Vagus nerve stimulation.
- SourceAvailable from: Samu Taulu[show abstract] [hide abstract]
ABSTRACT: Multichannel measurement with hundreds of channels oversamples a curl-free vector field, like the magnetic field in a volume free of sources. This is based on the constraint caused by the Laplace's equation for the magnetic scalar potential; outside of the source volume the signals are spatially band limited. A functional solution of Laplace's equation enables one to separate the signals arising from the sphere enclosing the interesting sources, e.g. the currents in the brain, from the magnetic interference. Signal space separation (SSS) is accomplished by calculating individual basis vectors for each term of the functional expansion to create a signal basis covering all measurable signal vectors. Because the SSS basis is linearly independent for all practical sensor arrangements, any signal vector has a unique SSS decomposition with separate coefficients for the interesting signals and signals coming from outside the interesting volume. Thus, SSS basis provides an elegant method to remove external disturbances. The device-independent SSS coefficients can be used in transforming the interesting signals to virtual sensor configurations. This can also be used in compensating for distortions caused by movement of the object by modeling it as movement of the sensor array around a static object. The device-independence of the decomposition also enables physiological DC phenomena to be recorded using voluntary head movements. When used with properly designed sensor array, SSS does not affect the morphology or the signal-to-noise ratio of the interesting signals.Brain Topography 02/2004; 16(4):269-75. · 3.67 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Limitations of traditional magnetoencephalography (MEG) exclude some important patient groups from MEG examinations, such as epilepsy patients with a vagus nerve stimulator, patients with magnetic particles on the head or having magnetic dental materials that cause severe movement-related artefact signals. Conventional interference rejection methods are not able to remove the artefacts originating this close to the MEG sensor array. For example, the reference array method is unable to suppress interference generated by sources closer to the sensors than the reference array, about 20-40 cm. The spatiotemporal signal space separation method proposed in this paper recognizes and removes both external interference and the artefacts produced by these nearby sources, even on the scalp. First, the basic separation into brain-related and external interference signals is accomplished with signal space separation based on sensor geometry and Maxwell's equations only. After this, the artefacts from nearby sources are extracted by a simple statistical analysis in the time domain, and projected out. Practical examples with artificial current dipoles and interference sources as well as data from real patients demonstrate that the method removes the artefacts without altering the field patterns of the brain signals.Physics in Medicine and Biology 05/2006; 51(7):1759-68. · 2.70 Impact Factor
J. Neurosurg.: Pediatrics / Volume 2 / November 2008
L e t t e r s t o t h e e d i t o r
yond the purview of a technical note describing a proce-
dure which they acknowledge as “good.”
1) They mention attempting MEG with the VNS pulse
generator turned off before resorting to explantation. The
magnetic source imaging facilities available to us have
proven unable to consistently obtain reliable seizure focus
localization or mapping of eloquent cortex with the pulse
generator implanted and turned off.
2) Surgical management of challenging patients al-
ways requires risk/benefit decisions. Fortunately, the need
for pulse generator pocket preservation seldom arises. The
technique we describe applies to the rare pediatric epilepsy
patient receiving VNS whose presurgical work-up, in our
opinion, would prove incomplete without MEG.
3) Our brief technical report does not attempt to ad-
dress MEG’s role in evaluating pediatric epilepsy patients,
a topic recently reviewed.1 (DOI: 10.3171/PED.2008.2.
DaviD Donahue, M.D.
Rosa sanchez, R.n.
angel heRnanDez, M.D.
saleeM Malik, M.D.
c. ThoMas Black, M.D.
Johnnie honeycuTT, M.D.
Cook Children’s Medical Center
Fort Worth, Texas
1. Schwartz ES, Dlugos DJ, Storm PB, Dell J, Magee R, Flynn
TP, et al: Magnetoencephalography for pediatric epilepsy:
how we do it. AJNR Am J Neuroradiol 29:832–837, 2008
To The eDiToR: We have read with interest the techni-
cal note by Donahue et al. (Donahue D, Sanchez R, Her-
nandez A, et al: Preservation of a subcutaneous pocket
for vagus nerve stimulation pulse generator during mag-
netoencephalography. J Neurosurg 107 (6 Suppl Pediat-
rics):519–520, December, 2007).
In this technical note, the authors state that the re-
moval of the VNS pulse generator is a prerequisite to
magnetoencephalographic investigation due to important
magnetic artifacts generated by the metallic stimulator
during breathing or the patient’s movements. Based on
this statement, they propose a new surgical technique to
avoid obliteration of the subcutaneous pocket by scar tis-
sue after temporary explantation of the pulse generator,
should the pulse generator be reimplanted after comple-
tion of the new presurgical evaluation.
