John R. Ives

MRI compatible EEG electrode system for routine use in the epilepsy monitoring unit and intensive care unit.

OBJECTIVEnWe report on the development of an electroencephalographic (EEG) recording system that is Magnetic Resonance Imaging (MRI) compatible and can safely be left on the scalp during anatomical imaging or used to obtain simultaneous EEG and metabolic or hemodynamic data using functional imaging techniques such as functional MRI or MR spectroscopy.nnnMETHODSnWe assembled a versatile EEG recording set-up with medically acceptable materials that contained no ferromagnetic components. It was tested for absence of excess heating and distortion of the image quality in a spherical phantom similar in size to average adult human head in a clinical 1.5 T GE scanner. After testing its safety in four volunteers, 100 consecutive patients from our epilepsy long-term monitoring unit were studied.nnnRESULTSnThere was no change in the temperature of the EEG electrode discs during the various anatomical MRI sequences used in our routine clinical studies (maximum temperature change was -0.45 degrees C with average head SAR<==1.6 W/Kg in the selected subjects) nor were there any reported complications in the others. The brain images were not distorted by the susceptibility artifact of the EEG electrodes.nnnCONCLUSIONSnOur MRI compatible EEG set-up allows safe and artifact free brain imaging in 1.5T MR scanner with average SAR<==1.6 W/Kg. This EEG system can be used for EEG recording during anatomical MRI studies as well as functional imaging studies in patients requiring continuous EEG recordings.

A comparison of subdermal wire electrodes with collodion-applied disk electrodes in long-term EEG recordings in ICU.

OBJECTIVEnTo compare long-term electroencephalographic (EEG) recordings of standard collodion-applied scalp disk electrodes (SDEs) with newly developed subdermal wire electrodes (SWEs) in comatose intensive care unit (ICU) patients.nnnMETHODSnTen comatose ICU patients had simultaneous recordings from 8 active SDEs and 8 active SWE for >24 h. The timing and number of 60 Hz and other electrode artifacts were compared for each set of electrodes by an EEGer who read the recordings in a blinded manner.nnnRESULTSnSixty Hertz artifact was seen in 16 of 80 SDE and one of 80 SWEs within the first 6 h (P=0.0002). Large, persistent artifacts occurred in 30/80 SDE and 8 of 80 SWE (P=0.0001). Motion artifact with chest physiotherapy was more common in SWEs.nnnCONCLUSIONSnSWE are less susceptible to artifacts and are more suitable for the long-term EEG monitoring in ICU.nnnSIGNIFICANCEnThis is the first controlled study that demonstrates the superiority of SWEs compared to SDEs in an ICU population.

Linear aspects of transformation from interictal epileptic discharges to BOLD fMRI signals in an animal model of occipital epilepsy

Epileptic disorders manifest with seizures and interictal epileptic discharges (IEDs). The hemodynamic changes that accompany IEDs are poorly understood and may be critical for understanding epileptogenesis. Despite a known linear coupling of the neurovascular elements in normal brain tissues, previous simultaneous electroencephalography (EEG)-functional magnetic resonance imaging (fMRI) studies have shown variable correlations between epileptic discharges and blood oxygenation level-dependent (BOLD) response, partly because most previous studies assumed particular hemodynamic properties in normal brain tissue. The occurrence of IEDs in human subjects is unpredictable. Therefore, an animal model with reproducible stereotyped IEDs was developed by the focal injection of penicillin into the right occipital cortex of rats anesthetized with isoflurane. Simultaneous EEG-fMRI was used to study the hemodynamic changes during IEDs. A hybrid of temporal independent component analysis (ICA) of EEG and spatial ICA of fMRI data was used to correlate BOLD fMRI signals with IEDs. A linear autoregression with exogenous input (ARX) model was used to estimate the hemodynamic impulse response function (HIRF) based on the data from simultaneous EEG-fMRI measurement. Changes in the measured BOLD signal from the right primary visual cortex and bilateral visual association cortices were consistently coupled to IEDs. The linear ARX model was applied here to confirm that a linear transform can be used to study the correlation between BOLD signal and its corresponding neural activity in this animal model of occipital epilepsy.

