Practical video demonstration of the technique to identify the posterior superior insula orthogonal scalp projection without neuronavigation: the Fast-PSI

| BACKGROUND: The posterior-superior insula (PSI) has emerged as a potential target for non-invasive brain stimulation (NIBS) in the treatment of peripheral neuropathic pain. However, current methods for identifying the PSI require expensive and time-consuming brain imaging and neuronavigation. Here, we propose the Fast-PSI method, a novel approach based on craniometry and intracranial Euclidean distances proportions comparisons between scalp landmarks, to swiftly and accurately locate the PSI projection on the scalp. METHODS: Eleven healthy participants underwent identification of the PSI and cranial landmarks (CL) MNI152 coordinates using neuronavigation. Euclidean distances between nasion and PSI (N-sPSI), nasion and inion (N-I), vertex and PSI (Cz-sPSI), and vertex and tragus (Cz-T) were calculated. Craniometric-based Euclidean distances between PSI and CL were also measured. Correction factors were developed based on the proportionality of distances. RESULTS: Mean distances’ proportions were consistent between stereotactic-based and craniometric-based measurements. Correction factors were determined as 0.67 for N-sPSI and 0.75 for Cz-sPSI. The Fast-PSI formula was established using these factors to swiftly locate the PSI projection on the scalp. Test-retest, intra-and inter-rater reliability coefficients were high, with no statistical difference compared to neuronavigated coordinates. The mean time for Fast-PSI determination was significantly shorter than traditional neuronavigation. CONCLUSION: The Fast-PSI method demonstrates high precision and reliability in identifying the PSI projection for NIBS in neuropathic pain treatment. Its rapid execution and accuracy make it a promising alternative to current techniques, potentially reducing time and resource burdens associated with neuronavigation. Further validation in clinical trials and everyday practice is warranted to assess its utility in clinical settings.


Introduction
The PSI is a safe potential target for non-invasive brain stimulation (NIBS) in peripheral neuropathic pain (NeP) treatment. 1Currently, its identification requires costly and time-consuming brain imaging and neuronavigation. 2 Knowing intracranial structure proportions are generally constant 3,4 , using the standardization of the insular subdivisions' coordinates proposed by Faillenot et al. (2017) 5 and based on previous strategies to identify non-motor cortical neuromodulation targets 6 , we proposed the FAST-PSI. 6 have recently developed and validated the "Fast-PSI": a novel method to identify the orthogonal scalp projection of the posterior-superior insula (sPSI) based on craniometry and intracranial Euclidian distance proportions comparisons between scalp landmarks. 7
Fast-PSI: the intersection points between nasion-sPSI and vertex-sPSI lines.To validate the approach, two blinded raters assessed the Fast-PSI test-retest, intra-and inter-rater reliability coefficients in five different healthy volunteers (four women, 31.0 ± 4.0 years) (ten hemispheres' measurements).They were compared with the neuronavigated method and the mean time to perform the fast-PSI and the neuronavigation technique measured by a third researcher and compared.

The Fast-PSI web-based version
A Fast-PSI electronic version was developed on Rstudio Shiny Server: available at https://juliocesar9999apps.shinyapps.io/fast_psi_app/.
There was no statistical difference between the intra-rater and inter-rater measurements, and Fast-PSI and neuronavigated coordinates.Cronbach's alpha was 0.91.The mean time to identify the PSI by neuronavigation was 41.2 ± 2.59 minutes (without MRI time).The mean time for the "Fast-PSI" determination was 1.33 ± 0.04 minutes. 7e technique was compared to the neuronavigated location method, showing very good precision and a good high intra-and inter-rater reliability (see numbers in original paper).These results support the reliability and accuracy of the Fast-PSI to perform PSI rTMS. 7

Conclusion
Future studies comparing the utility of this framework in clinical trials and in daily practice will help in its establishment as a neuronavigation-free method to perform rTMS of the posterior insula.

Figure 1 .
Figure 1.Practical video demonstration Fast PSI