Acoustic cardiac triggering: a practical solution for synchronization and gating of cardiovascular magnetic resonance at 7 Tesla.

Abstract

To demonstrate the applicability of acoustic cardiac triggering (ACT) for imaging of the heart at ultrahigh magnetic fields (7.0 T) by comparing phonocardiogram, conventional vector electrocardiogram (ECG) and traditional pulse oximetry (POX) triggered 2D CINE acquisitions together with (i) a qualitative image quality analysis, (ii) an assessment of the left ventricular function parameter and (iii) an examination of trigger reliability and trigger detection variance derived from the signal waveforms.
ECG was susceptible to severe distortions at 7.0 T. POX and ACT provided waveforms free of interferences from electromagnetic fields or from magneto-hydrodynamic effects. Frequent R-wave mis-registration occurred in ECG-triggered acquisitions with a failure rate of up to 30% resulting in cardiac motion induced artifacts. ACT and POX triggering produced images free of cardiac motion artefacts. ECG showed a severe jitter in the R-wave detection. POX also showed a trigger jitter of approximately Δt = 72 ms which is equivalent to two cardiac phases. ACT showed a jitter of approximately Δt = 5 ms only. ECG waveforms revealed a standard deviation for the cardiac trigger offset larger than that observed for ACT or POX waveforms.Image quality assessment showed that ACT substantially improved image quality as compared to ECG (image quality score at end-diastole: ECG = 1.7 ± 0.5, ACT = 2.4 ± 0.5, p = 0.04) while the comparison between ECG vs. POX gated acquisitions showed no significant differences in image quality (image quality score: ECG = 1.7 ± 0.5, POX = 2.0 ± 0.5, p = 0.34).
The applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACT's trigger reliability and fidelity are superior to that of ECG and POX. ACT promises to be beneficial for cardiovascular magnetic resonance at ultra-high field strengths including 7.0 T.

Full text

Page 1

RESEARCHOpen Access
Acoustic cardiac triggering: a practical solution
for synchronization and gating of cardiovascular
magnetic resonance at 7 Tesla
Tobias Frauenrath1*, Fabian Hezel1, Wolfgang Renz1,2, Thibaut de Geyer d’Orth1, Matthias Dieringer1,3,4,
Florian von Knobelsdorff-Brenkenhoff3,4, Marcel Prothmann3, Jeanette Schulz Menger1,3,4, Thoralf Niendorf1,4
Abstract
Background: To demonstrate the applicability of acoustic cardiac triggering (ACT) for imaging of the heart at
ultrahigh magnetic fields (7.0 T) by comparing phonocardiogram, conventional vector electrocardiogram (ECG) and
traditional pulse oximetry (POX) triggered 2D CINE acquisitions together with (i) a qualitative image quality analysis,
(ii) an assessment of the left ventricular function parameter and (iii) an examination of trigger reliability and trigger
detection variance derived from the signal waveforms.
Results: ECG was susceptible to severe distortions at 7.0 T. POX and ACT provided waveforms free of interferences
from electromagnetic fields or from magneto-hydrodynamic effects. Frequent R-wave mis-registration occurred in
ECG-triggered acquisitions with a failure rate of up to 30% resulting in cardiac motion induced artifacts. ACT and
POX triggering produced images free of cardiac motion artefacts. ECG showed a severe jitter in the R-wave
detection. POX also showed a trigger jitter of approximately Δt = 72 ms which is equivalent to two cardiac phases.
ACT showed a jitter of approximately Δt = 5 ms only. ECG waveforms revealed a standard deviation for the cardiac
trigger offset larger than that observed for ACT or POX waveforms.
Image quality assessment showed that ACT substantially improved image quality as compared to ECG (image
quality score at end-diastole: ECG = 1.7 ± 0.5, ACT = 2.4 ± 0.5, p = 0.04) while the comparison between ECG vs.
POX gated acquisitions showed no significant differences in image quality (image quality score: ECG = 1.7 ± 0.5,
POX = 2.0 ± 0.5, p = 0.34).
Conclusions: The applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACT’s
trigger reliability and fidelity are superior to that of ECG and POX. ACT promises to be beneficial for cardiovascular
magnetic resonance at ultra-high field strengths including 7.0 T.
Background
The challenge of synchronization of data acquisition
with the cardiac cycle constitutes a practical impediment
of cardiovascular magnetic resonance (CMR). Cardiac
motion has been addressed by synchronization strategies
exploiting (i) finger plethysmography [1], (ii) cardiac
activity related esophageal wall motion [2], (iii) invasive
left ventricular blood pressure gating [3], (iv) Doppler
ultrasound [4], (v) motion induced changes in the impe-
dance match of RF-coils [5], (vi) self gating techniques
[6-10] and optic acoustic methods [11] including human
and animal studies. In current clinical CMR, cardiac
motion is commonly dealt with using electrocardio-
graphic (ECG) or finger pulse oximetry (POX) trigger-
ing/gating techniques [12-14] to synchronize data
acquisition with the cardiac cycle. At higher magnetic
field strengths the artifact sensitivity of ECG recordings
and even of advanced vector ECG increases [13,15].
ECG, being an inherently electrical measurement with
electrically active components [12], does carry a risk of
surface heating of patients’ skin and even of skin burns
resulting from induction of high voltages in ECG hard-
ware [16-19]. As ultrahigh field CMR becomes more
widespread, the propensity of ECG recordings to
* Correspondence: Tobias.Frauenrath@mdc-berlin.de
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular
Medicine, Berlin, Germany
Full list of author information is available at the end of the article
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
© 2010 Frauenrath et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative
Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.

