Confounding effects of imaging gradients in stimulated echo: case of diffusion
exchange imaging
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1
Copenhagen University Hospital, Danish Research Centre for Magnetic Resonance, Denmark
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2
Lund University, Physical Chemistry, Sweden
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3
CR Development AB, -, Sweden
Diffusion-weighted stimulated echo (STE) sequences are often used in applications
where T2 is short compared to the diffusion times1, for example in q-space imaging
with single diffusion encoding (SDE)2 and especially at higher fields. The STE is also
common in double diffusion encoding (DDE) used to measure diffusional exchange3,4 or microscopic anisotropy5.
Compared with the Pulsed-Gradient-Spin-echo (PGSE) sequence where a 180-degree
RF pulse refocuses the diffusion-encoded signal, the STE uses two 90-degree RF
pulses separated by a mixing time (tm). The first 90-degree RF pulse flips the
transversal magnetization along the longitudinal axis affected by the slower T1-decay
which enables the longer diffusion times.
In practice, the RF pulses in imaging setups are imperfect which introduces unwanted
phase-coherence pathways that at readout will result in distorted images. Crushers
placed symmetric around the RF pulse effective eliminate the unwanted coherence
pathways6. However, the crushers together with the slice gradients (known as
butterfly gradients) introduce unwanted diffusion-weighting during the tm period. The
butterfly gradients may significantly disrupt the experimental design[7], for example
when STE is used in high angular resolution diffusion imaging experiments with
SDE. However, in this case we have shown that the effects of butterfly gradients can
be easily compensated7. On the other hand, in DDE experiments that rely on varying
the mixing time, tm, such as in filter exchange imaging (FEXI)8-10, the butterfly
gradients may introduce a bias that cannot be easily mitigated as for SDE. Here we
discuss the bias caused by butterfly gradients used in FEXI and their possible
confounding effect on the measurement of apparent exchange rate (AXR) in pre- and
clinical settings.
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weighted MRI. Magn. Reson. Med. 44: 713–722 (2000)
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local order and dynamics. J. Chem. Phys. 120, 4032–4038 (2004).
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determination of cell membrane permeability. J. Magn. Reson. 200, 291–295 (2009).
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matter revealed by angular bipolar double-PFG MR. Magn. Reson. Med. 65, 1216–27 (2011).
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7. Lundell, H., Alexander, D. C. & Dyrby, T. B. High angular resolution diffusion imaging with
stimulated echoes: compensation and correction in experiment design and analysis. NMR
Biomed. (2014). doi:10.1002/nbm.3137
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with diffusion MRI. Magn. Reson. Med. 66, 356–365 (2011).
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using filter-exchange imaging. Magn. Reson. Med. 69, 1572–80 (2013).
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imaging in anisotropic systems. Magn. Reson. Med. 72, 756–762 (2014).
Keywords:
Diffusion,
MRI,
dMRI,
multidimensional,
diffusion encoding
Conference:
New dimensions in diffusion encoding, Fjälkinge, Sweden, 11 Jan - 14 Jan, 2016.
Presentation Type:
Oral presentation
Topic:
New Dimensions in Diffusion Encoding
Citation:
Dyrby
TB,
Lundell
H,
Topgaard
D and
Lasič
S
(2016). Confounding effects of imaging gradients in stimulated echo: case of diffusion
exchange imaging.
Front. Phys.
Conference Abstract:
New dimensions in diffusion encoding.
doi: 10.3389/conf.FPHY.2016.01.00005
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Received:
07 Jul 2016;
Published Online:
07 Jul 2016.
*
Correspondence:
Dr. Samo Lasič, CR Development AB, -, Lund, 221 00, Sweden, samol@drcmr.dk