Despite movement-related interferences produced by
the metallic stimulator, removal of the pulse generator is
no longer a prerequisite to MEG in patients with epilepsy
who have implanted VNS units. Indeed, a new method of
artifact removal called spatiotemporal signal space sepa-
ration (SSSt) allows almost complete removal of artifacts
generated by metallic objects in movements close to the
sensor array.2 This method is therefore particularly appli-
cable to patients with implanted VNS units.2
Vagus Nerve Stimulation
To The eDiToR: We were interested by the recent
communication of Donahue et al. (Donahue D, Sanchez
R, Hernandez A, et al: Preservation of a subcutaneous
pocket for vagus nerve stimulation pulse generator dur-
ing magnetoencephalography. J Neurosurg 107 (6 Suppl
Pediatrics):519–520, December, 2007) describing a solu-
tion to the dilemma posed when a patient who has a vagal
nerve stimulation (VNS) device is thought to need mag-
netoencephalography (MEG). Two points seem to have
been missed by the writers. Firstly, MEG has not been
demonstrated to be superior to conventional epilepsy in-
vestigational tools. It is complementary, and not needed in
all patients. In the most comprehensive study published to
date, data presented by Knowlton et al.1 suggest that MEG
will add useful information that might alter decision mak-
ing or improve outcomes in ~ 10% of patients. Our data,
which have recently been submitted for publication, agree
and indicate that the percentage could be even higher. A
comment on the necessity of exposing a patient to the ex-
pense and risk of 2 additional surgeries, the first to re-
move and the second to replace the VNS device, would
have been welcome.
Magnetoencephalography is indeed a useful tool, and
our group has employed it in the work-up of many of our
epilepsy patients for several years.2 A quick review of our
records has shown that 2 of our patients with VNS devices
have been sent for MEG without having the device re-
moved. The VNS unit was simply turned off. We were not
able to obtain useful data in one case because of artifact
from the VNS device, but were able to obtain good data
from the other. A tacit implication of the article is that
the VNS pulse generator needs to be removed from all
patients who are being sent for MEG. This is not the case.
While the technical suggestion made in this article is a
good one, we believe that practitioners should resort to
this sequence of procedures only when it has been deter-
mined that the patient absolutely requires MEG and trial
MEG with the pulse generator turned off has failed.
ian B. Ross, M.D.
TaTiana Maleeva, M.D.
WilliaM W. suTheRling, M.D.
Huntington Memorial Hospital
1. Knowlton RC, Elgavish R, Howell J, Blount J, Burneo JG,
Faught E, et al: Magnetic source imaging versus intracra-
nial electroencephalogram in epilepsy surgery: a prospective
study. Ann Neurol 59:835–842, 2006
2. Mamelak AN, Lopez N, Akhtari M, Sutherling WW: Magne-
toencephalography-directed surgery in patients with neocor-
tical epilepsy. J Neurosurg 97:865–873, 2002
Response: We appreciate the interest of Drs. Ross et
al. in our technical note. Their valid concerns go well be-
376 J. Neurosurg.: Pediatrics / Volume 2 / November 2008
Basically, the SSSt method works in 2 steps. First,
it separates brain-related signals from those generated
by external interference. This procedure makes use of
the conventional signal space separation method, which
is based on sensor geometry and Maxwell’s equations.1,2
Then, the artifacts generated by nearby sources such as
the VNS pulse generator are extracted using statistical
analysis in the time domain, and projected out from the
To illustrate the power of the SSSt method, in par-
ticular in presence of VNS pulse generator, we report the
case of a 17-year-old girl with epilepsy and an implanted
VNS pulse generator who underwent MEG as part of a
presurgical evaluation for pharmacoresistant focal epi-
History and Presentation. An 8-year-old girl with a
previously unremarkable medical history presented with
2 episodes of secondarily generalized seizures. Cere-
bral MR imaging revealed the presence of a cavernous
hemangioma in the left inferior frontal region. Complete
resection of the lesion was then performed and the patient
became seizure-free. Four years after surgery, she devel-
oped pharmacoresistant epilepsy characterized by left
frontal seizures. At the age of 15 years, she underwent
invasive electroencephalographic monitoring combined
with language mapping using electrostimulations through
an 18-contact subdural grid centered over the left inferior
frontal region. This investigation revealed that the seizure
focus was centered on a brain region in which electrical
stimulation produced speech arrest. Resection was not
performed. A VNS pulse generator was then implanted
but had no effect on the seizures. In the context of an
evaluation for radiosurgery when the patient was 22 years
of age, MEG was undertaken. The patient gave informed
consent for this magnetoencephalographic investigation,
which was part of a research protocol approved by the
Erasme Hospital Ethical Committee.
