Fabian Küppers1, Seong Dae Yun1, and Nadim Jon Shah1,2,3,4
1Institute of Neuroscience and Medicine 4, Forschungszentrum Jülich, Jülich, Germany, 2Department of Neurology, RWTH Aachen, Aachen, Germany, 3Institute of Neuroscience and Medicine 11, Forschungszentrum Jülich, Jülich, Germany, 4JARA - BRAIN - Translational Medicine, Aachen, Germany
Synopsis
MRI
sequences with a
mixed
GE/SE signal have
been shown to be suitable for a wide range of applications due to the
wide variety
of accessible
parameters.
This work presents
in
vivo results of a
10-echo GESE sequence based on an
EPIK readout to
demonstrate
the feasibility of whole-brain acquisitions
below
5min, including
T2 and
T2*
maps;
comparison
is made to reference
methods. The generated
images are artefact
free and the T2*-maps
correspond
with conventional
multi-echo GE
results.
The T2
values
are higher than those from
conventional SE, but
are correctable
using
a reduced TE and
improved fitting.
Introduction
Multi-echo
acquisitions
based on EPI readouts
which
combine
pure SE, GE and mixed GE-SE signal have been used
recently.
Vessel
size
imaging1,
CBV analysis and morphology maps2,3
have been
performed with a 2-echo sequence and
T2
and T2*
mapping4
and
compensation for
off-resonance effects have been investigated5
based
on a 5-echo GRASE6
sequence.
Additionally,
OEF calculations using a 6-echo MASE sequence7
and
a 7-echo SAGE sequence with
slice
profile mismatch compensation have
been
published8.
The
potential of GE/SE acquisitions is underlined by these
numerous applications.
Multi-echo
sequences
based on an EPI
readout are often
plagued by
relatively long
TEs for late echoes, compromising
the SNR and slice coverage
for a
given TR. To overcome the aforementioned issues, a previous work
presented a five-echo
GESE sequence based on an
EPIK
(EPI with
a keyhole)
readout9,10.
The method was shown to outperform the
standard EPI-based method
in terms of SNR and slice coverage. In this work, five-echo GESE-EPIK
was further developed to acquire
10 GESE echoes (i.e. two with GEs, six with mixed GESE and two with
SE), enabling simultaneous quantification of T2
and
T2*.
The proposed method was verified with in
vivo T2
and T2*
maps at 3T and compared to
reference methods.Methods
The
10-echo GESE sequence diagram
including timing and signal
decay
is
presented in Fig.1.
The
EPIK
readouts
were
implemented
with 128$$$\times$$$128
matrix-size, 1.9$$$\times$$$1.9$$$\times$$$3mm3 resolution, an
EPIK
multi-shot
factor of
6 with
32
keyhole lines and GRAPPA11
factor 2 for
image acceleration.
Crusher gradient pairs
with different amplitudes
and opposite polarity were
placed
around both
refocusing
pulses to remove unwanted signal paths
from stimulated echoes12.
Spoiler gradients
on all gradient axes were
used and fat suppression was
applied before
each excitation. Three
preparatory
scans
were
used to
reach a
steady state.
A
volunteer
(male;
25years)
was
scanned with TR=15s, TE=17,$$$~$$$39,$$$~$$$81,$$$~$$$103,$$$~$$$125,$$$~$$$147,$$$~$$$169,$$$~$$$204,$$$~$$$226,$$$~$$$250ms, TA=3:15min and 20 slices.
Another data set (male; 28 years) was acquired with the same TEs, but
with TR=0.8s, a TA=26s and 4 slices.
T2
and
T2*
maps were
calculated by
fitting
the 5th
and 10th
echoes
for T2
and
1st
and
2nd
for T2*.
To
evaluate
the T2*
maps,
a conventional multi-echo (64) gradient-echo sequence13
was
employed to
derive reference
T2*
maps.
The
employed
TEs
range from 4.10
to 91.04ms with echo spacing of 1.38ms, 4 slices and TR=0.5s, yielding TA=1:29min. The reference T2
maps were
calculated from
5 single-echo
SE measurements
obtained
at TE=30,$$$~$$$80,$$$~$$$125,$$$~$$$180,$$$~$$$250ms, thus covering the same TE
range
as 10-echo
GESE-EPIK
but
with TA=13:10min.
All maps were fitted with
a least square algorithm, assuming
the following signal relation:
$$S(t)=S_{0}~e^{-{t}/{T_{2}^{*}}}$$ $$S(t)=S_{0}~e^{-{t}/{T_{2}}}$$Results
Fig.2
demonstrates the
brain coverage capability
of 10-echo GESE-EPIK
sequence, showing 20
slices
acquired at TE=17ms. The time course for all
ten acquired echoes, for three
exemplary
slices, is shown in Fig.3. The overall signal decay as well SNR
enhancement at the SE locations (5th
and 10th),
is
evident. A closer
look at the exact decay is depicted in Fig.4, showing the
signal of a single voxel
as a function of TE.
The expected behavior of the
combined T2
and T2*
decay and local maxima
at the SE positions is
confirmed. Fig.5
depicts
the resulting T2
and T2*
maps of 4 different slices
for 10-echo GESE-EPIK
and the corresponding
reference method
including histogram
distribution of the obtained T2
or T2*
maps. All images were
masked to
exclude background noise and the
skull.
