Werner Moshage, Kurt Bachmann,
MD MD
Stephan Achenbach Peter Wegener, MS
Biomagnetic Arrhythmias’
complexes
(PVCs),
three
with ventricular tachycardia four healthy subjects with induced paced beats were recorded for 2-15 minutes. After correction for superimposed repolanization activity, the site of origin of the arrhythmias was localized from the magnetic field and
distribution
at the
onset
of the
B
ec-
topic beats. The localization results of paced beats showed an error of a few millimeters in relation to the position of the catheter tip. The results of spontaneous PVC and VT were confirmed with endocardial mapping or associated with ischemic lesions. The authors conclude that multichannel magnetocardiographic studies enable the completely noninvasive localization of ventricular arrhythmias.
every electric a magnetic
causes
trical
excitation
specific
of the
magnetic
current field, the
heart
field
These
patterns can be measured without contact over the thorax to obtain the magnetocardiogram (MCG). These magnetic fields, unlike the common electrocardiogram, remain almost uninfluenced by the conductive properof the
tissues
cal sources system.
and
between
the
the
SUBJECTS Study
measurement
Therefore,
the
field
sequentially
over
many
points of a measuring grid to record sufficiently large field distribution. Localization could be done only in
a
the Wolff-Parkinson-White
1991;
180:685-692
in
syndrome
The recent development channel, superconducting interference-device
of multiquantum
systems
opened
I
From
Poliklinik, Oestliche
the
Medical
Clinic
II (Cardiology)
and
University of Erlangen-Nuremberg, Stadtmauerstrasse 29, 8520 Erlangen, Germany (W.M., S.A., KG., A.W., KB.); and the Medical Engineering Group, Siemens, Erlangen, Germany (P.W., S.S., W.H.). From the 1990 RSNA scientific assembly. Received November 29, 1990; revision requested February 11, 1991; revision received May 2; accepted May 9. Address reprint requests to W.M. C RSNA, 1991
as 37 sensors,
these
systems
can
record the magnetic field distribution over a sufficiently large area without repositioning
of the
system
or the
and
four
healthy
of these associated
10 patients, the extrasystoles with ischemic myocardial
were dam-
age documented by means of coronary angiography, cardiography of the left yentide, and MR imaging. The other six patients
presented
Three patient,
duced dial
parasystole.
with
patients had recurrent VT. In one several episodes of VT were inby means of programmed intracar-
stimulation
during
registration
MCG. Some of the episodes minated by burst stimulation; nonsustained. Two patients incessant
of the
had to be tersome were showed spon-
VT.
In four healthy subjects, paced beats were induced at a known site within the heart by use of the specially designed nonmagnetic pacing catheter (described subsequently) biomagnetic
to verify localization.
Registration
and
the accuracy
of
of the
Evaluation
MCG
new fields of clinical applications for the biomagnetic method (5). With as many
arrhythmias
i3
with paced beats (Table). All patients and healthy subjects had given informed consent after the nature of the study had been fully explained. Ten patients showed spontaneous, pnedominantly monomorphic PVCs. In four
taneous
repeated of accesdelta wave
a group of i7 individuals: spontaneous or induced
volunteers
distributions measured over the body surface permit the quantification and the three-dimensional localization of sources within the heart. They also permit the evaluation of the propagation of physiologic and pathologic heart excitation. Even though the magnetocardiographic method was described as early as 1963 by Baule and McFee (1), its clinical applicability was limited for a long time because measurements had to be obtained with systems composed of only one to seven channels. These systems had to be adjusted
METHODS
Group
ventricular
magnetic
AND
We studied patients with
electri-
(2-4). Radiology
MD H#{228}rer,MS
graphic localization in patients with premature ventricular contraction and ventricular tachycardia (VT) and the verification of the localization accuracy of the method.
elec-
creates
patterns.
disorders with periodically signals, such as localization sony pathways from the
Index terms: Biomagnetism #{149} Heart, experimental studies, 51.1299 #{149}Heart, function, 515.1429,524.1429 S Heart, rhythm
#{149} Andreas Weikl, PhD #{149} Wolfgang
of Ventricular
ECAUSE
ties
patients (VT),
G#{246}hl,MD Schneider,
#{149}
Localization
The magnetic fields caused by electrical activity of the human heart can be coherently measured with a highly sensitive, multichannel, superconducting quantum interference-device system and can enable noninvasive localization of the underlying electrical activity. The magnetocardiograms (MCGs) of 10 patients with spontaneous premature ventricular
#{149} Konrad Siegfried
#{149} #{149}
The MCG was registered over the thorax without physical contact by use of a planar
biomagnetic
(Krenikon; (5).
pa-
lying
The
Siemens, patients
on a wooden
and
multichannel
system
Erlangen,
Germany)
healthy
couch
subjects
inside
were
a magnet-
tient. Besides drastic shortening of the recording times, coherent registration of the data for the first time permits study of spontaneous on sporadic events.
The cal
goal
application
of this
study
was
of magnetocardio-
the clini-
Abbreviations: ECG = electrocardiogram, MCC = magnetocardiogram, PVC = premature ventricular complex, S/N = signal-to-noise ratio, VT = ventricular tachycardia.
685
ically
shielded
room.
recordings
The
ranged
addition channels,
duration
from
of the
2 to 15 minutes.
In
Examinations
ration
(ECC)
channel
were
leads
and
recorded
Subject No.
simulta-
To localize the magnetic
the electrical activity from field distribution, the PVC,
beats
during
tachycardia,
were
averaged
or paced
to reduce
beats
background
and thus improve localization (6). Only data sampled during
tion were averaged ence of respiratory localization
to minimize movements
results
(7).
to 50 expiratory tamed
in each
In most the
the onset by
two
be ob-
fields
of the
Digital the
of the PVC was
magnetic
repolarization
separate
study,
could
caused
previous
subtraction
fields
tracted
from
the
combination
to
(8):
was done
To compensate for any the electronic components recording
periods
channel
separately,
signal
during
tracted
15
For
mean
recorded
intervals
was
the measured
these
steps
of electrical
three
distributions
single finite
localization
imaging
(Figs
Paced
NA
Paced
65/M
NA
Paced
Note.-+ = performed, Echo = echocardiography, resonance (MR) imaging, tar tachycardia.
= not performed, EPS = electrophysical NA = not applicable,
matched
the
Nonmagnetic, Imaging-compatible Catheter
in
field
To
of a
MCG,
dipole in the inspace (9).
with
visible
marks
in MR
to the to the
sponding
plane
positioning support
the same
ECG
to
and
was
with
system fixed
position
cardiography netocardiography,
artifacts
and images
MR the
both imaging. support
corre-
of a special
(5): A T-shaped to the patient’s
and
gat-
were
of the
the help
during
by
respiration
of the MCG MR
plastic chest in
magnetoFor magcarried
four current-driven wire coils, which were replaced by agaroseor copper sulphatefilled cubes during MR imaging. The position of the wire coils in magnetocardiography was determined by measuring the magnetic
field
rent
run
686
#{149}
through
Radiology
caused
+
-
-
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
+
4-
+
+
+
+
+
+
+
+
+
+
-
-
+
+
-
-
+
+
-
-
+
+
-
-
+
+
im-
the
Pacing
localization
a bipolar
pacing
in a way that exactly known
accuracy
catheter
of the
was
permitted site within
de-
stimulation the heart The of tip,
two platinum electrodes apart. The copper lead-in
are fixed 10 mm wires are
twisted so that the from the electrodes
current not cause
turbing net the lead-in
electric does
by
the coils
the
and
known
could
cur-
be
to the
magnetic field. Additionally, wires detach easily from
radio-frequency
MR imaging visibility is filled
at MR imaging, with lyophilized
tetate
dimeglumine,
the
To imthe cathegadopen-
echo sequences planes. Several
during
an MR imaging
contrast agent. Preliminary examinations with the catheter placed in a tank full of saline solution confirmed the compatibility of the catheter with biomagnetic measurements.
