Adding electrodes to the Grid changes simulation results

[tbone]

Hello,

I was following the Tutorial:
F.Temporal Interference Stimulation with Ohmic Electro Quasi-Static Solver with a Personalized Head Model

Here I've noticed for the 'Grid' settings:

• The bounding box (or wire block?) for the head was used for manual gridding with 0.7 mm resolution
• Automatic Grid settings were set for the 4 TI electrodes

I had some questions in regard to this setup.

1. When I do not add the Automatic Grid settings for the 4 TI electrodes (so just one manual grid for the bounding box), the interfering E-field results change. I am confused why there is a change, as the bounding box still encompasses the head model and the electrodes.
2. Why not just include the entire head model instead of the bounding box? Would it not be more accurate to automatically grid the segmented surfaces of the head model?

Comparing the results for the different grid settings was done in the same manner by:
Overall Field - Normalize Frequency Domain Results (0.001 A) - Multiplier (EM x2) - Mask Filter to ROI - Max Modulation

Thank you for your time and help!

[carbonell]

Hello,

When you only use a single manual grid (0.7 mm) on the bounding box, the entire domain - including the electrodes - is discretized uniformly at that resolution. While 0.7 mm is reasonably fine, it may still be too coarse to accurately capture the very steep electric field gradients near the electrodes, especially for TI setups where interference patterns are sensitive to local field accuracy.

When you enable Automatic Grid settings for the electrodes, the solver applies local mesh refinement around those electrode regions. This typically results in a smaller minimum step size near the electrodes than your global 0.7 mm grid. That improved resolution leads to more accurate current density and E-field calculations at the source, which then propagates into differences in the resulting interfering field.

So the change you see is not an error - it’s actually a sign that the solution is sensitive to mesh resolution near the electrodes, which is expected.

Regarding your other question: Using a bounding box vs. explicitly listing all head tissues generally does not change the result if the grid settings are identical, because the box already encloses all tissues. It’s mostly a workflow/simplicity choice.

I hope this information helps.