Use of an intracranial near-infrared probe for localization during stereotactic surgery for movement disorders

Technical note

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✓ Localization of targets during stereotactic surgery is frequently accomplished by identification of the boundaries between the gray matter of various nuclei and the surrounding white matter. The authors describe an intracranial probe developed for this purpose, which uses near-infrared (NIR) light.

The probe fits through standard stereotactic holders and emits light at its tip. The scattered light is detected and analyzed by a spectrometer, with the slope of the trailing portion of the reflectance curve used as the measurement value.

Near-infrared readings were obtained during 27 neurosurgical procedures. The first three operations were temporal lobectomies, with values obtained from tracks in the resected specimen and resection bed. In the next five procedures, the probe was inserted stereotactically to a depth of 1 to 2 cm with measurements obtained every 1 mm. The probe was then used in 19 stereotactic procedures for movement disorders, obtaining measurements every 0.5 to 1 mm to target depths of 6 to 8 cm to interrogate subcortical structures. The NIR signals were correlated to distances beneath the cortical surface measured on postoperative computerized tomography or magnetic resonance imaging by using angle correction and three-dimensional reconstruction techniques.

The NIR values for white and gray matter obtained during the lobectomies were significantly different (white matter 2.5 ± 0.37, gray matter 0.82 ± 0.23 mean ± standard deviation). The NIR values from the superficial stereotactic tracks showed initial low values corresponding to cortical gray matter and high values corresponding to subcortical white matter.

There was good correlation between the NIR signals and postoperative imaging in the 19 stereotactic cases. Dips due to adjacent sulci, a plateau of high signal due to subcortical white matter, a dip in the NIR signal during passage through the ventricle, dips due to the caudate nucleus, and peaks due to the white matter capsule between ventricle and thalamus were constant features. The putamen—capsule boundary and the lamina externa and interna of the globus pallidus could be distinguished in three cases. Elevated signals corresponding to the thalamic floor were seen in 10 cases. Nuances such as prior lesions and nonspecific white matter changes were also detected. There was no incidence of morbidity associated with use of the probe. Data acquisition was straightforward and the equipment required for the studies was inexpensive.

The NIR probe described in this article seems to be able to detect gray—white matter boundaries around and within subcortical structures commonly encountered in stereotactic functional neurosurgery. This simple, inexpensive method deserves further study to establish its efficacy for stereotactic localization.

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Figures

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    Artist's depiction of an NIR probe in the brain. The actual illuminated area is smaller.

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    Graph depicting reflectance plotted against wavelength in white and gray matter. Each single measurement from the NIR probe yields a graph of reflectance compared with wavelength. The arrows indicate the slopes of the trailing segments used as the numerical representation of the corresponding graph.

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    Data obtained in a patient undergoing thalamotomy. Upper: Postoperative MR image revealing the probe track and the thalamic lesion. Lower: Graph of reflectance plotted against depth beneath the brain surface from the same track. Greater values of reflectance indicate white matter and smaller values indicate gray matter. The sites indicated by letters on the MR image are at depths indicated by the same letters on the graph. Subcortical white matter is seen as a “plateau” until a depth of 36 mm. The ventricle (V) is seen as a sharp dip in the reflectance curve, with adjacency to the caudate nucleus (C), which is seen as a mild reflectance increase from CSF levels. The white matter lamina (L) between the ventricle and thalamus is seen as a peak, as is the white matter comprising the thalamic floor (F). Depth (x axis) is measured in millimeters and reflectance (y axis) is measured in arbitrary normalized units. This also applies to the lower portions of Figs. 4 through 7.

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    Data obtained in a patient undergoing pallidotomy. Upper: Postoperative MR image revealing the probe track and the pallidal lesion. Lower: Graph of reflectance plotted against depth. The cortical gray—white matter junction (GW) is seen as a sharp rise in reflectance, beginning a plateau corresponding to subcortical white matter. The track passes by the caudate nucleus (C), seen as a dip in the reflectance curve at the corresponding depth. The boundary between the internal capsule and the putamen (P) is seen as a dip in the reflectance, and the lamina (L) between the putamen and the GP is seen as a peak (compare with the contralateral side in the MR image [upper]).

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    Data obtained in a patient undergoing pallidotomy. Upper: Four postoperative coronal MR images. The track to the lesion is seen as well as a track corresponding to the second pass of the NIR probe used to delineate the lateral margin and floor of the GP. Lower: Graph showing reflectance plotted against depth. The initial 21 mm shows low reflectance due to CSF in the adjacent sulcus. The junction of cortical gray matter and subcortical white matter is marked (W). Just deep to the subcortical white matter plateau is a peak (L) corresponding to the adjacency of prior stereotactic lesions (asterisk in upper panel) placed at another institution. The white matter pallidal floor (F) is also seen as a peak.

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    Data obtained in a patient undergoing pallidotomy. Upper: Postoperative MR image revealing the probe track and the pallidal lesion. Lower: Graph depicting reflectance plotted against depth. Note the low reflectance values at 1 to 16 mm superficial to the beginning of subcortical white matter (W), due to CSF in the sulcus just medial to the probe track. A downward trough is due to adjacency of the tip of the sulcus (S) lateral to the track. Also seen is a dip corresponding to the boundary between the internal capsule and the putamen (P), a peak corresponding to the lamina between the putamen and the GP (L1), and a peak corresponding to the lamina inside the GP (L2).

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    Data obtained in a patient undergoing placement of a thalamic DBS. Upper: Preoperative MR image demonstrating a thin white matter lamina between the ventricle and thalamus (arrow). Lower: Graph showing reflectance plotted against depth. A subcortical white matter plateau is again seen as well as a sharp dip corresponding to the ventricle (V). Arrow indicates a peak at the depth of the thin lamina, indicated on the MR image.

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