[Paper Review] Complementary structural and functional abnormalities to localise epileptogenic tissue

Jonathan Horsley, Rhys H. Thomas+11 more|arXiv (Cornell University)|Apr 6, 2023

Advanced Neuroimaging Techniques and ApplicationsMedicine3 citations

TL;DR

This study demonstrates that combining diffusion-weighted MRI (dMRI)-inferred structural connectivity abnormalities with intracranial EEG (iEEG)-derived functional abnormalities significantly improves localization of the epileptogenic zone in drug-resistant focal epilepsy. Resection of maximal abnormalities from both modalities increased seizure freedom by 15-fold (p=0.008), and a combined decision tree correctly predicted surgical outcome in 84% of 43 patients, showing complementary value for pre-surgical planning.

ABSTRACT

When investigating suitability for surgery, people with drug-refractory focal epilepsy may have intracranial EEG (iEEG) electrodes implanted to localise seizure onset. Diffusion-weighted magnetic resonance imaging (dMRI) may be acquired to identify key white matter tracts for surgical avoidance. Here, we investigate whether structural connectivity abnormalities, inferred from dMRI, may be used in conjunction with functional iEEG abnormalities to aid localisation and resection of the epileptogenic zone (EZ), and improve surgical outcomes in epilepsy. We retrospectively investigated data from 43 patients with epilepsy who had surgery following iEEG. Twenty five patients (58%) were free from disabling seizures (ILAE 1 or 2) at one year. For all patients, T1-weighted and diffusion-weighted MRIs were acquired prior to iEEG implantation. Interictal iEEG functional, and dMRI structural connectivity abnormalities were quantified by comparison to a normative map and healthy controls respectively. First, we explored whether the resection of maximal (dMRI and iEEG) abnormalities related to improved surgical outcomes. Second, we investigated whether the modalities provided complementary information for improved prediction of surgical outcome. Third, we suggest how dMRI abnormalities may be useful to inform the placement of iEEG electrodes as part of the pre-surgical evaluation using a patient case study. Seizure freedom was 15 times more likely in those patients with resection of maximal dMRI and iEEG abnormalities (p=0.008). Both modalities were separately able to distinguish patient outcome groups and when combined, a decision tree correctly separated 36 out of 43 (84%) patients based on surgical outcome. Structural dMRI could be used in pre-surgical evaluations, particularly when localisation of the EZ is uncertain, to inform personalised iEEG implantation and resection.

Motivation & Objective

• To investigate whether structural connectivity abnormalities from dMRI complement functional iEEG abnormalities in localizing the epileptogenic zone (EZ) in drug-resistant focal epilepsy.
• To assess whether combining dMRI and iEEG abnormalities improves prediction of surgical outcomes compared to either modality alone.
• To explore the utility of dMRI in guiding personalized iEEG electrode placement during pre-surgical evaluation.

Proposed method

• Retrospective analysis of T1-weighted and diffusion-weighted MRI data from 43 epilepsy patients prior to iEEG implantation.
• Quantification of structural connectivity abnormalities using a normative dMRI map derived from healthy controls.
• Functional iEEG abnormalities were quantified using interictal iEEG data compared to a normative functional map.
• A decision tree model combined dMRI and iEEG abnormalities to predict surgical outcome (seizure freedom at one year).
• Resection extent of maximal dMRI and iEEG abnormalities was correlated with surgical outcome.
• A patient case study illustrated how dMRI abnormalities could inform iEEG electrode placement.

Experimental results

Research questions

• RQ1 Does resection of maximal structural (dMRI) and functional (iEEG) abnormalities improve surgical outcomes in epilepsy?
• RQ2 Do dMRI and iEEG abnormalities provide complementary information for predicting post-surgical seizure freedom?
• RQ3 Can dMRI-derived structural connectivity abnormalities guide optimal iEEG electrode placement in cases with uncertain EZ localization?

Key findings

• Seizure freedom was 15 times more likely in patients who underwent resection of maximal dMRI and iEEG abnormalities (p=0.008).
• Both dMRI and iEEG abnormalities separately distinguished between seizure-free and non-seizure-free outcome groups.
• A combined decision tree model correctly classified 36 out of 43 patients (84%) based on surgical outcome using both modalities.
• dMRI abnormalities provided useful, independent information for pre-surgical planning, especially when EZ localization was uncertain.
• The integration of dMRI and iEEG data improved prediction accuracy beyond either modality alone.
• The case study demonstrated that dMRI abnormalities could inform targeted iEEG electrode placement to better localize the epileptogenic zone.