In the four magnetic
healthy
pacing
subjects,
catheter
was
the
non-
stably
posi-
tioned at the apex of the right ventricle via the brachial or subclavian vein. During registration of the MCG, paced beats were induced scribed
according hereafter.
to the
protocol
After magnetocardiography, wires were removed, with remaining in place, under guidance. the catheter MR imaging
fields
can be prevented.
prove ter tip
Figure 1. Schematic representation of digital subtraction performed in a PVC emerging from the repolarization of a normal beat. Averaged normal beats were subtracted from the combination of a normal beat followed by an extrasystole to obtain the isolated extrasystole. This procedure was carried out in each channel separately.
to and a dis-
due
of movement
coordinates
+
MR
during registration of the MCC (10). bipolar catheter (5 F) is made entirely nonmagnetic material. At the catheter
of MR
frontal,
verify
signed at an
MR
assigned
application
in axial,
with
and
triggering ing. The transferred
-
= angiography, CHD = coronary heart disease, study, MI = myocardialinfarction, MR.! = magnetic PVC = premature ventricular complex, VT = ventricu-
electrodes and can be removed with the catheter remaining in its intracardial position. In this way, heating of the electrodes
free
+
-
AC
-
sagittal planes. MR imaging was chosen because soft tissues can be pictured clearly and because cross-sections of the thorax can be obtained in every desired plane made
+
+
beats
the
model
were
by
2-7)
+
+
aging.
localized
the
results
structures
NA
50/M
17
at each
Assignment
anatomic
-
beats
each
processing,
was
current half
Topographic Imaging The
of data
applying
equivalent homogeneous
42,/M
16
sep-
from the magnetic
by
+
sub-
signal
activity
dimensions
+
beats
instant.
After
51/M 67/M 76/NI 1WM 41/M 65/M 41/F
MRI
during
activity.
the
these
from
performed
zero
41/1s4
PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC VT VT VT Paced
Echo
additional
was
of known
7
Parasystole Parasystole Parasystole Parasystole Parasystole Parasystole CHD, MI CHD, MI CHD, MI CHD Idiopathic CHD, MI Cardiomyopathy NA
AG
offset caused by of the measur-
systems,
correction
42/F 45/M 22/F 47/M 50/M 67/M
EPS
of a normal
line (Fig 1). This procedure arately in all channels.
and
I 2 3 4 5 6
PVC, VT, or Paced Beats
A
beat and the following PVC to obtain isolated PVC activity arising from zero base-
baseline
(y)/Sex
Multichannel
beats
subtraction template was obtained by averaging normal beats. After careful time alignment, the template was digitally sub-
site
Underwent
nor-
was done
superimposed
Age
8 9 10 11 12 13 14
subject.
cases,
mal beat.
accuexpira-
the influon the
In this
averages
superimposed
ing
Who
Abnormality of the Heart
a respi-
neously.
by
in 17 Subjects
to 32 of 37 available magnetic the signals of 12 standard elec-
trocardiographic
noise racy
Performed
de-
exclude within raphy,
Subsequently, tip could be by application in three measures
a movement the heart: the catheter
the lead-in the catheter fluoroscopic the position documented of gradientorthogonal were taken
orthogonal the
registration
of the views
to
of the catheter Before magnetocardiogwas firmly anchored
the trabecular system of the right cle, and the position of the catheter was documented with orthogonal graphs and markers on the body Fluoroscopy
of with
catheter was
of the
position
repeated MCC,
in
ventritip radiosurface. just
before
in before and
after MR imaging, and especially during removal of the lead-in wires. In no case was a movement or change of position
September
of
1991
a. Figure
b.
C.
2. Biomagnetic localization results in a patient with a parasystolic focus high in the interventricular septum, verified with an endocardial catheter mapping procedure. The magnetic field distribution recorded at the onset of the PVC and the magnetocardiographic localization result (arrow) projected on the corresponding cross section of an MR image (a) show that the ectopic focus (0 ) lies high in the interventricular septum. The localization results during the first milliseconds of the ectopic beat form a path that indicates the spreading excitation moving distally along the interventricular septum in frontal view (b) and in axial view (c).
cases
of programmed
ulation msec
the after
ration voltage
ECG-triggered
impulse
was
sensing
stim-
applied
of the
300-350
R wave.
The
du-
of the impulse was I msec, with twice the stimulation threshold
a
(0.8-1.2 V). The stimulation pulse was followed by a low-voltage compensation current
of 12-23
msec
to prevent
effects at the electrodes Induction of VT-VT
polarizing
(Ii).
was induced during registration of the MCG by means of the nonmagnetic catheter. The stimulation protocol included one to three extrastimuli at three basic cycle lengths (600, 500,
and 400 msec) in the right ventricular apex. If VT was sustained, it was terminated by ventricular burst stimulation. Endocardial mapping-To determine 4-
Figures
3, 4.
(3) Biomagnetic localization result of monomorphic PVC in a patient with akinesia of the left ventricular apex. The results of magnetocardiographic localization, obtained during the onset of the PVC transferred to the corresponding MR section, depict the excitation spreading from the margin of the postinfarction scar. (4) Biomagnetic localization result of the origin of monomorphic PVC in a patient with a large aneurysm of the left ventricle. The circle indicates the position of the arrhythmogenic focus, determined with the MCG, at the margin of the lesion. postinfarction
the
exit point of ventricular arrhythmias, endocardial pacing was performed at different sites in the right ventricle, left yentricle, or both. The site of stimulation was systematically
lead
changed
ECG,
paced
the QRS
beats
or PVC
until,
and those
were
very
in the
complexes
12-
of the
of spontaneous
VT
similar.
RESULTS the catheter served.
tip within
During
MR imaging,
of the catheter tal, axial, time
tip were
and
interval
the first and respiratory
the ventricle
sagittal
cross-sections,
of about
45 minutes
last sequences.
weighs
inspiration
Furthermore,
in fron-
QRS
between
physiologic
diastolic
of the right
ventricular
tamed
with
a trigger
msec,
according
during
ratio
an exact
delay
could
be ob-
exceeding
coupling
300
intervals
the nonmagnetic,
that
Volume
pacing rendered
180
presented
topographic possible
Number
#{149}
MR imag-
catheter
intracardial
and,
with programmed registration of the
the
such
if necessary, stimulation MCG.
ing
catheter
raphy:
during
In all four
the
3
referverifica-
beats by
were
ulation
was
during technique
intervals. 100
the site of stimula-
ventricular
induced
extrastimulus
coupling
magnetocardiog-
cases,
The stimuli
apex. sinus
Paced
rhythm
at different frequency of stimper
Localization
of Spontaneous
minute;
in
PVCs
At the onset of every singular extrasystole, bipolar magnetic field patterns were observed that permitted
the three-dimensional an equivalent electrical within the heart. The
Studies
In the course of this study, three different electrophysiologic procedures were performed: Induction of paced beats-Paced beats were induced with the nonmagnetic pac-
hon was the right
ing-compatible ence
to
stimulation.
In this way,
arrhythmias
be induced
Electrophysiologic
cross-sections
apex
to the
terminated during
accuracy.
expiratory
in magnetocardioexpiration out-
in the
complex,
a
localization
complex
as VT could
with
of 2:1. Because MR imaging was conducted with ECG gating and triggered the
of biomagnetic
To exclude only
were evaluated In MR imaging,
lion
coordinates
identical
movements,
events graphy.
ob-
magnetic fields and the localized dipole
Transfer ity,
obtained
of the
site
from
localization of current dipole strength of the the strength increased
of electrical the
magnetic
of quickly.
activfield
distributions during the first milliseconds of the PVC, to the MR image permitted the localization of the ongin of the ectopic beat with respect to morphology.
The
orientations
localized dipoles roughly the direction of propagation
of the
agreed with of the
Radiology
#{149} 687
b.
a.
c.
Figure 5. MCG and electrophysiologic images from a patient with cardiomyopathy and incessant VT. (a, b) Results of biomagnetic localization. The dipole localizations during the beginning of the VT were transferred to the frontal (a) and axial (b) MR images and show the exit point of the VT (0) in the proximal left interventricular septum. (c, d) Results of the electrophysiologic pace-mapping examination. The electrodes of the pacing catheter are placed in
the left ventricle tion that
at the interventricular
at this site creates close of paced beats (d).
excitation.
With
increasingly cardium, represent
and
can
fined
septum
similarity
between
the depolarization
of
large parts of the the localized dipoles a summation of the
no longer
anatomic
In only
myotend to activity
be related
to con-
structures.
two
cases
in this
ectopic beat by repolariza-
tion activity, and simple baseline connection before the onset of the ectopy was sufficient for localization of the equivalent current source. In the other eight cases, the onset of PVC was superimposed by the repolarization activity of the previous beat. Evaluation of these patients showed that correct posed fields
ization.
separation is crucial
Whenever
physiologic
performed known,
had
of superimfor correct
invasive
mapping
and the
had
the
been
focus
was
localization
of up to 80 mm
superimposition
local-
electro-
ectopic
biomagnetic
an error
if the
of magnetic
fields
was not eliminated by means of digital subtraction. Only after the superimposed fields were separated with digital subtraction, as previously descnibed,
subsequent
baseline
con-
rection was performed the onset of the PVC
directly to eliminate
before sys-
tem
and
offset
localized With
could
the
correctly. this method,
ectopic
focus
anatomically
be
and
pathophysiologically meaningful localization was obtained in nine of iO patients. In two patients, the noninvasive biomagnetic localization was con-
firmed by means mapping. In one 688
Radiology
#{149}
below ECC
of endocardial pacepatient, we still had
the aortic
valve
morphology
(circle,
c). Stimula-
of spontaneous
VT and
doubts about the accuracy of the magnetocardiographic localization. Figure 2 consists of images from a patient
with panasystole. magnetic field set
of PVC
study, the onset of the was not superimposed
right the
of the
Figure distribution
extrasystole
2a shows at the projected
d.
the ononto
the MR image of the patient. The position of the MR section corresponds with the depth of the biomagnetically localized equivalent current dipole, which is depicted by the arrow and represents the site of origin of the ectopic
beat.
