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ECG Triggering in Ultra-High Field
Cardiovascular MRI
Daniel Stäb
1,2
, Juergen Roessler
3
, Kieran O’Brien
4
, Christian Hamilton-Craig
5
, and Markus Barth
1
1
The Centre for Advanced Imaging, The University of Queensland, Brisbane, Queensland, Australia;
2
Department of Diagnostic and Interventional Radiology, University of
Würzburg, Würzburg, Germany;
3
Siemens Healthcare GmbH, Erlangen, Germany;
4
Siemens Healthcare Pty Ltd, Brisbane, Australia; and
5
Richard Slaughter Centre of
Excellence in CVMRI, The Prince Charles Hospital, Brisbane, Queensland, Australia
Corresponding Author:
Daniel Stäb
The Centre for Advanced Imaging,
The University of Queensland,
Brisbane St Lucia, QLD 4072, Australia;
E-mail: daniel.staeb@cai.uq.edu.au
Key Words: ECG, ultra-high field, magnetohydrodynamic effect, cardiac, MRI
Abbreviations: Magnetohydrodynamic (MHD), electrocardiogram (ECG), cardiovascular
magnetic resonance (CMR), vectorcardiography (VCG)
Cardiac magnetic resonance imaging at ultra-high field (B
0
ⱖ7 T) potentially provides improved resolution
and new opportunities for tissue characterization. Although an accurate synchronization of the acquisition to
the cardiac cycle is essential, electrocardiogram (ECG) triggering at ultra-high field can be significantly im-
pacted by the magnetohydrodynamic (MHD) effect. Blood flow within a static magnetic field induces a volt-
age, which superimposes the ECG and often affects the recognition of the R-wave. The MHD effect scales
with B
0
and is particularly pronounced at ultra-high field creating triggering-related image artifacts. Here, we
investigated the performance of a conventional 3-lead ECG trigger device and a state-of-the-art trigger algo-
rithm for cardiac ECG synchronization at 7 T. We show that by appropriate subject preparation and by in-
cluding a learning phase for the R-wave detection outside of the magnetic field, reliable ECG triggering is
feasible in healthy subjects at 7 T without additional equipment. Ultra-high field cardiac imaging was per-
formed with the ECG signal and the trigger events recorded in 8 healthy subjects. Despite severe ECG sig-
nal distortions, synchronized imaging was successfully performed. Recorded ECG signals, vectorcardio-
grams, and large consistency in trigger event spacing indicate high accuracy for R-wave detection.
INTRODUCTION
Cardiovascular magnetic resonance (CMR) is an important and
well-established clinical tool for the diagnosis and management
of cardiovascular diseases, and it is the standard of reference for
the evaluation of cardiac morphology and function (1-3). CMR
must overcome the challenges introduced because of cardiac
and respiratory motion. In the clinic, CMR relies on accurate
cardiac gating alongside parallel imaging (4,5), simultaneous
multi-slice imaging (6-8), or other acceleration methods (9,10)
to address limitations due to motion. However, the fact remains
that CMR must always make a tradeoff between spatiotemporal
resolution and signal-to-noise ratio.
The signal-to-noise ratio gain inherent at higher field
strengths (11) has recently led to an increased use of high field
systems with B
0
⫽3 T for clinical CMR (12), and moreover, it has
encouraged investigations into ultra-high field (B
0
ⱖ7T)CMR
(13-15). Apart from enabling spatial resolutions that exceed
today’s limits (16), CMR at ultra-high field offers new opportu-
nities for magnetic resonance-based tissue characterization (17,
18) or metabolic imaging (19).
