2.1 Electrode Selection, Application & Scalp Integrity
Key Takeaways
- Silver/silver-chloride (Ag/AgCl) cup electrodes represent the non-polarizable gold standard for multi-day LTM, providing reversible chloride ion exchange with negligible half-cell potential drift and superior baseline stability.
- Collodion application utilizing 100% cotton gauze squares dried with low-pressure compressed medical air or nitrogen provides maximum mechanical stability against hypermotor seizure displacement, whereas compressed oxygen is strictly contraindicated due to volatile ether explosion hazards.
- Controlled mechanical skin preparation with a mild abrasive agent thins the high-resistance stratum corneum to achieve target impedances (100 to 5,000 Ω with balance <2,000 Ω), while avoiding aggressive over-abrasion that causes capillary bleeding and galvanic potential drift.
- Scalp pressure injury prevention mandates 24-hour systematic site audits, gentle non-circumferential tubular elastic netting, pressure-relieving foam donut pads under dependent occipital leads, and immediate collodion removal/re-siting if Stage 1 non-blanchable erythema occurs.
- Special scalp conditions—including craniotomy bone defects, burr holes, and ICU trauma—require careful electrode siting to prevent breach rhythm overinterpretation and strict selection of MR-conditional, non-ferromagnetic electrodes to eliminate radiofrequency thermal burn hazards.
Last updated: August 2026
2.1 Electrode Selection, Application & Scalp Integrity
Long-Term Monitoring (LTM) for epilepsy and critical care continuous EEG (cEEG) requires recording stability across multi-day acquisition periods ranging from 24 hours to several weeks. Unlike routine 20- to 30-minute outpatient EEGs, an LTM recording must withstand continuous patient movement, sleep position changes, mechanical friction against bedding, diaphoresis, and violent hypermotor seizures without electrode dislodgement or baseline drift. Achieving pristine recording quality demands mastery of electrode electrochemistry, skin anatomy, secure application methodologies, proactive pressure injury prevention, and neuroimaging safety standards.
1. Electrode Electrochemistry & Material Selection
Electrode performance in electroencephalography is governed by the electrochemical interface established between the metallic conductor and the electrolyte-rich conductive gel. When a metallic sensor contacts an electrolyte containing chloride ions ($Cl^-$), chemical reactions create a charge separation layer known as the half-cell potential (or contact potential). In continuous recording environments, fluctuations in this half-cell potential manifest as severe low-frequency baseline drift, DC voltage shifts, or movement-induced voltage transients.
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| ELECTRODE-ELECTROLYTE INTERFACE & POLARIZATION |
| |
| [SKIN / STRATUM CORNEUM] <---> [CONDUCTIVE GEL (Cl-)] <---> [METALLIC ELECTRODE] |
| ^ ^ |
| | | |
| Epidermal Resistance Half-Cell Potential |
| (<5,000 Ohms) (Charge Double Layer) |
| |
| NON-POLARIZABLE (Ag/AgCl): |
| - Reversible Cl- ion exchange across interface -> Zero charge accumulation |
| - Stable DC baseline, lowest noise, ideal for multi-day LTM, qEEG, and slow DC shifts|
| |
| POLARIZABLE (Gold / Platinum / Stainless Steel): |
| - Behaves as an electrical capacitor -> Accumulates surface charge |
| - Prone to DC baseline drift, motion-induced voltage steps, and sweat artifacts |
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Silver/Silver-Chloride (Ag/AgCl) Cup Electrodes
- Electrochemical Classification: Non-polarizable (reversible) electrode.
- Chemical Mechanism: A solid silver substrate is coated with a porous layer of silver chloride. Chloride ions move freely across the interface according to the reversible chemical equilibrium:
- Clinical Performance: Because charge crosses the junction via actual chemical transformation rather than capacitive storage, Ag/AgCl exhibits minimal half-cell potential drift, negligible polarization artifact, and superior fidelity for low-frequency (<0.5 Hz) cerebral rhythms, slow cortical potentials, and quantitative EEG (qEEG).