Invasive
electrophys-
iologic studies performed have proved the localization ectopic
focus
high
tricular septum agreement with
in the
in this case of the interven-
in good topographic the biomagnetic
findings. The propagation of the electrical activity during the first milliseconds of the ectopic beat is shown in
Figure
2b and
clearly
follows
2c. The the
excitation
contours
of the
interventricular septum in a distal direction. Figure 3 shows the result of magnetocardiographic localization in a patient
with
PVC
after
farction.
Coronary
showed
a distal
myocardial
in-
occlusion
of the
left
ventricle was documented at candiography of the left ventricle, echocardiography, and MR imaging. At magnetocandiography, the onset of the ventricular extrasystoles was localized at the margin of the damaged myo-
cardium (Fig 3). In another patient disease,
an
6. The magnetic field distribution and biomagnetic localization (arrow), measured during intracardial stimulation with the amagnetic pacing catheter, projected onto the corresponding MR cross section. The gadolinium-filled catheter tip causes a circumscript dark area while the moving blood is depicted in light gray in gradientecho sequences.
angiography
anterior descending artery. An akinetic area at the apex of the left
heart
Figure
with
extensive
coronary infarction
of the
anterior
wall
had
led
to a large
aneurysm documented with cardiography of the left ventricle, echocardiography, and MR imaging. Two years
after
infarction
sodes tion,
of VT and the patient
On
the
MCG
and
the
after
ventricular presented
ectopic
localized at the margin rysm toward the distal lar septum (Fig 4).
several
epi-
fibrillawith PVC.
focus
was
of the aneuinterventricu-
September
1991
b.
c.
Figure 7. Results of biomagnetic localization during (ta), and sagittal (c) sections in MR images. The circular electrical stimulus (circle) was localized within 4 mm (cross) was localized 9 mm from the site of stimulation.
Localization
ent
field
of VT
For the multichannel
first
time, system
registration
the biomagnetic permitted coher-
of the
magnetic
fields
during times
VT (Fig 8) with recording shorter than 1 minute, which
were
well
tolerated
by the
With the nonmagnetic ten, VT could repeatedly
measurement. The result tion
patients.
pacing cathebe induced
in its intracandial interfering with
of biomagnetic
in all three
agreement
with
the
trophysiologic
result
In all cases,
netic
field
the
at the
the
a stable
showed
only
magnetic
bipolar
and
of
field
pattern.
The
the equivalent and orientation
slight
changes
maximum of the R wave (Fig 9a). After the R-wave orientation of the current quickly changed polarization, the
onset
(Fig field
di-
until
the
was reached peak, the dipole
9b). During minimum
reand
maximum were interchanged compared with those at depolarization; otherwise, the field pattern most identical and remained until the end of nepolarization
9c). Toward the
localized
the
end
was alstable (Fig
of repolarization,
equivalent
current
di-
pole approached the site of the onset of VT. The stability of the magnetic Volume
180
Number
#{149}
3
and toward facilitated of VT.
may
be the
signal-
onset
of de-
the end of localization
reason
why
ously and the repolarization last beats could thus serve template.
It was
the origin
accuracy
simple
of the as a subalso
of VT with
from
that was stimulation
high
unaveraged
MCG recording. agreement with sive
confirmed
tricular
point
part
by
prothe
pace-mapping
the of the
proximal septum
proce-
left yenas the
exit-
of VT (Fig 5c, 5d).
Localization During
of Paced intracardial
localization
with
magnetocardiog-
The stimuli applied after complete repolarization of the previous beat could be localized in three dimensions
after
baseline
correction:
stimulus
registered
in the
mitted the localization lus. The average error and z axes was 8 mm, respectively,
pacing,
prema-
the
MCG
per-
of the stimualong the x, y, 5 mm, and with
an
average
spatial error of 11 mm in all three dimensions. Figure 6 shows the magnetic field distribution during intracardial stimulation and the biomagnetic
localization
indicated
by
the
yellow arrow. Both are projected onto the MR section corresponding to the depth of the localized dipole. The cmdank
mark
is caused
by
the
cath-
eten tip. After the termination of the compensation current (13-24 msec aften the stimulation), the magnetic field pattern caused by the paced beats
was
localization
registered
and
of the
ten tip, the circle netic localization
tune paced beats were induced at the catheter tip. Because the position of the catheter tip could be exactly documented at MR imaging, the exact location of both the electrical stimulus and the following myocardial depolarization was known and thus permitted verification of the accuracy of
In all
four healthy subjects, the bipolar magnetic field distribution during
site
depolarization, shown planes in Figure 7. The responds to the position
Beats
(a), axial tip. The paced beat
raphy.
cular
In good topographic this result, an inva-
endocardial
dune
induced during
catheter transferred to frontal represents the site of the catheter after stimulation, the onset of the
2 mm,
possible
data. Averaging of about 50 beats in VT only slightly improves the localization result. Figure 5a and 5b shows the result of magnetocardiographic localization in a patient with spontaneous VT grammed
characteristic
9). Immediately
magnetic field pole localization
mag-
of VT showed
remarkable
depolarization,
elec-
a high
at the
baseline correction at the very onset of the R wave yielded the same localization result as digital subtraction. The latter could be performed if the VT episodes terminated spontane-
spatial
proce-
registered
distributions
a typical,
showed
of the
(S/N)
polarization repolarization of the origin
to localize
in good
pace-mapping
dune.
(Fig
was
and
ratio
traction
localiza-
patients
distributions
to-noise
This
and terminated during registration of the MCG after the previously mentioned stimulation protocol. The catheter could remain position without
intracardial stimulation with the nonmagnetic dark mark at the apex of the right ventricle from the catheter tip. Thirteen milliseconds
the
cross
depicts of the
represents
the
permitted
of myocardial in all three dark mark conof the cathe-
the biomagstimulus, and site
of electri-
cal activity caused by myocardial vation localized from the MCG msec
after
stimulation.
The
acti13
difference
of about 10 mm from the localization of the stimulus is in good agreement with the known conduction velocity (approximately 1 mm/msec) in the myocardium. Radiology
#{149} 689
In two programmed premature stimulation to apply stimuli during
examinations, was performed the repolar-
ization
beat
of the
previous
I’
--‘
V
y
,1V
y
V
v
y
V
:V
,V
to simu-
late the pathologic activity being superimposed by background activity. The localization result of premature stimuli showed inaccuracies of several centimeters in all directions, with an overall error of 100 mm and 73 mm in the two studies in comparison with stimuli applied at zero baseline if the superimposed repolarization was not correctly eliminated. After digital subtraction of the repolarization template, the localization error of the premature stimuli could be reduced to 7 mm and 9 mm, respectively.
,
1.-’
.#{248}#{149}’ i---
\
t.
‘
.
Jr
lLxj,e
,.,
.‘
vi
‘i
r
ii
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DISCUSSION Previous reports have indicated the possibility to biomagnetically localize the arrhythmogenic tissue in patients with ventricular extrasystoles and VT (12,13).
However,
these
#{149} Radiology
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measure-
ments were obtained with singlechannel systems. The magnetic fields could not be registered coherently, and the measurements required long recording times. To obtain a sufficient number of expiratory averages, the recording time had to be prolonged even further. Arrythmias cannot be tolerated for a long time by the patient, especially if they are to be hemodynamically effective and thus of clinical relevance. Therefore, such arrhythmias can be only insufficiently studied with single-channel systems. Coherent registration of the magnetic field with biomagnetic multichannel systems in connection with adequate imaging, such as MR imaging, is the technical prerequisite for precise localization of transient events like singular PVC (14,15) or hemodynamically effective arrhythmias such as VT that can only be tolerated for a limited time. In this way, recording times of less than 15 minutes for PVC and less than 30 seconds for VT are sufficient to obtain an acceptable number of expiratory averages. A high S/N provides for exact localization of the ectopic activity. The following factors are among the most important contributors to localization inaccuracies with the biomagnetic method: patient movements, inaccuracy of the transformation of localization results to the MR images, and insufficient source and torso modeling. The greatest localization inaccuracies were found in the examination of deep sources, which confirmed earlier observations in phantom studies with a dipolar 690
..
