Non-invasive imaging of epileptic spikes and seizures using by ddn50364

VIEWS: 35 PAGES: 34

									           Non-invasive imaging
      of epileptic spikes and seizures
            using EEG and MEG

          M. Scherg1,2, H. Bornfleth1, T. Bast3
         MEGIS Software GmbH, Munich, Germany 1 ,
         Depts. of Neurology 2 & Pediatric Neurology 3,
           University Hospital Heidelberg, Germany



                                                          www.besa.de




This presentation was held at the several meetings in September
2004 and provides an introduction to mapping, source analysis
and imaging in epilepsy and the use of source montages for EEG
review.
 Overview

 • Fundamentals
       – EEG: propagation to the scalp and whole head
        mapping
       – Spike propagation: multiple sources
       – Source montages and source localization
 • Application to interictal spikes & seizures
       – Rolandic spikes
       – Temporal lobe spikes & seizures
       – Frontal spikes: focal cortical dysplasia


                                                             www.besa.de




The main goal of this presentation is to give a thorough
  introduction into the basic principles of source analysis and
  localization, to point out benefits and pitfalls in the source
  analysis of spike and seizure onset, and to show the value
  of advanced tools like source montages, 3D whole head
  mapping and spectral analysis using brain source
  montages.
EEG examples from patients with temporal and extra-temporal
  epilepsy will be used in addition to simulations that illustrate
  the potential and problems of inverse modeling during
  routine EEG review.
 Primary neuronal currents flow inside the
 pyramidal cells, i.e. perpendicular to the cortex

 Activity can be described by equivalent dipole vectors


            radial                            radial


                                                       tangential
                     tangential

          oblique                            oblique




                                                            www.besa.de




Neuronal current in the cortex flows predominantly perpendicular to the
  cortical surface for two reasons: First, the pyramidal cells in the
  cortical columns are aligned perpendicular to the cortical surface.
  Second, the dendritic trees that are parallel to the cortical surface
  have near-rotational symmetry and the electric fields of the related
  intracellular currents cancel to a large degree.
The intracellular current vectors of nearby cortical columns sum linearly
  and can be represented very accurately by an equivalent, compound
  dipole current vector. The magnitude, or strength, of the equivalent
  dipole is proportional to the number of activated neurons and
  therefore correlates with the area of activation and the mean dipole
  current density per square cm. Areas with up to 3 cm in diameter (!)
  can be very accurately (>99%) modeled by a single equivalent dipole.
Currents at the cortical convexity have a predominantly radial orientation,
  currents in cortical fissures have predominantly tangential orientation.
  Generally, a patch of activated cortex in a sensory, motor or spiking
  area will have an oblique orientation depending on the net orientation
  of the activated cortex.
 Neuronal and secondary volume currents


                                                    cortical surface:
                                                    ~ radial current
                                                    ~ radial map

                                                    similar location, but
                                                    different orientation!

                                                    sulcal surface:
                                                    ~ tangential current
                                                    ~ tangential map




                       created using DipoleSimulator & BESA       www.besa.de




Current loops in a conductive medium like the head are closed.
  Therefore, the intracellular currents resulting from action and post-
  synaptic potentials are accompanied by secondary return currents in
  the head volume. Since the brain and scalp have a higher electrical
  conductivity as compared to the cranium, most currents return within
  the extracellular brain space. Only a very small fraction flows out
  through the poorly conducting cranium and along the scalp before
  returning to the brain.
An ideal patch of superficial cortex creates a net radial current flow that
  can be very accurately modeled by an equivalent dipole near its
  center. The volume conduction results in a widespread, smeared
  voltage topography over the whole scalp with a negative maximum
  over the activated superficial cortical sheet. A corresponding more
  widespread positivity appears on the other side of the head. By
  physics, any negativity has a corresponding positivity somewhere else
  over the head. A cortical patch in a fissure generates tangential
  currents. The small return currents through the scalp create a dipole
  map with symmetric positive and negative poles aligned in the
  direction of the dipole current. The voltage directly over the source is
  zero, but the voltage gradients are maximal. The source is below the
  site of the densest equipotential lines. These lines and the whole
  shape of the topography carry more information on the location of the
  underlying generators than the colorful peaks.
Temporal lobe: 3 aspects, 3 dipole field topographies