- Maintenance & Chloriding: The delicate AgCl coating degrades over time with repeated abrasive cleaning, exposing bare silver and transforming the sensor into a polarizable electrode. Re-chloriding via electrolytic baths (soaking in saline or hydrochloric acid while passing a small DC current) or routine replacement is necessary.
Gold-Plated (Au) Disc Electrodes
- Electrochemical Classification: Polarizable electrode.
- Chemical Mechanism: Gold is chemically inert and does not exchange ions with chloride electrolytes. Current transfer occurs purely through capacitive charging across the electrical double layer.
- Clinical Performance: Highly durable, chemically inert, and resistant to mechanical wear and tarnishing. While gold electrodes are prone to DC baseline drift during sudden patient head movements or temperature fluctuations, their exceptional physical longevity and low maintenance make them common in standard clinical Epilepsy Monitoring Units (EMUs).
Disposable Pre-Gelled & Single-Use Cup Electrodes
- Clinical Indications: ICU continuous EEG, trauma patients, open cranial wounds, infectious isolation rooms (e.g., MRSA, C. difficile), and neonatal intensive care.
- Advantages: Eliminates cross-contamination risks and reduces initial technologist application time.
- Limitations: Hydrogel adhesives may desiccate over 48 to 72 hours, causing gradual impedance escalation and requiring re-geling or sensor replacement.
Subdermal Needle & Flexible Wire Electrodes
- Composition: Stainless steel or platinum-iridium fine-gauge needles placed sub-dermally into the galea aponeurotica.
- Indications: Emergency ICU setups in comatose patients with severe scalp edema, extensive cranial burns, or open cranial trauma where collodion/paste cannot adhere.
- Limitations: Higher source impedance than cup electrodes; invasive nature introduces micro-bleeding and infection risks; strictly single-use; susceptible to mechanical displacement during patient turns.
Comparative Electrode Characteristics Matrix
| Electrode Type | Polarization Class | Baseline Stability | Mechanical Durability | Primary Clinical Setting | Infection Risk Profile |
|---|---|---|---|---|---|
| Silver/Silver-Chloride (Ag/AgCl) | Non-polarizable | Exceptional (Lowest DC drift) | Moderate (AgCl coating wears) | Multi-day EMU, ICU cEEG, qEEG, Slow potentials | Low if reprocessed per CDC; Single-use ideal |
| Gold-Plated Disc | Polarizable | Good (Slight drift on motion) | Exceptional (Very rugged) | Routine LTM, Standard EMU monitoring | Low if reprocessed per hospital policy |
| Disposable Pre-Gelled Ag/AgCl | Non-polarizable | Excellent (Factory fresh) | Single-use only | Trauma ICU, Neuro-ICU, Isolation rooms | Zero cross-contamination (Discarded after use) |
| Subdermal Needle/Wire | Polarizable | Moderate (High impedance) | Single-use only | Coma cEEG emergency, Cranial burns, Trauma | Requires strict sterile single-use insertion |
2. Application Methodologies: Collodion vs Paste & Adhesive Wraps
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| COLLODION GAUZE APPLICATION PROTOCOL |
| |
| 1. Measure & mark scalp per 10-20 / 10-10 system |
| 2. Part hair; degrease skin with alcohol or saline wipe |
| 3. Gently abrade stratum corneum with NuPrep on cotton-tip applicator |
| 4. Seat Ag/AgCl or Gold cup electrode firmly on marked scalp site |
| 5. Place 1-inch square 100% cotton gauze soaked in collodion over cup |
| 6. Dry immediately with compressed medical AIR or NITROGEN (NOT O2!) |
| 7. Inject conductive gel via blunt-tip syringe through central cup hole |
| 8. Verify impedance < 5,000 Ohms (and balanced within 2,000 Ohms) |
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Collodion Application Technique (Gold Standard for Multi-Day LTM)
- Chemical Composition: Collodion is a viscous solution composed of pyroxylin (nitrocellulose) dissolved in a solvent mixture of diethyl ether and ethanol. As the solvent evaporates, it forms a tough, clear, water-insoluble, flexible adhesive film.
