Week 4 - Charlie

This week has been my least productive of the summer, owing to a fever that incapacitated me on Monday and Tuesday alongside the 4th of July yesterday. That said, I still have made some progress in confirming the validity of my TMS-mapping software.

Until now, I had been running with the assumption that the distance from the magnetic coil to the brain (the "scalp-cortex" distance) was solidly 14.77mm at all areas along the scalp. I am throwing away this assumption. I took the MNI coordinates of the estimated stimulated areas of the brain generated by my software and compared them to a few websites that visualize these coordinates on an average brain model. I found that the area I assumed we were stimulated was actually about 14mm above the top of the brain! My guess at the cause of this error was my implicit assumption that all magnetic pulses would be aimed directly at the brain, which is definitely not true. In fact, most of the pulses are delivered at an angle, which I calculated to be about 16 degrees. Even accounting for this deviation, however, the gap was too large. After playing with the numbers, I found that the distance from coil to brain is around 28mm, assuming that the coordinates from my other sources are accurate. I expect that the extra distance originates from the patient's hair and the cap that they wear during the stimulation.

I also used the coordinates generated by my software to see what areas of the brain we are actually stimulating. According to a paper I found that maps out various areas of the brain observed with fMRI in MNI coordinates, we appear to be activating the mesial premotor cortex (MPMC). Brain anatomy not being my forte, I need to check with Dr. Marei or Dr. Lange to see if this is indeed the spot we want to hit.

The spot I believe we have stimulated in one of our patients. The MPMC is highlighted in red.

It's exciting to connect this collection of dots on a grid to real human anatomy. I can't wait to reach some conclusions next week!