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Tips for cardiac triggering in MRI

Application Tip
de Kok, Wendy Philips Healthcare • Netherlands
Holthuis, Lonneke Philips • Netherlands

These application tips on cardiac triggering in MRI apply to systems equipped with a wired or wireless triggering module. This information on cardiac triggering can be used as a guidance to help optimize the quality of cardiac triggering.

During cardiac examinations, the patient’s ECG signal is measured to trigger scans to the cardiac cycle. A four-lead VCG (VectorCardioGraphy) signal is measured to reduce the effects of the static magnetic field on the ECG-signal. The measured signals are displayed in the physiology display.

Systems types for which this document is applicable:
  • Systems that operate on software release 2.6.1 or lower;
    these are all equipped with a wired module.
  • Systems that operate on software release 2.6.3 or higher.


See also Instructions for Use:

Do not use the physiology signals for monitoring or diagnostic purposes. The physiology signals are distorted when the patient is inside the magnet.

Tip 1: Check system hardware

Before VCG-triggering is used, it is important to check whether the system’s hardware is up-to-date. A field service engineer should check if all FCO’s that are applicable to the system are installed.

Tip 2: Factors affecting the VCG signal

For successful cardiac triggering of MR scans, a good VCG signal with a clearly detectable R-peak is a prerequisite. However the VCG signal and the calibration of the VCG signal are affected by several factors:

  • Position of the electrodes and the distance between electrodes:

Position the electrodes correctly to decrease disturbance in the VCG signal.

 

  • Patient respiration:

Cardiac scans are usually acquired in expiration breathhold. However, inspiration breathhold are is sometimes required. Deep inspiration changes the position and angulation of the heart in the patient's chest in relation to the VCG-electrodes. This might result in deterioration of the VCG-signal and some trigger loss may occur. The severity of the trigger loss depends on the position of the electrodes.
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.

 

  • Flowing blood and magnetic field:

The ECG-signal is a measurement of an electrical differential potential on the patient’s skin.
An electrical current that is placed in a magnetic field will cause induced voltages. The moving electrolytes in the blood (especially in the aortic arch) will also induce voltages. These are super-imposed on the measured VCG-signal.

 

In summary: the VCG-signal will change in the presence of a static magnetic field. These alterations are mainly seen in the baseline of the VCG-signal and usually superimposed on the area of the T-wave. The distortion is referred to as the T-wave artifact or the magneto-hydrodynamic effect (MHD).

The strength of the induced voltages increases with increasing field strength.

 

Figure 1 is a graphical representation of VCG-signals in the absence of a magnetic field (0T) and at increasing field strengths of 1.0T, 1.5T and 3.0T respectively.

  • Magnetic field and motion:

    Triggering algorithms rely on the detection of the R-top. Amplitude-based algorithms might result in incorrect triggering (detection of T-wave artifact) if the amplitude of the T-wave artifact approaches the amplitude of the R-top. 
    VectorCardioGram measures the VCG-signal in two orthogonal directions from which it is possible to resolve the real R-top from a possible T-wave artifact, as is shown in Figure 2.

    Practical implications:
    Even if the patient’s VCG signal looks normal during patient preparation, the VCG signal can become distorted once the patient is moved into the bore:

      • The B0-disruption causes the signal to rapidly changed shape.
      • Patient movement also disrupts the VCG-signal.

     

    Inspect the signal on the physiology display for trigger markers. If these are displayed on the R-top after the calibration is done, the system is ready for triggering

      • As a final check, ask the patient to breathe in and out a few times while in the bore to check if the trigger markers remain visible during inspiration and expiration. 

     

    See Figures 3.1, 3.2 and 3.3 for examples of the effect of B0 and motion on the trigger marker detection.

    Figure 1 T-wave artifact (indicated by circle) in VCG. Most likely occuring with the patient inside the bore.Figure 2 VCG-trace measured in two orthogonal directions.
    Figure 1
    Figure 2
    T-wave artifact (indicated by circle) in VCG. Most likely occuring with the patient inside the bore.
    VCG-trace measured in two orthogonal directions.
    Figure 3 1.Lying still outside the bore Perfect signal.

2.Moving outside the bore. Disturbed signal.

3.Lying still inside the bore. Disturbed signal.
    Figure 3
    1.Lying still outside the bore Perfect signal. 2.Moving outside the bore. Disturbed signal. 3.Lying still inside the bore. Disturbed signal.

    Tip 3: Patient preparation

    Proper patient preparation is essential for circumventing skin burns and for a reliable VCG signal.

     

    See also Instructions for Use:
    Prepare the patient's skin according to the electrodes manufacturer's Instructions for Use. To reduce skin impedance and to ensure good contact between electrodes and skin, proper skin preparation is required. Proper skin preparation:

    • Prevents electrodes from coming loose and consequently avoids signal loss.
    • Prevents high skin impedance that leads to noise in the VCG and to warming of the skin.

     

    When positioning ECG electrodes for the VCG module:

    • Only use MR safe or MR Conditional electrodes and leads.
    • Make sure electrodes have not passed their expiration date. Electrodes can dry out resulting in bad electrical contact.
    • Properly prepare the patient and apply the electrodes correctly.
    • Never reposition or reuse electrodes. Always use new electrodes.
    • Always carefully read and strictly follow the Instructions for Use of the electrode manufacturer.

     

    On Examination level the patient’s heart rate must be entered. All triggering-related parameters are derived from the entered heart rate. The patient’s actual heart rate might increase during long breath-hold scans or during stress examinations. Missed triggers can be avoided in this case by entering a heart rate that is slightly higher than the patient’s actual heart rate.