Page 2

interference from electromagnetic fields and to magneto-
hydrodynamic effects is further pronounced [20-22].
Realizing the constraints of conventional ECG, a MR-
stethoscope which uses the phonocardiogram has been
proposed. Its feasibility for the pursuit of/prospectively
triggered and retrospectively gated cardiac imaging has
been demonstrated for field strengths up to 3.0 T
[20,21,23] The applicability and clinical efficacy of
acoustic cardiac triggering (ACT) has not been demon-
strated for imaging of the heart at ultrahigh magnetic
fields yet due to the lack of appropriate RF coils and
other practical obstacles of CMR at 7.0 T. Recently, R-
wave mis-registration has been consistently reported for
ECG triggered CMR at 7.0 T [24-26]. Consequently, in
one study approximately 20% of the healthy subjects
needed to be excluded from left ventricular function
assessment [24,25]. In another study 80% of the acquisi-
tions were gated using pulse oximetry due to ECG-
triggering problems [26]. Driven by the limitations and
motivated by the challenges of conventional ECG
together with the advantages of ACT, this study com-
pares phonocardiogram triggered, conventional vector
electrocardiogram triggered and traditional pulse oxime-
try triggered CMR cine imaging at 7.0 T in a pilot study
as precursor to a larger clinical study.
To accomplish this goal, the suitability, accuracy and
reproducibility of each cardiac triggering approach for
the assessment of left ventricular parameter at 7.0 T is
explored. For this purpose, breath-held 2D CINE ima-
ging in conjunction with a retrospective triggering
regime is conducted paralleled by real time logging of
the ECG, POX and ACT signal waveforms to track
(mis)synchronization between the cardiac cycle and data
acquisition. A qualitative and quantitative analysis of 2D
CINE images and signal waveforms is performed. The
merits and limitations of the acoustic cardiac gating
approach are discussed and its implications for other
ultrahigh field MR imaging applications are considered.
Materials and methods
Acoustic Noise Measurements at 7.0 T
During CMR, recordings of a phonocardiogram inside of
the magnet bore are paralleled by acoustic noise due to
gradient coil switching consisting of several very sharp
harmonic components, which are related to the echo
time TE and the repetition time TR. For this reason,
acoustic measurements were conducted to assess the
acoustic signal-to-noise ratio between the sound pres-
sure level induced by the cardiac activity and the sound-
pressure level generated by the gradient noise. Acoustic
measurements were conducted inside the 7.0 T magnet
bore. Two series of acoustic signals were acquired. The
first series was designed to record and characterize the
noise generated in the MR environment by a 2D CINE
FLASH sequence (TE = 2 ms, TR = 4 ms, pixel band-
width = 445 Hz, FOV = (32 × 32) cm2using an optical
microphone (MO 2000 set, Sennheiser, Wedemark,
Germany). For this purpose, the optical microphone was
positioned at the same position with respect to the scan-
ning table and the magnet bore as it was used in the
volunteer study. The second series was setup to collect
and analyze the phonocardiogram derived from the
heart sound of healthy subjects superimposed by the
environmental noise generated by the same 2D CINE
FLASH technique. For this purpose, an acoustic sensor
(diameter = 5 cm) covered by a membrane to generate
a pressure wave in a waveguide was attached to the
optical microphone. The acoustic signal was recorded
by means of an USB-soundcard chip (PCM2903;
Texas Instruments, Dallas, TX, USA). The optical
microphone was calibrated with a 94 dB test tone pro-
vided from a pistonphone (Voltcraft SLC-100, Conrad
electronics, Germany). After connecting the mem-
brane to the microphone the set-up was calibrated
manually to correct for the minor attenuation induced
by the acoustic sensor’s membrane. With this set-up
absolute sound pressure levels were obtained. Further-
more, this approach offers the benefit that the charac-
teristics of the acoustic signal measurements are given
by the environment and not by the signal processing
unit. Due to the use of this linear time invariant net-
work approach frequency-shifts between the acoustic
signal obtained from the MR environment and the
final power spectrum can be avoided. Also, the sound
pressure level can be normalized to the auditory
threshold.
Pulse Oximetry, ECG and Acoustic Triggering
For pulse oximetry, a commercial sensor (Siemens,
Erlangen, Germany) was placed on the tip of the right
index finger to track changes in the absorbance due to
the pulsing arterial blood. A wireless connection linked
the sensors output to the internal physiological signal
controller circuitry of the MR scanner.
For ECG recording and triggering, a commercial vec-
tor ECG module (Siemens, Erlangen, Germany) was
used. The electrodes (ConMed, Corp., Utica, NY, USA)
of the vector ECG were carefully placed at the anterior
chest wall, with one electrode on the sternum, one
on the left thorax, and one below the sternum following
the manufacturer’s patient preparation instructions.
For the vector ECG’s training period the patient table
was placed in the home position to eliminate major
interferences with the fringe field. The waveform was
delivered to the internal physiological signal controller
circuitry of the clinical MR scanner.
Unlike traditional ECG-triggering the acoustic
approach employs the phonocardiogram’s first heart
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 2 of 14

Page 3

tone for triggering instead of electrophysiological signals
[23]. The acoustic gating device comprises four main
components: an acoustic sensor made of synthetic mate-
rial placed on the subject’s anterior chest for phonocar-
diogram detection, an acoustic wave guide for signal
transfer and to ensure galvanic decoupling, a signal pro-
cessing unit and a coupler unit to the MR system. The
two former aspects have also safety implications since
the ACT approach galvanically isolates the subject and
hence eliminates the risk for patient burns. Signal pro-
cessing and conversion were conducted outside the
magnet room using dedicated electronic circuit [23].
Short rectangular shaped trigger pulses were generated
to provide an output trigger signal. This waveform was
delivered to the internal physiological signal controller
circuitry of the clinical MR scanner. This design was
chosen to meet the needs of cardiac gated/triggered
CMR: (i) maximum latency of 35 ms between the ECG’s
R-wave and phonocardiogram based trigger output
pulse, (ii) free of interference with electromagnetic fields
and (iii) immunity to magneto-hydrodynamic effects.
The current implementation connects the trigger signal
to the MR-scanner’s standard external trigger signal
input. Hence, no changes to the MR system’s hardware
or software are required [23]. The acoustic sensor was
positioned directly on the subject’s chest at the anterior
left fifth intercostals space and gently fixed with a strap
incorporated in the MR patient table. Note that neither
the limited ECG in the MR scanner nor the acoustic
waveforms reported here should be treated as reliable
indicators of patient emergency conditions.
Three patient table positions were selected to examine
the signal waveform as a function of the magnetic field
strength: (i) patient table in home position so that the
magnetic field strength at the ECG, POX and ACT sen-
sor position was approximately 0.3 T, (ii) ECG, POX
and ACT sensor position aligned with the front end of
the magnet with a fringe field of approximately 1.0 T
and (iii) ECG, POX and ACT sensor in the magnet’s
isocenter (B0= 7.0 T).
Logging and Analysis of Signal Waveforms at 7.0 T
For all subjects vector ECG, POX and ACT were con-
nected at the same time to record traces of waveforms
along with the trigger information simultaneously. This
information was extracted from the scanners central
physiological monitoring unit (CPMU) and simulta-
neously stored in log files with a sampling rate of 400
Hz (ECG), 50 Hz (POX) and 200 Hz (ACT). Also, the
trigger detection tickmarks generated for Vector ECG,
POX or ACT triggering by the scanners CPMU were
written into log files. This logging procedure was paral-
leled by storing the respiratory trace simultaneously
using a sampling rate of 50 Hz.
The recorded data were processed to analyze the trig-
ger information and to assess triggering efficiency and
temporal fidelity of synchronization with the cardiac
cycle for each trigger technique. Off-line analysis of the
log-files was performed using LabVIEW (National Instru-
ments, Austin, TX, USA). For this purpose a customized
post-processing algorithm was developed. The post-
processing procedure includes the following features:
• Identification of breath hold periods: Only portions
of the trigger signal traces which were acquired during
breath-held 2D CINE FLASH imaging were included
into the waveform analysis. For this purpose, the
respiratory trace was used.
• Segmentation and temporal realignment: The ECG
waveform is segmented into individual R-R intervals by
using cross correlation between R-R intervals. For this
purpose, one R-R interval acquired at the one breath-
held CINE series is taken as a reference while all other
R-R intervals are shifted along the time axis to achieve
maximum correlation with the reference. This R-R wave
segmentation mask is applied to the segmentation of the
POX and ACT waveforms, which were acquired simul-
taneously to the ECG waveform. Temporal realignment
was used to overcome the potential bias of (erroneous)
trigger detection provided by the scanners internal real-
time circuitry.
• Reassignment of the trigger detection tickmarks
derived from the scanners central physiological monitor-
ing unit to the realigned ECG, POX and ACT traces and
assessment of the trigger jitter across the cardiac cycle.
• Calculation of the mean value and the standard
deviation of the cardiac cycle for ECG, POX and ACT
as an objective measure for trigger reliability.
• Calculation of the offset between the ECG’s R-wave
and the trigger detection moment derived from the
ECG, POX and ACT waveform as an objective measure
for trigger reliability.
Cine CMR at 7.0 T
End-expiratory breath-hold short axis views of the heart
ranging from the atrioventricular ring to the apex were
acquired using retrospectively gated 2D FLASH CINE
on a 7.0 T whole body MR systems (Magnetom, Sie-
mens, Erlangen, Germany). A 4 element transmit/receive
coil was used for RF excitation and signal reception [27].
The coil was connected to the 7.0 T system via 4 trans-
mit/receive (T/R) switches and a 1 to 4 radio frequency
power splitter and combiner with a CP-like phase set-
ting for the four individual channels. The coil setup
consists of two identical coil subsets - one placed on the
subject’s anterior torso and one positioned posterior -
each containing two transmit/receive loop coil elements.
For breath-held 2D CINE FLASH imaging field
of view (FOV) was set to (340 × 308) mm2, data
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 3 of 14