Magnetoencephalogaphic Investigation. Magneto-
encephalographic measurement was performed using the
whole-head 306-channel Vectorview Elekta Neuromag
System (Elekta, Inc.) and light-weight magnetic shield
(MaxShield, Elekta, Inc.). Spontaneous magnetic brain
activity (eyes closed, rest, supine position) was recorded
during 1 hour (sampling frequency: 1 kHz, pass-band
0.1–300 Hz). The patient’s antiepileptic medication regi-
men was unchanged. The VNS pulse generator was off.
Continuous magnetoencephalographic data were pre-
processed off-line using the SSSt method.2 The data were
then band-pass filtered to 1–30 Hz and visually inspected
for epileptic events.
Source locations of epileptic events were obtained by
conventional dipole modeling tools (Elekta Neuromag)
using spherical conductor models determined from pa-
tient’s MR imaging study. Equivalent current dipoles
(ECD) were fitted at the onset and the maximum peak of
epileptic events using a selection of at least 40 channels.
Dipole fits were considered correct when the goodness-
of-fit was > 80% and the 95% confidence volume was <
20 mm³. The ECDs were then overlaid on the patient’s
MR image, which was co-registered with MEG using 3
fiducial landmarks and 4 head position indicator coils, the
locations of which were digitized prior to MEG. Addi-
tional points on the scalp were also digitized and overlaid
on the MR image to ensure accurate MEG/MR imaging
Fig. 1. A: A sample of MEG data processed with the conven-
tional signal space separation (SSS) method is shown on the left.
Artifacts produced by the movements of VNS pulse generator
during breathing can be observed. The right side shows the same
sample of MEG data processed with the SSSt method. The arti-
facts produced by the pulse generator are completely removed.
An interictal spike can be seen on the left frontal (LF) and tem-
poral (LT) MEG sensors. B: Left inferior frontal equivalent
current dipoles (blue) corresponding to the electrical sources of
the observed interictal spikes are overlaid on parasagittal (up-
per) and axial (middle) slices from 3D T1-weighted MR imag-
ing. Magnetic field patterns at the peak of a left inferior frontal
spike superimposed on a representation (left lateral view) of the
neuromagnetometer helmet (bottom). The sensor array is viewed
from the left. The squares show the sensor element locations.
The red contours indicate magnetic efflux and the blue contours
magnetic influx. The green arrow depicts the surface projection
of the ECD that best explains the field pattern. C: Drawing il-
lustrating the 18-contact subdural grid location during the inva-
sive electroencephalographic recording. The solid yellow circles
correspond to the seizure-onset zone. The red circles correspond
to the electrodes that produced speech arrest when stimulated.
The green circles correspond to the area of interictal spiking.
J. Neurosurg.: Pediatrics / Volume 2 / November 2008
Findings. The magnetoencephalographic investiga-
tion revealed the existence of frequent interictal spikes
in the left frontotemporal magnetoencephalographic sen-
sors with ECDs localized in the left inferior frontal re-
gion (Fig. 1).
This case illustrates that MEG can be performed in
patients with implanted VNS units. The new artifact re-
moval method called SSSt almost completely eliminates
the artifacts generated by the VNS pulse generator, lead-
ing to meaningful MEG data collection. In the case pre-
sented, we observed a very good colocalization between
interictal spikes, ECDs, and the results of invasive elec-
Thus, removal of an implanted VNS pulse generator
can be avoided for MEG in patients treated with VNS. This
surgical procedure should only be performed in patients in
whom artifact removal methods fail to ensure collection of
high quality magnetoencephalographic data.
Dr. De Tiège receives funding from the Fonds Erasme pour la
Recherche Medicale, Brussels, Belgium.
XavieR De Tiège, M.D.
BenJaMin legRos, M.D.
MaRc op De Beeck, M.s.
seRge golDMan, M.D.
paTRick van BogaeRT, M.D.
1. Taulu S, Kajola M, Simola J: Suppression of interference
and artifacts by the Signal Space Separation Method. Brain
Topogr 16:269–275, 2004
2. Taulu S, Simola J: Spatiotemporal signal space separation
method for rejecting nearby interference in MEG measure-
ments. Phys Med Biol 51:1759–1768, 2006
Response: Many thanks to De Tiège et al. for the sug-
gestions. Indeed multiple methods for artifact rejection
are being developed, including independent component
analysis and SSSt algorithms to reduce the effects of arti-
fact produced by the VNS apparatus. Multicenter testing
of the validity of such magnetoencephalographic studies
is still forthcoming.
We continue to assess the applicability of MEG and
the remote necessity for VNS explant on a case-by-case
basis. (DOI: 10.3171/PED.2008.2.11.377)
DaviD Donahue, M.D.
angel heRnanDez, M.D.
Cook Children’s Medical Center
Fort Worth, Texas