EEG TMS Reading · PDF fileEEG-TMS recordings by Pierluigi ... for any stimulation point....

EEG-TMS recordings

by Pierluigi Castellone, Electronics Engineer

Brain Products‘ General Manager

Contents of the presentation

(1) Why combining EEG and TMS?

(2) Why is this application technically challenging?

(3) What do we offer for EEG and TMS recording?

Reading (UK), July 7th to 8th, 2010 Workshop on EEG & TMS and EEG & fMRI

Why combining EEG and TMS?

This methodology provides very interesting applications, which can be

grouped as follows:

� Inductive approach:

The evoked potentials generated by TMS (TEPs) are measured at the cortical

level to asses cortical reactivity and connectivity.level to asses cortical reactivity and connectivity.

� Interactive approach:

To explore where, when and how TMS affects a functional network during a

cognitive task.

� Rhythmic approach:

To interact with oscillatory brain activity in order to understand causal relationship

between cortical rhythms and perceptual, cognitive or motor processes.


Why is this application technically challenging?

(1) The TMS stimulation induces a very strong electrical field that can

saturate the recording amplifiers for a long duration.

(2) The TMS pulse induces an artifact in the EEG data that can last for

hundreds of milliseconds.

(3) Mains noise (50 or 60 Hz) is normally induced in EEG leads by the (3) Mains noise (50 or 60 Hz) is normally induced in EEG leads by the

TMS stimulator.

(4) A recharging artifact induced by the TMS stimulator affects the EEG

data several milliseconds after the stimulation.


Amplifier saturation

� The TMS stimulation induces a very strong electrical field that can

saturate the recording amplifiers for a long duration.

� Until few years ago the only way to avoid this phenomenon consisted in

blocking the amplifier during TMS

stimulation.

� Using BrainAmps: if a positive TTL pulse

is sent to pin 23 and pin 24 is bridged to

ground (pin 1), the amplifier input is zeroed

for the pulse duration.



� Using BrainAmps: if a positive TTL pulse is sent to pin 23 and pin 24 is

bridged to ground (pin 1), the amplifier input is zeroed for the pulse

duration: In this example

the TTL pulse lasts for

200 msec.



Other amplifiers use the

sample and hold circuit by

means of which the signal is



duration:

held for a few milliseconds

immediately after the TMS.



Other amplifiers use the

sample and hold circuit by

means of which the signal is



duration:

held for a few milliseconds

immediately after the TMS

stimulation.

Nevertheless this methodology

causes a loss of information

for the whole time in which the

amplifier is blocked

.



� BrainAmps do not require the use of the blocking function. The large

dynamic range of these amplifiers allows continuous data recording:C4

0

2000

4000

C4

-300

-200


R128

-2 0 2 4 6 8 10 12

4000

6000

8000

10000

12000

14000

16000

18000R128

-5 0 5 10 15 20

-100

0

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TMS induced artifact

The TMS-induced artifact is very likely due to:

(1) Currents induced in the electrodes and

leadwires by each magnetic pulse

(also known as Eddy Currents)

(2) Decay of the TMS-induced charge

differences between the electrode and

C4

-600

-400

-200

differences between the electrode and

the skin, which depend on the resistive

and capacitive properties of the skin-

electrode junction.


R128

-5 0 5 10 15 20

0

200

400

600

800


The length of the artifact depends mainly on the impedance value:

� With impedances below 5 K Ohm the TMS artifact lasts approximately 5 msec.

� With higher impedances the artifact can last even more than 20 msec.

D. Veniero et al. /

Clinical Neurophysiology



� The length of the artifact doesn’t seem to be not affected by the intensity

or type of stimulation. Whereas the shape of the artifact depends on the

type of stimulator (biphasic or monophasic) and intensity of the stimulation.


D. Veniero et al. /


TMS induced artifact: Our solutions

The design of the BrainAmp DC and the BrainAmp MR plus allows optimal

adaptation to the TMS stimulus magnitude by the ability of adjusting amplifier

sensitivity and operational range.

Recommended amplifier setting for EEG and TMS co-registration:

� A/D resolution: 0.1 or 0.5 µV depending on the stimulus magnitude� A/D resolution: 0.1 or 0.5 µV depending on the stimulus magnitude

� Sampling rate: 5000 Hz/channel

� Hardware bandwidth: DC to 1000 Hz

As the TMS pulse consists of high frequency variations, the use of low

sampling rate or narrower filter can result in an increase of the artifact

duration.



� Brain Products’ electrode caps designed for EEG and TMS recordings

have very thin electrode holders (4 mm).

� This solution allows placing the TMS coil very close to the scalp and elicits

the evoked response with low magnitude stimulus.



� Phase synchronizing the TMS stimulator with the recording amplifier

opens up new scenarios for the removal of the TMS-induced artifact.

� 5Hz sine wave has been recorded during TMS stimulation. A TMS pulse

artifact is clearly visible in the data.



� Phase synchronizing the TMS stimulator with the recording amplifier

opens up new scenarios for the removal of the TMS-induced artifact.

� Subtraction of the TMS artifact averaged across all stimuli shows very

promising results.


Mains noise induced in EEG leads

� Mains noise induced by the TMS stimulator in EEG leads can affect the

data substantially.


Mains noise induced in EEG leads

� Applying a notch filter would introduce a rippling of the signal and therefore

a longer artifact.


Mains noise induced in EEG leads: Our solutions

The PowerMAG does not induce any mains in the EEG data by:

� using a fully-closed housing made of metal for the main unit

� filtering noise right inside the high-voltage supply unit


Mains noise induced in EEG leads: Our solutions

� A cleaner signal can be recorded by

orientating the cables away from the coil

or coil cable.

� On our caps for EEG and TMS, electrode

cables can be rotated 360 degrees and

always allow for the best cable geometry

for any stimulation point.


Recharging artifact

� Most of the commercially available TMS stimulators contaminate the EEG

data with a coil recharge artifact.

� As reported in the article published by D. Veniero et. Al on Clinical Neuro-

physiology, the latency of the recharging artifact increases with the in-

crease of the power strength of the stimulation. The shape of this artifact is

very similar to physiological activity and can cause misinterpretation of TEP.


D. Veniero et al. /


Recharging artifact

The PowerMAG does not induce any recharging artifact by:

� keeping high-frequency noise in the recharging circuit low

� having an additional high frequency filter at the coil output


D. Veniero et al. /


Conclusions

1. EEG and TMS co-registration is a very interesting methodology that can

open up new opportunities for understanding human brain functions.

2. EEG/TMS co-registration is technically challenging as TMS-related

artifacts are induced in the EEG data.

3. The BrainAmp amplifiers and the PowerMAG TMS stimulator are 3. The BrainAmp amplifiers and the PowerMAG TMS stimulator are

designed to minimize the impact of these artifacts in the EEG data.

4. It has been proved that with BrainAmp amplifiers physiological activity is

visible after 5 milliseconds from the TMS stimulation.

5. Phase synchronization between BrainAmp amplifiers and the PowerMAG

opens up new scenarios even for the removal of the electrical artifact

produced by the magnetic pulse.


EEG TMS Reading · PDF fileEEG-TMS recordings by Pierluigi ... for any stimulation point....

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