- Application Step-by-Step:
- Site Preparation: Part the hair cleanly at the measured 10-20 or 10-10 landmark. Cleanse the scalp with an alcohol prep or saline gauze to strip sebum and hair styling oils.
- Abrasion: Apply a small amount of pumice-based abrasive paste (e.g., NuPrep) using a sterile cotton-tipped applicator. Apply controlled, gentle rotational friction for 3 to 5 seconds to thin the stratum corneum.
- Placement: Seat the electrode cup flat against the prepared scalp.
- Gauze Attachment: Place a 1-inch square piece of 100% cotton gauze saturated with collodion liquid directly over the electrode cup and lead wire exit.
- Compressed Gas Curing: Direct a gentle stream of compressed medical air or nitrogen through a low-pressure delivery wand over the gauze until the collodion cures into a rigid, white matrix (10–20 seconds).
- Conductive Gel Injection: Insert a sterile, blunt-tipped needle/syringe filled with chloride-based conductive gel through the central hole of the electrode cup. Gently rotate the blunt tip to contact the skin, deliver gel to fill the cup cavity without creating air pockets, and seal the opening with a dab of heavy conductive paste or petroleum jelly.
[!CAUTION] Explosion and Fire Hazard with Collodion: NEVER use compressed oxygen ($O_2$) to dry collodion. Diethyl ether vapors released during collodion evaporation are highly volatile, heavier than air, and flammable. Directing pure oxygen over ether vapors creates an explosive atmosphere that can ignite from static electrical sparks or hospital electronic equipment. Only low-pressure compressed medical air or nitrogen may be used.
Conductive Paste & Tape (Ten20 / Grass Paste)
- Clinical Indications: Short-term diagnostic recordings (<24 hours), pediatric patients with respiratory hypersensitivity, neonates with fragile skin, or patients with documented collodion allergies.
- Method: The electrode cup is filled with heavy chloride paste (e.g., Ten20), seated on abraded scalp, and secured with surgical paper tape, self-adherent wrap (Coban), or elastic tubular netting (Surgilast).
- Limitations in LTM: Paste desiccates, softens with body heat, and liquefies during diaphoresis. Friction against pillows or vigorous seizure movements rapidly displaces paste-applied electrodes, generating major movement artifacts and necessitating frequent re-application.
3. Skin Preparation & Stratum Corneum Management
The outermost layer of human skin—the stratum corneum—consists of dead, flattened, keratinized corneocytes embedded in a dense lipid matrix. This layer acts as a high-resistance electrical barrier, exhibiting natural dry impedance ranging from 50,000 Ω to >200,000 Ω.
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| STRATUM CORNEUM ABRASION MECHANICS |
| |
| [HIGH IMPEDANCE (>50k Ohms)] -- Unprepared Stratum Corneum (Lipid barrier)|
| | |
| v (Targeted NuPrep abrasive scrub - gentle rotation) |
| [TARGET IMPEDANCE (<5k Ohms)] -- Thinned Keratin / Hydrated Viable Dermis |
| | |
| v (CLINICAL TRAP: Excessive scrubbing / High shear force) |
| [MICRO-TRAUMA / BLEEDING] -- Capillary tear -> Infection & Galvanic Cell|
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Controlled Mechanical Abrasion Technique
- Lipid Degreasing: Wipe the marked site with 70% isopropyl alcohol or sterile water to dissolve surface sebum, oils, and hair styling products.
- Targeted Mechanical Abrasion: Apply pumice-based abrasive paste (e.g., NuPrep) using a wooden cotton-tipped applicator. Apply firm, circular pressure for 3–5 seconds to strip dead corneocytes while leaving the underlying stratum granulosum intact.
- Debris Removal: Wipe away excess abrasive grit with a dry or saline-dampened gauze square to prevent abrasive particles from contaminating the conductive gel.
[!WARNING] Clinical Trap: Over-Abrasion & Capillary Bleeding: Aggressive scrubbing that produces bleeding or weeping serous fluid breaks the protective epidermal barrier, dramatically increasing the risk of local cellulitis, bloodborne pathogen exposure, and painful skin ulceration. Furthermore, exposed blood creates an unintended electrochemical battery (galvanic potential) between the blood proteins and electrode metal, generating persistent baseline drift.