    Tip 4: Wireless electrode positioning

    Electrode positioning or lead placement is crucial to obtain a good VCG signal for triggering. The recommended lead placement for wireless triggering is shown in Figure 4.


    Figure 4 Wireless electrode positioning.
    Figure 4
    Wireless electrode positioning.

     

    Position the electrodes as described in this section:

    • Position the first electrode below the first intercostal space, just left of the sternum.
    • Position the second electrode just left of xiphoid, approximately 10 cm to 15 cm below the first electrode.
    • Position the third electrode at the patient's left side, forming a 90 degree angle with the other two electrodes. If positioned correctly, the imaginary line (between the second and the third electrode) runs under the nipple line.
    • Position the fourth electrode just below the second electrode.
    • Connect the leads to the electrode as shown in figure 4.
    • Make certain that the cables are correctly inserted into the VCG module as shown in figure 5.
     

    The distances between white and black, and between black and red must be ~ 15 cm to obtain sufficient amplitude of the VCG-signal. A smaller distance between the electrodes might reduce the noise and artifact level in the VCG signal, but it will decrease the amplitude of the measured R-peak as well. 

    See also Instructions for Use:
     

    • Do not place the VCG battery module (VCG or PPU unit) directly on the patient's skin. Direct contact may cause heating of the skin. Keep a distance to the patient's skin of at least 1 cm using pads of the standard auxiliary set.
    • Do not place the VCG battery module (VCG or PPU unit) close to the imaging volume. This may cause image artifacts.
    Figure 5 How to correctly instert cables into the VCG module.
    Figure 5
    How to correctly instert cables into the VCG module.

    Tip 5: Wired electrode positioning

    The recommended lead placement for wired triggering is shown in Figure 6.
    The black and green electrodes are exchanged from wireless physiology to wired physiology, while white and red are identical.
    Figure 6 Wired electrode positioning.
    Figure 6
    Wired electrode positioning.

    Tip 6: PPU as alternative to VCG-triggering

    In some cases, where the VCG signal is affected too much by e.g. the influence of the static magnetic field or the breathing pattern of the patient, VCG triggering might still fail.

    PPU (Peripheral Pulse Unit) can be used in those cases to perform cardiac triggered sequences.

    Please note that the detection of the PPU trigger moment is delayed when compared to the VCG R-peak detection as the blood pressure wave must propagate to the finger. And additional signal processing is required.

     

    The delay can easily be recognized in a comparison of Q-flow scans through the ascending aorta, acquired with VCG-triggering and PPU-triggering respectively.

     

    • In the VCG-triggered scan (Figure 7a), peak velocity in the AAO is measured shortly after the detection of the R-peak in the heart, which marks the start of the flow curve. The flow curve represents one full cardiac cycle, and as such, the end of the flow curve indicates the detection of the next R-peak.

     

    • In the PPU-triggered scan (Figure 7b), peak velocity in the AAO is measured already before the detection of the R-peak in the finger, which marks the end of the flow curve.

     

    Figure 7a and 7b VCG-triggered Qflow scan (left) and PPU-triggered Qflow scan (right).
    Figure 7a and 7b
    VCG-triggered Qflow scan (left) and PPU-triggered Qflow scan (right).

    For retrospective triggered scans, the first image in a PPU-triggered scan is acquired once the R-peak is detected in the finger, and it will be delayed in comparison to VCG-triggering. Full coverage of the cardiac cycle is however preserved and all required functional information will be available.

     
    Figures 8a and 8b show first image of a retrospective cine scan acquired with VCG and PPU-triggering respectively on Ingenia 1.5T. The PPU-triggered cine scan starts in mid-systole instead of at end-diastole.

    Figure 8a First phase of a VCG-triggered cine scan.Figure 8b First phase of a PPU-triggered cine scan.
    Figure 8a
    Figure 8b
    First phase of a VCG-triggered cine scan.
    First phase of a PPU-triggered cine scan.

    The delay will also affect trigger delay timing in prospective triggered scans: As an example, trigger delay “mid diastole” will be measured with a delay and will result in images that are triggered towards systole in the next cardiac cycle.

     
     

    Practical implications

     

    Change the setting of the parameter “trigger device” on the motion page from ECG to PPU

    • Note that the displayed value for patient’s heart rate in the physiology display is taken from the VCG-module as long as the leads are connected; either disconnect the VCG-leads or remove the battery from the VCG-module to display the patient’s heart rate as measured by the PPU-module

     

    • For retrospective triggered scans, the first image of the cine loop will be acquired in systole
    • For prospective triggered scans, trigger delay “mid-diastole” will result in images in systole; change trigger delay time to shortest or to user defined (enter a value that is shorter than the calculated value as displayed on the info page) to correct for the delay in R-peak detection.

    Tip 7: Manual VCG calibration

    You can either perform manual or continuous VCG calibration. It is recommended to use manual VCG calibration for smooth cardiac triggering in all patients.

    Manual VCG calibration is possible for systems that operate on software release 3.2.3 SP3 and on software release 5.3 and higher.

    Manual VCG calibration:
    • Allows you to determine the conditions under which the VCG calibration is performed.
    • Analyzes 15 seconds of VCG data to ensure accurate detection of R-peaks in all patients, also in patients at low heart rate or in arrhythmic patients.
    • Can be initiated at any moment in the workflow. It is recommended to execute it before moving the patient to the isocenter.
    • gives you the possibility to adjust the trigger threshold during cardiac triggered scans to adjust the sensitivity for the detection of R-peaks.

     



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    Application Tip
    Cardiac, ECG, heart, triggering, VCG, wireless
     

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