Localization of neurosurgically implanted electrodes via photograph-MRI-radiograph coregistration

Abstract

Intracranial electroencephalography (iEEG) is clinically indicated for medically refractory epilepsy and is a promising approach for developing neural prosthetics. These recordings also provide valuable data for cognitive neuroscience research. Accurate localization of iEEG electrodes is essential for evaluating specific brain regions underlying the electrodes that indicate normal or pathological activity, as well as for relating research findings to neuroimaging and lesion studies. However, electrodes are frequently tucked underneath the edge of a craniotomy, inserted via a burr hole, or placed deep within the brain, where their locations cannot be verified visually or with neuronavigational systems. We show that one existing method, registration of postimplant computed tomography (CT) with preoperative magnetic resonance imaging (MRI), can result in errors exceeding 1cm. We present a novel method for localizing iEEG electrodes using routinely acquired surgical photographs, X-ray radiographs, and magnetic resonance imaging scans. Known control points are used to compute projective transforms that link the different image sets, ultimately allowing hidden electrodes to be localized, in addition to refining the location of manually registered visible electrodes. As the technique does not require any calibration between the different image modalities, it can be applied to existing image databases. The final result is a set of electrode positions on the patient's rendered MRI yielding locations relative to sulcal and gyral landmarks on individual anatomy, as well as MNI coordinates. We demonstrate the results of our method in eight epilepsy patients implanted with electrode grids spanning the left hemisphere.

Fig. 1

Complete procedure for registration of electrodes from photographs, MRI, and X-ray. (a) Photograph showing position of grid implant. Cables entering the bottom left and bottom right corners of the craniotomy connect to electrode strips. (b) Photograph taken immediately before grid placement clearly showing anatomical features. A projective transform with the photograph in (a) is computed with several manually selected point correspondences, such as blood vessel features, craniotomy edges, and stationary surgical hardware. The transform were then applied to electrode positions from (a) and superimposed. (c) MRI-based brain rendering with electrode positions transcribed from the photographs. (d) Lateral radiograph showing electrode positions (dark dots) along with electrode cabling. The green circles outline the electrodes with 3D locations known from the previous photograph-MRI registration step; these two sets of coordinates were used as control pairs to generate the MRI-radiograph projective transform. (e) The projective transform can be used to compute the location of the X-ray source and trace the path of X-rays that generated the image for each electrode location. Surface electrodes are known to be at the intersection of these rays with the cortical surface. (f) The final rendering showing electrode positions confirmed by the photograph (in white) and those determined solely by backprojection (grid electrodes in blue, strip electrodes in yellow). Note that the strip electrodes A1–A4 and B1–B4 are difficult to see at the contrast level chosen for the shown radiograph.

Fig. 2

Interactive navigation is possible with the photograph, MRI, and radiograph linked via projective transforms. The crosshairs on each image indicate the same brain location, in this case, the inferior tip of a large parietal encephalomalacia in GP4.

Fig. 3

The photographs, radiographs, and final renderings for all 8 of the included patients

Fig. 4

Evaluation of registration for the same patient shown in Figure 1 (GP6). Known electrode positions from the photograph are shown as red dots, and the results from using the projective transform with all known electrode positions are depicted in blue. Since it is not possible to validate the location of hidden electrodes, a subset of six visible electrodes (boxed in yellow) were chosen to compute another projective transform. This transform was then applied to all X-ray locations, with the backprojection solution shown in yellow. The results from just these six electrodes agree closely with the known photograph-based positions as well as the results from the complete projective transform (blue dots).

Fig. 5

Comparison of preoperative MRI and postimplant CT in two patients. First column: Coregistration of postimplant CT (in color) with preoperative MRI (grayscale); even though registration of the skull, scalp, and right hemisphere is good, many surface electrodes on the coregistered CT land more than 10 mm deep on the MRI in both patients. Second column: Another slice of the CT scan adjusted for contrast to visualize the ventricles. Third column: The corresponding MRI slice. Fourth column: The same MR (color) slice overlaid on the CT (grayscale); note the displacement of the whole brain, including a clear shift of the ventricle and midline, indicating significant brain deformation.

Fig. 6

(a) Comparison of electrode registration with the proposed technique and from CT-MRI coregistration for GP1. The red splotches are the CT-derived positions; their shape is determined from intensity-based segmentation of the CT. The translucent white disks indicate positions known from the surgical photographs, while the yellow disks indicate positions backprojected from the X-ray. The CT-derived positions are considerably displaced posteriorly relative to the photograph-verified positions. (b) Surgical photograph for this patient. (c) and (d) show similar images for GP2.