       basal                         polar                       lateral




 We can use 3 dipoles to model the topographies of the different aspects


                                                                      www.besa.de




Creation of a temporal lobe regional source montage
Anatomically, each temporal lobe has 3 major surfaces, or aspects:
  basal, polar and lateral. Larger spikes in the EEG (> 50 uV) are
  likely to be oriented prependicular to the gross area they originate
  from. Therefore, we can expect the dipole fields to match the
  cortical surfaces that have a net vertical (basal), anterior-posterior
  (pole), or radial (lateral) orientation.
Therefore, each temporal cortex can be modeled by 3 equivalent
  dipoles reflecting the basal, polar and lateral aspects, as illustrated
  above for the left temporal lobe. In addition, the large lateral region
  can be divided and represented by an antero-lateral and a postero-
  lateral dipole.
To some extent, the vertical basal dipole will also pick up source
  currents in the Sylvian fissure. The spike polarity can help
  dissociate which side of a lobe or gyrus is discharging.
Temporal lobe spike: 3D whole head mapping




            polarity
            reversal



                              radial field




                                                               www.besa.de




In the longitudinal bipolar montage (double banana) the largest polarity
   reversal can be seen in all 3 spikes between F7 and T7.
The rolandic spike S3 also shows a later polarity reversal between the
  right electrodes F8 and T8 seen clearly only after averaging.
The 3D whole head maps of S1 show a radial dipole field over the left
  temporal lobe corresponding to the polarity reversal.
 Patient 1: maps show propagation from basal to lateral




                        topographies at
                        spike onset and
                        peak are different




                                                                 www.besa.de




The temporal evolution of the serial maps shows a changing
  topography during the spike with an initial vertical dipole field during
  spike onset.
The initial negativity is below the temporal lobe and is picked up mainly
  be the inferior temporal electrodes. Using spherical spline
  interpolation the negative peak below the temporal lobe can be
  extrapolated with sufficient accuracy.
The negativity appears to rotate forward toward the temporal pole
  before changing into the radial pattern at spike peak.
The rotation of the map from the initial vertical topography to a radial-
  lateral pattern indicates propagation and overlap at the spike peak.
 Temporal lobe spike (patient 2): no propagation




                      polarity
                      reversal




                           topographies do
                           not change
                           significantly




                                                                    www.besa.de




Spike S2 from subject 2 having temporal lobe epilepsy shows a similar
  polarity reversal, but the radial map does not change much over
  time. Thus, the spike does not seem to propagate.
In particular, an initial vertical dipole field with a negativity below the
   left temporal is not observed in contrast to S1.
 Rolandic spike: averaging reveals propagation


                           ?                      !




   long. bipolar         single spike           8 averages

                                                            www.besa.de




The single rolandic spikes show a polarity reversal between F3-T7 and
  T7-P7 similar to typical temporal lobe spikes. The corresponding
  radial map at the spike peak has a maximum negativity over the
  temporal lobe.
Spike onset is unclear and varies between the single spikes. The
  single spike map during onset reflects mostly EEG background
  activity.
After averaging, the tangential topography during spike onset becomes
   apparent. The onset pattern shows a more superior negativity over
   the frontal cortex.
 3D whole head maps: visual ‘dipole localization’




  time course of 3D maps      connect +/- peaks     find max. gradient

                                                                www.besa.de




More detailed inspection of the whole head topography reveals an
  estimate of an 'equivalent dipole' above the Sylvian fissure (green
  arrow) for both the tangential onset map and the radial peak map.
How can we estimate this 'approximate source location' from the
  maps?
First, consider a line connecting the negative and positive maxima on
   the scalp (red arrows). This line follows the shortest connection
   having the highest voltage gradient (~ narrowest distance between
   equipotential lines).
Second, find the region of highest gradients.
In a close-to-tangential map (upper row), the equivalent location is
   approximately below the area of maximum gradient (green arrow).
In a close-to-radial map (lower row), the center location is shifted from
   the negative peak towards the positive pole along the region of
   largest gradient.
The equivalent centers of both maps are similar and point to a
  circumscribed region of origin in the parietal cortex above the
  Sylvian fissure.
Spike averaging: use brain source montage




