T2* decay

[cncastillo]

As it is right now, the simulator does not take into account T2* decay. This intra-voxel perturbation of spins' off-resonance could be simulated easily by generating randomly distributed spins with off-resonance following a Lorentzian distribution (so in the time-domain the decay is exponential). Nevertheless, this is very inefficient computationally and adds unnecessary complexity.

To simulate efficiently, we could track the "width" of the Lorentzian distribution through time and calculate a decay factor E_T2*. This is basically equivalent to changing the spin model to a spin distribution model, where instead of just storing Mxy and Mz we also stored the FWHM of the phase distribution dPhi.

Maybe we can combine that with EPGs to solve #52 as well.

Theory of T2:* Microstructural off-resonance distributions.pdf