Understanding Voltage Plot

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Grace Guo

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Mar 3, 2023, 5:26:48 PM3/3/23

to roast-users

Hello Sir/Madam,

Thank you for developing and maintaining ROAST! This software is amazing and it's super helpful for TES simulation and visualization. Since I'm very new to TES and ROAST, I have some naive questions regarding how we should interpret the voltage values.

After running ROAST on the MNI_152_1mm file and placing electrodes on FP1 (1mA) and FP2 (-1mA), ROAST gives me the simulated voltage plot as shown below.

Screen Shot 2023-03-03 at 4.54.23 PM.png

The color bar on the right shows the voltage ranging from 2044mV to 2056mV. I'm not very sure, but these look like pretty large values. And by looking at the vol_all output, it seems that there are some voxels that have very low voltage (10^-7 ish). If I understand it correctly, the reference is w.r.t. the lowest voltage value. If that's the case, how should I interpret the voltage plot? Instead of looking at the absolute values on the color bar, would it make more sense to think about the difference of voltage across voxels (i.e. the red regions have roughly 12mV higher voltage than the blue regions in this case) ?

Thank you so much and I'd really appreciate your help!

Best,

Grace

Yu (Andy) Huang

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Mar 4, 2023, 11:42:37 AM3/4/23

to Grace Guo, roast-users

Hi Grace,

Yes the voltage is re-referenced to the min value, see this line.

Note the simulation is current-controlled instead of voltage-controlled. In tech terms it's Newmann boundary condition instead of Dirichilet boundary condition. That's why you see randomly high values of voltage, but you're right that voltage is relative. And we mainly care about the E-field (or electric current) whose boundary condition is controlled (ie. current-controlled simulation).

Hope this helps.

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