Figure 8. Magnetocardiographic registration of nonsustained VT with the biomagnetic multichannel system. The magnetic signal recorded during 4.6 seconds is shown in eight of the 37 MCG channels (top). Three of the 12 simultaneously recorded ECG leads are shown beneath this signal. The bottom curve shows the respiration channel.
source filled
submerged with
in a plastic
saline
tank
solution.
The development of a nonmagnetic pacing catheter compatible with the exact three-dimensional imaging technique of MR imaging permitted verification of the localization accuracy of this method. The results of our pacing studies indicate a high precision of magnetocardiographic localization, even if data evaluation and localization are done with simplified modeling rounding
of the volume
future, contribute
the study
more
sources and conductors.
of paced
to the
events
development
sophisticated
Experiments
the surIn the
models. with
the
nonmagnetic
stimulation catheter permitted study and solution of a major lem in magnetocardiography, perimposition
may of
of magnetic
the probthe su-
fields:
If
several sources are active within the heart at the same time, the registered magnetic field always represents a summation of several components. In this study, superimposed fields were recorded in patients with ventricular extrasystoles arising from the repolarization activity of the previous beat. Our results show that the problem can be solved by applying digital subtraction if the disturbing background fields are known (eg, if normal beats not followed by a PVC are registered
and can be used as a subtraction ternplate). The study of VTs revealed some characteristic
properties:
The
localiza-
tion of the equivalent current dipole remained stable and showed a high S/N during the complete upstroke of the R wave. For this reason, the site of origin of the VT could be localized without
cases
digital
even
During
subtraction,
from
in some
unaveraged
repolarization,
the
data. magnetic
field map had reverse polarity and, again, remarkable stability. This observation might correspond to specific patterns and mechanisms of excitation during VT. In one patient with VT, interpretation of the data with simplified models
was
not
possible,
because
the
mag-
netic field did not show a typical bipolar pattern. The high density of isofleld lines in the cramo-left-lateral direction might have been caused by a border effect indicating insufficiency of the model of the infinite half space, which is usually applied to describe the thorax as a volume conductor. Interpretation
of this
with the commonly model of a single yielded centimeters
tours. The
a localization outside
correct
field
used current
distribution
source dipole
result several the heart con-
localization
of the September
site 1991
I
.
! -
.
.
..
00
.‘#{149}#{149}-
iPj
r1 .,
Figure 9. The magnetic field distribution registered coherently with the biomagnetic multichannel system during VT. The field maps stroke ization
during
stable . .
-
.
.
._.i_
--
.
during
;Lt:
.
.
7;;#{149}ir’
.,
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I
..
w’
L.
..
. ..
no
n
a
electrical
.
Volume
180
#{149} Number
3
Toward
and
dipole
of the
the
remain R wave.
orientation
at
(b) During the
dipole
ap-
end
of repo-
the field map shows a similar disinterchanged polarity with that of the onset of VT (c).
with
activity
of heart
arrhythmias
can be completely and noninvasively examined. This method not only enables localization of the site of origin of arrhythmias, but it also offers new insights in the mechanisms of ectopic
/\
w,
its onset.
repolar-
charactenis-
of origin of VT in this case could only be obtained by the application of a new localization algorithm that is based on the reconstruction of the current density distribution (“lead fields”) (16). A very high number of evenly distributed current dipoles is assumed, and the strength and direction of the dipoles are chosen to optimally describe the measured field distribution. The number of sources is then systematically reduced until a minimal number of dipolar sources still describing the measured field distribution within the S/N is reached. With this algorithm (which, in this study, was applied to clinical magnetocardiographic data for the first time, to our knowledge), a localization result in exact agreement with invasive pace-mapping was obtained. In eight other cases in this study, in which the localization of a single current dipole had yielded verified or physiologically meaningful results, the site of highest current density was found in the same area. These observations may suggest that localization could be improved in some cases if improved source models were used. Adequate evaluation of data recorded with a biomagnetic multichannel system combined with adequate imaging, such as MR imaging, is the first and only method with which electrical
.
changes
the equivalent
compared
1
the upstroke
quickly
repolarization,
tribution
:.ci t
current
of the R wave
larization,
:
down-
field distribution
the maximum proaches
:::
(a) and
wave and during a remarkable
equivalent
The dipole .
:[yT.
:
R
tic: The magnetic localized
..
the upstroke
(b) of the (c) display
:
heart
activity.
Because the annual mortality rate due to cardiac rhythm disorders is about 1,500 per 1 million inhabitants in industrialized nations, a simple and noninvasive method to investigate these disorders is of outstanding importance.
Epidemiologic
studies
with
broad application of this noninvasive method could lead to better underRadiology
691
#{149}
standing and classification of this inhomogeneous group of diseases. The results may, in turn, help to improve methods of treatment in the future. Biornagnetisrn
might
be
a key
5.
al. study
6.
7.
References 1.
2.
3.
4.
692
Baule C, McFee R. Detection of the magnetic field of the heart. Am Heart J 1%3; 66:95-96. Ern#{233} SN. High resolution magnetocardiography: modeling and sources localizalion. Med BiolEng Comput 1985; 23(suppl): 1447-1450. Fenici RR, Melillo C, Maselli M, Capeffi A. Magnetocardiographic three-dimensional localization of Kent bundles. In: Atsumi K, Kotani M, Ueno S, et al, eds. Biomagnetism ‘87. Tokyo: Tokyo Denki University Press, 1987; 140-141. Katila T, Montoneu L, Maekijaervi M, Nenonen J, Raivic M, Siltanen P. Localization of the accessory cardiac conduction pathway. In: AtSUnIi K, Kotani M, Ueno S, et al, eds. Biomagnetism ‘87. Tokyo: Tokyo Denki University Press, 1987; 430-433.
#{149} Radiology
S, Hoenig
Multichannel
E, Rechenberger
biomagnetic
system
H, et for and
of electrical activity in the brain Radiology 1990; 176:825-830. Abraham-Fuchs K, Schneider 5, Reichenberger H. MCG inverse solution: influence of coil size, grid size, number of coils, and SNR. IEEE Trans Biomed Eng 1988; 573-576. Abraham-Fuchs K, Weikl A, Schneider S. et al. Application of a biomagnetic multichannel system to the comparative localization of accessory conduction pathways in patients with WPW syndrome. In: Williamson SJ, Hoke M, Stroink C, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 369-372. Achenbach 5, Moshage W, Weild A, et al. Elimination of electronic offset and physiological background activity in magnetocardiographic localization. Biomed Tech (Berlin) 1990; 35(suppl 3):160-161. Williamson SJ, Romani CL, Kaufman L, Modena J. Biomagnetism: an interdisciplinary approach. New York: Plenum, 1982. Moshage W, Achenbach 5, BoLz A, et al. A non-magnetic, MR-compatible pacing catheter for clinical application in magnetocardiography. Biomed Tech (Berlin) 1990; 35(suppl 3):162-163. Schaldach M, Boheim C, Edelh#{228}user R. Em beitrag zur erkennung evozierter po-
8.
9.
10.
11.
Biomed Tech (Berlin) 1988; 2):343-344. Fenici RR, Melillo C, Capelli A, Deluca C, Maseffi M. Magnetocardiographic localization of a pacing catheter. In: Williamson SJ, Hoke M, Stroink G, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 361-364. Fenici RR, Melillo C, Capelli A, Deluca C, Maselli M. Atrial and ventricular tachycardias: invasive validation and reproducibility of magnetocardiographic imaging. In: Williamson SJ, Hoke M, Stroink C, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 441-444. Moshage W, Achenbach 5, Weikl A, et al. Magnetocardiography (MCG): Noninvasive localization of ventricular extrasystoles with a biomagnetic multichannel system. Presented at Update in Clinical Cardiology, Herzlya, Israel, May 20-22, 1990. Moshage W, Weild A, Abraham-Fuchs K, et al. Magnetokardiographie: erste klinische erfahrungen mit einem biomagnetischen vielkanalsystem. HerzfKreisl 1990; 22:245249. Joannides AA, Bolton JPR, Clarke CJS. Continuous probabilistic solutions to the biomagnetic inverse problem. inverse Problems 1990; 6:523-542. tentiale.
33(suppl 12.
heart.
to solv-
ing a problem that is of preeminent importance because of the high preyalence of life-threatening arrhythmias. #{149}
Schneider
13.
14.
15.
16.
