Snapshot‐CEST: Optimizing spiral‐centric‐reordered gradient
echo acquisition for fast and robust 3D CEST MRI at 9.4 T
High‐field Magnetic Resonance Center, Max
Planck Institute for Biological Cybernetics,
Department of Biomedical Magnetic
Resonance, Eberhard‐Karls University
Tübingen, Tübingen, Germany
M. Zaiss, High‐field Magnetic Resonance
Center, Max Planck Institute for Biological
Cybernetics, Max‐Planck‐Ring 11 72076
Number: ZA 814/2‐1; H2020 European
Institute of Innovation and Technology,
Grant/Award Number: 667510; European
Union's Horizon 2020 research and innovation
program, Grant/Award Number: 667510; Max
Planck Society, German Research Foundation,
Grant/Award Number: ZA 814/2‐1
Gradient echo (GRE)‐based acquisition provides a robust readout method for chemical exchange
saturation transfer (CEST) at ultrahigh field (UHF). To develop a snapshot‐CEST approach, the
transient GRE signal and point spread function were investigated in detail, leading to optimized
measurement parameters and reordering schemes for fast and robust volumetric CEST imaging.
Simulation of the transient GRE signal was used to determine the optimal sequence parameters
and the maximum feasible number of k‐space lines. Point spread function analysis provided an
insight into the induced k‐space filtering and the performance of different rectangular reordering
schemes in terms of blurring, signal‐to‐noise ratio (SNR) and relaxation dependence. Simulation
results were confirmed in magnetic resonance imaging (MRI) measurements of healthy subjects.
Minimal repetition time (TR) is beneficial for snapshot‐GRE readout. At 9.4 T, for TR = 4 ms
and optimal flip angle close to the Ernst angle, a maximum of 562 k‐space lines can be acquired
after a single presaturation, providing decent SNR with high image quality. For spiral‐centric
reordered k‐space acquisition, the image quality can be further improved using a rectangular spi-
ral reordering scheme adjusted to the field of view. Application of the derived snapshot‐CEST
sequence for fast imaging acquisition in the human brain at 9.4 T shows excellent image quality
in amide and nuclear Overhauser enhancement (NOE), and enables guanidyl CEST detection.
The proposed snapshot‐CEST establishes a fast and robust volumetric CEST approach ready
for the imaging of known and novel exchange‐weighted contrasts at UHF.
APT, CEST, chemical exchange saturation transfer, human brain, NOE, snapshot, UHF, 9.4 T
Chemical exchange saturation transfer (CEST) allows for indirect detection of diluted molecules via their saturation transfer to the abundant water
Many different diluted solutes have been reported to be detectable with CEST, such as peptides and proteins,
with dependence on pro-
and even injected solutes, such as iopamidol,
and glucose derivatives.
the detection of these different CEST effects, different saturation schemes are required, matching the saturation power and duration to the
exchange rate and frequency offset of the protons of the many different CEST sites. At ultrahigh fields (UHFs), the frequency separation of CEST
effects is much easier, allowing the reconstruction of several interesting CEST contrasts. However, this comes with the cost of longer measurement
times, to sample all required offsets, as well as stronger field inhomogeneities
and instabilities at UHF.
Thus, a robust and fast imaging readout
is needed to effectively exploit the high spectral resolution at UHF.
Abbreviations used: 3D, three‐dimensional; BW, bandwidth; CEST, chemical exchange saturation transfer; CNR, contrast‐to‐noise ratio; E, elongation factor of
rectangular spiral Cartesian readout; EPI, echo planar imaging; FA, flip angle α; FLASH, fast low‐angle shot; FOV, field of view; FWHM, full width at half‐maximum
of the PSF; GM, gray matter; GRASE, gradient and spin echo; GRE, gradient echo; MRI, magnetic resonance imaging; MT, magnetization transfer; n
, number of k‐
space lines when initial difference is decayed to 5%; NOE, nuclear Overhauser enhancement; rFWHM, relative FWHM: FWHM of the filtered PSF divided by
FWHM of the unfiltered PSF; rMag, relative magnitude of the filtered PSF compared with the unfiltered PSF; PSF, point spread function; R
, parallel imaging
acceleration factors in each direction; R, total parallel imaging acceleration factor R = R
; RF, radiofrequency; SNR, signal‐to‐noise ratio; T
relaxation time; TA, acquisition time (per offset); TE, echo time; TR, repetition time; T
, readout time; TSE, turbo spin echo; T
, total saturation time, duration of
saturation pulse train; UHF, ultrahigh field; WASABI, water shift and B
; WM, white matter
Received: 2 August 2017 Revised: 13 November 2017 Accepted: 14 November 2017
NMR in Biomedicine. 2018;31:e3879.
Copyright © 2018 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/nbm 1of14