Page 4

acquisition matrix size was set to 256 × 186 elements
(reconstruction matrix size 256 × 232 elements). 30 car-
diac phases (temporal resolution = 33 ms for a heart
rate of 60 bpm) were acquired using typically 18 slices
(slice thickness = 4 mm, slice gap = 2 mm). Slice order
was reversed from apex-base to base-apex throughout
the set of subjects to eliminate patient discomfort or
training effects. Image acquisition was confined to a sin-
gle slice per breath-hold. The flip angle was set to a =
35 for all subjects, resulting in TR = 5.5 ms and TE =
2.7 ms. Parallel imaging was applied (R = 2) using sensi-
tivity encoding based reconstruction.
Three sets of breath-held 2D CINE FLASH acquisi-
tions were performed. In one set ACT was employed.
For comparison, the other set made use of vector ECG
based cardiac triggering while another set used POX for
cardiac triggering. The use of ACT, ECG and POX was
swapped randomly to avoid systematic errors.
Image Analysis
For LV chamber quantification end-diastolic and end-
systolic volume (EDV, ESV), and left ventricular mass
(LVM) were calculated using commercial evaluation
software (CMR42®, Circle Cardiovascular Imaging,
Calgary, Canada) from images of all subjects using all
three triggering methods. CMR reading was performed
by one cardiologist with very profound expertise in clin-
ical CMR (>3000 CMR examinations), who was not
involved in the image acquisition at all.
For CINE image quality assessment two independent
observers reviewed and scored the images in a rando-
mized, blinded reading session. For this purpose, overall
image quality of end-diastolic and end-systolic images
was rated using a scale ranging from 0 to 3 for
each slice. The following scale was used for the blinded
reading:
0 - images with poor and non-diagnostic quality due
to cardiac motion induced blurring,
1 - image quality impaired by cardiac motion which
may lead to misdiagnosis,
2 - good image quality, cardiac motion artifacts hard
to recognize and
3 - excellent image quality, no cardiac motion arti-
facts observed.
After the independent image quality assessment was
completed both readers exchanged their rankings for
each case-, slice- and cardiac phase, and agreed on a
consensus score.
Endocardial border sharpness (EBS) of the 2D CINE
FLASH images derived from ACT, ECG POX triggered
acquisitions was determined through an objective mea-
surement of acutance using a dedicated algorithm [21].
Study Population
The study was designed as a comparative volunteer
study using healthy adult subjects with no history of
cardiovascular disease (n = 9). The mean age was 32 ±
10 years ranging 23-52 years. The average body surface
area was (2.0 ± 0.3) m2ranging from 1.8 m2to 2.7 m2.
The average body mass index was 24.8 ± 4.4 kg/m2
(range 20.9-35.9 kg/m2). Volunteers with contraindica-
tions to CMR were excluded. The study was carried
out according to the principles of the Declaration of
Helsinki and was approved by the local institutional
ethics committee. Informed written consent was obtained
from each volunteer prior to the study, in compliance
with the local institutional review board guidelines.
Statistical Analysis
All data are presented as mean ± standard deviation
(SD) unless stated otherwise. Statistical significance in
the difference of the image scores was analyzed using
Wilcoxon matched pairs test for the consensus scores
for ACT vs ECG, ACT vs POX and ECG vs POX trig-
gered data. Statistical significance in the difference of
the parameter derived from LV function assessment was
analyzed using t-test. A probability p ≤ 0.05 was consid-
ered statistically significant. All computations were per-
formed with Microsoft Excel (Microsoft, Redmond,
USA) and R (R Foundation for Statistical Computing,
Vienna, Austria). Comparison of the different triggering
techniques was carried out using the Bland and Altman
method [28] (GraphPad Software, Inc., La Jolla, CA,
USA). The confidence interval was set to the mean
value ± 1.96 of the standard deviation.
Results
Acoustic Signal-to-Noise Measurements at 7.0 T
Recordings of phonocardiograms inside the magnet bore
are paralleled by acoustic noise due to gradient coil
switching. High sound pressure levels (SPL) of up to
120 dB were induced by magnetic field gradients driven
by the switching scheme of a 2D FLASH CINE imaging
technique. Figure 1 illustrates the spectrogram which
shows the sound pressure level spectrum over time
obtained from a subject positioned at the magnet’s iso-
center during 2D CINE FLASH imaging (TE = 2.0 ms,
TR = 4.0 ms) at 7.0 T. In this case of the phonocardio-
gram being paralleled by the gradient switching noise
the sound pressure level (SPL) was tracked over a series
of 6 heartbeats. The power spectra show numerous
noise peaks including several very sharp harmonic com-
ponents at 1/TR, 2/TR, 3/TR and 1/TE which are
related to the gradient switching scheme of 2D FLASH
with maximum SPL close to 120 dB. The spectrogram
also revealed that the heart sound encompasses low-
frequencies.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 4 of 14