September
1991
MD MD
Stephan Achenbach Peter Wegener, MS
Biomagnetic Arrhythmias’
complexes
(PVCs),
three
with ventricular tachycardia four healthy subjects with induced paced beats were recorded for 2-15 minutes. After correction for superimposed repolanization activity, the site of origin of the arrhythmias was localized from the magnetic field and
distribution
at the
onset
of the
B
ec-
topic beats. The localization results of paced beats showed an error of a few millimeters in relation to the position of the catheter tip. The results of spontaneous PVC and VT were confirmed with endocardial mapping or associated with ischemic lesions. The authors conclude that multichannel magnetocardiographic studies enable the completely noninvasive localization of ventricular arrhythmias.
every electric a magnetic
causes
trical
excitation
specific
of the
magnetic
current field, the
heart
field
These
patterns can be measured without contact over the thorax to obtain the magnetocardiogram (MCG). These magnetic fields, unlike the common electrocardiogram, remain almost uninfluenced by the conductive properof the
tissues
cal sources system.
and
between
the
the
SUBJECTS Study
measurement
Therefore,
the
field
sequentially
over
many
points of a measuring grid to record sufficiently large field distribution. Localization could be done only in
a
the Wolff-Parkinson-White
1991;
180:685-692
in
syndrome
The recent development channel, superconducting interference-device
of multiquantum
systems
opened
I
From
Poliklinik, Oestliche
the
Medical
Clinic
II (Cardiology)
and
University of Erlangen-Nuremberg, Stadtmauerstrasse 29, 8520 Erlangen, Germany (W.M., S.A., KG., A.W., KB.); and the Medical Engineering Group, Siemens, Erlangen, Germany (P.W., S.S., W.H.). From the 1990 RSNA scientific assembly. Received November 29, 1990; revision requested February 11, 1991; revision received May 2; accepted May 9. Address reprint requests to W.M. C RSNA, 1991
as 37 sensors,
these
systems
can
record the magnetic field distribution over a sufficiently large area without repositioning
of the
system
or the
and
four
healthy
of these associated
10 patients, the extrasystoles with ischemic myocardial
were dam-
age documented by means of coronary angiography, cardiography of the left yentide, and MR imaging. The other six patients
presented
Three patient,
duced dial
parasystole.
with
patients had recurrent VT. In one several episodes of VT were inby means of programmed intracar-
stimulation
during
registration
MCG. Some of the episodes minated by burst stimulation; nonsustained. Two patients incessant
of the
had to be tersome were showed spon-
VT.
In four healthy subjects, paced beats were induced at a known site within the heart by use of the specially designed nonmagnetic pacing catheter (described subsequently) biomagnetic
to verify localization.
Registration
and
the accuracy
of
of the
Evaluation
MCG
new fields of clinical applications for the biomagnetic method (5). With as many
arrhythmias
i3
with paced beats (Table). All patients and healthy subjects had given informed consent after the nature of the study had been fully explained. Ten patients showed spontaneous, pnedominantly monomorphic PVCs. In four
taneous
repeated of accesdelta wave
a group of i7 individuals: spontaneous or induced
volunteers
distributions measured over the body surface permit the quantification and the three-dimensional localization of sources within the heart. They also permit the evaluation of the propagation of physiologic and pathologic heart excitation. Even though the magnetocardiographic method was described as early as 1963 by Baule and McFee (1), its clinical applicability was limited for a long time because measurements had to be obtained with systems composed of only one to seven channels. These systems had to be adjusted
METHODS
Group
ventricular
magnetic
AND
We studied patients with
electri-
(2-4). Radiology
MD H#{228}rer,MS
graphic localization in patients with premature ventricular contraction and ventricular tachycardia (VT) and the verification of the localization accuracy of the method.
elec-
creates
patterns.
disorders with periodically signals, such as localization sony pathways from the
Index terms: Biomagnetism #{149} Heart, experimental studies, 51.1299 #{149}Heart, function, 515.1429,524.1429 S Heart, rhythm
#{149} Andreas Weikl, PhD #{149} Wolfgang
of Ventricular
ECAUSE
ties
patients (VT),
G#{246}hl,MD Schneider,
#{149}
Localization
The magnetic fields caused by electrical activity of the human heart can be coherently measured with a highly sensitive, multichannel, superconducting quantum interference-device system and can enable noninvasive localization of the underlying electrical activity. The magnetocardiograms (MCGs) of 10 patients with spontaneous premature ventricular
#{149} Konrad Siegfried
#{149} #{149}
The MCG was registered over the thorax without physical contact by use of a planar
biomagnetic
(Krenikon; (5).
pa-
lying
The
Siemens, patients
on a wooden
and
multichannel
system
Erlangen,
Germany)
healthy
couch
subjects
inside
were
a magnet-
tient. Besides drastic shortening of the recording times, coherent registration of the data for the first time permits study of spontaneous on sporadic events.
The cal
goal
application
of this
study
was
of magnetocardio-
the clini-
Abbreviations: ECG = electrocardiogram, MCC = magnetocardiogram, PVC = premature ventricular complex, S/N = signal-to-noise ratio, VT = ventricular tachycardia.
685
ically
shielded
room.
recordings
The
ranged
addition channels,
duration
from
of the
2 to 15 minutes.
In
Examinations
ration
(ECC)
channel
were
leads
and
recorded
Subject No.
simulta-
To localize the magnetic
the electrical activity from field distribution, the PVC,
beats
during
tachycardia,
were
averaged
or paced
to reduce
beats
background
and thus improve localization (6). Only data sampled during
tion were averaged ence of respiratory localization
to minimize movements
results
(7).
to 50 expiratory tamed
in each
In most the
the onset by
two
be ob-
fields
of the
Digital the
of the PVC was
magnetic
repolarization
separate
study,
could
caused
previous
subtraction
fields
tracted
from
the
combination
to
(8):
was done
To compensate for any the electronic components recording
periods
channel
separately,
signal
during
tracted
15
For
mean
recorded
intervals
was
the measured
these
steps
of electrical
three
distributions
single finite
localization
imaging
(Figs
Paced
NA
Paced
65/M
NA
Paced
Note.-+ = performed, Echo = echocardiography, resonance (MR) imaging, tar tachycardia.
= not performed, EPS = electrophysical NA = not applicable,
matched
the
Nonmagnetic, Imaging-compatible Catheter
in
field
To
of a
MCG,
dipole in the inspace (9).
with
visible
marks
in MR
to the to the
sponding
plane
positioning support
the same
ECG
to
and
was
with
system fixed
position
cardiography netocardiography,
artifacts
and images
MR the
both imaging. support
corre-
of a special
(5): A T-shaped to the patient’s
and
gat-
were
of the
the help
during
by
respiration
of the MCG MR
plastic chest in
magnetoFor magcarried
four current-driven wire coils, which were replaced by agaroseor copper sulphatefilled cubes during MR imaging. The position of the wire coils in magnetocardiography was determined by measuring the magnetic
field
rent
run
686
#{149}
through
Radiology
caused
+
-
-
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
-
+
+
+
+
4-
+
+
+
+
+
+
+
+
+
+
-
-
+
+
-
-
+
+
-
-
+
+
-
-
+
+
im-
the
Pacing
localization
a bipolar
pacing
in a way that exactly known
accuracy
catheter
of the
was
permitted site within
de-
stimulation the heart The of tip,
two platinum electrodes apart. The copper lead-in
are fixed 10 mm wires are
twisted so that the from the electrodes
current not cause
turbing net the lead-in
electric does
by
the coils
the
and
known
could
cur-
be
to the
magnetic field. Additionally, wires detach easily from
radio-frequency
MR imaging visibility is filled
at MR imaging, with lyophilized
tetate
dimeglumine,
the
To imthe cathegadopen-
echo sequences planes. Several
during
an MR imaging
contrast agent. Preliminary examinations with the catheter placed in a tank full of saline solution confirmed the compatibility of the catheter with biomagnetic measurements.
