Defining the anterior nucleus of the thalamus (ANT) as a deep brain stimulation target in refractory epilepsy: Delineation using 3 T MRI and intraoperative microelectrode recording

Abstract

BackgroundDeep brain stimulation (DBS) is a minimally invasive and reversible method to treat an increasing number of neurological and psychiatric disorders, including epilepsy. Targeting poorly defined deep structures is based in large degree on stereotactic atlas information, which may be a major source of inconsistent treatment effects.Aim of the studyIn the present study, we aimed to study whether a recently approved target for epilepsy (anterior nucleus of thalamus, ANT) is visualized in clinically established 3 T MRI and whether ANT is delineated using intraoperative microelectrode recording (MER). We have especially focused on individual variation in the location of ANT in stereotactic space. We also aimed to demonstrate the role of individual variation in interpretation of MER data by projecting samples onto AC–PC (anterior and posterior commissure) and ANT-normalized coordinate systems.MethodsDetailed analysis of ANT delineations in 3 T MRI short tau inversion recovery (STIR) images from eight patients undergoing DBS for refractory epilepsy was performed. Coronal and sagittal cross-sectional models of ANT were plotted in the AC–PC coordinate system to study individual variation. A total of 186 MER samples collected from 10 DBS trajectories and 5 patients were analyzed, and the location of each sample was calculated and corrected accordingly to the location of the final DBS electrode and projected to the AC–PC or coordinate system normalized to ANT.ResultsMost of the key structures in the anatomic atlas around ANT (mammillothalamic tract and external medullary lamina) were identified in STIR images allowing visual delineation of ANT. We observed a high degree of anatomical variation in the location of ANT, and the cross-sectional areas overlapped by study patients decreased in a linear fashion with an increasing number of patients. MER information from 10 individual trajectories correlated with STIR signal characteristics by demonstrating a spike-negative zone, presumably white matter layer, at the lateral aspect of ANT in ANT-normalized coordinate system as predicted by STIR images. However, MER information projected to the AC–PC coordinate system was not able to delineate ANT.ConclusionsANT is delineated in 3 T MRI by visualization of a thin white matter lamina between ANT and other nuclear groups that lack spiking activity. Direct targeting in the anterior thalamic area is superior to indirect targeting due to extensive individual variation in the location of ANT. Without detailed imaging information, however, a single trajectory MER has little localizing value.

Outcome based definition of the anterior thalamic deep brain stimulation target in refractory epilepsy

“…Importantly, a high degree of anatomical variation has been reported in the location of ANT in stereotactic space in non-epileptic subjects [18]. We have recently confirmed this finding using 3T MRI in patients with epilepsy [19]. These data suggest that atlas based targeting may not be unquestionably feasible to ANT targeting.…”

“…The MRI sequences obtained for surgical planning included sagittal T1WI with thin slices for multiplanar reconstruction, axial T2WI, and STIR in all three orientations in majority of the patients [19]. All images were obtained using a 3T scanner (MAGNETOM Trio 3T, Siemens Healthcare Sector, Erlangen, Germany) with a 12-channel head matrix coil.…”

“…Contact locations were superimposed onto a sagittally oriented overlap model of left ANT delineations (n=8) based on 3T MRI STIR images described previously [19] to demonstrate the relationship between mean contact locations with respect to Schaltenbrant-Wahren atlas (SWA) [21] and 3T MRI based location of ANT.…”

“…These data suggest that atlas based targeting may not be unquestionably feasible to ANT targeting. Fortunately, recent developments in the imaging of the ANT suggest that ANT can be delineated using 3T MRI techniques visualizing the white matter structures enveloping ANT [19,20] enabling direct DBS targeting of ANT.…”

The Correlation between Intraoperative Microelectrode Recording and 3-Tesla MRI in Patients Undergoing ANT-DBS for Refractory Epilepsy

“…Reportedly, the ANT has marked anatomical variation between individuals in stereotactic space [10,11]. To overcome this anatomical variation, we decided to use the boundaries of the ANT in STIR images as the frame of the coordinate system instead of AC and PC.…”