7T versus 3T MRI in the presurgical evaluation of patients with drug-resistant epilepsy

Seizure-onset zone lateralization in temporal lobe epilepsy using 7T rs-fMRI: Direct comparison with 3T rs-fMRI

OBJECTIVE

Resting-state functional magnetic resonance imaging (rs-fMRI) at ultra-high field strengths (≥7T) is known to provide superior signal-to-noise to comparable acquisitions at lower field strengths. In this study, we provide a direct comparison of the seizure onset-zone (SOZ) lateralizing ability of 7T rs-fMRI and 3T rs-fMRI.

METHODS

We investigated a cohort of 70 patients with temporal lobe epilepsy (TLE). A paired cohort of 19 patients had 3T and 7T rs-fMRI acquisitions for direct comparison between the two field strengths. Forty-three patients had only 3T, and eight patients had only 7T rs-fMRI acquisitions. We quantified the functional connectivity between the hippocampus and other nodes within the default mode network (DMN) using seed-to-voxel connectivity, and measured how hippocampal-DMN connectivity could inform SOZ lateralization at 7T and 3T field strengths.

RESULTS

Differences in hippocampal-DMN connectivity ipsilateral and contralateral to the SOZ were significantly higher at 7T (Cohen's d = 0.51, p = 0.008) than at 3T (Cohen's d = 0.26, p = 0.68) when measured in the same subjects. We found that SOZ lateralization was superior at 7T (receiver-operating characteristic area under the curve [ROC AUC] = 0.97, 95% confidence interval [CI]: 0.92-1.00) than 3T (ROC AUC = 0.67, 95% CI: 0.36-0.98), for the same subjects scanned at both field strengths. Our findings were reproduced in extended cohorts of subjects scanned at either 3T or 7T. Our rs-fMRI findings at 7T, but not 3T, are consistent (Spearman ρ = 0.65, p = .01) with clinical fluorodeoxyglucose positron emission tomography (FDG-PET) lateralizing hypometabolism.

SIGNIFICANCE

We show superior SOZ lateralization in patients with TLE when using 7T relative to 3T rs-fMRI, supporting the adoption of high field strength functional imaging in the epilepsy presurgical evaluation.

Imaging the Internal Auditory Canal with an 8 × 2 Transceiver Array Head Coil at 7T

7T neuroimaging has known problems with B1 + strength, homogeneity and B0 susceptibility that make imaging in the inferior brain regions difficult. We investigated the utility of a decoupled 8 × 2 transceiver coil and shim insert to image the internal auditory canal (IAC) and inferior brain in comparison to the standard Nova 8/32 coil. B1 +, B0, and the T2 sampling perfection with application-optimized contrasts by using flip angle evolution sequence (SPACE) were compared by using research and standard methods in n = 8 healthy adults by using a Terra system. A T2 TSE was also acquired, and 2 neuroradiologists evaluated structures in and around the IAC, blinded to the acquisition, by using a 5-point Likert scale. The Nova 8/32 coil gave lower B1 + inferiorly compared with the whole brain while the transceiver maintained similar B1 + throughout. SPACE images showed that the transceiver performed significantly better, e.g., the transceiver scored 4.0 ± 0.8 in the left IAC, compared with 2.5 ± 0.8 with the Nova 8/32. With T2-weighted imaging that places a premium on refocusing pulses, these results show that with improved B1 + performance inferiorly, good visualization of the structure of the IAC and inferior brain regions is possible at 7T.

Proposal for local SAR safety margin in pediatric neuro-imaging using 7 T MRI and parallel transmission

Objective.Ultra-high field MRI with parallel transmission (pTx) provides a powerful neuroimaging tool with potential application in pediatrics. The use of pTx, however, necessitates a dedicated local specific absorption rate (SAR) management strategy, able to predict and monitor the peak local SAR (pSAR10g). In this work, we address the pSAR10gassessment for an in-house built 7 T 16Tx32Rx pediatric head coil, using the concept of virtual observation points (VOPs) for SAR estimation.Approach. We base our study on full-wave electromagnetic simulations performed on a database of 64 numerical anatomical head models of children aged between 4 and 16 years. We built VOPs on different subsets of this database ofN= 2 up to 30 models, and cross-validated the pSAR10gprediction using non-intersecting subsets, each containing 30 models. We thereby propose a minimum anatomical safety factor (ASF) to apply to the VOP set to enforce safety, despite intersubject variability. Our analysis relies on the computation of the worst case SAR to VOP-SAR ratio, independent of the pTx RF excitation.Main results.The interpolation model provides that the minimum ASF decreases as1+5.37⋅N-0.75withN. Using all 64 models to build VOPs leads to an estimated ASF of 1.24 when considering the VOP validity for an infinite number of subjects.Significance.We propose a general simulation workflow to guide ASF estimation and quantify the trade-off between the number of numerical models available for VOP construction and the safety factor. The approach would apply to any simulation dataset and any pTx setup.