                          mark a block of
                          300-500 ms as
                          spike template
  click to select a
  source channel
  for search




                                                              www.besa.de




Pattern search using linear correlation in a single channel
For source analysis of the spike onset, it is necessary to increase the
  spike signal over the EEG background. The lateral temporal
  channel of the TR brain source montage already improved the spike
  signal as compared to T7-avr or F7-T7, for example. Further
  improvement is obtained by averaging spikes with similar spatio-
  temporal patterns.
Spike detection and averaging can be done by pattern matching using
  temporal correlation with a template in one channel across the
  whole EEG and detecting a spike, e.g. if the linear correlation
  coefficient peaks and exceeds 80%.
Rolandic spike: compare averaged and single spikes




      average                          detected spikes




                                                               www.besa.de




Spatio-temporal pattern search
A more selective search can be done by using the whole pattern
  across all channels and by calculating spatio-temporal correlation
  with a threshold of 75%. In the case of S3 this means that the left
  lateral spike (trace 3 – upper arrow) must always be followed by a
  right lateral spike (trace 7 – lower arrow).
The average shows a magnitude similar to the single spikes detected
  both by the single channel and spatial-temporal pattern search.
  Furthermore, the single spikes exhibit similar patterns across
  channels. Thus , in this child with rolandic epilepsy spikes were quite
  uniform and had similar propagation patterns .
 Focal rolandic spike: no propagation

   local current        one spiking zone               single topography




   propagation          scalp waveforms                         similar maps


                         created using DipoleSimulator & BESA           www.besa.de




Using a simulated example, we will now learn how to discriminate the
  scalp waveforms and topographies due to a focal and a propagating
  rolandic spike. Furthermore, we will learn a basic strategy for the
  source analysis of spikes.
An assumed spiking patch in the right central sulcus produces a near-
  tangential dipole field with a positive peak over the mid-frontal
  (spike downward, e.g. at Fz and FC2=max.) and a negative more
  widespread peak over the right inferior parietal cortex (max. at P4).
  The patch is synchronously activated and there is no propagation.
  Accordingly, the net orientation remains the same. The waveforms
  at the different electrodes have different magnitudes but the same
  evolution over time. The topographic maps change only in
  magnitude but not in shape. Map polarity simply reverses in the
  small wave phase following the initial spike.
  Focal rolandic spike: propagation (red -> blue)

    local currents        overlap at scalp             topographies




    propagation           scalp waveforms            rotating maps !


                                                                  www.besa.de




Consider the situation of a spike propagating within a few milliseconds
  to a nearby area that is about 3 cm away. The propagation is not by
  lateral spreading (which should be slower) but by fibers connecting
  the two areas. Each of the areas has a spike pattern with onset,
  peak, and polarity reversal. However, the second spike is delayed
  according to the propagation time, is a little broader, and has a
  longer lasting wave phase.
The two patches have different orientations. This is the main cause for
  their very different scalp topographies. Due to the time difference in
  activation their maps overlap with continuously changing magnitudes
  according to the instantaneous strength of the 2 compound currents.
  This results in an apparent rotation of the maps over time and it
  becomes difficult to identify and select the 'spike peak', i.e. a single,
  most important peak.
Propagation: erroneous localization at the spike peak!

   local currents             scalp data            source waveform



                                                  single dipole error!




      forward                                            inverse
   propagation to scalp                           source localization


                                                                  www.besa.de




If we fit a single dipole at the 'spike peak' as determined by the
   electrode with the largest spike, we obtain an incorrect localization
   intermediate between both sources. The source waveform
   combines both underlying activities into a broader spike which has a
   latency intermediate between the original spikes.
In this example, the mislocalization is moderate because the two
   spiking regions are relatively close. However, in real frontal epileptic
   spikes propagation often occurs over larger distances and to
   several areas within 10-15 ms.
As a consequence, our goal should be to identify and localize the very
  onset of the spike before much propagation has occurred. However,
  this poses a severe problem because the onset signal is usually
  smaller or not much larger that the background EEG. Then
  mislocalization will occur because of the overlap with the concurrent
  EEG in other brain areas.
Therefore, we need to increase the spike signal amplitude
       a) by separating the activities from the different areas
       b) by averaging spikes with similar onset and propagation
 Localizing the first dipole at spike onset (0-50%)

   source model: 1                                     scalp waveforms
                           relocate dipole
                                                       model waveforms