Impedance Thresholds & Standards
- Target Impedance: 100 Ω to 5,000 Ω (5 kΩ) per ACNS guidelines.
- Inter-Electrode Balance: Impedance difference between any two paired recording electrodes must not exceed 2,000 Ω (2 kΩ) to maintain high Common Mode Rejection Ratio (CMRR).
- Salt Bridge Warning: Impedance <100 Ω suggests that conductive gel from adjacent electrodes has pooled together, creating a low-resistance short circuit that shunts and cancels genuine cerebral potentials.
4. Scalp Pressure Injury Prevention & Skin Integrity
Scalp pressure injuries represent one of the most common and severe preventable iatrogenic complications in the EMU and Neuro-ICU. In long-term recordings lasting 3 to 14 days, rigid electrode cups pressed continuously against the skull can induce severe tissue ischemia.
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| PATHOPHYSIOLOGY OF SCALP PRESSURE NECROSIS |
| |
| Rigid Electrode Cup + Tight Compression Wrap + Patient Head Weight |
| | |
| v |
| Focal Interfacial Pressure Exceeds Scalp Capillary Closing Pressure |
| (>32 mmHg) |
| | |
| v |
| Microvascular Occlusion -> Local Tissue Hypoxia & Ischemia (<2-4 Hours) |
| | |
| v |
| Stage 1: Non-blanchable erythema (Reversible with immediate relief) |
| Stage 2: Blistering, partial dermis breakdown |
| Stage 3/4: Full-thickness skin necrosis, subcutaneous ulceration, |
| permanent alopecia / exposed calvarium |
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Pathophysiology of Scalp Necrosis
- Capillary Closing Pressure: Scalp microvascular perfusion ceases when external focal pressure exceeds 32 mmHg.
- Vulnerable Anatomical Zones: Dependent posterior electrodes (O1, O2, Oz, T5/P7, T6/P8) bear the direct weight of the patient's head against the mattress, generating focal pressures that can exceed 100 mmHg if unmitigated.
- High-Risk Patient Populations: Comatose ICU patients, sedated individuals, neonates/infants, elderly patients with paper-thin atrophic skin, and patients receiving continuous vasopressor infusions (which induce peripheral vasoconstriction).
Pressure Injury Staging (NPUAP Criteria Adapted for Scalp)
- Stage 1: Non-blanchable erythema of intact skin over an electrode site. Area may be warm, indurated, or tender.
- Stage 2: Partial-thickness skin loss with exposed dermis; manifests as a shallow open ulcer, abrasion, or serum-filled blister beneath the cup rim.
- Stage 3: Full-thickness skin loss extending into subcutaneous tissue; slough and crusted exudate present.
- Stage 4: Full-thickness tissue loss with exposed galea aponeurotica, periosteum, or cranial bone; carries high osteomyelitis risk and causes permanent cicatricial alopecia.
Mandatory Pressure Injury Prevention Protocols
- Routine 24-Hour Scalp Audits: Technologists must systematically inspect and gently palpate all electrode sites at least once every 24 hours. Check for local edema, skin blanching, moisture accumulation, and patient-reported localized burning sensations.
- Head Wrap Mechanics: Never use tight, circumferential non-elastic tape or excessively tight Coban bandages. Utilize loose-fitting, expandable tubular elastic netting (e.g., Surgilast size 6–8) that secures wires without exerting downward compressive force.
- Pressure-Relieving Accessories:
- Place high-density foam donut rings, viscoelastic gel pads, or fluid-filled pillows beneath the occiput to distribute gravitational force away from dependent electrode cups.
- Ensure lead wires are dressed backward in smooth, non-overlapping bundles so patients do not lie directly upon wire intersections.
- Action Plan for Identified Skin Breakdown:
- If Stage 1 erythema or breakdown is detected, immediately dissolve the collodion using medical solvent (acetone or specialized citrus-based collodion remover) in a well-ventilated space.
- Clean the area with sterile saline and apply a hydrocolloid protective barrier or topical skin protectant.