Page 5

SNR was defined as the ratio between the sound pres-
sure level due to cardiac activity and the gradient
switching induced sound pressure level. For example,
for the frequency range between 10 Hz and 50 Hz a
minimum signal to noise ratio of SNR = 30 dB was
found for 2D CINE FLASH imaging at 7.0 T. To make
acoustic triggering immune to interference from acous-
tic noise generated by gradient switching, separation of
the acoustic cardiac activity from the higher frequency
gradient noise was carried out by means of a third order
inverse Chebychev filter using a UAF42 chip (Burr
Brown Products by TI, Dallas, Texas, USA). The cut off
frequency was set to fc= 105 Hz since the energy of the
first heart tone is mainly covered by frequencies ranging
from 1 to 100 Hz. With this filtering no gradient noise
peaks or peaks due to environmental noise were found
within the relevant frequency range between 0 Hz to
105 Hz. The filtering yielded an attenuation of the gra-
dient noise peaks found for frequencies above the cut-
off frequency fc= 105 Hz by at least 30 dB.
Assessment of ECG, POX and ACT Waveforms at 7.0 T
Off-line analysis of the log-files revealed that ECG wave-
forms were susceptible to severe distortions. Adverse
signal elevation was found for cardiac phases where nor-
mally the T-wave occurs; a magneto-hydrodynamic
effect which was pronounced at the isocenter of the
magnet as illustrated in Figure 2, which shows ECG,
POX and ACT traces derived from an individual subject
over 18 cardiac cycles. ECG waveform distortions
yielded an amplitude reaching the same order of magni-
tude or even larger than that of the R-wave. Pulse oxi-
metry waveforms were free of interference from
electromagnetic fields and magneto-hydrodynamic
effects. A rather flat plateau and a significant scatter in
the amplitude and width were observed for the peak in
the POX trace. The peak in the POX trace showed a
mean latency of approximately 350 ms with respect to
the R-wave of the ECG trace. The acoustic approach
provided waveforms free of interferences from electro-
magnetic fields or from magneto-hydrodynamic effects
even in the isocenter of the 7.0 T magnet as illustrated
in Figure 2. Off-line analysis of ECG and ACT traces
yielded an average delay of Δt = 29.65 ms ± 4.43 ms
between the R-wave and the first heart tone. This delay
is not detrimental for whole R-R coverage 2D CINE
FLASH using retrospective triggering.
Cardiac 2D CINE FLASH Imaging at 7.0 T
In the case of correct R-wave detection, ECG-gated 2D
CINE FLASH imaging was found to be immune to car-
diac motion effects as illustrated in figure 3 for one of
the 9 subjects (subject 1). However, frequent R-wave
mis-registration occurred in ECG-triggered acquisitions
with a failure rate of up to 30% which manifests itself in
a severe jitter of the R-wave detection tickmarks. Conse-
quently, ECG triggered 2D CINE FLASH imaging was
prone to severe cardiac motion artifacts if R-wave mis-
registration occurred. For example, an ECG cardiac trig-
gered whole heart coverage 2D CINE FLASH dataset
obtained at diastole is shown in the top row of Figure 4
for one subject of the 9 subjects (subject 2). Images suf-
fering from cardiac motion induced blurring are marked
with dotted lines. Unlike ECG, ACT triggering produced
images free of cardiac motion artefacts as illustrated in
the bottom row of Figure 4 for the same subject. POX
triggering 2D CINE FLASH acquisitions obtained from
the same subject also produced images free of blood
pulsation and cardiac motion artefacts as demonstrated
in the middle row of Figure 4.
Assessment of the Trigger Detection Variance
Figure 5 shows mid-ventricular short axis views of the
heart together with whole R-R interval time series of
one-dimensional projections, trigger detection tickmarks
and signal waveforms obtained at 7.0 T using ECG,
POX and ACT triggered 2D CINE FLASH acquisitions
for a midventricular slice derived from subject 1. In this
example of almost correct recognition of the onset of
cardiac activity, ECG, POX and ACT triggered 2D CINE
FLASH imaging were found to be rather immune to the
Figure 1 Acoustic Spectrogram obtained at 7.0 T. Spectrogram
obtained from a subject positioned at the 7.0 T magnet’s isocenter
during 2D CINE FLASH acquisitions (TE = 2.0 ms, TR = 4.0 ms). The
graphs show signal contributions from gradient switching
superimposed on the cardiac signals. The gradient switching
manifests itself by several very sharp harmonic components at 1/TR,
2/TR, 3/TR and 1/TE with maximum sound pressure level close to
120 dB. The spectrogram also shows the 1stand the 2ndheart tone
which are of low-frequency nature. Acoustic signal-to-noise ratio,
which is defined as the ratio between the sound pressure level due
to cardiac activity and the gradient switching induced sound
pressure level is approximately 30 dB for the frequency range
between 10 Hz and 70 Hz.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 5 of 14

Page 6

effects of cardiac motion. Consequently, the 2D CINE
FLASH images derived from ECG, ACT and POX
acquisitions together with the M-mode like whole R-R
interval time series of one-dimensional projections along
the cardiac phases safeguard recognition and delineation
of the ventricular blood/myocardium interface. In spite
of ECG’s severe signal distortion faultless ECG trigger-
ing was observed for this example, with the exception of
a scatter in the ECG trigger detection of approximately
Δt = 60 ms which might compromise the temporal fide-
lity and hence might constitute a synchronization pro-
blem. Please note that a variance was also observed for
the peak amplitude and peak width of the POX wave-
form as pointed out in Figure 5. This variance resulted
in a trigger detection jitter of approximately Δt = 65 ms,
which is equivalent to two cardiac phases. In compari-
son, ACT showed a 5 ms jitter which can be attributed
to the ACT waveform sampling rate of 200 Hz.
Figure 6 shows an example of erroneous ECG trigger-
ing for a midventricular slice derived from subject 2. In
this case, ECG triggered 2D CINE FLASH imaging was
prone to severe cardiac motion artifacts due to R wave
mis-registration. Trigger detection was found to be scat-
tered across several cardiac phases including early
Figure 2 ECG, pulse oximetry and acoustic trigger signal traces acquired at three different field strengths. ECG (top), pulse oximetry
(middle) and acoustic trigger (bottom) signal traces derived from a healthy subject. Signal traces were recorded with the ECG, ACT and POX
sensors located at the patient table home position (left), at the front end of the 7.0 T magnet (center) and in the isocenter of the 7.0 T magnet
(right) and post-processed and filtered by the scanners central physiological monitoring unit. Severe signal distortion occurred in the vector ECG
signal obtained at the magnet’s isocenter. A scatter in the amplitude and width was observed for the peak in the pulse oximetry trace. ACT is
free of interferences with electromagnetic fields and magneto-hydrodynamic effects.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 6 of 14

