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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
1.
2.
3.
4.

5.
6.
7.
8.

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
1.

Poor skin prep or electrode contact
 Shave hair, clean skin with NuPrep, and dry skin
 Always use fresh pre-gelled electrodes

2.

Improper electrode positioning
 Align with electrical axis
 Move around if necessary

3.

Gradient & RF pulse noise
 Use ECG cable with active noise filter
 Use Active Fiber Optic Cable or Wireless VCG System

4.

Noise from patient moving in static magnetic field
 Stabilize patient
 Ensure good electrode contact (tape if necessary)

5.

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)


ECG Triggering
Patient Motion
Pitfall: Patient motion may cause false triggers.

Solution: Stabilize and comfort patient, ensure good
electrode contact, use active fiber optic cable with
active noise filter, or use wireless VCG system.

Actrive Fiber Optic Leads

Wireless VCG Leads


ECG Triggering
Gradient Pulsing Noise
Pitfall: Noise from gradient pulses.

Solution: Use fiber optic cable with active noise
filter, or use wireless VCG system.

Actrive Fiber Optic Leads

Wireless VCG Leads


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
8 to 10 hours of use on a full charge


Wireless VCG Triggering
2 Channel ECG System

AVF

Lead 1

• 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 2 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 2 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 1
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 1 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 1

AVF

VCG / standard
Channel 1 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 1 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
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
1

2

3

4

5

N 1

6 .........

interpolated frames

1 2 3 4 5 6.......
measured frames

2

3

4

5

6 .........

N

interpolated frames

N

1 2 3 4 56.......

N

measured frames

Undersampled cardiac cycle = 10-20%


Retrospective Triggering Setup

1.

1st Signal Mode is the signal source and triggering method.

2.

Average Cycle is the average RR interval, and is determined by the patient.

3.

Target RR is the center of the RR acceptance range, and is set by clicking on the
button Captured Cycle.

4.

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)

5.

TR is the measured temporal resolution of each cardiac phase, and is directly
proportional to the number of Segments.

6.

Segments is the number of K-space lines collected per phase each heartbeat.

7.

Arrhythmia Detection determines if arrhythmia rejection mode is on or off.

8.

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

1.

1st Signal Mode is the signal source and triggering mode.

2.

Average Cycle is the average RR interval, and is determined by the patient.

3.

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.

4.

Trigger Pulse is the number of heartbeats included in each cine loop, and is
typically set to 1.