1.5T GP Flex Coil SNR Test and Troubleshooting

SNR measurements for the GP Flex Coil require a set of signal and noise scans. Refer to the Data Sheet in Table 5 to understand the data required to calculate the SNR. The image quality check uses two different protocols for signal and noise image acquisition:

• The Signal Scan is a Fast Spin Echo (FSE) sequence used to minimize susceptibility and B0 inhomogeneity effects. The signal scan must be run prior to the noise scan since the R1, R2, and TG values from the signal scan are used for the noise scan.

• The Noise Scan is a Gradient Echo (GRE) sequence that has a Control Variable (do_noise) to eliminate the transmit RF completely during the scan.

The following procedure is specific to the GE 1.5T Systems.

1. Make sure the Quad Head Coil is removed from the cradle before performing any body or surface coil scans. Failure to do this may result in damage to the Quad Head Coil T/R network.
2. Place the phantom with loader on the table; place the coil on top of the phantom loader, and plug the flex coil into Port A.
3. At the magnet, press the Alignment Light button to turn on the light.
   1. Move the cradle to align the coil to the alignment light as shown.
   2. Press the Landmark button to landmark the alignment.

      Figure 1. Landmark and Alignment

      

4. Move the coil to the scan position by pressing the Move to Scan button; ensure the cable does not get snagged.
5. At the console, set the protocols as described in the table below.

   Table 4 Signal Protocols

   Patient/Exam Information
   ID	geservice
   Name	GP Flex Coil
   Patient Weight	111 lbs. (50 kg)
   A/P Center, R/L Center	0.0
   S/I Start	S75
   S/I End	I75
   # Slices	5
   Table Delta	0.0
   Patient Position
   Patient Position	Supine
   Patient Entry	Head First
   Coil	GP Flex
   Series Description	Signal
   Imaging Parameters
   Plane	Axial
   Mode	2D
   Pulse Seq	FSE-XL
   Imaging Options	Fast
   PSD Name	leave blank
   Protocol	leave blank
   Scan Timing
   # TE(s) per Scan	1
   TE	17
   TR	500
   Echo Train Length	4
   Bandwidth	15.63
   Acquisition Timing
   Freq	256
   Phase	256
   NEX	1
   Phase FOV	1.00
   Freq. DIR	R/L
   Autoshim	On
   Phase Correct	On
   Auto Center Freq	Peak
   Scanning Range
   FOV	24
   Slice Thickness	5
   Spacing	32.5
   A/P Center, R/L Center	0.00
   S/I Start	S75
   S/I End	I75
   # Slices	5

6. Click Save RX to download the protocols, then Prepare to Scan.
7. Run Auto Prescan, and record the R1, R2, and TG values on the SNR Data Sheet below.

   Table 5 SNR Data Sheet

   Date	Comments
   Slice Location	R1	R2	TG	Signal Mean (A)	Noise Standard Deviation (B)	SNR (A/B)	SNR Specification Limit
   S37.5							72.5
   S0							106
   I37.5							72

   Date	Comments
   Slice Location	R1	R2	TG	Signal Mean (A)	Noise Standard Deviation (B)	SNR (A/B)	SNR Specification Limit
   S37.5							72.5
   S0							106
   I37.5							72

   Date	Comments
   Slice Location	R1	R2	TG	Signal Mean (A)	Noise Standard Deviation (B)	SNR (A/B)	SNR Specification Limit
   S37.5							72.5
   S0							106
   I37.5							72

8. Run Scan.

A signal scan must be run prior to the noise scan since the same R1, R2, and TG values must be used for both scans. Do not run an Auto Prescan prior to the noise scan as the values will change.

1. Copy and paste the signal scan series: Select Signal Series, right mouse click and select Copy Series and Paste Series in the Rx Manager.
2. Click View Edit and set the protocols as described in the table below.

   Table 6 Noise Protocols

   Patient Position
   Patient Position	Supine
   Patient Entry	Head First
   Coil	GP Flex
   Series Description	Signal
   Imaging Parameters
   Plane	Axial
   Mode	2D
   Pulse Seq	GRE
   Imaging Options	None
   PSD Name	leave blank
   Protocol	leave blank
   Scan Timing
   # TE(s) per Scan	1
   TE	Min Full
   TR	34
   Flip Angle	1
   Bandwidth	15.63
   Acquisition Timing
   Freq	256
   Phase	256
   NEX	1
   Phase FOV	1.00
   Freq. DIR	R/L
   Autoshim	On
   Phase Correct	On
   Auto Center Freq	Peak
   Scanning Range
   FOV	24
   Slice Thickness	5
   Spacing	32.5
   A/P Center, R/L Center	0.00
   S/I Start	S75
   S/I End	I75
   # Slices	5
   Table Delta	0.00

3. Click Save RX and select Prepare to Scan.
4. Select the arrow next to the Scan button, select Research Options, then set rhformat and do_noise CVs to 1.
5. Run Manual Prescan. Verfiy R1, R2, and TG values are the same as recorded for signal scan. Do not make any changes.
6. Click Done.
7. Run Scan.

Regions of Interest (ROIs) in both signal and noise images can be measured directly in the image browser. A circular ROI should be used for the signal and a rectangular ROI for noise measurements centered at the phantom.

1. Click the Measure button, select the circular or rectangular shape, and adjust its size and orientation when the shape is displayed in the selected image. Mean, Standard Deviation, and area of the ROI will display in the lower right corner of the image.
   note:

   The SNR calculation uses the Mean of the signal image and Standard Deviation of the noise image.

2. For the signal measurement, choose an ROI covering approximately 80% of the phantom. Also measure the noise with an ROI covering 80% of the Field of View (FOV). Examples of typical ROIs are shown below.

   Figure 2. Signal and Noise Scan ROI Examples

   

The SNR measurements must be greater than or equal to the specifications in the table below.

• Images 1 and 5 are collected to verify that the phantom is correctly centered. They should be the same size.

• If they are not the same size, the phantom should be repositioned relative to the coil center.

  Table 7 SNR Specifications

  Slice Location	Image Number	SNR
  S37.5	2	72.5
  S0	3	106
  I37.5	4	72

There is only one pin diode in this antenna. This procedure indicates if the pin diode is defective and if the external cable is defective.

1. notice Due to safety hazards associated with soldering in the field, pin diodes are no longer FRUs. If the coil has a defective pin diode, the coil must be replaced.

   Select the Diode Test function on the Digital Multimeter (DMM).
2. Connect the positive lead of the DMM to the center pin of the BNC on the external cable. Connect the negative lead to the main housing/ground of the BNC.
3. Flex the external cable, especially near the connectors and the strain relief. A reading of 0.400 to 0.600 should remain on the DMM with no instability or fluctuations.

   • If the reading is below 0.400 in either direction, the output cable is shorted or the PIN diode is defective.

   • If the reading is above 0.600, the PIN diode is defective.

4. Connect the positive lead of the DMM to the main housing/ground of the BNC on the external cable, and connect the negative lead to the center pin of the BNC as shown below.

   Figure 3. BNC Connector

   

5. Flex the external cable, especially near the connectors and the strain relief. A reading of INFINITY should remain on the DMM with no instability or fluctuations.

   • If the reading is INFINITY in both directions, the output cable is open or the pin diode is open.

   • If the cable fails any of the above tests, refer to Flex Coil Replacements to replace the cable.

Use Table 5 to record the calculated signal to noise ratio (SNR) data obtained from the Functional Checks.