Electrophysiologic Validation of Diffusion Tensor Imaging Tractography during Deep Brain Stimulation Surgery

Abstract

Background and purpose: Diffusion tensor imaging fiber tractography-assisted planning of deep brain stimulation is an emerging technology. We investigated its accuracy by using electrophysiology under clinical conditions. We hypothesized that a level of concordance between electrophysiology and DTI fiber tractography can be reached, comparable with published modeling approaches for deep brain stimulation surgery.

Materials and methods: Eleven patients underwent subthalamic nucleus deep brain stimulation. DTI scans and high-resolution T1- and T2-weighted MR imaging was performed at 3T. Corticospinal tracts were traced. We studied electrode positions and current amplitudes that elicited corticospinal tract effects during the operation to determine relative corticospinal tract distance. Postoperatively, 3D deep brain stimulation electrode contact locations and stimulation patterns were applied for the same corticospinal tract distance estimation.

Results: Intraoperative electrophysiologic (n = 40) clinical effects in 11 patients were detected. The mean intraoperative electrophysiologic corticospinal tract distance was 3.0 ± 0.6 mm; the mean image-derived corticospinal tract distance (DTI fiber tractography) was 3.0 ± 1.3 mm. The 95% limits of agreement were ±2.4 mm. Postoperative electrophysiology (n = 44) corticospinal tract activation effects were encountered in 9 patients; 39 were further evaluated. Mean electrophysiologic corticospinal tract distance was 3.7 ± 0.7 mm; for DTI fiber tractography, it was 3.2 ± 1.9 mm. The 95% limits of agreement were ±2.5 mm.

Conclusions: DTI fiber tractography depicted the medial corticospinal tract border with proved concordance. Although the overall range of measurements was relatively small and variance was high, we believe that further use of DTI fiber tractography to assist deep brain stimulation procedures is advisable if inherent limitations are respected. These results confirm our previously published electric field simulation studies.

© 2016 by American Journal of Neuroradiology.

Figures

Fig 1.

Flow chart of the procedures. Post OP indicates postoperative; preOP, preoperative; DOF, deformation; EPio, intraoperative electrophysiology; EPpo, postoperative electrophysiology; Navi, Navigation (Sequence); pre, before; post, after; VAT, volume of activated tissue; highres, high-resolution.

Fig 2.

3D renditions of the corticospinal tract. A, Depiction of a left CST (red) in the fiber-tracking software (StealthViz DTI; Medtronic) but already depicted as a DICOM hull structure. B, Bilateral visualization of the transferred DICOM structure in the planning software (FrameLink 5.0; Medtronic Surgical Navigation). Blue probe simulations indicate intraoperatively tested electrode positions (test el.). PG indicates precentral gyrus; test el. (sim.), simulated test electrode position.

Fig 3.

Evaluation based on intraoperative electrophysiology: corticospinal tract depiction in axial, coronal, and sagittal (A–C) planes. A, Red dot indicates post hoc simulation of the intraoperative position of the test electrode in the planning software (Framelink 5.0; Medtronic Surgical Navigation) according to microTargeting Drive settings. In this example, 5 mA of intraoperative stimulation resulted in “gaze palsy” as capsular effect. The shortest spatial distance to the medial CST border of 5.5 mm is indicated with a blue circle. Both coordinates (electrode tip, medial border of CST) were recorded and later plotted (Fig 4). Note that the CST is located posterior and lateral relative to the positon of the electrode (A).

Fig 4.

Postoperative electrophysiologic evaluation by using CT depiction of the DBS electrode artifacts. The 3D helical postoperative CT is superimposed on the planning data. Reconstruction along the main DBS electrode (white) axis, quasiaxial (A) and coronal (B). The minimal spatial distance to the medial border of the CST is 4.0 mm. B, The DBS electrode (inset; geometry; DBS lead model 3389; Medtronic) is seen as a white structure in the STN region. In this example, electrode contact 0 (EC0, deepest contact, 2.3 mm from the electrode tip) elicited capsular effects during postoperative clinical testing. C, 3D rendering of the right (rt) and left (lt) CSTs with DBS electrode artifacts from helical CT.

Fig 5.

Graphic depiction of the EPio experiment results. Atlas templates in axial (A, 6.2 mm below the midcommissural point [MCP]) and coronal sections (B, 2 mm behind the MCP) (idealized according to Schaltenbrand and Wahren). Intraoperative electrode positions are represented by black dots. Black lines represent the shortest distance in space to the CST as depicted with the DTI technology. Blue dots show the individual CST penetration in space. Shaded circles indicate estimated volumes of activated tissue around a test electrode, specific to the current that was applied to elicit an electrophysiologic CST response according to Ranck (1975), (Fig 1). CST (blue dots) corresponds nicely with electric field borders, indicating that medial CST definition with DTI reliably predicts the CST border as measured with electrophysiology. RN indicates red nucleus. (Of note in A, the CST is always located posterior and lateral to the STN region.)

Fig 6.

A, Bland-Altman plot, intraoperative measurements. Gray labels indicate individual patients. B, Bland-Altman plot, postoperative measurements. Gray labels indicate individual patients.