In the four magnetic
healthy
pacing
subjects,
catheter
was
the
non-
stably
posi-
tioned at the apex of the right ventricle via the brachial or subclavian vein. During registration of the MCG, paced beats were induced scribed
according hereafter.
to the
protocol
After magnetocardiography, wires were removed, with remaining in place, under guidance. the catheter MR imaging
fields
can be prevented.
prove ter tip
Figure 1. Schematic representation of digital subtraction performed in a PVC emerging from the repolarization of a normal beat. Averaged normal beats were subtracted from the combination of a normal beat followed by an extrasystole to obtain the isolated extrasystole. This procedure was carried out in each channel separately.
to and a dis-
due
of movement
coordinates
+
MR
during registration of the MCC (10). bipolar catheter (5 F) is made entirely nonmagnetic material. At the catheter
of MR
frontal,
verify
signed at an
MR
assigned
application
in axial,
with
and
triggering ing. The transferred
-
= angiography, CHD = coronary heart disease, study, MI = myocardialinfarction, MR.! = magnetic PVC = premature ventricular complex, VT = ventricu-
electrodes and can be removed with the catheter remaining in its intracardial position. In this way, heating of the electrodes
free
+
-
AC
-
sagittal planes. MR imaging was chosen because soft tissues can be pictured clearly and because cross-sections of the thorax can be obtained in every desired plane made
+
+
beats
the
model
were
by
2-7)
+
+
aging.
localized
the
results
structures
NA
50/M
17
at each
Assignment
anatomic
-
beats
each
processing,
was
current half
Topographic Imaging The
of data
applying
equivalent homogeneous
42,/M
16
sep-
from the magnetic
by
+
sub-
signal
activity
dimensions
+
beats
instant.
After
51/M 67/M 76/NI 1WM 41/M 65/M 41/F
MRI
during
activity.
the
these
from
performed
zero
41/1s4
PVC PVC PVC PVC PVC PVC PVC PVC PVC PVC VT VT VT Paced
Echo
additional
was
of known
7
Parasystole Parasystole Parasystole Parasystole Parasystole Parasystole CHD, MI CHD, MI CHD, MI CHD Idiopathic CHD, MI Cardiomyopathy NA
AG
offset caused by of the measur-
systems,
correction
42/F 45/M 22/F 47/M 50/M 67/M
EPS
of a normal
line (Fig 1). This procedure arately in all channels.
and
I 2 3 4 5 6
PVC, VT, or Paced Beats
A
beat and the following PVC to obtain isolated PVC activity arising from zero base-
baseline
(y)/Sex
Multichannel
beats
subtraction template was obtained by averaging normal beats. After careful time alignment, the template was digitally sub-
site
Underwent
nor-
was done
superimposed
Age
8 9 10 11 12 13 14
subject.
cases,
mal beat.
accuexpira-
the influon the
In this
averages
superimposed
ing
Who
Abnormality of the Heart
a respi-
neously.
by
in 17 Subjects
to 32 of 37 available magnetic the signals of 12 standard elec-
trocardiographic
noise racy
Performed
de-
exclude within raphy,
Subsequently, tip could be by application in three measures
a movement the heart: the catheter
the lead-in the catheter fluoroscopic the position documented of gradientorthogonal were taken
orthogonal the
registration
of the views
to
of the catheter Before magnetocardiogwas firmly anchored
the trabecular system of the right cle, and the position of the catheter was documented with orthogonal graphs and markers on the body Fluoroscopy
of with
catheter was
of the
position
repeated MCC,
in
ventritip radiosurface. just
before
in before and
after MR imaging, and especially during removal of the lead-in wires. In no case was a movement or change of position
September
of
1991
a. Figure
b.
C.
2. Biomagnetic localization results in a patient with a parasystolic focus high in the interventricular septum, verified with an endocardial catheter mapping procedure. The magnetic field distribution recorded at the onset of the PVC and the magnetocardiographic localization result (arrow) projected on the corresponding cross section of an MR image (a) show that the ectopic focus (0 ) lies high in the interventricular septum. The localization results during the first milliseconds of the ectopic beat form a path that indicates the spreading excitation moving distally along the interventricular septum in frontal view (b) and in axial view (c).
cases
of programmed
ulation msec
the after
ration voltage
ECG-triggered
impulse
was
sensing
stim-
applied
of the
300-350
R wave.
The
du-
of the impulse was I msec, with twice the stimulation threshold
a
(0.8-1.2 V). The stimulation pulse was followed by a low-voltage compensation current
of 12-23
msec
to prevent
effects at the electrodes Induction of VT-VT
polarizing
(Ii).
was induced during registration of the MCG by means of the nonmagnetic catheter. The stimulation protocol included one to three extrastimuli at three basic cycle lengths (600, 500,
and 400 msec) in the right ventricular apex. If VT was sustained, it was terminated by ventricular burst stimulation. Endocardial mapping-To determine 4-
Figures
3, 4.
(3) Biomagnetic localization result of monomorphic PVC in a patient with akinesia of the left ventricular apex. The results of magnetocardiographic localization, obtained during the onset of the PVC transferred to the corresponding MR section, depict the excitation spreading from the margin of the postinfarction scar. (4) Biomagnetic localization result of the origin of monomorphic PVC in a patient with a large aneurysm of the left ventricle. The circle indicates the position of the arrhythmogenic focus, determined with the MCG, at the margin of the lesion. postinfarction
the
exit point of ventricular arrhythmias, endocardial pacing was performed at different sites in the right ventricle, left yentricle, or both. The site of stimulation was systematically
lead
changed
ECG,
paced
the QRS
beats
or PVC
until,
and those
were
very
in the
complexes
12-
of the
of spontaneous
VT
similar.
RESULTS the catheter served.
tip within
During
MR imaging,
of the catheter tal, axial, time
tip were
and
interval
the first and respiratory
the ventricle
sagittal
cross-sections,
of about
45 minutes
last sequences.
weighs
inspiration
Furthermore,
in fron-
QRS
between
physiologic
diastolic
of the right
ventricular
tamed
with
a trigger
msec,
according
during
ratio
an exact
delay
could
be ob-
exceeding
coupling
300
intervals
the nonmagnetic,
that
Volume
pacing rendered
180
presented
topographic possible
Number
#{149}
MR imag-
catheter
intracardial
and,
with programmed registration of the
the
such
if necessary, stimulation MCG.
ing
catheter
raphy:
during
In all four
the
3
referverifica-
beats by
were
ulation
was
during technique
intervals. 100
the site of stimula-
ventricular
induced
extrastimulus
coupling
magnetocardiog-
cases,
The stimuli
apex. sinus
Paced
rhythm
at different frequency of stimper
Localization
of Spontaneous
minute;
in
PVCs
At the onset of every singular extrasystole, bipolar magnetic field patterns were observed that permitted
the three-dimensional an equivalent electrical within the heart. The
Studies
In the course of this study, three different electrophysiologic procedures were performed: Induction of paced beats-Paced beats were induced with the nonmagnetic pac-
hon was the right
ing-compatible ence
to
stimulation.
In this way,
arrhythmias
be induced
Electrophysiologic
cross-sections
apex
to the
terminated during
accuracy.
expiratory
in magnetocardioexpiration out-
in the
complex,
a
localization
complex
as VT could
with
of 2:1. Because MR imaging was conducted with ECG gating and triggered the
of biomagnetic
To exclude only
were evaluated In MR imaging,
lion
coordinates
identical
movements,
events graphy.
ob-
magnetic fields and the localized dipole
Transfer ity,
obtained
of the
site
from
localization of current dipole strength of the the strength increased
of electrical the
magnetic
of quickly.
activfield
distributions during the first milliseconds of the PVC, to the MR image permitted the localization of the ongin of the ectopic beat with respect to morphology.
The
orientations
localized dipoles roughly the direction of propagation
of the
agreed with of the
Radiology
#{149} 687
b.
a.
c.
Figure 5. MCG and electrophysiologic images from a patient with cardiomyopathy and incessant VT. (a, b) Results of biomagnetic localization. The dipole localizations during the beginning of the VT were transferred to the frontal (a) and axial (b) MR images and show the exit point of the VT (0) in the proximal left interventricular septum. (c, d) Results of the electrophysiologic pace-mapping examination. The electrodes of the pacing catheter are placed in
the left ventricle tion that
at the interventricular
at this site creates close of paced beats (d).
excitation.
With
increasingly cardium, represent
and
can
fined
septum
similarity
between
the depolarization
of
large parts of the the localized dipoles a summation of the
no longer
anatomic
In only
myotend to activity
be related
to con-
structures.
two
cases
in this
ectopic beat by repolariza-
tion activity, and simple baseline connection before the onset of the ectopy was sufficient for localization of the equivalent current source. In the other eight cases, the onset of PVC was superimposed by the repolarization activity of the previous beat. Evaluation of these patients showed that correct posed fields
ization.
separation is crucial
Whenever
physiologic
performed known,
had
of superimfor correct
invasive
mapping
and the
had
the
been
focus
was
localization
of up to 80 mm
superimposition
local-
electro-
ectopic
biomagnetic
an error
if the
of magnetic
fields
was not eliminated by means of digital subtraction. Only after the superimposed fields were separated with digital subtraction, as previously descnibed,
subsequent
baseline
con-
rection was performed the onset of the PVC
directly to eliminate
before sys-
tem
and
offset
localized With
could
the
correctly. this method,
ectopic
focus
anatomically
be
and
pathophysiologically meaningful localization was obtained in nine of iO patients. In two patients, the noninvasive biomagnetic localization was con-
firmed by means mapping. In one 688
Radiology
#{149}
below ECC
of endocardial pacepatient, we still had
the aortic
valve
morphology
(circle,
c). Stimula-
of spontaneous
VT and
doubts about the accuracy of the magnetocardiographic localization. Figure 2 consists of images from a patient
with panasystole. magnetic field set
of PVC
study, the onset of the was not superimposed
right the
of the
Figure distribution
extrasystole
2a shows at the projected
d.
the ononto
the MR image of the patient. The position of the MR section corresponds with the depth of the biomagnetically localized equivalent current dipole, which is depicted by the arrow and represents the site of origin of the ectopic
beat.