- With physician approval, relocate the electrode 1 cm adjacent to the standard 10-20 position, annotating the montage adjustment in the acquisition log.
5. Special Scalp Considerations: Craniotomy Defects, Burr Holes & Trauma
Patients undergoing LTM in the EMU or ICU frequently have altered cranial anatomy due to prior neurosurgery, penetrating trauma, or intracranial disease.
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| CRANIAL DEFECTS & BREACH RHYTHM |
| |
| INTACT SKULL (Normal High-Frequency Bone Attenuation): |
| - Bone acts as high-pass spatial filter & high electrical resistor |
| - Attenuates beta (>13 Hz) and sharpens dipole fields |
| |
| BONE DEFECT (Craniotomy Flap, Craniectomy, Burr Holes): |
| - Absence of high-resistance bone -> Unimpeded electrical conduction |
| - Produces BREACH RHYTHM: Focal high-voltage sharp beta & mu activity |
| - CLINICAL PITFALL: Do NOT misinterpret breach sharp activity as focal |
| epileptogenesis without true evolving ictal morphology! |
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Craniotomy Defects & Bone Flaps
- Breach Effect: The skull bone provides substantial electrical resistance, attenuating high frequencies and smoothing voltage gradients. When a bone flap is removed (hemicraniectomy) or replaced with fibrous union, electrical resistance drops dramatically. The overlying electrodes record breach rhythm—focal high-amplitude activity characterized by spiky, arch-shaped rhythms, prominent beta activity, and sharp-contoured alpha/mu rhythms.
- Electrode Placement Rules: Never place rigid electrode cups directly over fresh, unhealed surgical incisions, cranial burr holes, or tense craniectomy defects. Place electrodes at least 1–2 cm away from incision borders and document the exact anatomical offset.
ICU Head Trauma & Severe Edema
- In patients with extensive scalp lacerations, subgaleal hematomas, or open wounds, standard cup electrodes with collodion cannot adhere. Technologists should deploy sterile subdermal needle electrodes or flexible wire electrodes placed into uninjured scalp regions, adjusting montages to capture viable cerebral activity while strictly avoiding infected or contaminated margins.
6. MRI and CT Neuroimaging Compatibility & Safety
Patients undergoing LTM frequently require urgent or elective neuroimaging during their hospital stay, including 3T structural MRI, functional MRI (fMRI), or head CT scans.
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| MRI COMPATIBILITY & THERMAL HAZARDS |
| |
| RADIOFREQUENCY (RF) PULSES (128 MHz at 3T): |
| - Induces eddy currents in conductive metal lead wires |
| - Loop formation (coiled leads) creates resonant antenna -> SEVERE BURNS |
| |
| STATIC MAGNETIC FIELD (1.5T / 3T): |
| - Ferromagnetic attraction (projectile hazard) & torque forces |
| - Magnetic susceptibility artifact -> Distorts anatomical MR images |
| |
| SAFETY PROTOCOL: |
| - Use ONLY labeled "MR Conditional" electrodes (conductive plastic/carbon)|
| - Straighten lead wires (NO wire loops or overlapping coils) |
| - Thermal barrier padding between leads and skin |
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Thermal Burn Hazards & Antenna Effects in MRI
- Mechanism: The pulsed radiofrequency (RF) magnetic fields transmitted by MRI scanners induce electrical currents in conductive metal lead wires. If lead wires are coiled, cross one another, or form closed loops, resonant antenna effects occur, generating intense resistive heating at the electrode-scalp junction. Scalp temperatures can exceed 60°C within seconds, causing full-thickness third-degree burns.
- MRI-Conditional Electrode Requirements:
- Electrodes must be certified MR Conditional with specified parameters (e.g., maximum 1.5T or 3T, maximum Specific Absorption Rate [SAR] limits, maximum scan duration).
- Sensors are typically constructed from conductive polymers, carbon fiber, or micro-thin non-ferromagnetic silver/silver-chloride coatings.
- Lead wires must be routed in straight, parallel paths extending directly out of the bore without coiling, looping, or overlapping.