Cardiac gating is usually performed using electrocardio-
gram (ECG) triggering. In general, vectorcardiography (VCG)-
based QRS detection algorithms (20) are used, which aim to
detect the R-wave in their peak by recognizing the R-wave’s
rising edge. However, ECG signal distortions from several effects
have been challenging at ultra-high field. The interaction of the
conductive fluid blood with the static magnetic field B
0
, for
instance, induces a voltage perpendicular to B
0
and the direction
of flow that superimposes on the ECG signal (21). This so-called
magnetohydrodynamic (MHD) effect is particularly large during
the early systolic phase, when the blood is ejected from the left
ventricle into the aortic arch. Hence, it mainly affects the T-
wave of the ECG signal (22-24). The probability that a rising
edge of an MHD artifact is similar to the rising edge of the
R-wave is generally nonzero. Consequently, deteriorated cardiac
synchronization is likely in the presence of strong MHD artifacts
that are similar to the R-wave’s rising edge. Problems have been
observed at clinical field strengths such as3T(
25,26), and
because the MHD effect scales with B
0
, distortions have been
reported to be worse at ultra-high field (27-29). In addition, the
time-varying magnetic gradient fields, which induce voltage
perturbations in the ECG leads, also distort the ECG signal. To
avoid motion artifacts, the lengthening of scan times and scan
repetitions that result from poor ECG triggering, the establish-
ADVANCES IN BRIEF
ABSTRACT
© 2016 The Authors. Published by Grapho Publications, LLC This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
ISSN 2379-1381 http://dx.doi.org/10.18383/j.tom.2016.00193
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ment of a stable cardiac synchronization technique is essential
to advance ultra-high field CMR.
Pulse triggering is often used in cases were the conventional
ECG approach fails (26). However, being derived from softly
shaped peaks in the pulse wave signal, the trigger events are
subject to immanent enhanced jittering (28,30), which com-
monly introduces trigger-related image artifacts. Because the
trigger events are also delayed with respect to the R-wave, pulse
triggering is unsuitable for modalities like myocardial tagging
that require an accurate detection of the R-wave.
Doppler ultrasound (30) and acoustic trigger devices (28,31)
as well as self-navigation (32-34) and pilot tone navigation (35)
have recently been explored as alternative tools to conventional
ECG triggering. In addition, advanced ECG detection algorithms
(36-40) have been proposed, and promising results have been
shown in initial studies.
Here, we explore the technical capabilities ofa7Tresearch
MRI system and state-of-the-art 3-lead ECG equipment for car-
diac synchronization at ultra-high field. Our initial study shows
that by including an appropriate ECG learning phase outside of
the magnetic field, existing ECG trigger technology in 7 T
research systems allows for generating stable and reliable ECG
trigger signals.
METHODOLOGY
All measurements were performed on a noncommercial 7 T
whole-body research MRI scanner (Siemens Healthcare GmbH,
Erlangen, Germany) under institutional review board permis-
sion. The gradient system provided a maximum gradient
strength of 70 mT/m and a slew rate of 200 T/m/s. A dedicated
7 T cardiac Tx/Rx array with 8 transmit and 32 receive channels
(MRI Tools GmbH, Berlin, Germany) was used for radiofre-
quency transmission and signal reception. The coil array was
operated in single-channel transmit mode. To improve the B
0
field homogeneity, third-order shimming was used. For all hu-
man in vivo experiments, written informed consent was ob-
tained before the examination as approved by the local ethics
committee.
Cardiac Synchronization
For cardiac synchronization, a 3-lead ECG trigger device
(Siemens Healthcare GmbH, Erlangen, Germany) using wire-
less signaling was used in conjunction with the standard trigger
algorithm provided by the device manufacturer. The basic prop-
erties of this algorithm are briefly described in the following
paragraph. For a detailed description, we refer the interested
reader to work by Frank et al. (41).
To ensure an accurate detection of the peak of the R-wave,
the trigger algorithm learns the shape of the rising edge of the
R-wave during an initial learning phase in both ECG channels.
Learning is performed while the subject is lying on the patient
table outside of the magnet bore, where the MHD effect is
typically negligible.
