Edited by: Hermann J. Mueller, University of Munich, Germany
Reviewed by: Toemme Noesselt, Otto-von-Guericke-University, Germany; Patrizia Fattori, University of Bologna, Italy
*Correspondence: Thomas Talbot, Laboratory of Cellular and Synaptic Neurophysiology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, 13 South Drive Room G360, Bethesda, MD 20892-5712, USA. e-mail:
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In order to examine a wide range of neuronal functions
Traditionally, a chamber is chronically affixed to the skull above the ROIs via surgical screws and acrylic dental cement. During the subsequent experimental sessions (e.g., electrophysiological recording/microstimulation/focal drug delivery), a removable grid, with holes parallel to the walls of the recording chamber, is used to guide one or multiple electrodes/cannulae through the craniotomy and the dura mater into the targeted tissue (see also Crist et al.,
There are, however, several disadvantages to this traditional approach. First, on the skull, the space for placing the chamber is limited. This limited space may be further reduced by other associated mechanical attachments, such as the headpost. As a result, it may not be possible to place the chamber at the most advantageous position, nor to have a single chamber be as large as the study might dictate (e.g., covering both hemispheres and both medial and lateral target areas). Second, in studies that require reaching the outermost lateral regions of the brain, the chamber must be implanted in a vicinal region. This necessitates larger muscle retractions in order to place the chamber, which increases the risk of collateral damage to the animal (e.g., damaging the temporalis muscle). Third, as in the case where a vessel passes through the top of the ROI, if using a straight grid, the investigators must either take the risk of hitting the vessel and damaging the brain tissue, or relocate the cannula/electrode a sufficient number of grid holes away to avoid the vessel, but possibly missing the critical ROI; clearly both of these solutions are less than ideal.
Although there are some angled guide grids commercially available, most of them have only one specified angle, which may allow the investigator to reach a single ROI but at the same time possibly preclude reaching other ROIs (e.g., when reaching the different ROIs requires different angles, as might occur with bilateral ROIs). For such cases, the investigator would have to do the experiment in serial fashion, changing to differently angled grids to access each ROI, prohibitively protracting the overall experimental time, and rendering simultaneous study of multi-ROI neuronal activities impossible. Furthermore, a single angle grid actually only shifts the reach of the chamber but cannot increase it. Actually, it will even decrease it in some cases because the wall of the chamber may block some grid holes from reaching the target.
In order to solve the problems described above, we pursued development of a novel angled guide grid system. This new guide grid system would permit each point of interest to be reached at the same time, irrespective of the angle(s) required to target that point. Additionally, it would permit a larger target area to be reached by the same size of chamber.
One adult male macaque monkey (
After a two-week recovery period, we inserted the traditional straight guide grid (52 × 25 × 10 mm, made of Ultem®) into the chamber and filled the chamber with gadolinium (Magnevist, Berlex Pharmaceuticals; 1:1200 dilution in sterile saline, pH 7.0–7.5) to illuminate the grids holes in the MR images (Figure
Five ROIs were selected from each hemisphere. Only the right side is shown for clarity (see Figure
For the example presented here, 10 points were determined by depth, distance right to left, and distance anterior to posterior referenced to the top center of the guide grid. Each point was assigned a color to aid in identification. A three-dimensional model of the guide grid, recording chamber, and targets of interest was created using the software SolidWorks® (Dassault Systemes SolidWorks, Concord: MA). Projection axes were created between each of the 10 target points and corresponding points on the surface of the grid. Sketch planes were created perpendicular to each axis at the depth of the target points (Figure
This varied angle guide grid was saved in the stereolithography “.STL” file format and printed using a Dimension Elite 3D printer (Stratasys, Eden Prairie: MN). This is a significant salient feature that rendered this method feasible as conventional machining of such a grid insert would have been prohibitively expensive. Three-dimensional printing is readily available and affordable as an outside service for those institutions that do not have their own printer.
Small guide grids, 5 mm by 5 mm by 10 mm were fabricated from Ultem® (Figure
The goal of our initial study was to assess targeting accuracy through this new type of grid. As described in Sections “Solid Modeling and Master Grid Fabrication” above, a three-dimensional model was designed, based on AFNI mapping of target regions, for the master varied angled grid with ten individual square cutouts to house the individual small Ultem® guide grids. Once this master grid was fabricated, and the small guide grids had been inserted, we placed the grid system into the chamber and filled the chamber with gadolinium (1:1200 dilution in sterile saline) to illuminate the grids holes in the MR images (Figure
In the present study, a new type of 3D printed grid insert system, which is capable of accommodating multiple angles simultaneously, was designed and proven to be a productive scheme for expanding the reach of the electrodes and/or injection cannulae aimed at sites deep in the brain. This type of grid insert system would have applications in a variety of experiments (see target areas in Hernández et al.,
An equally valid but different approach to the problem of simultaneous multi-angle targeting of multiple brain regions would be the use of an array of permanent indwelling individual guide cannulae (as available through Plastics One, Roanoke: VA, for instance). Such cannulae can be obtained in MR compatible materials (fused silica), allowing for anatomical scanning to validate target acquisition, as well as functional imaging experiments. One caveat, however, is that this method is far less flexible than a grid based system, as modification of the target acquisition may require additional intervals for placement surgeries. Both methods could be combined effectively of course, for targeting of tissues by trajectories originating from more caudal or lateral points. For example, such combination would retain the capacity for flexible, simultaneous bilateral targeting of ROIs in frontal as well as temporal cortices via this new multi-angle grid system, while hippocampal tissue could be approached longitudinally (see Hampton et al.,
Future work will test the feasibility of adapting this technique for use with recording electrodes. This will involve modification of a microdrive (Nichols et al.,
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We thank the NIF facility including Dr. Frank Ye and Charles Zhu for assistance with scanning. This project was fully funded by the Intramural Research Program of NIMH, NICHD, and NINDS/NIH/DHHS.