A new, open-source 3D-printed transcranial magnetic stimulation (TMS) coil tracker holder for double blind, sham-controlled neuronavigation studies

To the Editor

Transcranial magnetic stimulation (TMS) is a form of noninvasive neuromodulation that can be combined with neuronavigation to target specific brain regions and/or ensure reliable application to a given brain region. Current neuronavigation systems use a coil tracker holder that attaches around the handle of a TMS coil, and a coil tracker that inserts into the slot of the coil tracker holder (Figure 1A). This commercially available TMS coil tracker holder design is one-sided, meaning it contains a slot for a coil tracker holder on only one side. Thus, to flip between two sides of the coil, the TMS operator must unscrew/flip the coil tracker holder and recalibrate the coil (Figure 1A and 1C). This recalibration procedure can also cause unblinding of the TMS operator, particularly in new high-dose protocols that deliver multiple sessions per day that can include both active and sham stimulation within an individual (1–3). In this study, we invented an open-source, two-sided, 3D-printed TMS coil tracker holder that has a slot for a TMS coil tracker on both sides (Figure 1B). This two-sided coil tracker holder allows the user to quickly and easily switch between coil sides by flipping the TMS coil and inserting the coil tracker into the slot on the opposite side (Figure 1B and 1C). Notably, moving the coil tracker holder or recalibration is not necessary with this new design. We predicted that this 3D-printed coil tracker holder would be significantly easier and faster to use while remaining just as accurate as a commercially available TMS coil tracker holder requiring separate recalibration on each side.

Figure.

1A: Picture of a commercial, one-sided TMS coil tracker holder + coil tracker. 1B: Picture of our 3D-printed, two-sided TMS coil tracker holder + coil tracker 1C: Overview of the recalibration procedure between coil sides using a commercial, one-sided TMS coil tracker holder vs. our 3D-printed, two-sided TMS coil tracker holder. While it takes 3 steps and approximately 6–7 minutes to switch between sides using the commercial TMS coil tracker holder, it takes just one step and approximately 5 seconds for our 3D-printed coil tracker holder. Note: Double-blind TMS coils look the same on each side; the colors used here are to illustrate the difference between Sides A and B on the coil. 1D: Accuracy differed between TMS coils/coil tracker holders but there was no significant interaction between coil tracker and side of coil. 1E: Participants took significantly less time to flip the 3D-printed coil tracker holder (***p < 0.001). 1F: Participants rated the 3D-printed coil tracker holder as significantly easier to use (***p < 0.001).

We recruited 11 TMS-trained operator participants (5 women, average TMS experience = 20.1 months, range = 1–72 months; average neuronavigation experience = 15.0 months, range = 0–72 months) for this study who were blinded to the purpose of the experiment. Prior to operator participants entering the room, we registered a confederate’s head to the 6^th Generation MNI-152 template brain in the Brainsight neuronavigation system (Rogue Research; Montreal, Canada) and calibrated two TMS coils (a double-blind capable MagVenture Cool-B65 A/P coil with our 3D-printed TMS coil tracker holder and MagVenture C-B60 coil with the commercially available TMS coil tracker holder). Two different TMS coils were used due to the commercial TMS coil tracker holder not fitting on the MagVenture Cool-B65 A/P coil.

To test accuracy, participants were asked to hold each TMS coil at a location that we marked on the confederate’s swim cap for left dorsolateral prefrontal cortex (identified via 10–20 EEG probabilistic placement to F3). Notably, operator participants were not allowed to look at the neuronavigation computer monitor as the distance between the initial target and position (in millimeters) was used to assess accuracy. Operator participants held the first TMS coil over the F3 spot and we sampled this location. Next, participants were asked to move the TMS coil at least 1 meter on and off F3 5 times, taking 5 or more seconds between each measurement.

To test time intensiveness, participants flipped the TMS coil to mimic a double-blind protocol requiring the TMS coil to be flipped. The methodology differed between the two coil trackers. With the commercial tracker, participants performed a conventional, 3 step recalibration procedure. First, participants were instructed to: 1) unscrew the coil tracker holder 2) flip the coil and screw the coil tracker holder onto the opposite coil side, 3) recalibrate the coil (Figure 1C). With the 3D-printed tracker, participants were instructed to perform a 1 step recalibration procedure. Participants were asked to: 1) take the coil tracker out, flip the coil, and reinsert the coil tracker into the other, symmetrically placed slot (Figure 1C). Notably, the 3D-printed tracker is two-sided and has a slot on each side, precluding the need for unscrewing the coil tracker holder, flipping the coil tracker holder, or recalibrating the coil. In both cases, we recorded the amount of time it took to flip and recalibrate the coil. After flipping the coil, we again recorded the distance between each target and the initial sampled F3 location 5 times.

Operator participants each performed the protocol for both TMS coils in a counterbalanced order (6 used the 3D-printed coil tracker holder first). Following the experiment, participants filled out a questionnaire that assessed the ease of use between each coil tracker holder on scales of 0 (easiest) to 10 (hardest). We used repeated-measures ANOVAs in SPSS 25.0 (Armonk, NY, IBM Corp.) to assess the differences in accuracy, time to flip the coil, and ease of use between TMS coil tracker holders.

Accuracy: There was a significant main effect of coil type (F,1,10) = 11.18, p = 0.007, ɳ_p² = 0.528), showing that our 3D-printed coil tracker holder on a MagVenture Cool-B65 A/P coil was more accurate than the commercial coil tracker holder on a MagVenture C-B60 coil (Figure 1D). However, there was no main effect of coil side, F(1,10) = 4.26, p = 0.066, ɳ_p² = 0.299, and no interaction between coil type and side. F(1,10) = 2.49, p = 0.146, ɳ_p² = 0.199. Thus, while our 3D-printed coil tracker holder was more accurate, conclusions are limited by the confound of needing to use two different coils in addition to the two coil tracker holders.

Time Intensiveness: Participants flipped our 3D-printed coil tracker significantly faster than flipping and recalibrating the commercially available coil tracker, F(1,10) = 126.4, p < 0.001, ɳ_p² = 0.927 (Figure 1E). Participants took an average of 4.75 seconds to flip our 3D-printed coil tracker (SEM = 0.47 seconds, range = 2.3–7.3 seconds) compared to 384.5 seconds (SEM = 33.9 seconds, range = 228–610 seconds) for the commercial TMS coil tracker holder.

Ease of Use: Participants rated our 3D-printed coil tracker holder as significantly easier to use, F(1,10) = 119.7, p < 0.001, ɳ_p² = 0.923 (Figure 1F). On a scale from 0 (easiest to use) to 10 (hardest to use), participants rated our 3D-printed tracker an average of 1.20 (SEM = 0.41, range = 0–4.4) and the commercial coil tracker with recalibration an average of 8.05 (SEM = 0.32, range = 6.3–10).

In conclusion, using our 3D-printed coil tracker holder to switch between two sides of a double-blind TMS coil is faster and easier, while just as accurate, as a commonly used commercially available coil tracker holder that requires recalibration between coil sides. Studies, in particular, that necessitate 1) precise and 2) reliable stimulation and blinding of both 3) participant and 4) operator in the context of delivery protocols that may include sham and active stimulation in the same day and/or protocol may benefit from utilizing this 3D-printed coil tracker holder. This 3D-printed tracker holder may also be useful for TMS operators who are unfamiliar with neuronavigation as it precludes the need for recalibration between coil sides. Our 3D-printed coil tracker holder is open-source and available online for free (https://www.thingiverse.com/thing:4077389).