Hey Xiaoxiao,

sorry for my late reply!
The tissue probabilities are part of the input that is provided to the simulator, i.e. p_csf.mha, p_white.mha, p_grey, ...
You need to multiply each one of these probabilities with the corresponding texture image and add the results of the multiplications.
Multiplication and addition of images can easily be done with any program where you have straight access to the image values, e.g. Matlab.

Greetings,

Tom