EEG-fMRI 1 - Good quality data and safety

See also: 
EEG-fMRI 2 - head coils, protection & sequences

EEG-fMRI 3 EEG, Exg and Checklist

These techniques obtain data on brain activity in different ways. Whether through hemodynamic or bioelectrical aspects:

Hemodynamic 

Near infrared spectroscopy (NIRS)

Functional Magnetic Resonance Imaging (fMRI)

Bioelectrical activity
Electroencephalography (EEG)

Biomagnetic activity

Magnetoencephalography (MEG)

 

In this knowledge base we will be introducing the methodology to use simultaneous EEG-fMRI

These EEG devices are designed to use with simultaneous fMRI:
BRAINAMP MR
BRAINAMP MR PLUS

And for acessories:
BRAINAMP EXG MR 

 

Measure synchronous activation of the dendritic tree of a large population of neurons (pyramidal cells, oriented perpendicularly to the scalp surface) in response to exogenous stimulation. 

EEG methods present several limitations. One major limitation is that only scalp signals can be detected. 

EEG-fMRI is a multimodal neuroimaging technique which enables the acquisition of EEG and fMRI data synchronously. The technique is being used worldwide to evaluate normal brain function as well as brain function associated with recognized brain disorders.

Scalp EEG reflects the brain‘s electrical activity, more specifically it represents post-synaptic potentials in the cerebral cortex. fMRI detects haemodynamic changes throughout the brain known as the BOLD effect (Blood Oxygen Level Dependent). EEG-fMRI therefore enables the direct correlation of these two important measures of brain activity

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.

 

Simultaneous acquisition of EEG and fMRI data of good quality is a difficult task. Challenges with regards to subject safety and data quality cannot be underestimated. 

 

Careful preparation of the subject in accordance with established safety guidelines for EEG examination in the scanner is mandatory. One such possible health and safety threat is the precipitation of induced currents dangerous to the subject.

- Local MR safety rules must be observed at all times.

- The instructions in this document are intended to complement the safety instructions of the scanner manufacturer, they do not replace them.

- It is recommended that all people involved in using the BrainAmp MR / MR plus / ExG MR in an MR environment have previously acquired the status of an MR authorized person or MR operator.

 

The image shows an example of a scanner facility layout. The details below provide a recommendation for where to place the various components of the Brain Products EEG system. 

An item that poses no known hazards resulting from exposure to any MR environment. MR Safe items are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic.

An item with demonstrated safety in the MR environment within  defined conditions. At a minimum, address the conditions of the static magnetic field, the switched gradient magnetic field and the radio frequency fields. Additional conditions, including specific configurations of the item, may be required.

 

An item which poses unacceptable risks to the patient, medical staff or other persons within the MR environment.

Product labeling

All components of the MR series of amplifiers carry a label related to their safety properties in the MR environment. ASTM 2503-13 distinguishes three categories regarding the safety properties of items taken into the MR environment.

 

- Unless otherwise stated all standard BrainCap MRs are designed and approved for field strengths up to 3T.

- For applications in environments >3T please contact the Brain Products Technical Support team for advice.

- No general approval is provided for custom cap designs. All nonstandard BrainCap MR layouts are subject to a liability waiver.

 

Approved human whole body MR scanners: 

Static gradient field 

Siemens: 1.5 - 7 T* 

Philips: 1.5 - 7 T* 

General Electric: 1.5 - 7 T* 

Bruker: up to 4 T* 

 

- If you have a scanner from a manufacturer not listed, please contact the Brain Products Technical Support team. 

* Over 3 Tesla: Only possible after dedicated setup customization and site specific training by the Brain Products Technical Support team.

The choice of the head coil is not only important for the MRI measurements and quality of the images but also for safety. The head coil is the most important determinant of the work space for EEG measurements.

The suitability of a head coil for EEG measurements is determined by considering the presence of appropriate options for straight EEG cable routing and the RF-transmit capabilities. 

 

- EEG optimized head coils provide both transmit capabilities and a cable duct of sufficient diameter to feed EEG cables in line with the Zaxis of the scanner to the amplifier.

- Head coils that do not permit straight EEG cable routing from the cap to an amplifier behind the head are EEG incompatible and should not be used.

 

Schematic representation of an RF field in a body transmit coil Figure 5.1 and a head transmit coil Figure 5.2. If a body coil is used as the RF transmitter coil, the equipment used for recording the EEG and/or peripheral physiology are exposed to a considerably stronger and more widespread RF field, Figure 5.1. If a local (head) transmit coil is used, Figure 5.2, only the electrodes and cables are exposed to the RF field. The choice of RF transmitter coil is therefore a crucial factor when setting up a simultaneous EEG-fMRI study.

 

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

 

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.

 

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