Invasive
electrophys-
iologic studies performed have proved the localization ectopic
focus
high
tricular septum agreement with
in the
in this case of the interven-
in good topographic the biomagnetic
findings. The propagation of the electrical activity during the first milliseconds of the ectopic beat is shown in
Figure
2b and
clearly
follows
2c. The the
excitation
contours
of the
interventricular septum in a distal direction. Figure 3 shows the result of magnetocardiographic localization in a patient
with
PVC
after
farction.
Coronary
showed
a distal
myocardial
in-
occlusion
of the
left
ventricle was documented at candiography of the left ventricle, echocardiography, and MR imaging. At magnetocandiography, the onset of the ventricular extrasystoles was localized at the margin of the damaged myo-
cardium (Fig 3). In another patient disease,
an
6. The magnetic field distribution and biomagnetic localization (arrow), measured during intracardial stimulation with the amagnetic pacing catheter, projected onto the corresponding MR cross section. The gadolinium-filled catheter tip causes a circumscript dark area while the moving blood is depicted in light gray in gradientecho sequences.
angiography
anterior descending artery. An akinetic area at the apex of the left
heart
Figure
with
extensive
coronary infarction
of the
anterior
wall
had
led
to a large
aneurysm documented with cardiography of the left ventricle, echocardiography, and MR imaging. Two years
after
infarction
sodes tion,
of VT and the patient
On
the
MCG
and
the
after
ventricular presented
ectopic
localized at the margin rysm toward the distal lar septum (Fig 4).
several
epi-
fibrillawith PVC.
focus
was
of the aneuinterventricu-
September
1991
b.
c.
Figure 7. Results of biomagnetic localization during (ta), and sagittal (c) sections in MR images. The circular electrical stimulus (circle) was localized within 4 mm (cross) was localized 9 mm from the site of stimulation.
Localization
ent
field
of VT
For the multichannel
first
time, system
registration
the biomagnetic permitted coher-
of the
magnetic
fields
during times
VT (Fig 8) with recording shorter than 1 minute, which
were
well
tolerated
by the
With the nonmagnetic ten, VT could repeatedly
measurement. The result tion
patients.
pacing cathebe induced
in its intracandial interfering with
of biomagnetic
in all three
agreement
with
the
trophysiologic
result
In all cases,
netic
field
the
at the
the
a stable
showed
only
magnetic
bipolar
and
of
field
pattern.
The
the equivalent and orientation
slight
changes
maximum of the R wave (Fig 9a). After the R-wave orientation of the current quickly changed polarization, the
onset
(Fig field
di-
until
the
was reached peak, the dipole
9b). During minimum
reand
maximum were interchanged compared with those at depolarization; otherwise, the field pattern most identical and remained until the end of nepolarization
9c). Toward the
localized
the
end
was alstable (Fig
of repolarization,
equivalent
current
di-
pole approached the site of the onset of VT. The stability of the magnetic Volume
180
Number
#{149}
3
and toward facilitated of VT.
may
be the
signal-
onset
of de-
the end of localization
reason
why
ously and the repolarization last beats could thus serve template.
It was
the origin
accuracy
simple
of the as a subalso
of VT with
from
that was stimulation
high
unaveraged
MCG recording. agreement with sive
confirmed
tricular
point
part
by
prothe
pace-mapping
the of the
proximal septum
proce-
left yenas the
exit-
of VT (Fig 5c, 5d).
Localization During
of Paced intracardial
localization
with
magnetocardiog-
The stimuli applied after complete repolarization of the previous beat could be localized in three dimensions
after
baseline
correction:
stimulus
registered
in the
mitted the localization lus. The average error and z axes was 8 mm, respectively,
pacing,
prema-
the
MCG
per-
of the stimualong the x, y, 5 mm, and with
an
average
spatial error of 11 mm in all three dimensions. Figure 6 shows the magnetic field distribution during intracardial stimulation and the biomagnetic
localization
indicated
by
the
yellow arrow. Both are projected onto the MR section corresponding to the depth of the localized dipole. The cmdank
mark
is caused
by
the
cath-
eten tip. After the termination of the compensation current (13-24 msec aften the stimulation), the magnetic field pattern caused by the paced beats
was
localization
registered
and
of the
ten tip, the circle netic localization
tune paced beats were induced at the catheter tip. Because the position of the catheter tip could be exactly documented at MR imaging, the exact location of both the electrical stimulus and the following myocardial depolarization was known and thus permitted verification of the accuracy of
In all
four healthy subjects, the bipolar magnetic field distribution during
site
depolarization, shown planes in Figure 7. The responds to the position
Beats
(a), axial tip. The paced beat
raphy.
cular
In good topographic this result, an inva-
endocardial
dune
induced during
catheter transferred to frontal represents the site of the catheter after stimulation, the onset of the
2 mm,
possible
data. Averaging of about 50 beats in VT only slightly improves the localization result. Figure 5a and 5b shows the result of magnetocardiographic localization in a patient with spontaneous VT grammed
characteristic
9). Immediately
magnetic field pole localization
mag-
of VT showed
remarkable
depolarization,
elec-
a high
at the
baseline correction at the very onset of the R wave yielded the same localization result as digital subtraction. The latter could be performed if the VT episodes terminated spontane-
spatial
proce-
registered
distributions
a typical,
showed
of the
(S/N)
polarization repolarization of the origin
to localize
in good
pace-mapping
dune.
(Fig
was
and
ratio
traction
localiza-
patients
distributions
to-noise
This
and terminated during registration of the MCG after the previously mentioned stimulation protocol. The catheter could remain position without
intracardial stimulation with the nonmagnetic dark mark at the apex of the right ventricle from the catheter tip. Thirteen milliseconds
the
cross
depicts of the
represents
the
permitted
of myocardial in all three dark mark conof the cathe-
the biomagstimulus, and site
of electri-
cal activity caused by myocardial vation localized from the MCG msec
after
stimulation.
The
acti13
difference
of about 10 mm from the localization of the stimulus is in good agreement with the known conduction velocity (approximately 1 mm/msec) in the myocardium. Radiology
#{149} 689
In two programmed premature stimulation to apply stimuli during
examinations, was performed the repolar-
ization
beat
of the
previous
I’
--‘
V
y
,1V
y
V
v
y
V
:V
,V
to simu-
late the pathologic activity being superimposed by background activity. The localization result of premature stimuli showed inaccuracies of several centimeters in all directions, with an overall error of 100 mm and 73 mm in the two studies in comparison with stimuli applied at zero baseline if the superimposed repolarization was not correctly eliminated. After digital subtraction of the repolarization template, the localization error of the premature stimuli could be reduced to 7 mm and 9 mm, respectively.
,
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-
DISCUSSION Previous reports have indicated the possibility to biomagnetically localize the arrhythmogenic tissue in patients with ventricular extrasystoles and VT (12,13).
However,
these
#{149} Radiology
1
..
1.51S
3’C1’?A.P*T2)b4.#{248}1
e.500
2.SSS
.EXT2.
3.SSS
3.51S
4.SSS
4.511
(SECI
S
measure-
ments were obtained with singlechannel systems. The magnetic fields could not be registered coherently, and the measurements required long recording times. To obtain a sufficient number of expiratory averages, the recording time had to be prolonged even further. Arrythmias cannot be tolerated for a long time by the patient, especially if they are to be hemodynamically effective and thus of clinical relevance. Therefore, such arrhythmias can be only insufficiently studied with single-channel systems. Coherent registration of the magnetic field with biomagnetic multichannel systems in connection with adequate imaging, such as MR imaging, is the technical prerequisite for precise localization of transient events like singular PVC (14,15) or hemodynamically effective arrhythmias such as VT that can only be tolerated for a limited time. In this way, recording times of less than 15 minutes for PVC and less than 30 seconds for VT are sufficient to obtain an acceptable number of expiratory averages. A high S/N provides for exact localization of the ectopic activity. The following factors are among the most important contributors to localization inaccuracies with the biomagnetic method: patient movements, inaccuracy of the transformation of localization results to the MR images, and insufficient source and torso modeling. The greatest localization inaccuracies were found in the examination of deep sources, which confirmed earlier observations in phantom studies with a dipolar 690
..
