Spike Noise Check

Prerequisites

Table 2. Tools and test equipment
Item	Quantity	Effectivity	Part number	Manufacturer
100 MHz storage oscilloscope, Tektronix	1	-	46-183029P61 46-183029P64	-
Twinax to BNC adapter	1	-		-

Table 3. Safety

warning

POISON HAZARD!
THE PHANTOM CONTAINS NICKEL, A SUSPECTED CARCINOGEN.
DO not INGEST. DISPOSE OF AS A HAZARDOUS WASTE ACCORDING TO STATE AND FEDERAL REGULATIONS.

notice

Possible Equipment Damage! Remove the Head Coil from the Patient Table before performing this procedure.

This procedure describes the check for spike noise. The following functions are a part of the check:

See Hardware Preparation for Spike Noise Data Collection for “Hardware Preparation for Spike Noise Data Collection.”
See Spike Noise Scan Preparation for “Spike Noise Scan Preparation.”
See Sample Noise Data Collection and Analysis for “Sample Noise Data Collection and Analysis.”

1 Hardware Preparation for Spike Noise Data Collection

Procedure

Connect the oscilloscope power plug to the service outlet or power strip inside the System cabinet to prevent ground loops that may cause higher floor noise.
Connect a BNC cable to J109 of the Mega Switch. Connect the other end of this cable to an open port on the 2nd Pen Panel.

note:
An extra length of BNC-to-BNC cable may be needed to reach the PEN panel.

Figure 1. J109, Mega SW Connection
With a coaxial cable connect the 2nd PEN panel port to channel 1 (CH1) on the oscilloscope (o-scope).
Connect another coaxial cable to J5 on the Exciter panel. Connect the other end of the coaxial cable to channel 2 (CH2) on the o-scope.

Figure 2. J5, Exciter
Connect another coaxial cable to J11 on the Exciter panel. Connect the other end of the coaxial cable to the Trigger Input on the o-scope.

Figure 3. J11, Exciter
Set up the oscilloscope as follows:
1. Channel 1 - 100 mV/div, 1 mW input, AC coupled, 1 meg ohm impedance.
2. Channel 2 - 5 V/div, 1 mW input, DC coupled, 1 meg ohm impedance.
3. Time Base - 2 msec/div.
4. Trigger - auto, external.

2 Spike Noise Scan Preparation

Procedure

At the operator workspace, select the Scan icon on the desktop control panel.
If necessary, exit out of any previous exams by selecting End Exam.
Click New Pt and enter the following:
1. Name: spike noise
2. ID: geservice
3. Weight (lb.): 111
Remove all phantoms from the patient table. This is an empty bore test.
Place the head coil without a phantom on the cradle. Landmark on the center of the head coil. At the keypad on the front magnet enclosure, press LANDMARK and MOVE TO SCAN.

Click Start Exam and then press Move to Scan.

note:

Use Series 2-8 per later procedure steps.

Table 4. Series 1 - 8 Scan Parameters
	Series 1:	Series 2:	Series 3:	Series 4:	Series 5:	Series 6:	Series 7:	Series 8:
	Head	Head	Head	Head	Body	Body	Body	Body
SCAN PARAMETER
Plane:	Axial	Axial	Sagittal	Coronal	Coronal	Sagittal	Axial	Axial
Sequence:	2D GRE	2D GRE	2D GRE	2D GRE	2D GRE	2D GRE	2D GRE	2D GRE
ACQUISITION TIMING
Freq:	256	256	256	256	256	256	256	256
Phase:	256	256	256	256	256	256	256	256
NEX:	1	1	1	1	1	1	1	1
Phase FOV:	1	1	1	1	1	1	1	1
Locs Before Pause:	0	0	0	0	0	0	0	0
Freq Dir:	A/P	R/L	A/P	R/L	R/L	S/I	R/L	A/P
SCAN TIMING
TE:	10	10	10	10	10	10	10	10
TR:	20	20	20	20	20	20	20	20
Flip Angle:	10	10	10	10	10	10	10	10
Bandwidth:	15.63	15.63	15.63	15.63	15.63	15.63	15.63	15.63
SCANNING RANGE
FOV:	24	24	24	24	24	24	24	24
SL THK:	5	5	5	5	5	5	5	5
Spacing:	1.5	1.5	1.5	1.5	1.5	1.5	1.5	1.5
Start:	I60	I60	R60	P60	P60	R60	I60	I60
End:	S60	S60	L60	A60	A60	L60	S60	S60
# Of Slices:	20	20	20	20	20	20	20	20

