MultiGradientEchoSimulator

class torchsim.simulators.MultiGradientEchoSimulator(*args, **kwargs)

Bases: Simulator

Water and fat, dephasing and decaying across a gradient-echo train.

Fat is a multi-peak spectrum [1] at a fixed set of chemical shifts, so its signal at each echo is a known complex modulation and what is left to fit per voxel is how much fat there is, the transverse decay and the off-resonance. Each model BART’s multi-echo fits use is a subset of the properties: water and fat with a shared or separate T2*, water alone with T2*, and off-resonance alone.

fat_fraction and fat_phase span the water and fat signals: with a complex scale in front, water is (1 - fat_fraction) of it and fat fat_fraction of it rotated by fat_phase. Leaving T2star out means no decay, and leaving fat_T2star out gives fat water’s T2*.

References

[1]

Hamilton, G., Yokoo, T., Bydder, M., et al., “In vivo characterization of the liver fat 1H MR spectrum”, NMR in Biomedicine 24.7 (2011), pp. 784-790. https://doi.org/10.1002/nbm.1622

Examples

from torchsim.simulators import MultiGradientEchoSimulator

sequence = MultiGradientEchoSimulator(TE=(1.2, 2.4, 3.6, 4.8, 6.0, 7.2))
signal = sequence.simulate(fat_fraction=(0.0, 0.2), T2star=30.0, B0=15.0)
print(signal.shape)

torch.Size([2, 6])

Methods

bind	This simulator with more fixed on it, values or settings alike.
describe	Return the description this protocol plays.
evaluate	Evaluate the closed form, no state machine and no description.
from_description	Return a simulator over a stream someone else assembled.
from_pulseq	Return a simulator over one repetition of a Pulseq sequence.
jacobian	Return the signal and its derivative with respect to diff.
layout	Return the operators of one repetition, in the order they play.
played	Return the protocol as it will be laid out.
repetition_s	Return how long one repetition lasts, given what the layout played.
simulate	Return the recorded signal.

evaluate(properties, **sequence)

Evaluate the closed form, no state machine and no description.