EEG Source Imaging

EEG source imaging (ESI) uses mathematical modelling to estimate where epileptic electrical activity recorded at the scalp originated inside the brain.

It can add localisation information beyond ordinary visual EEG interpretation and is especially useful during presurgical evaluation of drug-resistant focal epilepsy.

What ordinary EEG shows

Scalp EEG records voltage differences between electrodes placed on the head.

A conventional EEG report describes the distribution of epileptiform discharges across those electrodes, but the electrode showing the largest signal is not necessarily directly above the true cortical source.

What source imaging adds

ESI solves an “inverse problem”: given the electrical potentials measured at the scalp, what cortical source or source distribution best explains them?

The estimated source can then be displayed on the person’s MRI.

High-density EEG

Source localisation is generally improved when more scalp electrodes are used.

High-density EEG systems may use 64, 128 or more electrodes, providing better spatial sampling than standard clinical montages.

Interictal source imaging

Most ESI is performed using interictal epileptiform discharges—spikes or sharp waves recorded between seizures.

Repeated similar spikes can be averaged to improve signal quality.

Ictal source imaging

Seizure-onset EEG activity can also be modelled.

Ictal source imaging can be technically harder because muscle artefact, movement and rapid seizure propagation may obscure the earliest signal.

Why individual MRI matters

A personalised head model based on the patient’s MRI improves anatomical accuracy.

Models estimate how electrical currents travel through brain tissue, cerebrospinal fluid, skull and scalp.

What the evidence shows

Systematic reviews support ESI as a useful component of multimodal presurgical assessment.

A meta-analysis of operated patients found relatively high sensitivity for electrical and magnetic source imaging, but lower specificity—meaning localisation must be interpreted with other tests rather than used alone.

MRI-negative epilepsy

ESI can be particularly valuable when structural MRI is negative or when several possible abnormalities exist.

It can help identify which cortical region should receive closer MRI review, PET analysis or invasive EEG coverage.

Planning SEEG

ESI can contribute to the hypothesis used to place stereoelectroencephalography electrodes.

It does not replace the clinical hypothesis or other investigations.

Concordance matters

Confidence increases when ESI agrees with seizure semiology, MRI, PET, MEG and other localisation data.

Discordance is not automatically an error—it may reveal propagation, multifocal disease or an incomplete hypothesis.

Limitations

Accuracy depends on electrode coverage, signal quality, the head model, the inverse algorithm and correct selection of epileptic events.

Deep generators and some basal or mesial structures are more difficult to localise from scalp EEG.

ESI is not a picture of the seizure focus

The displayed source is a mathematical estimate, not a direct image of abnormal tissue.

Its value comes from integration into the whole presurgical evaluation.

Worldwide access

EEG itself is much more widely available than MEG or PET, so ESI has potential as a comparatively scalable localisation tool.

However, high-density systems, validated software and specialist expertise remain unevenly available.

The central point

EEG source imaging extracts spatial information already hidden inside scalp EEG signals. It can sharpen a presurgical hypothesis, particularly in MRI-negative focal epilepsy, but should never be interpreted in isolation.

Sources

Evidence checked: September 2026.