Cardiac Triggering Basic Concepts Cardiac Function Exam (ecg-triggered cine technique) Cardiac Triggering Signal Source & Detection Two Pacemakers: • SinoAtrial Node (SA) • AtrioVentricular Node(AV) Conduction Network: • Bundle Branches • Purkinje Fibers in Muscle Current Propagates: • Cause heart contractions • Cause voltages at skin Cardiac Triggering Signal Source & Detection “Electrical Axis” is the direction of the net sum of all voltages, and generally points caudal and to the left Direction varies based upon patient body habitus, more vertical for tall patients or horizontal for short patients Direction varies cyclically during RR cycle Cardiac Triggering Signal Source & Detection 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 Cardiac Triggering Patient Preparation Shave chest hairs if necessary Clean and roughen skin with NuPrep abrasive gel Thoroughly dry skin with soft clean cloth Use MR-compatible pre-gelled electrodes with adhesive pads Attach electrode pads on chest in optimal locations (elecrical axis) Route cable straight out of 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 Standard 4-leads Active Fiber Optic 3-leads Wireless VCG Cardiac Triggering MR-Compatible Electrodes  Mfg’s of MR-compatible Electrodes  Conmed Cleartrace 2700 for wireless VCG system  InVivo Tritrode for active fiber optic ECG 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 Cardiac Triggering Adverse Effects of Poor ECG Signal • Missed R-waves • Poor image quality • Triggering intermittently on flow artifact • Miscalculated heart rate ECG Triggering Causes of Poor ECG Signal Poor skin prep or electrode contact  Shave hair, clean skin with NuPrep, and dry skin  Always use fresh pre-gelled electrodes Improper electrode positioning  Align with electrical axis  Move around if necessary Gradient & RF pulse noise  Use ECG cable with active noise filter  Use Active Fiber Optic Cable or Wireless VCG System Noise from patient moving in static magnetic field  Stabilize patient  Ensure good electrode contact (tape if necessary) Flow Effects (magneto-hydrodynamic)  Use Vector Cardiography (VCG) signal analysis ECG Triggering MR-related Signal Noise  B0 related (main static field)  magnetohydrodynamic effect  flux change due to respiratory motion  flux change due to cable motion  B1 related (radio-frequency field)  RF pulses  electrode and ECG amplifier related  G related (gradient switching) Cardiac Triggering Wireless VCG What is it ? Why is it useful ? How does it work ? Wireless VCG Triggering • Wireless BlueTooth Technology reduces patient motion artifacts • Vector Cardiography (VCG) signal processing reduces flow artifacts • Active RF and Gradient noise suppression • Wall mounted battery charger lasts to 10 hours of use on a full charge Wireless VCG Triggering Channel ECG System AVF Lead • Fixed lead placement with no guesswork • One vertical signal (AVF) • One horizontal signal (Lead I) Wireless VCG Triggering Magnitude Signal AVF z-direction Lead I √ x2 + z2 Magnitude Signal x-direction • 1-dimensional magnitude signal is generated from channels • Trigger detection algorithm is applied to the magnitude signal Wireless VCG Triggering Phase Signal AVF z-direction Lead I x-direction Phase Signal • 2-dimensional phase signal is generated from channels • True R-waves are clustered and spatially separate from artifacts • Directional information is used to eliminate false-triggers Arterial Flow Pulse Magneto-Hydrodynamic Effect Outside Bore (0 Tesla) Inside Bore (1.5 Tesla) • 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 VCG requires a “learning phase” ECG AVF • Learning phase obtains a clean VCG signal with patient outside bore • Measured during last 10 seconds prior to moving table into bore • Verify both Lead and AVF are clean and triggering correctly while patient lies quietly on table (at home position) during learning phase • Monitor both signals throughout the entire exam to ensure they both continue to trigger correctly Three Triggering Modes Available ECG AVF VCG / standard Channel only Channel AVF only • VCG / standard mode is usually best, but requires that both signals are clean and triggering correctly • Alternatively, single-channel mode may be used if only signal is clean and triggering correctly Ecg-Triggering Strategies Prospective Triggering • Measures less than entire cardiac cycle • Very sensitive to arrhythmias and variable heartrates • Acquisition Window manually adjusted • Cine frame-rate determined by # of measured segments Retrospective Triggering • Measures through entire cardiac cycle • Arrhythmia rejection is available • Acquisition Window automatically adjusted • Interpolates to any desired cine frame-rate Most Common Ecg-Triggering Strategies ECG Trigger Pulses Measured Echoes Retrospective (most common) Prospective a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | N interpolated frames measured frames N interpolated frames N 56 N measured frames Undersampled cardiac cycle = 10-20% Retrospective Triggering Setup 1st Signal Mode is the signal source and triggering method Average Cycle is the average RR interval, and is determined by the patient Target RR is the center of the RR acceptance range, and is set by clicking on the button Captured Cycle Calculated Phases is the number of reconstructed cardiac phases in the cine loop, and is typically set from 20 to 30 Retrospective Triggering Setup (cont) TR is the measured temporal resolution of each cardiac phase, and is directly proportional to the number of Segments Segments is the number of K-space lines collected per phase each heartbeat Arrhythmia Detection determines if arrhythmia rejection mode is on or off Trigger Window is the range of acceptable RR values, where half is prior to Target RR and half is after Target RR; use larger values for arrhythmias or smaller values for consistent heartbeats Prospective Triggering Setup 1st Signal Mode is the signal source and triggering mode Average Cycle is the average RR interval, and is determined by the patient Acquisition Window is the period for data collection, and is typically set to about 90% of the Average Cycle or even less for highly variable heartrates Trigger Pulse is the number of heartbeats included in each cine loop, and is typically set to Prospective Triggering Setup (cont) Trigger Delay is the delay after the R wave, and is typically set to zero TR is the measured temporal resolution of each cardiac phase, and is directly proportional to the number of Segments Segments is the number of K-space lines collected per phase each heartbeat Phases is the number of measured frames in the cine loop ... quality Cardiac Triggering Adverse Effects of Poor ECG Signal • Missed R-waves • Poor image quality • Triggering intermittently on flow artifact • Miscalculated heart rate ECG Triggering Causes... Alternatively, single-channel mode may be used if only signal is clean and triggering correctly Ecg -Triggering Strategies Prospective Triggering • Measures less than entire cardiac cycle • Very sensitive... charge Wireless VCG Triggering Channel ECG System AVF Lead • Fixed lead placement with no guesswork • One vertical signal (AVF) • One horizontal signal (Lead I) Wireless VCG Triggering Magnitude