Real‐time display of artifact‐free electroencephalography during functional magnetic resonance imaging and magnetic resonance spectroscopy in an animal model of epilepsy

Simultaneous recording of electroencephalogram (EEG) and functional MRI (fMRI) or MR spectroscopy (MRS) can provide further insight into our understanding of the underlying mechanisms of neurologic disorders. Current technology for simultaneous EEG and MRI recording is limited by extensive postacquisition processing of the data. Real‐time display of artifact‐free EEG recording during fMRI/MRS studies is essential in studies that involve epilepsy to ensure that they address specific EEG features such as epileptic spikes or seizures. By optimizing the EEG recording equipment to maximize the common mode rejection ratio of its amplifiers, a unique EEG system was designed and tested that allowed real‐time display of the artifact‐free EEG during fMRI/MRS in an animal model of epilepsy. Spike recordings were optimized by suppression of the background EEG activity using fast‐acting and easily controlled inhalational anesthesia. Artifact suppression efficiency of 70–100% was achieved following direct subtraction of referentially recorded filtered EEG tracings from active electrodes, which were located in close proximity to each other (over homologous occipital cortices) and a reference electrode. Two independent postacquisition processing tools, independent component analysis and direct subtraction of unfiltered digital EEG data in MATLAB, were used to verify the accuracy of real‐time EEG display. Magn Reson Med 53:456–464, 2005.

MR compatibility of EEG scalp electrodes at 4 tesla

To design and apply a method to quantitatively evaluate the MR compatibility of electroencephalographic (EEG) scalp electrodes based on pulse sequence‐independent metrics.

Usefulness of a 1.5 T MRI-compatible EEG electrode system for routine use in the intensive care unit of a tertiary care hospital

BACKGROUNDnContinuous electroencephalogram (cEEG) recordings are being increasingly used in intensive care units (ICUs) to detect epileptic seizures and other changes. MRI scans can interrupt such recordings if the EEG electrodes need to be removed and important data can be missed.nnnMETHODSnWe retrospectively examined EEG recordings from ICU patients who underwent MRI scans, comparing those from patients with the MRI-compatible EEG electrodes with those who had to have the EEG electrodes removed before scanning. We also examined technical aspects of the recording and scalp abrasions in both groups.nnnRESULTSnFourteen of 31 (45%) EEG recordings with the MRI-compatible electrode system in patients that underwent MRI scans between 03:00 p.m. and 07:00 a.m. (when technologists were not available) captured seizures. In contrast, all of the six EEG recordings with the MRI-incompatible electrode system in patients that underwent MRI scanning during the same interval were interrupted and had no data. Recording characteristics of the EEGs were comparable between the two groups and none had scalp abrasions.nnnCONCLUSIONnA significant proportion of patients undergoing MRI scans with the MRI-compatible EEG electrodes had seizures that would have been missed if the MRI-incompatible EEG electrodes had been used.

Improving Functional Magnetic Resonance Imaging Motor Studies Through Simultaneous Electromyography Recordings

Specially designed optoelectronic and data postprocessing methods are described that permit electromyography (EMG) of muscle activity simultaneous with functional MRI (fMRI). Hardware characterization and validation included simultaneous EMG and event‐related fMRI in 17 healthy participants during either ankle (n = 12), index finger (n = 3), or wrist (n = 2) contractions cued by visual stimuli. Principal component analysis (PCA) and independent component analysis (ICA) were evaluated for their ability to remove residual fMRI gradient‐induced signal contamination in EMG data. Contractions of ankle tibialis anterior and index finger abductor were clearly distinguishable, although observing contractions from the wrist flexors proved more challenging. To demonstrate the potential utility of simultaneous EMG and fMRI, data from the ankle experiments were analyzed using two approaches: 1) assuming contractions coincided precisely with visual cues, and 2) using EMG to time the onset and offset of muscle contraction precisely for each participant. Both methods produced complementary activation maps, although the EMG‐guided approach recovered more active brain voxels and revealed activity better in the basal ganglia and cerebellum. Furthermore, numerical simulations confirmed that precise knowledge of behavioral responses, such as those provided by EMG, are much more important for event‐related experimental designs compared to block designs. This simultaneous EMG and fMRI methodology has important applications where the amplitude or timing of motor output is impaired, such as after stroke. Hum Brain Mapp 2006.