                         source waveform


                                       minimize
                                       difference
                                       -1
                 forward                     inverse
                 model                       model



   head model

                                                                 www.besa.de




The fit procedure:
We assume that a single dipole will explain the early onset phase (i.e.
 initial source model hypothesis is a single equivalent dipole). Using
 the head model the forward model topography is estimated. The
 inverse of the topography matrix is applied to the data to estimate
 the source waveform. The source waveform is projected back to the
 scalp using the forward coefficients of the map to estimate the
 model signals (blue). Measured and modeled data are subtracted to
 estimate the residual waves. In an interactive process, dipole
 location and orientation is adjusted and the calculation process is
 repeated until the residual difference between scalp and model
 waveforms is minimized. The equivalent dipole locates in or near
 the active cortex if the hypothesis, head model, and data are
 sufficiently accurate.

Fitting strategy for multiple activities – step 1 :
Use the 3D maps to define the fit interval from the time when a clear
  dipole field emerges until it starts changing. Performing a principal
  components analysis over this interval should show one dominant
  component. The percentage of variance it explains should
  decrease, if the interval is extended further. Fit the first dipole over
  this interval.
 Localizing the 2nd dipole around the spike peak

   source model: 2          source waveforms           scalp / residual
                                                        model ~ scalp




                  forward               -1 inverse
                  model                    model



    head model

                                                                  www.besa.de




Fitting strategy for multiple sources – step 2 :
Display the residual waves and maps. Perform a PCA on the residual
  waves and repeat the same procedure to mark the next onset
  interval in the residual data. Fit a second dipole to this interval while
  keeping the first dipole fixed in location and orientation.
In the simulated example with good signal-to-noise, this results in the
   separation of the underlying active areas and their source
   waveforms.
Finally, we should check the homologous brain region in the other
   hemisphere for a potential spread of activity using a probe source at
   the mirror location of dipoles 1 and 2.
       Multiple sources separate the activities from different
       brain regions
       Source topographies            EEG data           Source waveforms
       source          -1
       montage
       filter
                      L                    D                       S
       operator

       source montage                red active    blue active    red -> blue


                           (L-1) 1

                           (L-1) 2

                           (L-1) 3


                                                                       www.besa.de




The above example demonstrates the full separation of the source activities 1
& 2 in our simulation and illustrates the absence of activity in source area 3
since its source waveform shows only EEG background signal.
Thus, multiple sources can mutually contrast and separate the activities
of the brain areas that they represent.
The displayed circles on the left illustrate that separation of the activity from
several brain areas is principally possible, if they are sufficiently remote from
each other (> 3 cm). However, precise localization within each region is not
possible in typical data because of the EEG background noise.
In the overdetermined case, i.e. if there are less sources than measured
channels, the linear inverse multiple source operator is constructed to fully
separate the different source activities. The vector operator for source 1 will
fully recover source activity 1, but not see and suppress any contribution from
sources 2, 3... and vice versa. This sharp separation has a drawback, if some
of the sources have a high spatial correlation in the sensor space. Then the
inverse operator will have large entries and the noise will be amplified
accordingly. However, this problem is easily handled by modest regularization
(BESA default = 1%) when calculating the inverse of the topography matrix.
 Rolandic spike propagation: regional source analysis



                                                                  1
                                                          3
                                                                    2


                                                         1
                             R                  L
                                                         2

                                                         3
     coregistration of
     electrodes with         3D view with               source activity
     surface mesh            regional sources

