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ECG TIMING DELAY
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10.15.2014
Application Note 281:
ECG Timing Delay Associated with Amplifier Filter Selections
This application note is concerned with the nature of ECG100C and ECG100C-MRI timing delays. In particular, the focus
is on ECG data collected in fMRI or MRI for the express purpose of triggering magnet scans during imaging. This
procedure is important when attempting to image physiological processes in synchronization with the heart.
Typically, ECG data is collected via the ECG100C-MRI and this data is piped out of an MP System, to drive a Digital
Trigger Unit (DTU300), which in turn drives the magnet scanning process, via magnet scan trigger input. This entire
process occurs in real time, not subject to any computer processing delay
The signal sequence would appear as follows:
ECG100C-MRI  UIM100C  DTU300  MAGNET SCAN TRIGGER INPUT
Figure 1: Data Collected with ECG100C-MRI
in a 1.5T MRI
Figure 2: Close-up view of ECG Channels at point
of EPI Start
Note that EPI starts at roughly 41.40 seconds into the recording. Note that magnetohydrodynamic (MHD) effects are
apparent on the ECG data, as a consequence of the heart’s mechanical action in a 1.5T static magnetic field.
Furthermore, as typical for ECG recording in the MRI or fMRI, artifact is evident as a consequence of EPI generated
magnetic field gradient changes during imaging. These gradient changes induce voltages in the body of the subject and
are recorded in superposition with the subject’s ECG.
These artifacts can be greatly reduced by
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using the ECG100C-MRI MRI Smart Amplifier
using the respective cable/filter set (MECMRI-BIOP)
placing ECG leads close to the heart
observing good electrode attachment methods, including scrubbing skin with prep gel (ELPREP)
keeping carbon composition leads short (LEAD108B preferred)
checking electrode impedances - under 5 kΩ (EL-CHECK)
Figure 2 Channel 1 (blue) is the raw ECG100C-MRI data, Channel 2 (green) is the Channel 1 data processed by the
ECG100C-MRI amplifier module filter selection of 150 Hz LP, and Channel 3 (cyan) is the Channel 1 data processed by
the module filter selection of 35 Hz LPN. Note that the bottom two waveforms are slightly delayed with respect to the raw
ECG data collected in the magnet.
The 150 Hz LP filter introduces a delay of 1.5 ms and the 35 Hz LPN filter introduces a delay of 6.5 ms. Accordingly, if the
magnet scan is triggered when either of these amplifier module filters is selected then the scan will be delayed, by this
specified amount, in relation to the actual R-wave peak timing.
ECG Timing Delay in MRI or fMRI
BIOPAC Systems, Inc.
This data demonstrates that it is practical to collect ECG data in the fMRI or MRI and use that real-time data to reliably
trigger a magnet scan. Although generated magnet artifact does not materially impact the ability of the ECG100C-MRI
amplifier’s output to drive the DTU300 (in this data set), note that a synchronized scan will also be completely devoid of
artifact before and after the scan period. Accordingly, so long as blanking is employed on the DTU300, any artifact
generated in the ECG data will not affect DTU300 triggering operation, despite possibly egregious artifact in the collected
ECG data.
Figure 3: Close-up view of ECG Channels overlapped
Figure 4: Close-up view of ECG Channels with Triggers
Figure 4 indicates the associated trigger timing relationships depending on the ECG100C-MRI amplifier module’s filter
switch options of 150 Hz LP or 35 Hz LPN. Note that depending on the value of the trigger threshold, the respective
trigger position is considerably affected. In this case, the trigger threshold on the DTU300 was set to 1 mV. The 4th
channel (brown) is the generated trigger associated with the module’s 150 Hz LP filter setting. The 5th channel (red) is the
generated trigger associated with the module’s 35 Hz LPN filter setting. The DTU300 will output TTL compatible (0-5 V)
trigger output levels to control the fMRI or MRI magnet scanning process.
Related reading:
Nijm, G.M.; Swiryn, S.; Larson, A.C.; Sahakian, A.V., “Characterization of the Magnetohydrodynamic Effect as a
Signal from the Surface Electrocardiogram during Cardiac Magnetic Resonance Imaging,” Computers in Cardiology,
2006 , vol., no., pp.269,272, 17-20 Sept. 2006
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