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JuliaHealth / KomaMRI.jl

Koma is a Pulseq-compatible framework to efficiently simulate Magnetic Resonance Imaging (MRI) acquisitions. The main focus of this package is to simulate general scenarios that could arise in pulse sequence development.
https://JuliaHealth.github.io/KomaMRI.jl/dev/
MIT License
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T2* decay #62

Open cncastillo opened 2 years ago

cncastillo commented 2 years ago

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

cncastillo commented 2 years ago

This abstract from the ISMRM 2022 does something similar to what I have been thinking: Phase distribution graphs for differentiable and efficient simulations of arbitrary MRI sequences

cncastillo commented 2 years ago

More information: https://ismrm-esmrmb-ismrt2022.us3.pathable.com/meetings/virtual/84Zujvqyot5eiu6Ze

image

They are storing the dephasing "time" of the distribution as an extra parameter!

The main problem with this method is that the number of states increases as $3^{N_\mathrm{RF}}$.