magnitude error.

The RFA Loopback Test can be used to measure deviations in magnitude and phase for channel 1 and channel 2 anywhere in the transmit chain between the DTX exciter and the DTRSW. It cannot be used to accurately measure magnitude and phase for channel 1 and channel 2 from the RF body coil.

It is recommended that RFA be executed first at the output of the DTRSW. This will determine if an issue is either with the system electronics leading up to the DTRSW or if the issue is with the body coil. If the failure is determined to be before the DTRSW, RFA can be used to isolate the failure location.

If the RF transmission from channel 1 and channel 2 is good, order a Network Analyzer (part #5336593-2). Follow the procedure for MR750w Body Coil Tuning to check and/or adjust the tuning on the body coil.