[Nirfast] Positive Jacobian Values

[Thomas Johnson]

Hello,

I am performing single wavelength forward simulations (800nm) on a custom 6-layer head mesh that models skin, skull, meninges, CSF, WM and GM. At a new different fibre separation lengths I end up with a Jacobian that has mostly positive values, while most other separation lengths give negative values. To me, this indicates the tissue is generating light! Can anyone help me out on this one? Here are things I am thinking:

- I have given each tissue type a separate index of refraction. Could this be reflection coming off the tissue boundary?

- There is a large region of CSF in my model. Is this an artefact of the diffusion approximation being unable to accurately simulate low scattering media?

Thank you,

Thomas Johnson