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MR Cardiac ECG Triggering

This Web Based Training discusses the physiological basis of ECG gating methods. We will explore techniques such as patient preparation, electrode and lead positioning, as well as other factors that will improve your ECG gating which can improve your image quality.

Objectives
- Identify the physiology of the heart
- Describe the patient preparation for cardiac triggering
- Identify the primary causes of poor ECG signal
- Perform learning phase of ECG triggering
- Perform prospective and retrospective triggering

 

This concludes the course MR Cardiac ECG Triggering. You should now be able to: Identify the physiology of the heart Describe the patient preparation for cardiac triggering Identify the primary causes of poor ECG signal Perform learning phase of ECG triggering Perform prospective and retrospective triggering By the end of this course you should be able to: Identify the physiology of the heart Describe the patient preparation for cardiac triggering Identify the primary causes of poor ECG signal Perform learning phase of ECG triggering Perform prospective and retrospective triggering Click the tabs below to view information about the physiology of the heart as well as using ECG triggering to synchronize the MRI data acquisition. Place ECG leads on skin:   - Detect ECG voltages   - Trigger MRI data Characteristic ECG Waveform:   - P – atria contract   - QRS – atria relax & ventricles contract   - T wave – ventricles relax Electrical Axis is the direction on the net sum of all voltages Points caudal and to the left Direction based patient body habitus Tall patients - more vertical Short patients - more horizontal Shave chest hairs if necessary Clean and roughen skin with NuPrep abrasive gel Thoroughly dry skin with soft clean cloth Use MR-Safe pre-gelled electrodes with adhesive pads   Siemens Approved, MR Safe Electrodes Conmed Cleartrace 2700 for wireless VCG system Application Hints Verify MR-compatibility with electrode vendor Verify electrodes are within the expiration date Do not leave pre-gelled electrodes exposed to air, they will dry out Consequences of using non-MR-compatible Electrodes Worst Case:  Patient burns Typically:  ECG Interference causes poor image quality   Click the tabs below to view the cardiac ECG positioning options based on your software. Position the PERU in the appropriate sponge   Route the cable straight out of the magnet with no loops or kinks   Check ECG waveform integrity both in and out of magnet bore   Reposition electrodes or change lead selection if necessary   Wireless technology reduces patient motion artifacts Vector Cardiogram (VCG) signal processing reduces flow artifacts Active RF and Gradient noise suppression filters Wall mounted battery charger     2-channel system has 3 electrodes 1 vertical signal (AVF) 1 horizontal signal (Lead I) 3-channel system has 4 electrodes: 1 vertical signal (Lead 1) 2 horizontal signals (Leads 2,3) 1-dimensional magnitude signal is generated from 2 channels Trigger Detection Algorithm is applied to the magnitude signal   2-dimensional signal is generated from 2 channels  True R-waves are clustered and spatially separate from artifacts  Proximity to cluster is used to eliminate false-triggers   Aortic blood flow causes an artifactual pulse on ECG signal (arrows) Flow pulse occurs only when patient is inside the magnetic field VCG signal processing eliminates effects of flow pulse on triggering   Learning phase obtains a clean VCG signal with patient outside bore   Measured during last 10 clean heartbeats prior to moving table into bore   Learning phase continues until table is more than 300mm into bore        Visually verify all VCG signals are clean for at least 10 heart beats while patient lies quietly on the table is at its home position Terminate the Learning Phase by moving the table into the bore at least 300mm before making any last minute adjustments Continue monitoring all VCG signals during the entire exam to ensure they both continue to trigger correctly Note:  If a patient is moved out during the exam, repeat the Learning Phase within the 300 mm zone B-Line Software (VB17 & up) 4 different algorithms D-Line Software (VD11 & up) 5 different algorithms Relearn VCG function also available All options are user-selectable via a pull-down menu on Physio Display Good ECG signals are seen but no trigger pulses Use a different triggering algorithm  -OR- Use relearn VCG function with patient at isocenter                                                   Click the tabs below to view key information about the triggering modes for 2-channel systems. This mode is optional Use if the advanced VCG algorithm is not working Uses slope detection to compare the slope of the R-wave of both signals  If R-wave slope matches closely enough, a trigger event is issued These modes are optional If the R-wave slope of Lead 1 or Lead AVF exceeds a preset thresh-hold, a trigger event is issued This should only be used if both VCG algorithms are not working and a single channel is best Click the tabs below to view key information about the triggering modes for 3-channel systems. This should be used if the advanced VCG algorithm is not working Slope detection is used to compare the slope of the R-wave of the best 2 channels    If the R-wave slope matches closely enough, a trigger event is issued This should only be used if both VCG algorithms are not working and a single channel is best If the R-wave slope of Lead 1, Lead 2, Lead 3  exceeds a preset thresh-hold, a trigger event is issued Since this is based on a single signal, no prior information from the Learning Phase is applied Retrospective Triggering Measures through entire cardiac cycle Arrhythmia rejection is available            Acquisition Window automatically adjusted Interpolates to any desired cine frame-rate Prospective Triggering Measures less than entire cardiac cycle Very sensitive to arrhythmias and variable heart rates Acquisition Window manually adjusted Cine frame-rate determined by # of measured segments Average Cycle is the average RR interval determined by the patient’s heart rate Calculated Phase is the number of reconstructed cardiac phases in the cine loop Segments is the number of shared views per segment Arrhythmia Detection By Time accepts only RR values falling into the acceptable range Trigger Window is the width of the acceptable range of RR values Target RR is the center of the acceptable range of RR values (click Capture Cycle) 1st Signal/Mode is the signal source and triggering method, set to ECG/Trigger Average Cycle is the average RR interval, determined by the patient’s heart rate Acquisition Window is the period for data collection, set to about 90% of the Average Cycle or even less for highly variable heart rates Trigger Pulse is the number of heart beats included in each cine loop, set to 1 Missed R-wave triggers Triggering intermittently on flow artifact or T wave Miscalculated heart rate Poor image quality                                Poor skin prep or electrode contact Shave hair if needed Clean skin with NuPrep and dry skin Always use fresh pre-gelled electrodes Noise from patient motion Stabilize patient in a comfortable position Ensure good electrode contact (tape if necessary) Improper electrode positioning Reposition if necessary   Non-MR Compatible electrodes Use Conmed Cleartrace 2700   Gradient & RF pulse noise Use Wireless VCG system   Flow Effects (magneto-hydrodynamic) Relearn VCG Triggering Use optimal Triggering Algorithm