technique and safety of performing eeg/fmri … and safety of performing eeg/fmri measurements ......

Technique and Safety of performing EEG/fMRI measurementsperforming EEG/fMRI measurements

by Pierluigi Castellone, Electronics Engineer

Brain Products‘ General Manager

Contents of the presentation

� Why recording simultaneous EEG and fMRI?

� Challenges in recording EEG in the MRI: Safety issues

� Challenges in recording EEG in the MRI: Technical issues� Challenges in recording EEG in the MRI: Technical issues

� Brain Products solutions and recommendations

� Four steps to successful EEG/fMRI measurement

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

Why recording EEG?

� The activation of neurons produces an electrical signal which can be

detected by using electrodes placed on the scalp.

� Being an electrical signal, any change in the neural activation is

detected very quickly. The EEG has time resolution in the order of

milliseconds but bad spatial resolution (inverse problem not solvable).


Why recording fMRI?

� Stimulation of the brain causes a local increase in blood flow. To the

increase of oxygen in the activated areas does not correspond an

increase of the used oxygen. This results in a net increase of oxygen in

the area activated.

� We can recognize two phases related to the brain activation:

(1) Deoxygenated blood (Paramagnetic – distortions in the MRI signal)

increases due to areas activated. T2 and T1 are shorter.

(2) Oxygenated blood (Diamagnetic – no distortions in the MRI signal)

flows in the activated areas. T2 and T1 are longer.

� By scanning the brain over time repeatedly, it’s possible to detect the

BOLD (Blood Oxygenation Level Dependent) signal.


Why recording fMRI?

� The fMRI can detect local brain activations with an extremely fine

spatial resolution (order of millimetres). Nevertheless it has bad

temporal resolution (1 sec in the best case).


Why recording simultaneously EEG and fMRI?

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Why recording simultaneously EEG and fMRI?

� In every cognitive experiment, the EEG/fMRI co-registration offers the

unique chance to record the brain activity originated by a specific

stimulation under the SAME experimental conditions!

� The validity of data acquired for the EEG and fMRI domains in separate

sessions is not only dependent on aspects related to the experimental

paradigm, but also depends on differences in the measurement environment. paradigm, but also depends on differences in the measurement environment.

� The restricted space available in the scanner bore, the position of the

experiment subject during the simultaneous recording (supine rather than

sitting upright) and the loud noise caused by the MRI gradient system are all

factors altering the experimental effects.


Why recording simultaneous EEG and fMRI?

� In every cognitive experiment, the EEG/fMRI co-registration offers the

unique chance to record the brain activity originated by a specific

stimulation under the SAME experimental conditions!

� To reduce the costs related to the experiments� To reduce the costs related to the experiments

� To reduce the working time


Challenges in recording EEG in the fMRI: Safety IssuesEEG in the fMRI: Safety Issues

The MRI scanner


The MRI scanner


EPI sequence design

Challenges in recording EEG in the MRI: Safety issues

(1) Static magnetic field

(2) Gradients

(3) Radio frequency pulses

The strength of the magnetic field of a scanner is measured in units of


The strength of the magnetic field of a scanner is measured in units of

“Tesla”. Today, a normal magnet would be a 1.5 or 3T scanner, where 3T

means that the magnetic field is roughly 60.000x stronger than the earth’s

natural magnetic field of .00005T!

This static magnetic field is ALWAYS on!

It is normally not possible to

remove such an object since

simple human power and force is

insufficient for removal.


Complete patient monitoring system wedged in the opening of a 1.5 T MR scanner system

In most of the cases the scanner

has to be “quenched”, completely

shutting off the power and the

Helium cooling system.

The cost of this is approximately

250.000 $!



� Acoustic noise:

During MR imaging, the slowly-varying gradient fields

are continuously turned on and off at variable rates and cause a loud

noise (mechanical resonance) .

Ear plugs and passive noise reduction are needed.


� Neural activation:

The gradients being switched are so strong that they can lead to

peripheral nerve stimulation and peripheral muscle contraction.

According to the Faraday‘s Law, the RF field used in MR imaging induces

currents into an electrically conductive object. The induced current also

changes its direction continuously.


The RF energy emitted during slice

acquisition is coupled onto the acquisition is coupled onto the

electrode leads and dissipated at the

points of highest thermal resistance,

which is typically the contact point

between the electrode pin and the

scalp across the conductive gel.


The RF energy is dissipated also on the first stage of the amplifier.

Brain Products solutions and recommendations

BrainAmp MR systems are the only commercially available solutions

designed to work directly INSIDE the MRI bore:

� There is no ferrite in the amplifier

� The system is powered by MRI-compatible batteries

� The first stage of the amplifier filters out high frequencies potentially � The first stage of the amplifier filters out high frequencies potentially

very dangerous for the hardware

� Most of the more sensitive electronics circuitry needed to operate the

system are in the control room (USB interface)



� The electrical connections from the cap to the amplifier’s input stage are

shorter and thereby the influence of the MRI gradient switching system

on the acquired data is minimized.