Page 7

systole and diastole as demonstrated by the tickmarks
depicted in Figure 6. R-wave mis-registration induced
reduction in myocardium/blood contrast and image
sharpness as illustrated by the short axis views together
with whole R-R interval time series of one-dimensional
projections. In comparison, ACT triggered 2D CINE
FLASH imaging provided faultless trigger detection,
accurate to the peak induced by the 1stheart tone and
hence produced images free of motion artifacts. Please
note the scatter in the POX peak amplitude and peak
width, causing a jitter (Δt = 72 ms) in the pulse-oxime-
try trigger detection.
Mean R-R interval lengths deduced from the signal
waveforms of ECG, POX and ACT triggered acquisi-
tions are surveyed for each subject in Table 1 together
with the standard deviation of the R-R interval length,
which is a measure of the trigger detection accuracy.
A close match in the mean R-R interval length and in
the standard deviation of the R-R interval length derived
from the assessment of the ECG, ACT and POX wave-
forms signifies correct trigger detection. A significant
difference in the standard deviation of the R-R interval
length deduced from ECG, ACT and POX waveforms
indicates trigger (mis)registration at cardiac phases other
than that which mark the onset of cardiac activity. Four
out of nine healthy subjects showed a standard deviation
for the cardiac cycle derived from ECG waveforms
which was at least 1.5 times larger (SDECG≥ 1.5 *
SDACT) than that obtained from ACT or POX
waveforms.
Table 1 also surveys the standard deviation of the off-
set between the ECG’s R-wave and the trigger detection
moment which was derived from the ECG, POX and
ACT waveforms and which was applied for synchroniza-
tion. Four out of nine healthy subjects showed a stan-
dard deviation for the cardiac trigger offset derived from
ECG waveforms which was at least four times larger
(SDECG≥ 4 * SDACT) than that observed for ACT or
POX waveforms.
Left Ventricular Parameter Assessment
Left ventricular volumes, mass and ejection fraction
deduced from ECG, POX and ACT triggered acquisi-
tions are surveyed in Figure 7. For each subject, the
mean of the two measurements (ACT vs. ECG and
ACT vs. POX) and the difference between the LV para-
meter obtained for ECG, POX and ACT triggered 2D
CINE FLASH acquisitions are shown using Bland-Alt-
man plots. The mean LV parameter derived from ECG
triggered 2D CINE FLASH acquisitions were ESV =
74 ± 22ml, EDV = 170 ± 43ml, LVM = 130 ± 17) g, EF =
57 ± 8%. The mean LV parameter obtained from POX
triggered 2D CINE FLASH acquisitions showed ESV =
67 ± 19 ml, EDV = 168 ± 44 ml, LVM = 130 ± 15g, EF =
60 ± 5%. In comparison, the ACT-triggered acquisitions
yielded: ESV = 66 ± 20 ml, EDV = 173 ± 43 ml, LVM =
130 ± 15g, EF = 62 ± 3%. T-test revealed no significant
differences for LV parameter derived from ACT, POX
and ECG triggered acquisitions (all p > 0.05).
Image Quality Assessment
The application of acoustic triggering substantially
improved the 2D CINE FLASH image quality as com-
pared to the conventional approach. The blinded read-
ing yielded a mean image quality score of 1.7 ± 0.5 for
end-diastolic and 1.3 ± 0.6 for end-systolic cardiac
images derived from ECG gated acquisitions. In compar-
ison POX gated acquisitions yielded a mean image qual-
ity score of 2.0 ± 0.5 for end-diastolic and of 1.6 ± 0.5
for end-systolic cardiac phases. ACT triggered images
showed a mean image quality score of 2.4 ± 0.5 for end-
diastolic and of 2.0 ± 0.5 end-systolic cardiac phases as
summarized in Table 2. A comparison between the
image quality score obtained for ACT, POX and ECG
gating using Wilcoxon paired test revealed significant
differences between ACT vs. ECG gated acquisitions
(p = 0.10 for endsystole, p = 0.04 for enddiastole) and
for ACT vs. POX gated acquisitions (p = 0.03 for end-
systole, p = 0.01 for enddiastole). The image quality
comparison between ECG vs. POX gated acquisitions
showed no significant differences in image quality
(p = 0.40 for endsystole, p = 0.34 for enddiastole).
In case of faultless gating the EBS analysis revealed
similar results for all synchronisation techniques. ECG
Figure 3 Short axis diastolic views free of cardiac motion
effects. Short axis diastolic views obtained from breath-held base-
to-apex 2D CINE FLASH acquisitions using (top) vector ECG,
(middle) ACT and POX (bottom) gating. In this case of correct
R-wave detection (subject 1), ECG-gated 2D CINE FLASH imaging
was found to be immune to cardiac motion effects, as were ACT
and POX gated acquisitions.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 7 of 14

Page 8

gated acquisitions showed an average EBS of (2.2 ± 0.3)
pixels. ACT gated acquisitions yielded an average EBS
of (2. 1 ± 0.2) pixels, and POX gated acquisitions
showed an average EBS of (2.1 ± 0.2) pixels. In case of
erroneously ECG gated acquisitions EBS analysis was
challenging due to heavily reduced contrast to noise
ratio (CNR) between blood and myocardium. The CNR
degradation was caused by severe signal blurring across
the endocardial border, as demonstrated in Figure 4.
Discussion
CMR at 7.0 T is still in its infancy and needs to con-
tinue to be very carefully validated against CMR applica-
tions very well established at 1.5 T and 3.0 T [29]. In
current basic and clinical research practice some of the
traits of ultrahigh field CMR are offset by challenges
intrinsic to the use of ultrahigh magnetic field strength
such as the synchronization of data acquisition with car-
diac motion using traditional electrocardiographic
(ECG) techniques. To address this issue, this study
examines the applicability of acoustic cardiac triggering
for CMR at 7.0 T in healthy volunteers, as a precursor
to a larger clinical study. The acoustic approach was
found to be immune to interference from environmental
and gradient switching induced acoustic fields plus to
be free of interference from electromagnetic fields and
magneto-hydrodynamic effects. The efficacy and reliabil-
ity of acoustic triggering is demonstrated by eliminating
the frequently-encountered difficulty of mis-triggering
due to ECG-waveform distortions or temporal jittering
in the pulse-oximetry synchronization. R-wave mis-
registration occurred in ECG-triggered acquisitions with
a failure rate of up to 30% which manifest itself in
severe cardiac motion induced image blurring.
It should be noted, that ECG trigger mis-registration
was not equally distributed across the entire cardiac
cycle but occurred at cardiac phases with large ampli-
tude or up-slope in the ECG waveform including (i) an
Figure 4 Short axis diastolic views showing cardiac motion artifacts when using ECG gating. Short axis diastolic views obtained from
breath-held apex-to-base 2D CINE FLASH acquisitions using (top) vector ECG, (middle) ACT and POX (bottom) gating. In this subject (subject
2) vector ECG triggered 2D CINE FLASH imaging was prone to severe cardiac motion artifacts if R-wave mis-registration occurred. Images
suffering from cardiac motion induced blurring are marked with grey bars. Acoustic triggering and POX provided image quality free of
interferences from cardiac motion effects.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 8 of 14

