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Tips for cardiac triggering in MRI

Application Tip
de Kok, Wendy Philips Healthcare

In cardiac exams, the patients' ECG-signal is measured to trigger scans to the cardiac cycle of the patient. A four-lead VCG signal is measured to reduce the effects of the static magnetic field on the ECG-signal. Recommendations for electrode positioning vary depending on the available triggering module (non-wireless vs wireless).

Tip 1: electrode positioning for non-wireless triggering module

Systems that operate on SW release 2.6.1 or lower, are all equipped with a non-wireless triggering module.


Recommended lead positioning for the
non-wireless triggering module is displayed in the image below and is set-up according to these guidelines:

  • white electrode at first intercostal space, just left of the sternum

  • green electrode at the level of the xyphoid, just left of the sternum

  • red electrode on the patient's left side, horizontally aligned with the green electrode

  • black electrode in vincinity of green


  • white, red and green electrode make 90-degree angle

  • distance white / green ~ 15 cm

  • distance red / green ~ 15 cm


Green is common active, black is common ground in this set-up

 Recommended electrode positioning for non-wireless VCG-module
Recommended electrode positioning for non-wireless VCG-module

Tip 2: electrode positioning for wireless triggering module

Systems that operate on SW release 2.6.3 or higher are either equipped with a wireless triggering module or with a non-wireless triggering module. The recommendations for non-wireless triggering module are described in the previous section.


Recommended lead positioning for the
wireless triggering module is displayed in the image below and is set-up according to these guidelines:


  • white electrode at first intercostal space, just left of the sternum

  • green electrode at the level of the xyphoid, just left of the sternum

  • red electrode on the patient's left side, horizontally aligned with the green electrode

  • black electrode close to the left armpit


  • white, red and green electrode make 90-degree angle

  • black electrode completes the "square"

  • distance white / green ~ 15 cm

  • distance red / green ~ 15 cm


Black is common active, green is common ground in this set-up


Using this electrode set-up allows for interoperability with InVivo Precess monitoring equipment.

 Recommended electrode positioning for wireless VCG-module
Recommended electrode positioning for wireless VCG-module

Tip 3: alternative electrode positioning for wireless triggering module

Alternative electrode positioning using the wireless triggering module is possible if interoperability with the Invivo Precess monitoring equipment is not required. The alternative lead positioning is displayed in the image below and set-up according to these guidelines:


  • white electrode at first intercostal space, just left of the sternum

  • black electrode at the level of the xyphoid, just left of the sternum

  • red electrode on the patient's left side, horizontally aligned with the black electrode

  • green electrode in vincinity of black


  • white, red and black electrode make a 90-degree angle

  • distance white / black ~ 15 cm

  • distance red / black ~ 15 cm


Black is common active, green is common ground in this set-up
Please note that the color coding is different for non-wireless triggering!

 Alternative electrode positioning for wireless VCG-module
Alternative electrode positioning for wireless VCG-module

Tip 4: VCG-triggering in combination with inspiration breath-hold

Cardiac triggered scans are usually acquired in expiration breath-hold. However, inspiration breath-holds are sometimes required. Deep inspiration changes the position and orientation of the heart in the patient's chest in relation to the VCG-electrodes. This might result in deterioration of the VCG-signal.


The VCG signal can be improved by positioning all electrodes a few cm below their original position. Always apply new electrodes in this case, never re-use electrodes.

Tip 5: Use of PPU-triggering for retrospective triggered scans

In patients where VCG-triggering is not possible, PPU-triggering can be used as an alternative. Detection of the QRS-complex in the finger is delayed however so it might be required to adapt the trigger delay time of the scans.


For retrospective triggered scans:

  • change the "trigger device" on the contrast tab from ECG to PPU.

Changes to trigger delay time are not required.

The first image in the PPU-triggered scan is acquired at the detection of the QRS-complex in the finger and is delayed in comparison to VCG-triggering. Full coverage of the cardiac cycle is however preserved and all required functional information will be available.

Delay in detection of QRS-complex: The red arrow indicates the detection of the QRS-complex in the PPU-signal. The QRS-complex was detected earlier in the VCG-signal First phase of a retrospective triggered cine scan using VCG. The image is acquired in end-diastole First phase of a retrospective triggered cine scan using PPU. The image is acquired in mid-systole
Delay in detection of QRS-complex:
The red arrow indicates the detection of the QRS-complex in the PPU-signal. The QRS-complex was detected earlier in the VCG-signal
First phase of a retrospective triggered cine scan using VCG. The image is acquired in end-diastole
First phase of a retrospective triggered cine scan using PPU. The image is acquired in mid-systole

Tip 6: Use of PPU-triggering for prospective triggered scans

In patients where VCG-triggering is not possible, PPU-triggering can be used as an alternative. Detection of the QRS-complex in the finger is delayed however so it might be required to adapt the trigger delay time of the scans.


For prospective triggered scans:

  • change the "trigger device" on the contrast tab from ECG to PPU.

  • change the "trigger delay time" on the contrast tab from e.g mid-diastole to user defined

  • enter a user defined time that is ~ 250 ms shorter than the desired trigger delay time (for heart rates 60 - 90 bpm)

 The red arrow indicates a standard trigger delay time in a two-beat sequence. Using PPU-triggering, the timing towards the VCG-signal is incorrect (blue arrow) The red arrow indicates the user-defined trigger delay time in a two-beat sequence. Using PPU-triggering, the timing towards the VCG-signal is correct (blue arrow)
The red arrow indicates a standard trigger delay time in a two-beat sequence. Using PPU-triggering, the timing towards the VCG-signal is incorrect (blue arrow)
The red arrow indicates the user-defined trigger delay time in a two-beat sequence. Using PPU-triggering, the timing towards the VCG-signal is correct (blue arrow)
 VCG-triggered black blood image. Trigger delay time = mid-diastole PPU-triggered black blood image. Trigger delay time = mid-diastole. The heart is in systole due to delay in the detection of the QRS-complex PPU-triggered black blood image. Trigger delay time = user defined, shorter than mid-diastole. The heart is in mid-diastole after compensating for the delay
VCG-triggered black blood image. Trigger delay time = mid-diastole
PPU-triggered black blood image. Trigger delay time = mid-diastole. The heart is in systole due to delay in the detection of the QRS-complex
PPU-triggered black blood image. Trigger delay time = user defined, shorter than mid-diastole. The heart is in mid-diastole after compensating for the delay

Tip 7: Entered heart rate

All triggering-related parameters are derived from the heart rate that is entered at ExamCard level at start of scan. It is important to make sure that the entered heart rate matches the patient's actual heart rate.


The patient's actual heart rate might increase during long breath-hold scans or during stress
examinations. Missing triggers can be avoided in this case by entering a heart rate that is slightly higher than the patient's actual heart rate.



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May 9, 2011

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Application Tip
Achieva 1.5T, Achieva 3.0T, Ingenia 1.5T, Ingenia 3.0T
Cardiac, Cardiac function, Cardiac function cine, Cardiac Morphology, dStream, Heart, triggering, VCG
 

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