Once learning is completed, the trigger algorithm continu-
ously compares different derived entities (ie, derivatives, filtered
versions of derivatives) [for details refer Frank et al.’s study (41)]
of the incoming ECG signal with the corresponding entities of
the learned shape in real time. The comparisons are mainly
based on 2 filter functions. The first is a matched filter, which is
widely used in telecommunications (42) and mathematically
corresponds to forming the correlation of the 2 signals. The filter
function is given by the following equation:
mj(
)⫽aj·兺i⫽0
⌬t
sj(
⫺⌬t⫹t)·rj
*(t), (1)
where ais a normalization factor, ⌬tdepicts the period of
comparison, and jrefers to the signal entity for comparison. The
incoming and the reference signal entities are depicted by s(t)
and r(t), respectively. Both signal entities are complex with the
real and imaginary components derived from the 2 ECG chan-
nels. The second filter function sums up the squared differ-
ences between the incoming ECG signal and the learned
signal shape according to the following equation:
qj(
)⫽bj·兺t⫽0
⌬t
(ⱍsj(
⫺⌬t⫹t)·rj(t)ⱍ)2(2)
with baccounting for normalization. Trigger events are initiated
by thresholding the filtered signals, m
j
and q
j
. In addition to
using those filters, the angle of the VCG vector, which is spanned
by the signal in the 2 ECG channels at each time instant, is used
for R-wave detection. This angle criterion is implemented as a
necessary, but not sufficient, condition for a trigger generation.
The trigger algorithm showed an overall high performance at 1.5
T(
43).
In Vivo Measurements
To evaluate the performance of the underlying ECG trigger
algorithm, cardiac cine imaging at ultra-high field was per-
formed in 8 healthy volunteers. Before starting the imaging
procedure, ECG electrodes were placed onto the chest of the
subject, following the instructions of the trigger device manu-
facturer, and in conjunction with a senior electrophysiology
cardiac scientist. In 2 volunteers, the chest hair was removed in
the target area before electrode placement to ensure good cou-
pling at the body/electrode interface. After positioning the sub-
ject on the patient table, the ECG trigger device leads were
connected to the electrodes, which automatically started the
learning phase of the ECG algorithm. A pulse sensor (Siemens
Healthcare GmbH, Erlangen, Germany) was attached to the
subject’s index finger as a backup trigger device. After fin-
ishing all other preparatory steps, the learning phase of the
ECG algorithm was stopped to initiate the R-wave detection
mechanism. The signals detected in both available ECG chan-
nels and the generated trigger events were recorded through-
out all examinations.
Cardiac cine imaging was performed using a high-resolu-
tion breath-held ECG retro-gated segmented 2-dimensional
spoiled gradient echo (FLASH) sequence with the following
acquisition parameters: field of view ⫽360 ⫻270 (360 ⫻338)
mm
2
, matrix ⫽256 ⫻192 (256 ⫻240), slice thickness ⫽4.0
mm, echo time ⫽3.13 milliseconds, repetition time ⫽6.1
milliseconds, receiver bandwidth ⫽592 Hz/px, flip angle ⫽60°,
phases ⫽20, segments ⫽7, and temporal resolution ⫽43
milliseconds. Parallel imaging was used with an acceleration
factor of R ⫽2. All images were reconstructed online using
GRAPPA with 34 reference lines for weight set calculation.
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Qualitative Evaluations
The performance of the underlying trigger algorithm and the
impact of the MHD effect onto the performance was qualita-
tively evaluated based on the recorded ECG signals. Signal time
curves for the individual ECG channels were visually assessed.
VCG plots were generated and examined to identify mistrigger-
ing. Finally, the time intervals between succeeding trigger
events were analyzed to obtain an estimate on the amount of
false positive and false negative trigger events.
RESULTS
A general overview of the MHD effect at ultra-high field can be
gained from Figure 1A. It shows the change of the detected
signal in one of the 2 ECG channels over several RR-intervals
during the transition of the examined subject into the isocenter
of the magnet. Outside of the magnet bore, the signal is
generally smooth and undistorted, and the R-wave is easy to
distinguish as the highest peak. With increasing magnetic flux
density, the MHD-related alterations of the ECG become increas-
ingly pronounced and result in a significant distortion of the
ECG signal at the magnet’s isocenter. In the depicted case, the
distorted T-wave clearly exceeds the R-wave.