Magnetic Susceptibility & CT Beam Hardening Artifacts
- MRI Susceptibility Artifacts: Metallic materials distort the local magnetic field ($B_0$), causing signal voids and geometric distortion that obscure underlying brain parenchyma. Conductive plastic and thin-film electrodes minimize susceptibility artifact on T1, T2, and FLAIR sequences.
- CT Beam Hardening: High-density metals (such as solid gold or silver cups) absorb low-energy X-ray photons, generating intense starburst or streak artifacts across the brain on CT scans. Low-density radiolucent electrodes (carbon/conductive plastic) prevent CT diagnostic degradation.
7. Infection Prevention & Spaulding Classification
Electrodes contact abraded skin and are exposed to sweat, sebum, and occasional serosanguinous fluid. Adherence to hospital infection control standards prevents transmission of multi-drug resistant pathogens (MRSA, VRE, C. difficile), hepatitis B/C, HIV, and fungal organisms.
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| CDC / SPAULDING CLASSIFICATION FOR EEG REPROCESSING |
| |
| NON-CRITICAL ITEMS (Intact Skin Contact): |
| - Lead wires, headboxes, pulse oximeter probes |
| - Requirement: Low- to intermediate-level disinfection (CaviWipes / Quat) |
| |
| SEMI-CRITICAL ITEMS (Abraded Skin / Non-Intact Mucosa Contact): |
| - Scalp cup electrodes placed on abraded skin |
| - Requirement: High-Level Disinfection (HLD) with EPA-registered |
| sterilant (Glutaraldehyde, Hydrogen Peroxide, or 1:10 Bleach Dilution) |
| |
| CRITICAL ITEMS (Vascular / Sterile Tissue Penetration): |
| - Subdermal needle electrodes, Depth / Grid intracranial electrodes |
| - Requirement: Sterile Single-Use ONLY (or Autoclave per IFU) |
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Reusable Cup Electrode Reprocessing Protocol
- Pre-Cleaning: Immediately post-removal, submerge electrodes in warm water with an enzymatic detergent to dissolve residual paste, collodion, and organic bioburden. Mechanically clean cups using a soft nylon brush (never metal wire brushes that strip plating).
- High-Level Disinfection (HLD): Immerse cleaned electrodes in an EPA-registered high-level chemical disinfectant (such as accelerated hydrogen peroxide, orthophthalaldehyde, or a freshly prepared 1:10 sodium hypochlorite bleach dilution) for the manufacturer-validated contact time.
- Rinsing & Drying: Thoroughly rinse electrodes with sterile or deionized water to remove toxic chemical residues that could cause chemical dermatitis on subsequent patients. Hang vertically in a clean, dust-free cabinet to air dry.
Prion Disease (Creutzfeldt-Jakob Disease - CJD) Protocol
- Transmission Risk: Prions are infectious proteinaceous particles that resist standard hospital autoclaving, ethylene oxide gas, alcohol, and conventional chemical disinfectants.
- Mandatory Procedure: If an LTM or cEEG recording is performed on a patient with suspected or confirmed CJD (or rapidly progressive dementia under evaluation), all reusable scalp electrodes and subdermal needles MUST be strictly single-use and incinerated immediately following the recording as hazardous biological waste.
Test Your Knowledge
Which electrode material represents the non-polarizable gold standard for multi-day LTM recordings requiring quantitative analysis and baseline stability, and what electrochemical property explains this superiority?
A
B
C
D
Test Your Knowledge
An adult patient on day 3 of EMU video-EEG monitoring complains of localized burning pain under occipital electrode O1. Upon physical inspection, the technologist observes persistent, non-blanchable erythema of intact skin beneath the electrode rim. What is the most appropriate clinical action?
A
B
C
D
Test Your Knowledge
During the setup of a multi-day video-EEG monitoring study using the collodion technique, which compressed gas source is safe to use for drying collodion-soaked gauze, and why?
A
B
C
D
Test Your Knowledge
A patient in the EMU with continuous scalp EEG electrodes is scheduled for an urgent 3T brain MRI. What is the primary safety hazard associated with scanning a patient with conventional metallic EEG electrodes and coiled lead wires?
A
B
C
D