� Furthermore, shorter cables massively reduce the likelihood to spoil the

data acquisition with artifacts induced by cable motion and other types


of noise in the MRI environment.


The BrainCap MR transmits the electrophysiological signals from the

subject‘s head through a short distance of about 50 cm right into the

BrainAmp MR/MR plus which is also placed directly in the scanner.

BrainCap MRBrainCap MR

BrainAmp MR BrainVisionprofessional

Recorder

Stimulation



BrainCap MR features (1):

� In the combined EEG/fMRI acquisition the electrodes should never touch the skin.

Our electrodes are pin type sensors which are placed inside a plastic holder

mounted on the cap. Gel will be filled into the holder to reduce skin conductance

and to establish a contact between the sensor and the subject's skin.

� Every electrode contains safety resistors between the sensor and the connection � Every electrode contains safety resistors between the sensor and the connection

wire. Connection between the components is performed by gluing, not soldering.

� Additional safety resistors are placed inside the cap connector, acting like an

additional RF-filter.

� Wires are located at the outside of the cap to ensure isolation between skin and

wire according to the FDA patient safety regulations.



BrainCap MR features (2):

� High temperature isolating tubes wrapped around the ECG electrode cable avoid

creating contact between skin and wire.

� Drop-down electrodes contain higher resistors than normal electrodes to

compensate the technical characteristics of longer wires.

� All wires are fixed onto the cap to avoid loops.

� Wire length from electrode to the amplifier’s input is fixed to a maximum of 1.5m

not to match the Larmor frequency.

� Wire outlets for the cable tree at central positions avoid creating loops due to

cable routing.



Picture from Veera Merilainen’s thesis –Helsinki University of Technology

MRI sequence: The picture shows temperature changes induced by three

different MR sequences in a frontopolar EEG electrode on a sheep’s head.

MR scanning was performed for 900 seconds with each sequence.



� Strict restriction to low SAR sequences

� GE-Localizer

� GE-Structural sequences (MP-RAGE)

� GE-EPI

� Whenever the subject wears a EEG cap:� Whenever the subject wears a EEG cap:

� No Spin-Echo

� No Turbo-Spin-Echo

� No Spiral-EPI

� Avoid Body-Coil Tx !!

� Use whenever available head coil TxRx


What not to do…


Otherwise…


Challenges in recording EEG in the fMRI: Technical IssuesEEG in the fMRI: Technical Issues

Challenges in recording EEG in the MRI: Technical issues

� When an object with the susceptibility* different from that of surrounding

tissues is placed in a homogeneous magnetic field, it distorts the field

and causes local inhomogeneties.

*In physics and electrical engineering, the magnetic susceptibility is the degree of magnetization

of a material


� Two clear artifacts are visible in the EEG during the combined

experiment:

(1) Gradient artifact

(2) Pulse artifact



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� The gradient artefact is caused by the variation over time in the

strength and polarity of the electromagnetic field used in MR imaging.

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� If the position of the conductor changes relatively to the magnetic field

(motion artifact, ballistocardiogram artifact)

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Four steps to successful EEG/fMRI measurement …

(1) Safe Subject Preparation

(2) Trigger Setup / Synchronization

(3) Proper Measurement Settings

(4) Correct Data Preparation(4) Correct Data Preparation



(1) Safe subject preparation

� Informed consent must be obtained from all subjects!

� It must be established that the subject does not have any implants that are

ferrous or magnetizable.

� The subject must be free from claustrophobia.

� The subject has to fully understand the procedure and also any potential

causes of harm such as the ambient noise, gradient induced peripheral

effects and the difficulty of fast exiting.



(2) Trigger Setup

� Scanner artifacts are technical in nature, meaning that they are always the

same from acquisition to acquisition. In order to correct the EEG data for

these artifacts we must find the onset of each of these episodes very exactly.

� This can either be done in software by detecting features or values or it can

be done with the help of the scanner system, which can normally issue a


be done with the help of the scanner system, which can normally issue a

trigger at the exact time point of slice or volume acquisition.

� On BrainAmp MR systems, the normal stimulation system trigger cable is

complemented with a BNC trigger input that is internally connected to Bit 15,

thus giving a trigger of type “Response” and with the code “R128” for every

pulse the scanner sends.

� Most scanners are set up to send such a pulse with every slice or with every

volume onset by default.


(2) Synchronization

� Most scanners provide the service technicians with a diagnostic output that is

phase synchronous with the gradient clock itself.

� This mechanism allows for easy synchronization between the scanner

gradient system and the BrainAmp sample clock.


� The scanner has to provide a signal that is an integer multiple of 5000 Hz,

such as 20 kHz, 1 MHz, 20 MHz, …


(3) Proper Measurement Settings

� The sample rate has to be set to 5000.

� The amplifier bandwidth should be adjusted according to the characteristics

of the system used [DC or 0.1 - 250 Hz].

� The vertical resolution should be set at 0.5 µV/LSB.


Only if it is known that the system has weak and low gradients 0.1µV/LSB

should be used.

technique and safety of performing eeg/fmri … and safety of performing eeg/fmri measurements ......

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