Individual ECG channels can be affected in different ways,
as shown in Figure 1B, which compares the MHD effect on the
signals of the 2 different ECG channels. To reduce the influence
of inter-RR signal fluctuations, the ECG signals were averaged
over 40 consecutive RR-intervals. The signals detected in both
channels experience distortions at the isocenter of the magnet.
However, only in channel 2 does the overall shape of the signal
considerably change, and the R-wave is exceeded by
the distorted ECG segments. Despite the significant impact of the
MHD effect, the QRS complex is clearly identifiable in both
channels and—as can be seen from the accurate alignment of the
individual RR-intervals— has been accurately detected by the
trigger algorithm for each of the displayed cardiac cycles.
Exemplary VCG plots obtained in 3 volunteers outside and
at the isocenter of the magnet are given in Figure 2. In each
vectorcardiogram, the ECG signal recorded in channel 2 is plot-
ted against the signal measured in channel 1 over several RR-
intervals. Associated trigger events are superimposed (black
circles). The data were collected outside of the magnet bore (first
row, blue), at the isocenter in the absence of gradient activity
during free breathing (second row, red) and during a breath-held
cine scan (third row, yellow). The MHD effect-related increase of
the ECG signal variations within each RR-interval is apparent when
comparing the vectorcardiograms obtained outside and inside of
the magnet. In addition, the large magnetic field introduces con-
siderable changes in the shape of the vectorcardiograms. Altera-
tions can also be observed, when comparing data collected during
free breathing and breath-hold periods. The characteristic VCG
curves including the trigger events tend to be dispersed along the
vertical axis, when obtained during free breathing (second row). As
depicted by the enlarged section in Figure 2C (third row), gradient
activity seems to introduce only tiny additional deflections in the
VCG signal. For each subject, the location of the trigger events in
the VCG plots is preserved in the vast majority of cases when
exposing the subject to the ultra-high static magnetic field and
dynamic gradient fields.
The accuracy of the trigger algorithm is shown in more
detail in Figure 3. For one of the volunteers, ECG signal curves
obtained outside of the magnet bore (Figure 3A) and at the
magnet’s isocenter (Figure 3, B and C) are compared with each
other. The MHD effect-related distortions of the ECG signal are
clearly recognizable. Despite these distortions, trigger events are
typically placed accurately. Only on rare occasions, false nega-
Figure 1. Electrocardiogram (ECG) signal time curves obtained in a healthy subject. Evolution of the ECG signal in
channel 2 across consecutive RR-intervals during the transition from outside into the isocenter of the magnet (A). All
curves were aligned based on observed trigger events. RR-intervals recorded outside and at the isocenter of the magnet
are marked by blue and red lines, respectively. During the transition into the magnet (RR-intervals 30–100), the magne-
tohydrodynamic (MHD) effect increasingly impacts the ECG signal. Signal time curves averaged over 40 consecutive
RR-intervals (solid) and corresponding standard deviation (dotted) following alignment by the observed trigger events
observed outside (blue) and at the isocenter (red) of the magnet (B).
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tive and misplaced trigger events were observed. False positive
events were extremely rare. This is the case even in presence of
severe additional distortions (Figure 3C), which, in this case, can
be attributed to enhanced inhaling and exhaling in preparation
of a scan-related breath-hold. As seen in the enlarged section,
the gradient activity of the scan causes smaller variations of the
ECG signal. The start of the scan is marked by the dashed line.
An impression of the high trigger accuracy can also be
gained from Figure 4, which depicts histograms of the time
intervals between consecutive trigger events in 3 healthy sub-
jects. While the width of the histogram peak reflects the varia-
tion of the subject’s heart rate, outliers indicate false positive
trigger events and undetected RR-intervals. The spacing be-
tween almost all trigger events is in the range of a single
RR-interval. Only a few counts are spread out along the hori-
zontal axis of the histogram.
