EEG-fMRI 2 - head coils, protection & sequences
Brain Products has offered state-of-the-art systems for EEG & fMRI since the year 2000, i.e. providing
BrainAmp MR series amplifiers - from non magnetic materials - dedicated for use inside the scanner bore.
offline as well as real-time gradient and pulse artifact correction software (BrainVision Analyzer 2 and BrainVision RecView) for EEG-fMRI artifact correction.
special "MR conditional" electrode caps - preserving subject safety and image quality
... resulting in hundreds of EEG-fMRI publications using Brain Products Hard- and Software.
Head coils for combined EEG-fMRI recordings must have one of the following characteristics:
- Ducts or openings for the EEG cables in Z axis direction.
- In the absence of a suitable cable duct or opening the coil must consist of a top and bottom part where the coil can be operated without the top part.
Some example of suitable and unsuitable head coils are given on the following slides.
Open head coil
An open head coil makes it possible to route the connecting cables between the EEG cap and the amplifier in a straight line, thus preventing any loops.
Example: Siemens 8Ch CP Tx/Rx.
Head coil for combined EEG-fMRI with cable duct
This head coil makes it possible to route the connecting cables between the EEG cap and the amplifier in a straight line, thus preventing any loops or curves.
Examples: Philips 32- channel, MR Instruments 32- channel Rx, QED 32- channel TxRx, Siemens Head/Neck 64-channel, GE 48 Channel Head Coil.
Head coil with usable side opening
This head coil allows one to route the connecting cables between the EEG cap and the amplifier through the side of the coil. The cables exit at the side of the coil close to the midline and then bend before continuing to the amplifier. Loops are prevented. Models with a side opening are not as well suited to combined measurements as open head coils or head coils that have been specially adapted for EEG recordings. However, they offer an adequate level of safety provided that the connecting cables can be led out of the head coil without any loops forming.
Examples: Siemens Invivo 8Ch Rx SENSE, Siemens 12- channel Head Matrix Rx, GE 8- channel, GE 16- channel Head- Neck, Siemens 8- channel SENSE Rx, Siemens mMR Head/Neck, Siemens Head/Neck 20.
Closed head coil
Do not use closed head coils for simultaneous EEG-fMRI recordings. With closed head coils cables cannot be routed in a straight line. Extreme curves or bends will be formed in the cable because the bundled connecting cable must be routed back along the electrode cap inside the coil, exit at the bottom and then be routed back up the outside of the head coil. This also results in the cable being very close to the face of the subject, this is an unacceptable risk.
Examples: Nova Medical Head Coil 32RX, Siemens 32-channel Rx.
Protecting the amplifier from damage
Even though our primary goal is to protect the subject from electrode or cable heating, care has to be taken to protect the EEG equipment from damage due to RF-heating.
The system components that are particularly sensitive to RF-heating include:
- EEG electrode protection resistors
- Protection resistors in the cap connecting box
- The protection circuits at the amplifier input stage.
Protecting the amplifier from damage
The mechanisms of RF overload include:
- MR sequences using inappropriate parameters
- Inappropriate setup geometry, e.g. off center or orthogonal cable routing, cable loops
- Electrically un-terminated channels
Overloading at the amplifier input by voltages in open channels or high-impedance channels
Electrodes with high impedance act as antennas during scanning and pick up RF energy. The safety circuit at the input is designed to dissipate a certain amount of power, however, when the capacity of the safety circuit is exceeded, the amplifier input is overloaded and damage to the circuit board can occur.
Do the following to prevent damage to the amplifier:
- Minimize the impedances of all electrodes.
- Even the impedance of unused electrodes must be minimised, there should be no electrodes that are not connected to the subject. Deactivating unused channels in the Recorder workspace does not provide protection against voltage overload.
- Take special care of the ECG and EOG electrodes. These electrodes must not become detached during patient positioning as this will result in high impedance.
- Always check the impedance of all available channels.
- Make sure that the ribbon cable is securely attached to the amplifier and the cap connector box.
- If you are using a BrainAmp ExG, when you install the ExG AUX Box make sure that all electrode leads are securely plugged into the appropriate ports to avoid antenna effects.