Physiological monitoring of small animals during magnetic resonance imaging

Maintaining a stable physiologic state is essential when studying animal models of epilepsy with simultaneous electroencephalograph (EEG) and functional magnetic resonance imaging (fMRI) or EEG and magnetic resonance spectroscopy (MRS). To achieve and maintain such stability in rats in the MRI environment, a minimally invasive but comprehensive system was developed to monitor body temperature, heart rate, blood pressure, blood oxygen saturation and end-tidal CO2 (ETCO2) of expired gas. All physiologic parameters were successfully monitored in Sprague-Dawley rats without interfering with EEG recordings during simultaneous fMRI and MRS studies. Body temperature, heart rate, blood pressure, blood oxygen saturation, and ETCO2, were maintained between 36.5 and 37.5 degrees C, 250-450 beats/min, 136+/-17 mmHg, >90%, and 20-35 mmHg, respectively for 6-8 h under inhalational anesthesia. This set-up could be extended to study in vivo applications in other laboratory animals with only minor modifications.

RF Heating of Gold Cup and Conductive Plastic Electrodes during Simultaneous EEG and MRI

ABSTRACT Purpose: To investigate the heating of EEG electrodes during magnetic resonance imaging (MRI) scans and to better understand the underlying physical mechanisms with a focus on the antenna effect. Materials and Methods: Gold cup and conductive plastic electrodes were placed on small watermelons with fiberoptic probes used to measure electrode temperature changes during a variety of 1.5T and 3T MRI scans. A subset of these experiments was repeated on a healthy human volunteer. Results: The differences between gold and plastic electrodes did not appear to be practically significant. For both electrode types, we observed heating below 4°C for straight wires whose lengths were multiples of ½ the radiofrequency (RF) wavelength and stronger heating (over 15°C) for wire lengths that were odd multiples of ¼ RF wavelength, consistent with the antenna effect. Conclusions: The antenna effect, which has received little attention so far in the context of EEG-MRI safety, can play as significant a role as the loop effect (from electromagnetic induction) in the heating of EEG electrodes, and therefore wire lengths that are odd multiples of ¼ RF wavelength should be avoided. These results have important implications for the design of EEG electrodes and MRI studies as they help to minimize the risk to patients undergoing MRI with EEG electrodes in place.

A New Ground and Reference Technique for Invasive EEG Recordings

ABSTRACT. Objective: To demonstrate a subdermal wire electrode technique for establishing a ground (GND) and reference (REF) during long-term EEG monitoring (LTM) with intracranial electrodes. Usually, a separate GND and REF are required and this GND&REF pair can be selected contacts in the invasive electrode arrays themselves, special invasive electrodes, or Standard surface disc electrodes which require frequent maintenance. We investigated the use of a pair of chronic Subdermal Wire Electrodes (SWE) for use as GND&REF. Methods: A pair of SWEs as GND&REF was tested in nine patients undergoing invasive EEG monitoring. SWE impedances were monitored in two patients and compared to disc electrode impedances. Results: Without maintenance, SWE impedances remained low and stable during the entire recording period (up to 20 days), whereas disc electrodes showed rapid impedance increase όfter the first day. In all nine patients, the consistent and durable integrity of the GND&REF pair of SWE allowed for a good quality EEG recording. No local skin complications were observed. Conclusions: A pair of SWE electrodes provides a GND&REF system that is easy to place, maintain, and provides a high quality recording with long-term stability when couplecl with referential based EEG recording system from invasive electrodes. Significance: A more efficient means of establishing a GND&REF pair during the monitoring of patients with invasive electrodes is described.