3 Sample Noise Data Collection and Analysis

Procedure

At the Operator Workspace, click Manual Prescan and set R1 = 11, R2 = 15, TG =0 and select Scan TR.
Go to the oscilloscope and adjust the scope trigger to display one cycle of the A/D window on the scope channel 2.

Figure 4. Oscilloscope Display For A/D Window
Press the PAUSE SCAN button on the console keyboard. Press the FAN button on the magnet enclosure to turn off the patient comfort fan (the fan turns on automatically with scan software).

note:
Ignore the first 30 seconds of data.
The Mulit-Generational Data acquisition/Radio Frequency (MGD/RF) are now in the receive mode and any noise can be observed at the Mega Switch Spike Detect port. The scope must be in auto trigger mode.
Put the scope in continuous acquisition mode. The method for doing this varies with the type of scope. Clear the oscilloscope display and ensure the trigger is in the auto position.
Observe the scope display for about two minutes. The noise envelope (channel 1) is typically 200 mV p–p. Check that there are no spikes >0.5 Nenv (where Nenv is the noise envelope peak to peak value) above the noise envelope.

Figure 5. Spike Noise Scope Waveform (Paused Prescan)

note:
Ideally, there should be no spikes beyond the envelope. Any spikes present indicate that a connection is loose, or a component is bad (e.g., intermittent connection of power cable to a shield cooler cold head, or copper fingers on exam room door are damaged). Noise spikes that are greater than the spike spec create images with corduroy-like artifacts. Noise spikes that are presently less than the spike spec get worse over time.

note:
If there are no spikes present and there I sdoubt whether the setup is working, simulate an RF leak by opening the exam room door, or by turning the room lights on or off if the site is RF quiet. Clear the oscilloscope display and observe the new waveform on the display. Noise spikes should display beyond the waveform envelope. Do not forget to close the exam room door when finished.
Press the FAN button on the magnet enclosure to turn on the patient comfort fan. Clear the oscilloscope display and ensure that the trigger is in the Auto position. Observe the oscilloscope display for about two minutes. The noise envelope (channel 1) should remain as described in Step 6 above.
Clear the oscilloscope display and change the trigger from Auto to Normal.
Press the FAN button on the magnet enclosure to turn off the patient comfort fan.
Press the START SCAN button on the console keyboard to resume pre-scan. Click on Scan T/R.
Observe the scope display after two minutes. The noise envelope (channel 1) should be approximately 200 mV. Check that there are no spikes >0.5 Nenv (where Nenv is the noise envelope peak-to-peak value) above the noise envelope in the region of interest (except for maybe a beginning and ending transition spike). See Figure 5.

note:
The Region of Interest extends beyond when the observed Data In Window signal is high because the Data In Window position varies with Excitation Time (TE). The Transition Spike (not always present) occurs when the RF subsystem transitions from transmit to receive frequency; it occurs ~1 msec prior to the Data In Window. (The frequency transition point can be confirmed by viewing the DDS OUT signal on the Exciter Board.)

note:
If the spike noise appears only during pre-scan, the problem is more likely to be a loose connector, freely hanging cables near the magnet, poorly soldered components, or coil mounting hardware. Investigate anything that is subject to vibration from gradient activity.
Click Done.
Click New Series. Set up scan prescription for series #2 scan per notes and Table 4, then repeat steps Sample Noise Data Collection and Analysis.
Repeat the previous step until all head pre-scans (series #1 through #4) are complete.

4 Finalization

Procedure

When you’re done with the loopback testing then restore the cabling to the default clinical state.
Perform a Check Scan to check the system operation.