Page 9

initial peak which covers the R-wave and (ii) major
waveform distortions at systole and end-diastole. This
mis-triggering behavior forces the k-space segmentation
and retrospective reconstruction strategy to combine
and assign k-space data which were acquired at different
phases of myocardial contraction and relaxation to the
same cardiac phase used to form a final image. This
out-of-sync assignment causes severe degradation in
image quality and image sharpness. Please note that this
study did not yield missed ECG triggers. Missed triggers
Figure 5 Example of correct ECG trigger detection. Cardiac images, trigger detection tickmarks and signal waveforms obtained at 7.0 T using
vector ECG (left), pulse oximetry (center) and acoustically triggered (right) 2D CINE FLASH acquisitions. All signal waveforms show the output of
the scanners central physiological monitoring unit (including processing of the ECG, POX and ACT signal) as displayed at the scanners user
interface. Top: Mid-ventricular, short axis views of the heart together with whole R-R interval time series of one-dimensional projections along
the profile (dotted line) marked in the short axis view. Middle: Trigger detection tickmarks obtained from a single subject over 18 cardiac cycles
after temporal realignment using cross correlation and reassignment. Bottom: Signal waveforms obtained from a single subject (subject 1) over
18 cardiac cycles. In spite of vector ECG’s severe signal distortion faultless vector ECG triggering was observed for this example. Hence, in this
example of correct recognition of the onset of cardiac activity, vector ECG, POX and ACT triggered 2D CINE FLASH imaging were found to be
immune to the effects of cardiac motion. Please note the jitter in the vector ECG (Δt = 60 ms) and in the pulse oximetry trigger (Δt = 65 ms)
detection.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 9 of 14

Page 10

would not compromise image quality but lengthen the
acquisition time. Acoustically triggered 2D CINE
FLASH imaging at 7.0 T produced images free of
motion artifacts, as did pulse oximetry triggered 2D
CINE FLASH imaging. The latter showed a scatter in
the POX peak amplitude and peak width, causing a jitter
of approximately Δt = 72 ms in the pulse-oximetry
based trigger detection. This might be tolerable for a
temporal resolution of 30 ms to 50 ms commonly used
in conventional CINE imaging since data acquisition is
distributed over several cardiac cycles which might
result in averaging of motion effects. However, it stands
Figure 6 Example of erroneous ECG trigger detection. Cardiac images, trigger detection tickmarks and signal waveforms obtained at 7.0 T
using vector ECG (left), pulse oximetry (center) and acoustically triggered (right) 2D CINE FLASH acquisitions. All signal waveforms show the
output of the scanners central physiological monitoring unit (including processing of the ECG, POX and ACT signal) as displayed at the scanners
user interface. Top: Mid-ventricular, short axis views of the heart together with whole R-R interval time series of one-dimensional projections
along the profile (dotted line) marked in the short axis view. Middle: Trigger detection tickmarks obtained from a single subject over 18 cardiac
cycles after temporal realignment using cross correlation and reassignment. Bottom: Signal waveforms obtained from a single subject (subject 2)
over 18 cardiac cycles. In this example vector ECG triggered CINE imaging was prone to severe cardiac motion artifacts due to R wave mis-
registration which induced reduction in myocardium/blood contrast and image sharpness. ACT triggered 2D CINE FLASH imaging provided
faultless trigger detection and hence produced images free of motion artifacts. Please note the scatter in the POX peak amplitude and peak
width, causing a jitter (Δt = 72 ms) in the pulse oximetry trigger detection which constituted a synchronization problem.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 10 of 14

Page 11

to reason that the enhanced temporal resolution of
CINE imaging facilitated by the signal-to-noise benefit
at high and ultra-high fields [30] combined with the
speed gain of parallel cardiac imaging [31] can be used
to generate highly accurate time-series curves for wall
motion tracking (i) to determine the exact time point of
maximal systolic contraction and diastolic filling includ-
ing assessment of mechanical dyssynchrony and (ii) to
visualize small rapidly moving structures. Taking the
underlying physiological temporal resolution into
account it is fair to assume that a jitter in the POX trig-
ger detection larger than 2-3 cardiac phases might
diminish the temporal fidelity needed to characterize
those physiologic phenomena within the cardiac cycle.
Hence, the trigger detection jitter observed for pulse
oximetry can constitute a challenge for reliable synchro-
nization of data acquisition with the cardiac cycle.
Another drawback of pulse oximetry is the latency
between cardiac activity and trigger registration caused
by the travel time of blood between the heart and the
sensor position which can be in the order of several
hundreds of milliseconds but which can also depend on
(patho)physiology.
Acoustic triggering presents no risk of high voltage
induction and patient burns, patient comfort and ease of
clinical use, which all have, patient comfort, safety and
practical implications. With reliable ACT triggering
available, a positively-inclined practitioner might envi-
sage using the merits of ACT to further simplify clinical
CMR. For example, LV assessment is routinely con-
ducted using 2D CINE imaging encompassing a stack of
end-expiratory breath-held short axis views of the heart
ranging from the atrioventricular ring to the apex. In
current clinical practice, it is common to plan and scan
each slice in an independent series instead of scanning
all slices in a single series because of the frequently
encountered risk of mis-triggering. This practical work
around bears the advantage that only the most recently
acquired slice needs to be re-scanned in case of non-
diagnostic image quality due to mis-synchronization.
Overcoming the hassle of ECG triggering, reliable trig-
gering using ACT holds the promise to obviate the need
for creating an independent series for each slice. Hence
ACT, may help to streamline CINE imaging by using a
single series for the entire stack of slices which has
practical, data storage, data handling and data mining
implications.
Admittedly, acoustic cardiac gating shares an apparent
drawback of conventional ECG and pulse oximetry
based cardiac triggering/gating that extra hardware is
required for signal detection and processing, although
the current ACT setup does not disturb the scanners
certification. To overcome the constraint of using ancil-
lary hardware various self-gating methods have been
proposed and it’s feasibility has been demonstrated for
CMR [7,8,10,32]. However, self-gated CMR of small dis-
placements such as vessel wall motion or MR angiogra-
phy (MRA) remains a challenge due to the small
changes in blood volume, low changes in vessel size and
small vessel displacements throughout the cardiac cycle.
The acoustic cardiac gating approach reported here is
conceptually appealing for the pursuit of vascular CMR
since the acoustic sensor can be used to directly detect
vessel pulsation from larger vessels included in the tar-
get vessel territory.
It should be noted that the work reported here is lim-
ited to using vector ECG recordings and retrospective
reconstruction techniques implemented on a clinical
platform. Further fine-tuning of the post-processing and
image reconstruction procedures used in current clinical
Table 1 Synopsis of the trigger detection variance assessment
Mean cardiac interval
length
Subject(ms)
ACTECGPOX
Standard deviation of the mean cardiac interval
length
(ms)
ACT ECG
Standard deviation of the cardiac trigger
offset
(ms)
ACTECG POXPOX
1
2
3
4
5
6
7
8
9
869
870
875
1064
926
841
848
1110
762
865
795
845
999
990
939
839
886
576
840
1103
834
1079
923
898
876
1091
660
89
83
48
76
49
95
79
37
81
54
126
28
143
54
69
62
174
168
45
87
34
52
31
35
48
33
144
78
76
13
44
14
149
75
1
112
10
477
3
456
56
52
257
1047
488
11
101
19
456
23
15
67
14
331
Synopsis of the mean R-R interval lengths, standard deviation of the mean R-R interval lengths and standard deviation of the cardiac trigger detection offset
deduced from the signal waveforms of ECG, POX and ACT triggered acquisitions. A significant difference in the standard deviation of the R-R interval length
deduced from ECG, ACT and POX waveforms indicates trigger misregistration. A large standard deviation of the offset between the ECG’s R-wave and the trigger
detection which was derived from the ECG, POX and ACT waveforms indicates trigger misregistration.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 11 of 14

