Improved Calibration of TMS Coil
Developed with Dustin Martin and Bryan Wilcox
Written by Dustin Martin
The precision offered by frameless stereotactic systems makes them ideally suited to track the position of a transcranial magnetic stimulation (TMS) coil in 3-dimensional space and relative to the brain. Such a system, properly set up, can provide the researcher with exact location of interest, down to a fraction of a millimeter, when applying TMS. This precision is necessary when attempting to stimulate specific locations of, for example, the visual cortex, as a change of only a few millimeters in the position of the TMS coil can alter whether and where a TMS-induced effect is experienced.
The stereotactic system used in our lab (Brainsight 2 neuronavigation system developed by Rogue Research Inc) relies on reflection of infrared light from trackers attached to the TMS coil and to the subject; the system must be calibrated prior to each session. Calibration of the subject tracker (“registration”) relies on fiducials on the subject’s head, and is thus fairly consistent across multiple sessions. However, calibration of the coil uses a calibration block, on which the coil is balanced on three collinear posts. The location of the coil’s “hot spot” (in the case of the figure-eight coil, the point at which the two magnetic fields sum) is registered by placing it directly on the center post. As the coil obscures the view of the posts, this positioning must be done by estimation (the Brainsight manual recommends using a small drill bit to create a divot under the center of the coil, but we found that the divot loses precision through frequent use). Even if the coil’s “hot spot” is positioned perfectly on the center post of the calibration frame, the coil can still move in left-right direction because the sides of the coil are not stabilized by the current calibration block. Hence, because the precision of the system is dependent on the quality of the calibration, and because the location of brain to be stimulated needs to stay the same across days and across calibrations, we decided to slightly improve the calibration method. **We note that as of late 2012, Rogue Research Inc has also developed calibration aids to improve consistency across sessions. Please contact the company for details
We developed a frame which, when placed flush with the Brainsight calibration block, positions the center of the coil exactly over the center post during calibration. The coil rests on a platform enclosed on three sides, allowing the coil to be pressed against the sides as a reference point. The platform is suspended over the calibration block by two L-shaped upright pieces; these pieces are precisely measured to raise the coil and offset the platform so that the coil’s “hot spot” is exactly over the center post of the calibration block. When used during calibration, this frame ensures that the coil position is the same across sessions, virtually eliminating any deviation along the X, Y, and Z planes and ensuring that the coordinates of coil’s location across sessions are consistent.
Our frame was assembled from ¼” thick acrylic, cut using an Epilog Mini 24 laser cutter. The pieces were designed and laid out using computer-aided drafting (CAD) software. The frame consists of a platform [A] on which the coil rests during calibration, surrounded by one front [B] and two side [D] stops to provide stability for the coil. Ideally, the coil is held tight by all three sides; however, resting the coil against the front and one side stop will provide the same stability and consistent coil position during calibration. The platform is elevated above the calibration block by two upright blocks [C], which are precisely measured to position the coil’s “hot spot” over the center post of the calibration block. These uprights perform two functions: the long portion of the upright is measured to position the underside of the coil just over the center post on the calibration block (taking into account the thickness of the platform), while the short portion positions the platform such that the coil’s “hot spot” rests directly over the center calibration post. Finally, four platform brackets [E] and two rear brackets [F] add rigidity to the joint between the upright blocks and the platform and to the joint between the side stops and the platform, respectively. The design and dimensions of each of these components may be adjusted to complement different applications, calibration methods, TMS coils, stereotactic systems, or for use with other equipment that may need to be tracked. All the pieces also contain index markers and notches to ensure a precise, flush fit.
The majority of the parts were assembled using acrylic cement from TAP Plastics, a solvent adhesive that forms a strong bond between two pieces of acrylic. This method requires clamping together the two pieces to be joined at a right angle, with the adhesive applied along the seam. Capillary action pulls the adhesive into the joint, as the solvent properties of the adhesive “welds” the two pieces together. For a few joints, clamping was difficult or unnecessary, and so Super Glue (specifically designed for plastics) was used instead. The steps of assembly are detailed below.
- The side stops [D] are glued to the platform [A]. These are notched so that there is only one orientation in which they will fit.
- The rear brackets [F] are attached to add stability to the side stops. These will fit only one way, with the extended flat piece against the side stop.
- The platform brackets [E] are attached to the upright blocks [C]; if the uprights are attached to the platform first, the brackets will not fit. The brackets for one piece must also be attached in the opposite direction from the brackets on the other, otherwise they will not fit into the corresponding holes on the platform.
- Attach the upright blocks [C] to the platform [A]. Each of these should only fit in one position and orientation, with the index markers on the uprights fitting into the notches near the middle of the platform, and the brackets fitting into the holes in the platform surface. The vertical portion of the blocks should be at the front of the platform.
- Attach the front stop [B] to the platform [A]. This should fit flush against the platform and side stops, and the notches on the front stop should be aligned with the index markers on the platform and upright blocks.
- Because the calibration block used by the Brainsight 2 system has a fillet at the corner the frame pushes up against, the interior corner of each upright block must be filed slightly. Since the radius of the fillet is less than the width of the material, only a portion of each upright’s corner must be filed away. This allows the frame to fit flush with the calibration block and provides a precise and consistent calibration of the coil, vital to obtaining consistent coordinates of the coil’s location across multiple sessions.
Using the Calibration Frame
During use, the frame is pressed against the flat edges of the calibration block, such that the open portion of the coil platform is directly over the posts on the block. It is important that the frame is positioned flat against the block to minimize calibration errors. The coil is then placed on the platform and pressed against the front and at least one of the sides (if assembled properly, the coil should fit snugly between the sides of the platform); this should position the coil’s “hot spot” directly over the center post on the calibration block. Calibration is then performed using the Brainsight software.
From an engineering perceptive, the calibration frame design has room for improvement. However, given limited resources for specialized and precise tool production, we think this frame serves its function well. The current calibration procedure used with Brainsight 2 may be sufficient and work well in many settings, while the precision afforded by the calibration frame presented here may be relevant only to a small number of special cases, when TMS is applied to very small, focal areas of interest across multiple sessions. Regardless of the setting of TMS application — may it go smoothly!
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