Overheating of the amplifier or PowerPack by strong electromagnetic fields and radio-frequency fields
All amplifiers are shielded to protect the sensitive electronic components. If the operating temperature of the amplifier (40 °C) is exceeded, thermal overload protection is triggered and error messages are sent to the recording software. However, this is not a fail-safe mechanism, if overheating continues, it results in irreversible thermal destruction of the amplifier.
Observe the following rules to avoid overheating of the amplifier and the PowerPack and to ensure stable operation:
- Use a head coil as the RF transmitter coil where possible.
- Only use allowed sequences with an inherently low specific absorption rate (SAR). Refer to MRI Sequences on page 26 for information on SAR and which sequences are allowed.
- Position the amplifier and cables in accordance with the instructions
MRI Sequences
The BrainAmp MR / MR plus system is MR conditional, refer to Definitions from ASTM international standard F2503-13 on page 10 . One of the conditions for safe use is to use only MRI sequences that meet the criteria specified in this chapter.
Permitted sequences: specification of sequence conditions
We recommend a maximum B1+rms for MRI sequences used for simultaneous EEG. You will find the recommended thresholds in the following table and this chapter has further information on what B1+rms is and why we use it.
furthermore, all other specifications described in the user manual still apply e.g. position of the amplifier, cable routing, and a head coil with an appropriate option for routing the EEG cables.
What is B1+rms?
In short, B1+rms is a metric used to quantify the amount of radio frequency (RF) magnetic field that is generated by the RF transmit coil for a specific pulse sequence and is expressed in units of µT
Why B1+rms?
Historically we have recommended using low Specific Absorption Rate (SAR) sequences, however, SAR is dependent not only on the imaging parameters but also on the volunteer’s weight. B1+rms on the other hand is volunteer independent and is determined by basic MRI parameters.
It is a known value based on the pulse sequence parameters, unlike SAR which is an estimated value. Once a sequence has been adjusted to the required B1+rms, it is saved to the scanning protocol and will remain that way as long as relevant parameters are not changed.
B1+rms has been recommended as an alternative metric to SAR for limiting the amount of RF power during scanning (for an overview see Faulkner, 2016).
Why B1+rms?
The B1+rms limit that we specify protects the volunteer and the amplifier from excessive RF power while allowing the user more flexibility with their sequence parameters. By specifying a maximum B1+rms for the BrainAmp MR system, the user is free to determine sequence parameters as they see fit, as long as the B1+rms remains below the specified threshold and as long as all other safety guidelines are followed
Test conditions
Testing for the new B1+rms limits was done under the conditions recommended in this manual. Measurements were done in a Siemens Prisma (3T) scanner using a Siemens Head/Neck 64 head coil. Temperature measurements were made using a Neoptix Reflex fiber optic temperature thermometer; the positions of the probes were determined by previous measurements using an infrared camera.
Note: The tests were limited to the specific conditions described here, all combinations of manufacturer and head coil could not be individually investigated. However, while the specific parameters affecting the B1+rms may vary between MR system vendors, B1+rms itself is a precise RF exposure metric and the threshold specified here can be applied across 3 T scanner platforms.
Preparing the Subject
Before starting the measurement, explain to the test subject what they should do if they begin to feel uncomfortable or perceive unexpected sensations during the examination. In line with regular MR procedures, show the test subject how to operate the alarm bell.
Attaching the ECG cable
the ECG electrode leaves the cap at the occipital pole. You can only attach the ECG electrode to the back of the test subject. You will achieve the best possible ECG signal quality if you attach the ECG electrode as far down as permitted by the cables, on the back of the test subject along the paravertebral line.
Positioning the ECG electrode medially in this manner reduces the amplitude of the scanner artifacts, increases the amplitude of the R peaks and prevents the electrode leads from forming any loops.
References
Performing simultaneous EEG-fMRI measurements, Conditions for the safe use of BrainAmp MR amplifiers and accessories in the MR environment, Document version: 002, Publishing date: 4/9/2020. Brain Products GmbH.
Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment, ASTM F2503-13, ASTM International, West Conshohocken, PA; 2013.
International Electrotechnical Commission. IEC 60601-2-33:2010 + COR1:2012 + A1:2013 + A2:2015 + COR2:2016. Medical electrical equipment. Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis.
Faulkner W. (2016) New MRI Safety Labels & Devices. https://www.ismrm.org/smrt/E-Signals/2016FEBRUARY/eSig_5_1_hot_2.htm
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