                                                                  www.besa.de




Results of source modeling
With several strategies, the left supra-sylvian rolandic region was
  identified as the origin of the spike onset. The initial source region
  (red circle, L) was identified in the individual MRI after fitting the
  electrodes and fiducials to the 3D rendered scalp surface
  (coregistration).
Because of the local ambiguity of the solution and due to the limits in
  source localization based on 27 electrodes, it cannot be decided in
  which of the 2 prominent fissures on the left the spikes originated
  (central or pre-central sulcus?). However, the regional sources
  identified the left rolandic region as the initial spiking zone followed
  by the right rolandic region (green circle, R). The 6 source
  waveforms (see next figures) of the regional sources specify the
  timing and propagation of the spike from right to left and dissociate
  the fissural from the superficial cortical activity.
Note: Principally, spatial resolution is in the order of ½ of mean
  electrode distance in a 2D array of electrodes. I.e. spatial resolution
  better than ~2-2.5 cm cannot be expected with 33 electrodes used
  is the original recording of this case. This is indicated by the circles
  marking the left and right supra-sylvian rolandic cortex.
Creating a source montage: the principle
                                                                 1
                                                          1
                                              3                  2
                                                          2
    maps at peak          scalp                                  3
    show overlap           EEG
           spike propagates
                     1               inverse model
                     2
                     3     brain
                          source                     -1
                         montage
      source
     waveforms
                                                              + add. sources


                                                                     www.besa.de




Creation of a brain source montage to review the on-going EEG
Imagine three spiking areas: 1- left rolandic fissure, 2 - left rolandic
  cortical surface, 3 - right rolandic cortical surface. According to the
  timing of propagation, the compound spike activities 2 & 3 are
  delayed by 10 & 20 ms relative to 1. During the spike peak at the
  scalp, the large negativity of the left cortical surface is already
  overlapped with the right negativity (top maps). The longitudina l
  bipolar montage shows polarity reversal over the left (large) and
  right (very small) temporal lobes (upper right).
A dipole model can be created from the averaged spikes by fitting one
  regional source in the left and one in the right rolandic region. The 3
  dipoles of the left source are then oriented to match the initial
  tangential activity (1, blue) and the later radial activity (2, red). The
  right regional source has only one major activity and its first dipole
  (3, green) is oriented accordingly.
Using a dipole model for the 2 regions plus additional regional sources
  in other brain regions, an inverse (-1) linear operator can be
  calculated from the maps of all sources (lower right) and applied to
  the original EEG data. This results in the source waveforms (lower
  left) that depict the estimated activities of the 3 cortical surfaces
  plus the other source regions as a new 'brain source montage'.
Single rolandic spikes in individual source montage


    1
    2

    3




                                                                  www.besa.de




Rolandic source montage with individual dipoles (1-6) and (other)
  standard brain regions:
This brain source montage shows all three dipole source waveforms of
  the left and right rolandic regions and highlights the traces showing
  propagating spike activity (1 - left rolandic fissure, 2 - left rolandic
  cortical surface, 3 - right rolandic cortical surface). The 3 dipoles of
  each regional source are orthogonal and depict the current flow in
  all directions. For the other brain regions, only the activity in the
  direction of maximum power is shown.
All individual spikes show the same pattern of propagation with similar
   time lags from left fissural to left surface, followed by right surface.
   In some spikes the initial tangential activity is quite small. This
   suggests variable depth of onset in the sulcus. Propagation to the
   right with a constant time delay is quite consistent across all spikes.
Note that the on-going slow vertex waves are well separated by the
  sources CM and PM which model the parasagittal central and
  parietal activities.
Single rolandic spikes in longitudinal bipolar montage




   3




                                                                www.besa.de




Longitudinal bipolar montage (doube banana):
Polarity reversal can only be seen clearly over the left temporal lobe.
  The spikes could be misinterpreted as temporal lobe spikes. The
  propagation to the right side is barely seen in this montage
In contrast, the longitudinal bipolar montage (double banana) did not
   reveal the propagation from sulcus to surface and across
   hemipsheres. Only for some spikes, the later polarity reversal o ver
   the right temporal electrodes could be seen.
The traces were generated on the basis of virtual electrodes calculated
  using spherical splines interpolation over all digitized electrode
  positions. Interpolation was used, because the electrodes were
  placed using a cap that was a bit too small for the head of this child
  and therefore had electrodes above the standard 10-20 locations
  (see previous figure with MRI).
How to analyze spikes: summary

• Spike averaging
• Interpreting the time course of 3D maps
• Localizing the onset–peak interval
• Localizing using regional sources
• Creating a source montage
• Evaluating the single spikes
• Multiple source imaging (new!)