ECG-triggered cardiac cine imaging worked generally well
with the used synchronization setup. Representative exam-
ples of cardiac cine images obtained at 7 T are depicted in
Figure 5. Shown are diastolic and systolic time frames of a
short-axis (top row) and a 4-chamber long-axis view (bottom
row) of a healthy subject’s heart (see online Supplemental
Video 1 PLAY VIDEO and Video 2 PLAY VIDEO ). The myocardial walls
are well delineated, and the images are free of visible artifacts
that could be related to unsuccessful ECG triggering. The long-
axis views show slight signal inhomogeneities introduced by
Figure 2. Vectorcardiograms obtained in 3 healthy subjects outside of the magnet (first row, blue), at the isocenter of
the magnet during free breathing (second row, red), and during a breath-held cine acquisition (third row, yellow), each
within a time-frame of 20 seconds (A–C). The depicted scales reflect the actual relative signal amplitudes between the
channels. The generally small high-frequency signal variations induced by the imaging gradients can be seen in the en-
larged section in the third row for subject (C).
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destructive B1 interferences (arrows). Moreover, flow effects are
pronounced because of the high flip angle used.
DISCUSSION
For the medical application of CMR, the accurate synchroniza-
tion of the imaging protocol to the cardiac cycle is essential to
achieve high image quality and accurate results in functional
evaluations. In this initial study, we explored the applicability of
existing and state-of-the-art 3-lead ECG trigger technology for
cardiac synchronization at the ultra-high field strength of 7 T.
Without using additional hardware, the underlying trigger al-
gorithm generated reliable ECG trigger signals and provided the
Figure 3. ECG signal over time measured in one of the volunteers outside of the magnet (A) and at the isocenter of the
magnet (B–C) in the first (red) and second (blue) channel of the ECG. Trigger events (green vertical lines) are typically
placed accurately at the peak of the R-wave. The larger distortions in (C) can be attributed to deep breathing preceding
a breath-held cine acquisition. The enlarged section visualizes the start of the sequence (dashed line) and the effect of
the imaging gradients on the ECG signal (arrows).
Figure 4. Histograms analyzing the apparent time interval between succeeding trigger events in 3 healthy subjects
(A–C) outside (blue) and at the isocenter (red) of the magnet. The bins along the horizontal axis are separated by 100
milliseconds. For almost all events, the time elapsed with respect to the previous event is in the range of 1 RR-interval.
Counts at considerably larger or shorter intervals indicate false negative and false positive triggering.
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basis for high-fidelity cardiac imaging. Despite severe ECG sig-
nal distortions due to the MHD effect and breathing motion,
synchronized imaging was feasible without severe disruptions
in all healthy volunteers. The attached pulse sensor was not
required in any case as a substitute trigger device, and signifi-
cant synchronization-related image artifacts were not observed.
In general, ECG signal distortions can be introduced by
various motions and imaging gradients. In this initial study, the
distorting effect of imaging gradients turned out to be small,
although, as expected, considerable distortions were caused by
the MHD effect. Particularly irregular and, in some cases, severe
distortions could be attributed to breathing and subject motion.
Even in presence of these adverse effects, the utilized trigger
algorithm allows for an accurate R-wave detection, provided a
learning phase in the absence of the magnetic field has been
executed.
In the absence of the large magnetic field, ECG triggering
was found to be highly accurate with the used setup. In the
presence of the ultra-high field, the MHD effect led to significant
distortion of the ECG signal time curves in all volunteers. Nev-
ertheless, the overall synchronization accuracy remained high.
As indicated by the histogram analysis, the length of almost all
RR-intervals is within the range of a single RR-interval. The
relatively high accuracy can be explained by the fact that the
QRS complex of the ECG is typically only marginally impacted
by the MHD effect, and the used trigger algorithm relies on
real-time detection of the shape of the rising edge of the R-wave,
rather than signal thresholding of the R-wave. In this way, the
misplacement of trigger events and the generation of false
positive events is minimized. Consequently, even in cases where
the distorted T-wave of the ECG clearly exceeds the targeted
R-wave, accurate cardiac synchronization is feasible.