Advances in MRI-based diagnosis of temporal lobe epilepsy: Correlating hippocampal subfield volumes with histopathology

Epilepsy, affecting 0.5%-1% of the global population, presents a significant challenge with 30% of patients resistant to medical treatment. Temporal lobe epilepsy, a common cause of medically refractory epilepsy, is often caused by hippocampal sclerosis (HS). HS can be divided further by subtype, as defined by the International League Against Epilepsy (ILAE). Type 1 HS, the most prevalent form (60%-80% of all cases), is characterized by cell loss and gliosis predominantly in the subfields cornu ammonis (CA1) and CA4. Type 2 HS features cell loss and gliosis primarily in the CA1 sector, and type 3 HS features cell loss and gliosis predominantly in the CA4 subfield. This literature review evaluates studies on hippocampal subfields, exploring whether observable atrophy patterns from in vivo and ex vivo magnetic resonance imaging (MRI) scans correlate with histopathological examinations with manual or automated segmentation techniques. Our findings suggest only ex vivo 1.5 Tesla (T) or 3T MRI with manual segmentation or in vivo 7T MRI with manual or automated segmentations can consistently correlate subfield patterns with histopathologically derived ILAE-HS subtypes. In conclusion, manual and automated segmentation methods offer advantages and limitations in diagnosing ILAE-HS subtypes, with ongoing research crucial for refining hippocampal subfield segmentation techniques and enhancing clinical applicability.

Flexible metasurface for improving brain imaging at 7T

PURPOSE

Ultra-high field MRI offers unprecedented detail for noninvasive visualization of the human brain. However, brain imaging is challenging at 7T due to the B  1 + $$ {}_1^{+} $$ field inhomogeneity, which results in signal intensity drops in temporal lobes and a bright region in the brain center. This study aims to evaluate using a metasurface to improve brain imaging at 7T and simplify the investigative workflow.

METHODS

Two flexible metasurfaces comprising a periodic structure of copper strips and parallel-plate capacitive elements printed on an ultra-thin substrate were optimized for brain imaging and implemented via PCB. We considered two setups: (1) two metasurfaces located near the temporal lobes and (2) one metasurface placed near the occipital lobe. The effect of metasurface placement on the transmit efficiency and specific absorption rate was evaluated via electromagnetic simulation studies with voxelized models. In addition, their impact on signal-to-noise ratio (SNR) and diagnostic image quality was assessed in vivo for two male and one female volunteers.

RESULTS

Placement of metasurfaces near the regions of interest led to an increase in homogeneity of the transmit field by 5% and 10.5% in the right temporal lobe and occipital lobe for a male subject, respectively. SAR efficiency values changed insignificantly, dropping by less than 8% for all investigated setups. In vivo studies also confirmed the numerically predicted improvement in field distribution and receive sensitivity in the desired ROI.

CONCLUSION

Optimized metasurfaces enable homogenizing transmit field distribution in the brain at 7T. The proposed lightweight and flexible structure can potentially provide MR examination with higher diagnostic value images.

The impact of referring patients with drug-resistant focal epilepsy to an epilepsy center for presurgical diagnosis

BACKGROUND

Epilepsy surgery is an established treatment for drug-resistant focal epilepsy (DRFE) that results in seizure freedom in about 60% of patients. Correctly identifying an epileptogenic lesion in magnetic resonance imaging (MRI) is challenging but highly relevant since it improves the likelihood of being referred for presurgical diagnosis. The epileptogenic lesion's etiology directly relates to the surgical intervention's indication and outcome. Therefore, it is vital to correctly identify epileptogenic lesions and their etiology presurgically.

METHODS

We compared the final histopathological diagnoses of all patients with DRFE undergoing epilepsy surgery at our center between 2015 and 2021 with their MRI diagnoses before and after presurgical diagnosis at our epilepsy center, including MRI evaluations by expert epilepsy neuroradiologists. Additionally, we analyzed the outcome of different subgroups.

RESULTS

This study included 132 patients. The discordance between histopathology and MRI diagnoses significantly decreased from 61.3% for non-expert MRI evaluations (NEMRIs) to 22.1% for epilepsy center MRI evaluations (ECMRIs; p < 0.0001). The MRI-sensitivity improved significantly from 68.6% for NEMRIs to 97.7% for ECMRIs (p < 0.0001). Identifying focal cortical dysplasia (FCD) and amygdala dysplasia was the most challenging for both subgroups. 65.5% of patients with negative NEMRI were seizure-free 12 months postoperatively, no patient with negative ECMRI achieved seizure-freedom. The mean duration of epilepsy until surgical intervention was 13.6 years in patients with an initial negative NEMRI and 9.5 years in patients with a recognized lesion in NEMRI.

CONCLUSIONS

This study provides evidence that for patients with DRFE-especially those with initial negative findings in a non-expert MRI-an early consultation at an epilepsy center, including an ECMRI, is important for identifying candidates for epilepsy surgery. NEMRI-negative findings preoperatively do not preclude seizure freedom postoperatively. Therefore, patients with DRFE that remain MRI-negative after initial NEMRI should be referred to an epilepsy center for presurgical evaluation. Nonreferral based on NEMRI negativity may harm such patients and delay surgical intervention. However, ECMRI-negative patients have a reduced chance of becoming seizure-free after epilepsy surgery. Further improvements in MRI technique and evaluation are needed and should be directed towards improving sensitivity for FCDs and amygdala dysplasias.

