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  Joint optimization of flip angle design and DenseNet parameters for reduced T2 blurring in TSE sequences

Dang, H., Endres, J., Weinmüller, S., Glang, F., Loktyushin, A., Dörfler, A., et al. (2022). Joint optimization of flip angle design and DenseNet parameters for reduced T2 blurring in TSE sequences. In 24. Jahrestagung der Deutschen Sektion der ISMRM (DS-ISMRM 2022) (pp. 14-15).

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 Creators:
Dang, HN, Author
Endres, J, Author
Weinmüller, S, Author
Glang, FM1, Author                 
Loktyushin, A1, Author                 
Dörfler, A, Author
Maier, A, 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
In fast imaging techniques, like single-shot TSE sequences [1], the short T2 values of different tissues of interest are
comparable to the echo-train duration, which results in significant T2 decay during the k-space acquisition. This leads to a
voxel-T2-dependent k-space filter, which results in a complex relaxation-dependent blurring or ringing.
We propose an end-to-end optimization approach to reduce T2 blurring in TSE sequences by performing a joint optimization
of Flip Angle (FA) design and deblurring CNN.
Methods
Signal simulation and optimization were performed using a Phase Distribution Graph algorithm [2] implemented in the
MRzero framework [3]. For maximal blurring, a single-shot 2D TSE sequence (matrix:128x128, FOV:200mm, GRAPPA2, TEeff =
11.8ms) was used with centric reordering. The forward simulation outputs TSE images (blurry: input to NN) and the
transversal magnetization (sharp: target for NN), generated from synthetic brain samples. MRzero optimizes sequence
parameters (refocusing FA magnitudes/phases) and DenseNet [4] parameters simultaneously. The final sequence was
exported with Pulseq [5] for in vivo measurements of a healthy subject at a Siemens Prisma 3T scanner using a 20ch head
coil, comparing to a 16-shot TSE sequence (matrix:128x128, FOV:200mm, GRAPPA2, TR = 12s, TEeff = 11.8ms).
Results & Discussion
The optimized FA deviate strongly from 180° refocusing FA train with a varying phase pattern (Fig. 2a). In vivo measurements
of the final sequence with and without DenseNet processing are shown in Figure 2b and compared to a single-shot and 16-
shot TSE sequence with 180° flip angle train, respectively. The T2 blurring in the phase encoding direction (anterior-
>posterior) is strongly reduced by optimized FA design and NN. Compared to a 16-shot TSE sequence with 3:12min
acquisition time the optimized single shot sequence provides a similar contrast in below 1s. Still, some mismatches e.g. in the
thalamus region are visible. Differences in FA trains compared to previous work [6] hints to a new optimal sequence design,
when sequence and NN reco are optimized simultaneously.

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 Dates: 2022-09
 Publication Status: Published online
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Event

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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
 Creator(s):
Affiliations:
Publ. Info: -
Pages: - Volume / Issue: - Sequence Number: V006 Start / End Page: 14 - 15 Identifier: ISBN: 978-3-948023-28-7