                                        www.besa.de
Rol. spike propagation: Multiple Source Probe Images
                  -30 ms           -15 ms           0 ms       +15 ms       +30 ms


  L        R


                        L               L                  L        R              R




                 R         L   R        L       R          L   R    L       R      L




                                                                        z

                       x 2.5            x 1.5



                                                                            www.besa.de




Multiple source probe imaging (MSPI)
Multiple source probe imaging (MSPI) is based on a multiple source
  model and a probe source scanning the whole brain on a 4-5 mm
  grid according to a principle published by Scherg, Brain Topogr.
  1992. The images show statistical z-values of the signal-to-noise
  ratio for the estimated source activity throughout the brain.
MSPI can be used to refine multiple source models and provides an
  intrinsic confirmation of the source model.
In this case of a rolandic spike, MSPI documents the origin in the left
   post-central gyrus (face area) and the propagation locally within the
   post-central gyrus, to more frontal areas and to the contralateral
   hemisphere.
FCD frontal spike - propagation - EEG 32                             MSPI
Bast et. al. Epilepsia 2004              Focal cortical dysplasia
                                       14-yr.old boy, seizure free

                                       36 spikes averaged, 10-45 Hz




                              -9 ms




                               0 ms


                                                                www.besa.de




3D Mapping reveals frontal onset
The map at the spike peak (0 ms) could be misinterpreted as a
  temporal lobe spike.
The averaged spikes onset (-9 ms) shows a lateral frontal origin.
FCD spike propagation – EEG32 / MEG122                                       MSPI
    EEG onset to peak propagation            MEG onset to peak propagation
                      single dipole                         multiple
                   source waveform                       source model
                  (fit: -20 to -10 ms)                (regional sources)
     -9 ms       0 ms          +3 ms         -19 ms         -9 ms            0 ms



             z          z                z            z             z               z




R            L




                                                                           www.besa.de




Multiple source probe imaging (MSPI)
MSPI confirms a sequential model with one regional source during the
  EEG onset phase (left) and suggests rapid propagation to more
  anterior fronto -polar and more inferior temporal-polar areas.
MSPI of the simulatenously recorded MEG using a multiple regional
  source model confirms the onset region consistently with the EEG
  and invasive recordings and documents the rapid propagation to
  multiple left frontal and temporal areas.
 Temporal lobe seizure

 • Temporal lobe source montage reveals right basal
   seizure onset.
 • Spectral analysis and FFT phase maps show a
   basal-to-polar propagation
 • Source localization of averaged seizure onset
   cycles with bilateral regional sources confirms
   results of brain source montage.




                                                               www.besa.de




Temporal lobe source montage applied to seizures
Next, the temporal lobe regional source montage will be
  applied to analyze the seizure onset of a patient with
  intractable temporal lobe epilepsy.
The brain source montage will be used in conjunction with 3D
  whole head mapping, spectral analysis, FFT and phase
  maps to identify the brain area (region) of seizure onset, and
  to assess the propagation pattern within each seizure cycle
  during the onset phase.
Source localization will support the interpretation of right
  mesial-temporal lobe epilepsy.
 Horizontal bipolar montage & 3D whole head maps




                   Hemispheric difference
                   is best seen in the
                   horizontal rows




                                                                  www.besa.de




Compare montages and map seizure onset cycles
Onset of the second seizure in this patient. The 3D whole head maps
  show a vertical dipole field with a prominent, widespread positivity
  over the central scalp, and an inferior negativity below both
  temporal lobes which is more pronounced on the right.
The best conventional scalp montage to show a hemispheric
  difference of vertical dipole fields is the horizontal-bipolar montage.
  However, the difference between the right (red) and left (blue)
  bipolar signal is less pronounced as compared to the temporal lobe
  brain source montages as shown in the next figures.
The vertical dipole field is typical for right basal-temporal activity and
  illustrates the need to record from a sufficient number of inferior
  electrodes bilaterally for a better diagnosis of temporal lobe
  epilepsy.
 TR source montage reveals hemispheric difference




                                                 DSA, TR montage,
                                                 Filter 1.6 – 30 Hz