Improved Seizure Onset-Zone Lateralization in Temporal Lobe Epilepsy using 7T Resting-State fMRI: A Direct Comparison with 3T

Objective

Resting-state functional magnetic resonance imaging (rs-fMRI) at ultra high-field strengths (≥7T) is known to provide superior signal-to-noise and statistical power than comparable acquisitions at lower field strengths. In this study, we aim to provide a direct comparison of the seizure onset-zone (SOZ) lateralizing ability of 7T rs-fMRI and 3T rs-fMRI.

Methods

We investigated a cohort of 70 temporal lobe epilepsy (TLE) patients. A paired cohort of 19 patients had 3T and 7T rs-fMRI acquisitions for direct comparison between the two field strengths. Forty-three patients had only 3T, and 8 patients had only 7T rs-fMRI acquisitions. We quantified the functional connectivity between the hippocampus and other nodes within the default mode network (DMN) using seed-to-voxel connectivity, and measured how hippocampo-DMN connectivity could inform SOZ lateralization at 7T and 3T field strengths.

Results

Differences between hippocampo-DMN connectivity ipsilateral and contralateral to the SOZ were significantly higher at 7T (pFDR=0.008) than at 3T (pFDR=0.80) when measured in the same subjects. We found that our ability to lateralize the SOZ, by distinguishing subjects with left TLE from subjects with right TLE, was superior at 7T (AUC = 0.97) than 3T (AUC = 0.68). Our findings were reproduced in extended cohorts of subjects scanned at either 3T or 7T. Our rs-fMRI findings at 7T, but not 3T, are consistent and highly correlated (Spearman Rho=0.65) with clinical FDG-PET lateralizing hypometabolism.

Significance

We show superior SOZ lateralization in TLE patients when using 7T relative to 3T rs-fMRI, supporting the adoption of high-field strength functional imaging in the epilepsy presurgical evaluation.

Pearls & Oy-sters: Harnessing New Diagnostic and Therapeutic Approaches to Treat a Patient With Genetic Drug-Resistant Focal Epilepsy

Focal cortical dysplasia (FCD) is a congenital developmental malformation and is one of the leading causes of drug-resistant focal epilepsy (DRFE). Although focal epilepsies traditionally have been regarded as acquired disorders, increasing evidence suggests a substantial genetic contribution to the pathogenesis of focal structural epilepsies, including FCDs. Variations in the Dishevelled, Egl-10, and domain-containing protein 5 (DEPDC5) have recently emerged as a causative gene mutation in familial focal epilepsies associated with FCD type 2a, including bottom-of-sulcus dysplasia (BOSD). We present the case of a 20-year-old man with DRFE, positive for DEPDC5 c.1555C>T (p.GIn519*) heterozygous pathogenic variant. Initial 3T brain MRI was unrevealing, but subsequent 7T MRI including 7T edge-enhancing gradient echo revealed a left superior frontal sulcus BOSD concordant with the electroclinical data. The patient underwent treatment with MR-guided laser interstitial thermal ablation of the left frontal BOSD without intracranial EEG monitoring (skipped candidate), resulting in a seizure-free outcome of 9 months since the last follow-up. Our case highlights the real-world application of summative information obtained through advancements in epilepsy genetic testing, minimally invasive surgeries, and ultra-high field MRI, allowing us to provide a safe and effective treatment for a patient with a genetic DRFE.

Recent advances in epilepsy surgery

PURPOSE OF REVIEW

Technological innovations in the preoperative evaluation, surgical techniques and outcome prediction in epilepsy surgery have grown exponentially over the last decade. This review highlights and emphasizes relevant updates in techniques and diagnostic tools, discussing their context within standard practice at comprehensive epilepsy centres.

RECENT FINDINGS

High-resolution structural imaging has set an unprecedented opportunity to detect previously unrecognized subtle abnormalities. Machine learning and computer science are impacting the methodologies to analyse presurgical and surgical outcome data, building more accurate prediction models to tailor treatment strategies. Robotic-assisted placement of depth electrodes has increased the safety and ability to sample epileptogenic nodes within deep structures, improving our understanding of the seizure networks in drug-resistant epilepsy. The current available minimally invasive techniques are reasonable surgical alternatives to ablate or disrupt epileptogenic regions, although their sustained efficacy is still an active area of research.

SUMMARY

Epilepsy surgery is still underutilized worldwide. Every patient who continues with seizures despite adequate trials of two well selected and tolerated antiseizure medications should be evaluated for surgical candidacy. Collaboration between academic epilepsy centres is of paramount importance to answer long-standing questions in epilepsy surgery regarding the understanding of spatio-temporal dynamics in epileptogenic networks and its impact on surgical outcomes.