In the case
of T2*,
the histogram distribution from the two methods is nearly identical
and is observed to
have similar T2* ranges as reported in previous literature,
considering the fact that the reported T2* values for WM/GM are 48.8/49.3ms14.
The
T2
values
resulting
from
the proposed method are higher and are
distributed more
widely
compared to the reference method and the literature (83.9/85.6ms
for WM/GM14).Discussion and Conclusion
The suitability
of 10-echo GESE-EPIK
for larger
brain coverage has been
demonstrated
and the resulting decay
curves are in good
agreement with the
expected complex decay behaviour of T2
and T2*.
Whole-brain coverage was
not shown,
however,
the sequence parameters allow 80
slices within 5min for the shortest TR,
yielding a
coverage of 24cm. Initial
T2
and T2*
maps were
extracted and compared with
reference methods. T2*
results are in good
agreement with the literature and reference method,
while
T2
values are
overestimated.
This mismatch is likely to
be caused by the larger
TE for the second SE at 250ms, where most
of the WM/GM
signal
is already decayed. The
desired TE
reduction will be facilitated
by a higher
GRAPPA factor and the use
of the partial
Fourier technique.
Furthermore,
the fitting routines were
based
on 2-point least-square fitting
and thus, improvements
in robustness and accuracy
are expected to
be obtained by including
matching pursuit15
algorithms to fit the complex dependencies of all the
10 acquired
echoes. It is anticipated
that the sequence could
also be employed for the
quantification of other
interesting parameters such as vessel
size, OEF
and
T2’.
In conclusion, 10-echo GESE-EPIK
is a very useful technique for multi-parametric imaging of the brain,
yielding T2 and T2* maps; the acquired
information can easily facilitate T2’/OEF mapping, as
well as vessel-size imaging (with CA injection).Acknowledgements
Hereby, I would like to show my gratitude to my supervisor Prof. N.J. Shah and coauthor Dr. S. Yun for their expertise and assistance. Next to this, we thank our colleagues, the MTAs and the organizational team from the INM-4 institute for their support.References
1. V. G. Kiselev et al., (2005),
‘Vessel Size Imaging in Humans’, Magnetic Resonance in Medicine
53:553–563
2. K. Donahue et al., (1998),
‘Simultaneous Gradient-Echo/Spin-Echo EPI of Graded Ischemia in
Human Skeletal Muscle’, JMRI 8:1106-1113
3. K. Donahue et al., (2000),
‘Utility of Simultaneously Acquired Gradient-Echo and Spin-Echo
Cerebral Blood Volume and Morphology Maps in Brain Tumor Patients’,
Magnetic Resonance in Medicine 43:845– 853
4. K. Oshio et al., (1991), ‘GRASE
(Gradient- and Spin-Echo) Imaging: A Novel Fast MRI Technique’,
Magnetic Resonance in Medicine 20:344-349
5. R. D. Newbould et al., (2007),
‘Simultaneous T2 and T2* dynamic susceptibility contrast perfusion
imaging using a multi-echo parallel imaging approach’, Proc. Intl.
Soc. Mag. Reson. Med. 15
6.
H. Schmiedeskamp et. al., (2009), ‘Multiple gradient- and spin-echo
EPI acquisition technique with z-shimming to compensate for
susceptibility-induced off-resonance effects’, Proc. Intl. Soc.
Mag. Reson. Med. 17
7.
Yayan Yin, Yaoyu Zhang, and Jia-Hong Gao, (2018), ‘Dynamic
Measurement of Oxygen Extraction Fraction Using a Multiecho
Asymmetric Spin Echo (MASE) Pulse Sequence’, Magnetic Resonance in
Medicine 80:1118–1124
8. H. Schmiedeskamp et al., (2012),
‘Compensation of Slice Profile Mismatch in Combined Spin- and
Gradient-Echo Echo-Planar Imaging Pulse Sequences’, Magnetic
Resonance in Medicine 67:378–388
9.
M. Zaitsev, K. Zilles, N.J. Shah, (2001), ‘Shared k-space echo planar
imaging with keyhole’, Magnetic Resonance in Medicine 45(1):109-17
10.
N.J. Shah, N.A. da Silva,S. Yun, (2019), ‘Perfusion weighted imaging
using combined gradient/spin echo EPIK: Brain tumour applications in
hybrid MR-PET.’, Hum Brain Mapp.
11.
M.A. Griswold, P.M. Jakob,R.M. Heidemann et al., (2002),
'Generalized autocalibrating partially parallel acquisition (GRAPPA)’,
Magnetic
Resonance in Medicine
47:1202-1210
12.
M. A. Bernstein, K. F. King and X. J. Zhou, (2004),
‘ Handbook of MRI pulse sequences’, Elsevier.
13.
T. Dierkes,H. Neeb, N.J. Shah, (2004),‘Distortion correction in
echo-planar imaging and quantitative T2*
mapping’, International
Congress Series 1265:181-185
14.
A.M. Oros-Peusquens, M. Laurila, N.J. Shah, (2008),
‘Magnetic
field dependence of the distribution of NMR relaxation times
in the living human brain’,
Magnetic Resonance Mater
Phy 21:131–147
15.
G. M. StCphane and Zhifeng Zhang, (1993), ‘Matching Pursuits
With Time-Frequency Dictionaries’, IEEE TRANSACTIONS ON SIGNAL
PROCESSING VOL 41, NO 12