Technique and Safety of. by Pierluigi Castellone, Electronics Engineer Brain Products General Manager

Transcription

1 Technique and Safety of performing EEG/fMRI measurements by Pierluigi Castellone, Electronics Engineer Brain Products General Manager

2 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 Brain Products solutions and recommendations Four steps to successful EEG/fMRI measurement

3 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).

4 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.

5 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).

6 Why recording simultaneously EEG and fmri? Oz [µv] [ms]

7 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. 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.

8 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 working time

9 Challenges in recording EEG in the fmri: Safety Issues

10 The MRI scanner

11 The MRI scanner EPI sequence design

12 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 Tesla. Today, a normal magnet would be a 1.5 or 3T scanner, where 3T means that the magnetic field is roughly x stronger than the earth s natural magnetic field of.00005t! This static magnetic field is ALWAYS on!

13 Challenges in recording EEG in the MRI: Safety issues Complete patient monitoring system wedged in the opening of a 1.5 T MR scanner system It is normally not possible to remove such an object since simple human power and force is insufficient for removal. 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 $!

14 Challenges in recording EEG in the MRI: Safety issues 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.

15 Challenges in recording EEG in the MRI: Safety issues 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 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.

16 Challenges in recording EEG in the MRI: Safety issues

17 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 very dangerous for the hardware Most of the more sensitive electronics circuitry needed to operate the system are in the control room (USB interface)

18 Brain Products solutions and recommendations 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.

19 Brain Products solutions and recommendations 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 MR BrainAmp MR BrainVision professional Recorder Stimulation

20 Brain Products solutions and recommendations 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 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.

21 Brain Products solutions and recommendations 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.

22 Brain Products solutions and recommendations 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.

23 Brain Products solutions and recommendations Strict restriction to low SAR sequences GE-Localizer GE-Structural sequences (MP-RAGE) GE-EPI 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

24 What not to do

25 Otherwise

26 Challenges in recording EEG in the fmri: Technical Issues

27 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

28 Challenges in recording EEG in the MRI: Technical issues Two clear artifacts are visible in the EEG during the combined experiment: (1) Gradient artifact (2) Pulse artifact

29 Challenges in recording EEG in the MRI: Technical issues The gradient artefact is caused by the variation over time in the strength and polarity of the electromagnetic field used in MR imaging. Fp1 Fp2 F3 F4 C3 C4 P3 P4 O1 O2 F7 F8 T7 T8 P7 P8 Iz Cz Pz FC1 FC2 CP1 CP2 FC5 FC6 CP5 CP6 TP9 TP10 Eog Ecg 100 µv S 2 S 1 S 2 S 2 S 1 S 3 S 1 S 2 S 3 S 1 S 3

30 Challenges in recording EEG in the MRI: Technical issues If the position of the conductor changes relatively to the magnetic field (motion artifact, ballistocardiogram artifact) Fp1 Fp2 F3 F4 C3 C4 P3 P4 O1 O2 F7 F8 T7 T8 P7 P8 Fz Cz Pz Oz FC1 FC2 CP1 CP2 FC5 FC6 CP5 CP6 TP9 TP10 EOG ECG Scan Start Scan Start Scan Start Scan Start

31 Challenges in recording EEG in the MRI: Technical issues Four steps to successful EEG/fMRI measurement (1) Safe Subject Preparation (2) Trigger Setup / Synchronization (3) Proper Measurement Settings (4) Correct Data Preparation

32 Challenges in recording EEG in the MRI: Technical issues (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.

33 Challenges in recording EEG in the MRI: Technical issues (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 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.

34 Challenges in recording EEG in the MRI: Technical issues (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,

35 Challenges in recording EEG in the MRI: Technical issues

36 Challenges in recording EEG in the MRI: Technical issues (3) Proper Measurement Settings The sample rate has to be set to The amplifier bandwidth should be adjusted according to the characteristics of the system used [DC or 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.