The generally high trigger accuracy is also recognizable in
the calculated vectorcardiograms. The location and the spread
of the trigger events were not significantly affected by the
exposure of the subject to the high magnetic field, despite the
considerable increase in signal fluctuations. Inside and outside
of the magnetic field, the VCG plots show different characteristic
patterns. Although the pattern change itself is governed by the
MHD effect, the additional dispersion of this pattern (Figure 2)
can be attributed to the breathing motion and the associated
movement of the chest with the electrodes through the static
magnetic field. In this study, imaging gradients did not have a
large influence on the VCG signal. As shown in Figures 2 and 3,
they only caused tiny and high-frequency signal deflections.
To achieve high trigger accuracy at ultra-high field, careful
subject preparation and electrode placement are essential to
ensure good connection of the electrodes with the subject’s skin
and to achieve high input signals in all ECG channels. As
depicted in Figures 2 and 3, gradient-induced ECG signal dis-
tortions can thus be kept small. A good preparation is particu-
larly important for applications such as retro-gated cine CMR,
where imaging gradients interfere with the QRS complex. Given
the additional effect of breathing motion corrupting the ECG
signal, properly instructing subjects on breathing techniques
might be helpful in achieving accurate triggering. This is par-
ticularly the case for free-breathing imaging applications.
In this work, accurate ECG triggering was achieved by fully
exploiting the technical capabilities of the ultra-high field scan-
ner and trigger equipment. Apart from conducting an appropri-
ate ECG learning phase, further hardware or software modifica-
tions were not required. Thus, the presented approach is widely
available and ready to use. Recently, published studies indicate
that using a larger number of ECG leads, the use of specifically
tailored trigger algorithms or combinations of both can be
advantageous in terms of trigger accuracy (36-40). Based on
this, we assume that the use of more leads or further refinements
of the algorithm could improve our results. However, the use of
a large number of leads also adds to patient discomfort and
preparation times.
Within the scope of this initial study, only a limited number
of healthy subjects could be examined. Thus, it is important to
note that the overall number of RR-intervals that could be
analyzed outside of the magnet bore is rather low. Moreover, it
is well-known that the success of ECG triggering can be highly
subject-dependent, and certainly, a larger number of subjects
need to be examined in future work to fully reveal the perfor-
mance of the trigger technology at hand. Apart from that, ECG
triggering can be particularly challenging in patient cohorts
with cardiac arrhythmia, where the MHD effect can be more
severe and variable.
In conclusion, we have shown that reliable cardiac
ECG triggering is feasible in healthy volunteers at ultra-high
Figure 5. Images obtained using fully ECG-trig-
gered cardiac cine imaging at ultra-high field.
Shown are diastolic (left) and systolic (right) time
frames of a short-axis (top) and horizontal long-
axis (bottom) view. Myocardial walls are accu-
rately delineated. Signal inhomogeneities induced
by destructive B1 interferences are depicted
(arrows).
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field by using a state-of-the-art 3-lead trigger device. The
used trigger algorithm provided sufficient accuracy for high-
fidelity cardiac cine imaging, despite severe ECG signal dis-
tortions due to the MHD effect. Future work will need to
further evaluate the algorithm in larger cohorts and pa-
tients with cardiac arrhythmia. Apart from CMR, other ultra-
high field imaging applications such as human brain func-
tional MRI with physiological noise correction may benefit
from the easy instrumentational setup and robust ECG
triggering.
Supplemental Materials
Video 1: http://dx.doi.org/10.18383/j.tom.2016.00193.vid.01
Video 2: http://dx.doi.org/10.18383/j.tom.2016.00193.vid.02
ACKNOWLEDGMENTS
CHC would like to acknowledge funding by the University of Queensland Academic Title
Holder Research Grant (ATHRF). The authors would like to thank Daniel Smith and Haris
Haqqani (Cardiac Electrophysiology Unit, The Prince Charles Hospital, Brisbane,
Queensland Australia) for their helpful advice on ECG devices, ECG triggering, and
ECG electrode placement. The authors acknowledge the facilities of the National
Imaging Facility (NIF) at the Centre for Advanced Imaging, University of Queensland,
and support by the NIF fellow Steffen Bollmann.
Disclosures: KO and JR disclose being employed by Siemens Healthcare. JR holds a
patent related to the work presented.
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