Page 12

practice including cross-correlation of ECG signals
obtained for different R-R intervals together with retro-
spective image reconstruction is conceptually appealing
to enhance ECG’s trigger detection accuracy, albeit this
change in the manufacturers ECG processing/recon-
struction methodology is beyond the scope of the work
reported here.
Conclusion
The applicability of acoustic triggering for cardiac CINE
imaging at 7.0 T was shown. The intrinsic insensitivity
of the MR-stethoscope to interference from electro-
magnetic fields renders it suitable for left ventricular
parameter assessment at 7.0 T due to its excellent trig-
ger reliability, which is superior to that of traditional
ECG, VCG and conventional pulse oximetry. Acoustic
cardiac triggering promises to be beneficial for ultra-
high field strengths including 7.0 T and beyond, which
is an important but challenging development looming
on the pre-clinical research horizon. Although the full
Figure 7 Comparison of cardiac chamber quantification parameters obtained for three triggering methods. Bland-Altman plots showing
for each subject the mean of two measurements (ACT vs ECG, ACT vs POX and ECG vs POX) and the difference in the left ventricular parameter
derived from vector ECG, ACT and POX gated 2D CINE FLASH acquisitions at 7.0 T. Dashed black lines in the Bland-Altman plots represent the
mean difference while the dotted lines embody the confidence interval which was set to the mean value ± 1.96 of the standard.
Table 2 Synopsis of the image quality assessment
End-diastolic Phase End-systolic Phase
Subject ACTECG POXACTECG POX
1
2
3
4
5
6
7
8
9
2,4
2,6
2,9
2,9
2,5
1,6
1,5
2,8
2,1
2,2
0,9
2,7
1,7
1,9
2,1
1,6
1,1
0,7
1,8
1,9
2,5
2,8
1,9
1,8
1,1
2,4
1,3
1,5
2,1
2,6
2,6
2,1
1,5
1,1
2,4
1,9
1,6
0,5
2,4
1,2
1,7
1,5
1,6
0,8
0,5
1,7
1,6
2,1
2,4
1,7
1,4
1,0
1,5
0,9
Mean2,4 1,72,0 2,0 1,31,6
SD (+/-)
0,50,70,50,50,60,5
Mean values of image quality scores for each subject and trigger approach.
Overall image quality of end-diastolic and end-systolic images was rated using
a scale ranging from 0 to 3 for each slice (0 - images with poor and non-
diagnostic quality due to cardiac motion induced blurring, 1 - image quality
impaired by cardiac motion which may lead to misdiagnosis, 2 - good image
quality, cardiac motion artifacts hard to recognize and 3 - excellent image
quality, no cardiac motion artifacts observed). Please note that the poor
scores obtained for subject 7 are due to respiration induced motion artifacts.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 12 of 14

