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<p class="MsoNormal"><span style="color:#1F497D">Hello Fieldtrippies,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Regarding a question I asked some months ago about the physical units of the lead field in Fieldtrip, I wanted to share the fact that I was wrong, and that it might be good to either document this in the code
and possibly to add a factor. When using 'cm' units, the lead field is in Wb/cm^2/(A*m) =
<b>1e4 T/Am</b> = 1e2 T/(A*cm). I started to realize my original assumption was wrong while working on my Biomag poster since I was getting very low SNR values (around 1) for typical source and system parameters. So I looked closer at the single sphere code
and my own old spherical forward solution code, and here is where the extra factor comes in:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Skipping to the last step, my formula is:
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="FR-CA" style="color:#1F497D">B = UnitConversion * (1e-7) * ( (QxrQ/F - (QxrQ * rv') * GradF/F^2) * SensorOrient' );<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">The second factor is the constant mu0/(4*pi) = 10^-7 Wb/Am.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Assuming a source (Q) in units of Am and distances in cm, the complicated bit has units of Am*cm/cm^3.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Thus without the first factor, B would have units of Wb/cm^2 = 1e4 T, and so I was using UnitConversion = 1e4 in my code, to get B in Tesla. If we don't put in a specific source amplitude (as in Fieldtrip),
and still using UnitConversion, then B would be in T/(Am).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">I guess units can be different than 'cm' though, so the factor would have to take that into account. Note that the single shell forward solution gives the same units as single sphere and would need the same
factor. I haven't looked at other forward methods. I also haven't verified if this is correctly dealt with when obtaining reconstructed source amplitudes, say with a beamformer.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Cheers,<o:p></o:p></span></p>
<p class="MsoNormal">Marc Lalancette<o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Arial Narrow","sans-serif";mso-fareast-language:EN-CA">Lab Research Project Manager
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Arial Narrow","sans-serif";mso-fareast-language:EN-CA">The Hospital for Sick Children, Toronto, Canada</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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