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  Fast analytical simulation of pulseq MRI sequence definitions

Endres, J., Dang, H., Glang, F., Herz, K., Loktyushin, A., Weinmüller, S., et al. (2022). Fast analytical simulation of pulseq MRI sequence definitions. Poster presented at 24. Jahrestagung der Deutschen Sektion der ISMRM (DS-ISMRM 2022), Aachen, Germany.

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 Creators:
Endres, J, Author
Dang, HN, Author
Glang, FM1, Author                 
Herz, K1, Author                 
Loktyushin, A1, Author                 
Weinmüller, S, Author
Zaiss, M1, Author                 
Affiliations:
1Department High-Field Magnetic Resonance, Max Planck Institute for Biological Cybernetics, Max Planck Society, ou_1497796              

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 Abstract: Introduction
Bloch simulations based on phase distribution graphs (PDG) provide a fast and exact simulation of arbitrary MRI imaging
sequences, outperforming classical isochromat based approaches1. Pulseq2 is an open-source project that defines a file
format for sequence definitions with implementations for scanners of different vendors, which makes reproducible in vivo
measurements possible. The presented simulation can as well interpret pulseq files and thus execute the exact same
sequence in silicio.
Methods
A pulseq interpreter capable of reading pulseq 1.2.0 to 1.4.0 files was written. The imported sequence is then converted into
the internal representation used by the simulation. As the focus of simulation lies in imaging techniques, the sequence is
additionally simplified by using instantaneous pulses and merging events if it does not change the measurement. The
simulation then executes the sequence on a virtual phantom to produce a complex signal that can then be reconstructed in
the same way as a signal obtained by a physical scanner. All code is written in python, a schematic of the whole process can
be seen in figure 1.
Results
The interpreter was successfully tested on the example sequences provided by the official pulseq GitHub repository3. The
duration of the simulation of the tested sequences ranged between 70 ms and 16 s, or between 0.5 and 13 times the
duration of the sequence itself. The resolution of the sequences ranged from 128² to 256² while the virtual phantom was
fixed to 128². Figure 2 shows the reconstruction of an imperfect spin echo sequence with spiral trajectory. PDG allows
splitting of the signal into different parts like FID or stimulated echoes to obtain additional insight into the inner workings of
the sequence.
Discussion
A fast simulation of arbitrary MRI sequences defined by pulseq files is a universal tool usable by any project that allows the
use of pulseq sequences. In contrast to measurement on physical systems, simulations are readily available. Furthermore,
PDGs can provide additional information about the acquired signal that cannot be obtained in a measurement1 and the
simulation is fully analytically differentiable, which together makes it a valuable tool in the research of novel imaging and
reconstruction techniques, or end-to-end optimization approaches like MRzero4.

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 Dates: 2022-09
 Publication Status: Published online
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Title: 24. Jahrestagung der Deutschen Sektion der ISMRM (DS-ISMRM 2022)
Place of Event: Aachen, Germany
Start-/End Date: 2022-09-21 - 2022-09-24

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Title: 24. Jahrestagung der Deutschen Sektion der ISMRM (DS-ISMRM 2022)
Source Genre: Proceedings
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Pages: - Volume / Issue: - Sequence Number: P006 Start / End Page: 82 - 83 Identifier: ISBN: 978-3-948023-28-7