Page 13

range of ultrahigh field CMR is untapped yet, it is
expected to drive future technological developments.
With appropriate ancillary triggering hardware, RF-coil
design and imaging techniques/protocols customized for
7.0 T applications, LV assessment and other CMR appli-
cations are feasible. While this is, for the moment,
merely a start, it continues to motivate new basic and
clinical research on ultrahigh field CMR, including extra
efforts towards the development of a wireless signal
transmission version of the acoustic triggering approach.
Author details
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrueck Center for Molecular
Medicine, Berlin, Germany.2Siemens Healthcare, Erlangen, Germany.
3Working Group on Cardiovascular Magnetic Resonance, Medical University
Berlin, Charité Campus Buch, HELIOS-Klinikum Berlin-Buch, Dept. of
Cardiology and Nephrology, Berlin, Germany.4Experimental and Clinical
Research Center, Charité - Campus Buch, Humboldt-University, Berlin,
Germany.
Authors’ contributions
TF build the acoustic cardiac triggering device, performed the experiments,
collected the data and was significantly involved in writing the manuscript.
FH performed data evaluation and statistical analysis and was involved in
editing the manuscript.TGO conducted analysis of the waveforms obtained
for ACT, ECG and POX and was involved in editing the manuscript. MD has
setup the CMR protocols for this study. FvK was responsible for the
volunteer care, data documentation, left ventricular function analysis and
image quality assessment. He was involved in editing the manuscript. MP
performed image quality assessment. He was involved in editing the
manuscript. JSM was significantly involved in the study and imaging
protocol design. She was involved in editing the manuscript. TN was
responsible for the overall concept, was significantly involved in the study
and protocol design. He was also significantly involved in writing the
manuscript.
Competing interests
Wolfgang Renz is a full-time employee of Siemens (Erlangen, Germany).
Received: 25 June 2010 Accepted: 16 November 2010
Published: 16 November 2010
References
1.Lanzer P, Botvinick EH, Schiller NB, Crooks LE, Arakawa M, Kaufman L,
Davis PL, Herfkens R, Lipton MJ, Higgins CB: Cardiac imaging using gated
magnetic resonance. Radiology 1984, 150:121-127.
2. Brau AC, Wheeler CT, Hedlund LW, Johnson GA: Fiber-optic stethoscope: a
cardiac monitoring and gating system for magnetic resonance
microscopy. Magn Reson Med 2002, 47:314-321.
3.Pattynama PM, van der Velde ET, Steendijk P, Lamb HJ, Baan J, de Roos A:
Cardiovascular MR imaging: pressure-gating using the arterial pressure
signal from a conventional ferromagnetic micromanometer-tip catheter.
Magn Reson Imaging 1994, 12:531-534.
4. Rubin JM, Fowlkes JB, Prince MR, Rhee RT, Chenevert TL: Doppler US
gating of cardiac MR imaging. Acad Radiol 2000, 7:1116-1122.
5.Buikman D, Helzel T, Roschmann P: The rf coil as a sensitive motion
detector for magnetic resonance imaging. Magn Reson Imaging 1988,
6:281-289.
6.Buehrer M, Curcic J, Boesiger P, Kozerke S: Prospective self-gating for
simultaneous compensation of cardiac and respiratory motion. Magn
Reson Med 2008, 60:683-690.
7. Larson AC, White RD, Laub G, McVeigh ER, Li D, Simonetti OP: Self-gated
cardiac cine MRI. Magn Reson Med 2004, 51:93-102.
8. Larson AC, Kellman P, Arai A, Hirsch GA, McVeigh E, Li D, Simonetti OP:
Preliminary investigation of respiratory self-gating for free-breathing
segmented cine MRI. Magn Reson Med 2005, 53:159-168.
9. Nijm GM, Sahakian AV, Swiryn S, Carr JC, Sheehan JJ, Larson AC:
Comparison of self-gated cine MRI retrospective cardiac synchronization
algorithms. J Magn Reson Imaging 2008, 28:767-772.
Crowe ME, Larson AC, Zhang Q, Carr J, White RD, Li D, Simonetti OP:
Automated rectilinear self-gated cardiac cine imaging. Magn Reson Med
2004, 52:782-788.
Rengle A, Baboi L, Saint-Jalmes H, Sablong R, Beuf O: Optical cardiac and
respiratory device for synchronized MRI on small animal. Conf Proc IEEE
Eng Med Biol Soc 2007, 2046-2049.
Lanzer P, Barta C, Botvinick EH, Wiesendanger HU, Modin G, Higgins CB:
ECG-synchronized cardiac MR imaging: method and evaluation.
Radiology 1985, 155:681-686.
Fischer SE, Wickline SA, Lorenz CH: Novel real-time R-wave detection
algorithm based on the vectorcardiogram for accurate gated magnetic
resonance acquisitions. Magn Reson Med 1999, 42:361-370.
Chia JM, Fischer SE, Wickline SA, Lorenz CH: Performance of QRS detection
for cardiac magnetic resonance imaging with a novel
vectorcardiographic triggering method. J Magn Reson Imaging 2000,
12:678-688.
Stuber M, Botnar RM, Fischer SE, Lamerichs R, Smink J, Harvey P,
Manning WJ: Preliminary report on in vivo coronary MRA at 3 Tesla in
humans. Magn Reson Med 2002, 48:425-429.
Kugel H, Bremer C, Puschel M, Fischbach R, Lenzen H, Tombach B, Van
Aken H, Heindel W: Hazardous situation in the MR bore: induction in ECG
leads causes fire. Eur Radiol 2003, 13:690-694.
Shellock FG, Kanal E: Burns associated with the use of monitoring
equipment during MR procedures. J Magn Reson Imaging 1996, 6:271-272.
Shellock FG, Crues JV: MR procedures: biologic effects, safety, and patient
care. Radiology 2004, 232:635-652.
Lange S, Nguyen QN: Cables and electrodes can burn patients during
MRI. Nursing 2006, 36:18.
Frauenrath T, Hezel F, Heinrichs U, Kozerke S, Utting JF, Kob M,
Butenweg C, Boesiger P, Niendorf T: Feasibility of cardiac gating free of
interference with electro-magnetic fields at 1.5 Tesla, 3.0 Tesla and 7.0
Tesla using an MR-stethoscope. Invest Radiol 2009, 44:539-547.
Becker M, Frauenrath T, Hezel F, Krombach GA, Kremer U, Koppers B,
Butenweg C, Goemmel A, Utting JF, Schulz-Menger J, Niendorf T:
Comparison of left ventricular function assessment using
phonocardiogram- and electrocardiogram-triggered 2D SSFP CINE MR
imaging at 1.5 T and 3.0 T. Eur Radiol 2009.
Snyder CJ, DelaBarre L, Metzger GJ, van de Moortele PF, Akgun C,
Ugurbil K, Vaughan JT: Initial results of cardiac imaging at 7 Tesla. Magn
Reson Med 2009, 61:517-524.
Frauenrath T, Niendorf T, Kob M: Acoustic Method for Synchronization of
Magnetic Resonance Imaging (MRI). Acta Acustica united with Acustica
2008, 94:148-155.
Brants A, Versluis M, de Roos A, Westenberg J, Webb A: Quantitative
comparison of left ventricular cardiac volume, mass and function
obtained at 7 Tesla with “gold standard” values at 1.5 Tesla. Proc Intl Soc
Mag Reson Med; Stockholm, SE 2010, 1299.
Brandts A, Westenberg JJ, Versluis MJ, Kroft LJ, Smith NB, Webb AG, de
Roos A: Quantitative assessment of left ventricular function in humans at
7 T. Magn Reson Med 2010.
Maderwald S, Nassenstein K, Orzada S, Schäfer LC, Oehmigen M, Bitz AK,
Kraff O, Brote I, Ladd SC, Ladd ME, Quick HH: MR imaging of cardiac wall-
motion at 1.5T and 7T: SNR and CNR comparison. Proc Intl Soc Mag
Reson Med; Stockholm, SE 2010, 1299.
Renz W, Lindel T, Seifert F, Hoffmann W, Schulz-Menger J, Ittermann B: A 4
channel TX/RX decoupled loop array for cardiac/body imaging at 7T.
26th Annual Scientific Meeting of the European Society for Magnetic
Resonance in Medicine and Biology; Antalya, TR 2009, 476.
Bland JMAD: Statistical method for assessing agreement between two
methods of clinical measurement. Lancet 1986, 307-310.
von Knobelsdorff-Brenkenhoff F, Frauenrath T, Prothmann M, Dieringer MA,
Hezel F, Renz W, Kretschel K, Niendorf T, Schulz-Menger J: Cardiac chamber
quantification using magnetic resonance imaging at 7 Tesla-a pilot
study. Eur Radiol 2010, 20(12):2844-52.
Gutberlet M, Noeske R, Schwinge K, Freyhardt P, Felix R, Niendorf T:
Comprehensive Cardiac Magnetic Resonance Imaging at 3.0 Tesla:
Feasibility and Implications for Clinical Applications. Invest Radiol 2006,
41:154-167.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 13 of 14

Page 14

31.Niendorf T, Sodickson DK: Highly accelerated cardiovascular MR imaging
using many channel technology: concepts and clinical applications. Eur
Radiol 2008, 18:87-102.
Nijm GM, Sahakian AV, Swiryn S, Carr JC, Sheehan JJ, Larson AC:
Comparison of self-gated cine MRI retrospective cardiac synchronization
algorithms. Journal of magnetic resonance imaging: JMRI 2008, 28:767-772.
32.
doi:10.1186/1532-429X-12-67
Cite this article as: Frauenrath et al.: Acoustic cardiac triggering: a
practical solution for synchronization and gating of cardiovascular
magnetic resonance at 7 Tesla. Journal of Cardiovascular Magnetic
Resonance 2010 12:67.
Submit your next manuscript to BioMed Central
and take full advantage of:
• Convenient online submission
• Thorough peer review
• No space constraints or color figure charges
• Immediate publication on acceptance
• Inclusion in PubMed, CAS, Scopus and Google Scholar
• Research which is freely available for redistribution
Submit your manuscript at
www.biomedcentral.com/submit
Frauenrath et al. Journal of Cardiovascular Magnetic Resonance 2010, 12:67
http://www.jcmr-online.com/content/12/1/67
Page 14 of 14