Figure 8. Magnetocardiographic registration of nonsustained VT with the biomagnetic multichannel system. The magnetic signal recorded during 4.6 seconds is shown in eight of the 37 MCG channels (top). Three of the 12 simultaneously recorded ECG leads are shown beneath this signal. The bottom curve shows the respiration channel.
source filled
submerged with
in a plastic
saline
tank
solution.
The development of a nonmagnetic pacing catheter compatible with the exact three-dimensional imaging technique of MR imaging permitted verification of the localization accuracy of this method. The results of our pacing studies indicate a high precision of magnetocardiographic localization, even if data evaluation and localization are done with simplified modeling rounding
of the volume
future, contribute
the study
more
sources and conductors.
of paced
to the
events
development
sophisticated
Experiments
the surIn the
models. with
the
nonmagnetic
stimulation catheter permitted study and solution of a major lem in magnetocardiography, perimposition
may of
of magnetic
the probthe su-
fields:
If
several sources are active within the heart at the same time, the registered magnetic field always represents a summation of several components. In this study, superimposed fields were recorded in patients with ventricular extrasystoles arising from the repolarization activity of the previous beat. Our results show that the problem can be solved by applying digital subtraction if the disturbing background fields are known (eg, if normal beats not followed by a PVC are registered
and can be used as a subtraction ternplate). The study of VTs revealed some characteristic
properties:
The
localiza-
tion of the equivalent current dipole remained stable and showed a high S/N during the complete upstroke of the R wave. For this reason, the site of origin of the VT could be localized without
cases
digital
even
During
subtraction,
from
in some
unaveraged
repolarization,
the
data. magnetic
field map had reverse polarity and, again, remarkable stability. This observation might correspond to specific patterns and mechanisms of excitation during VT. In one patient with VT, interpretation of the data with simplified models
was
not
possible,
because
the
mag-
netic field did not show a typical bipolar pattern. The high density of isofleld lines in the cramo-left-lateral direction might have been caused by a border effect indicating insufficiency of the model of the infinite half space, which is usually applied to describe the thorax as a volume conductor. Interpretation
of this
with the commonly model of a single yielded centimeters
tours. The
a localization outside
correct
field
used current
distribution
source dipole
result several the heart con-
localization
of the September
site 1991
I
.
! -
.
.
..
00
.‘#{149}#{149}-
iPj
r1 .,
Figure 9. The magnetic field distribution registered coherently with the biomagnetic multichannel system during VT. The field maps stroke ization
during
stable . .
-
.
.
._.i_
--
.
during
;Lt:
.
.
7;;#{149}ir’
.,
#{149}
I
..
w’
L.
..
. ..
no
n
a
electrical
.
Volume
180
#{149} Number
3
Toward
and
dipole
of the
the
remain R wave.
orientation
at
(b) During the
dipole
ap-
end
of repo-
the field map shows a similar disinterchanged polarity with that of the onset of VT (c).
with
activity
of heart
arrhythmias
can be completely and noninvasively examined. This method not only enables localization of the site of origin of arrhythmias, but it also offers new insights in the mechanisms of ectopic
/\
w,
its onset.
repolar-
charactenis-
of origin of VT in this case could only be obtained by the application of a new localization algorithm that is based on the reconstruction of the current density distribution (“lead fields”) (16). A very high number of evenly distributed current dipoles is assumed, and the strength and direction of the dipoles are chosen to optimally describe the measured field distribution. The number of sources is then systematically reduced until a minimal number of dipolar sources still describing the measured field distribution within the S/N is reached. With this algorithm (which, in this study, was applied to clinical magnetocardiographic data for the first time, to our knowledge), a localization result in exact agreement with invasive pace-mapping was obtained. In eight other cases in this study, in which the localization of a single current dipole had yielded verified or physiologically meaningful results, the site of highest current density was found in the same area. These observations may suggest that localization could be improved in some cases if improved source models were used. Adequate evaluation of data recorded with a biomagnetic multichannel system combined with adequate imaging, such as MR imaging, is the first and only method with which electrical
.
changes
the equivalent
compared
1
the upstroke
quickly
repolarization,
tribution
:.ci t
current
of the R wave
larization,
:
down-
field distribution
the maximum proaches
:::
(a) and
wave and during a remarkable
equivalent
The dipole .
:[yT.
:
R
tic: The magnetic localized
..
the upstroke
(b) of the (c) display
:
heart
activity.
Because the annual mortality rate due to cardiac rhythm disorders is about 1,500 per 1 million inhabitants in industrialized nations, a simple and noninvasive method to investigate these disorders is of outstanding importance.
Epidemiologic
studies
with
broad application of this noninvasive method could lead to better underRadiology
691
#{149}
standing and classification of this inhomogeneous group of diseases. The results may, in turn, help to improve methods of treatment in the future. Biornagnetisrn
might
be
a key
5.
al. study
6.
7.
References 1.
2.
3.
4.
692
Baule C, McFee R. Detection of the magnetic field of the heart. Am Heart J 1%3; 66:95-96. Ern#{233} SN. High resolution magnetocardiography: modeling and sources localizalion. Med BiolEng Comput 1985; 23(suppl): 1447-1450. Fenici RR, Melillo C, Maselli M, Capeffi A. Magnetocardiographic three-dimensional localization of Kent bundles. In: Atsumi K, Kotani M, Ueno S, et al, eds. Biomagnetism ‘87. Tokyo: Tokyo Denki University Press, 1987; 140-141. Katila T, Montoneu L, Maekijaervi M, Nenonen J, Raivic M, Siltanen P. Localization of the accessory cardiac conduction pathway. In: AtSUnIi K, Kotani M, Ueno S, et al, eds. Biomagnetism ‘87. Tokyo: Tokyo Denki University Press, 1987; 430-433.
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E, Rechenberger
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of electrical activity in the brain Radiology 1990; 176:825-830. Abraham-Fuchs K, Schneider 5, Reichenberger H. MCG inverse solution: influence of coil size, grid size, number of coils, and SNR. IEEE Trans Biomed Eng 1988; 573-576. Abraham-Fuchs K, Weikl A, Schneider S. et al. Application of a biomagnetic multichannel system to the comparative localization of accessory conduction pathways in patients with WPW syndrome. In: Williamson SJ, Hoke M, Stroink C, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 369-372. Achenbach 5, Moshage W, Weild A, et al. Elimination of electronic offset and physiological background activity in magnetocardiographic localization. Biomed Tech (Berlin) 1990; 35(suppl 3):160-161. Williamson SJ, Romani CL, Kaufman L, Modena J. Biomagnetism: an interdisciplinary approach. New York: Plenum, 1982. Moshage W, Achenbach 5, BoLz A, et al. A non-magnetic, MR-compatible pacing catheter for clinical application in magnetocardiography. Biomed Tech (Berlin) 1990; 35(suppl 3):162-163. Schaldach M, Boheim C, Edelh#{228}user R. Em beitrag zur erkennung evozierter po-
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Biomed Tech (Berlin) 1988; 2):343-344. Fenici RR, Melillo C, Capelli A, Deluca C, Maseffi M. Magnetocardiographic localization of a pacing catheter. In: Williamson SJ, Hoke M, Stroink G, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 361-364. Fenici RR, Melillo C, Capelli A, Deluca C, Maselli M. Atrial and ventricular tachycardias: invasive validation and reproducibility of magnetocardiographic imaging. In: Williamson SJ, Hoke M, Stroink C, Kotani M, eds. Advances in biomagnetism. New York: Plenum, 1989; 441-444. Moshage W, Achenbach 5, Weikl A, et al. Magnetocardiography (MCG): Noninvasive localization of ventricular extrasystoles with a biomagnetic multichannel system. Presented at Update in Clinical Cardiology, Herzlya, Israel, May 20-22, 1990. Moshage W, Weild A, Abraham-Fuchs K, et al. Magnetokardiographie: erste klinische erfahrungen mit einem biomagnetischen vielkanalsystem. HerzfKreisl 1990; 22:245249. Joannides AA, Bolton JPR, Clarke CJS. Continuous probabilistic solutions to the biomagnetic inverse problem. inverse Problems 1990; 6:523-542. tentiale.
33(suppl 12.
heart.
to solv-
ing a problem that is of preeminent importance because of the high preyalence of life-threatening arrhythmias. #{149}
Schneider
13.
14.
15.
16.
September
1991