                        7/11 Hz right onset


                                                                      www.besa.de




Density spectral arrays of source montages identify seizure onset
DSA is particularly helpful when used in conjunction with brain source
  montages. The temporal regional source montage separates right
  and left temporal activities and shows an almost complete
  lateralization of the seizure to the right in this patient (red traces 5 -8:
  right; blue traces 1 -4: left temporal lobe).
The most prominent signal can be seen in the right temporal-basal
  channel. The red nose tips in the DSA channel R+ (excess spectral
  amplitude of the right hemisphere) show 3 lateralized seizure
  onsets around 7 Hz. The first nose tip (green arrow) was used to
  navigate to the displayed second seizure segment.
Filtering (1.6 – 30 Hz, zero-phase) is useful to enhance the seizure
    cycles over the on-going EEG activity and muscle artifacts.
For more seizure examples showing the dissociation of right and left
  temporal lobe seizures by brain source montages see Assaf, B.A.,
  Ebersole, J.S. (1999). Visual and quantitative ictal EEG predictors
  of outcome after temporal lobectomy. Epilepsia 1: 52-61.
 FFT, TR montage, and 3D whole head maps




                              FFT analysis
       7.15 Hz peak          over first 5 sec
     right basal temp.      of seizure onset




                                                                  www.besa.de




Spectral analysis of seizure onset using FFT
Simply mark the initial 4-5 sec of the seizure onset phase and tpye F
  to perform a Fast Fourier Transform (FFT) of the displayed source
  channels. Click on the spectral peak value to obtain a map of
  spectral amplitude at the peak frequency of 7.2 Hz.
The spectral amplitude map depicts the maximum of activity below the
  right temporal lobe. The spectral amplitudes in the FFT (left) show
  high focal right basal activity, indicating clearcut lateralization to the
  right.
 FFT phase maps show cyclic propagation




                          Propagation from
                              basal to
                            antero-lateral




                                                               www.besa.de




Phase maps derived from FFT over single data block
Since we calculated the FFT over a single data block, the cosine and
   sine parts of the prominent frequency are known and their relati ve
   amplitudes at the different phases can be used to plot a voltage
   map at each phase angle.
The phase maps show a propagation from the initial vertical dipole
  field pattern to a radial-lateral pattern over 30-40 ms. This suggests
  sequential coupling and propagation from basal to antero-lateral
  during each seizure cycle.
 Source analysis of averaged seizure onset cycles




                                                                www.besa.de




Source localization using a pair of regional sources (right and left):
For seizure and spike analysis of adults, a realistic head model with
  anisotropic bone conductivity based on an averaged brain (24
  subjects) and the finite element method (FEM) was used.
A single regional source localized to the right basal-polar region. For the
   control of lateralization, a mirror probe source was added in the
   contralateral hemisphere. Similar to the brain source montages, the
   source waveforms show prominent right basal and lateral temporal
   activities (red traces) with a time difference between basal (1st trace)
   and lateral (2nd trace).
Accordingly, the initial orientation of the right regional source shows a
  vertical pattern corresponding to basal-temporal activity. The second
  activity reflects polar and antero-lateral activity. This pattern was
  similar in all seizures of this patient and is quite typical in mesial
  temporal lobe epilepsy.
Localization alone exhibits substantial uncertainty (red circle). Only
  when a careful interpretion of the gross localization is combined with
  an interpretation of the orientations and the sequence of activations,
  can the origin and path of propagation be defined more accurately.
Outlook
• Whole head mapping and source montages
  enhance the diagnostic potentials of the non-
  invasive EEG considerably.
• New automated procedures are beginning to
  emerge for a comprehensive analysis and for the
  imaging of epileptiform EEG activity.
• Improved EEG analysis software will enable a faster
  and more precise diagnosis of the non-invasive EEG
  in the near future. In particular, this will improve the
  planning of invasive recordings.



                                                       www.besa.de




More detailed lectures, tutorials and movies showing the
analysis of epileptic spikes and seizures can be found on:
                     www.besa.de
Acknowledgment

We are grateful for the collaborative support of:
• K. Hoechstetter, D. Weckesser, N. Ille, Munich
• John and Susan Ebersole, Chicago
• Rainer Boor, Mainz
• P. Berg, Konstanz



More details can be found in the lectures and movies
on www.besa.de


                                                    www.besa.de

								
To top