In this pre-computed simulated brain database (SBD), the parameter settings are fixed to 3 modalities, 5 slice thicknesses, 6 levels of noise, and 3 levels of intensity non-uniformity. You can also request simulations done with arbitrary parameters from the BrainWeb custom MRI simulations interface.
The voxel values in each image are magnitude values, rather than complex, real or imaginary. For more information, see the FAQ.