Yu Zhao1, Daniel F. Gochberg2, and Jianqi Li1
1Shanghai Key Laboratory of Magnetic Resonance, Shanghai, China, 2Vanderbilt University Institute of Imaging Science, Nashville, TN, United States
Synopsis
The purpose of this study was to characterize the contribution of fat to
the CEST signal when using a water-selective binomial-pulse excitation and the effects of multiple fat peaks and B0 inhomogeneity.
A CEST sequence with binomial-pulse excitation and a modified PRESS
localization was applied to in vivo experiments to determine signal
contributions of lipid resonances. Water excitation using
a {1-3-3-1} pulse provided a broad signal suppression, which provided
robustness against B0 inhomogeneity. Significant fat
signal contributions to CEST imaging of hydroxyl and amine were
unavoidable, while much smaller contamination was seen when imaging amide sites,
limited by B0 inhomogeneity.
Introduction
Artifacts caused by strong lipid signals
pose challenges in body CEST imaging. One approach to fat suppression is water-selective
excitation (WE) using a binomial RF pulse 1-4.
The binomial RF pulse approach is designed to emphasize the suppression of the
main fat peak 5,6,
ignoring the inferior fat peaks. Furthermore, body MRI is more prone to B0
inhomogeneity than brain MRI, and WE is relatively sensitive to such inhomogeneities. To characterize the contribution
of fat to the CEST signal In the WE-based CEST MRI and the effects of multiple fat peaks and B0
inhomogeneity, a modified point-resolved spectroscopy
(mPRESS) is used to acquire signals after CEST preparation.
Methods
Simulation
A simulation based on the numerical solution of Bloch equation was
conducted to investigate the performance of WE in the type of {1-3-3-1}. The resulting excitation
profile was defined as the magnitude of the transverse magnetization.
Pulse
sequences
The CEST-mPRESS sequence (Figure 1)
consists of three sections: the CEST preparation, WE excitation using a {1-3-3-1}
pulse, and the mPRESS acquisition. The CEST preparation contains a train of Gaussian-shaped
RF pulses and spoiling gradients. In the timing of mPRESS, the first slice-selective
90° RF pulse in the conventional PRESS was replaced by a slice-selective
180° RF pulse. Additionally, the frequency offset of
numerically controlled oscillator (NCO) during WE was used to generate an
off-resonance effect that was equivalent to the off-resonance effect induced by
B0 inhomogeneity.
In
vivo experiments
One male volunteer, 57-year-old, was recruited.
The volume of interest (VOI) with high proton-density fat fraction (PDFF) for the CEST-mPRESS
sequence was located at the spinal cord, which was determined by the
measurement of PDFF using HISTO 7. The
CEST preparation consisted of 6 Gaussian-shaped pulses, each 200 ms long, with
an average B1 = 1.6 μT. Frequency offsets were acquired in
Z-spectrum from −5 ppm to 5 ppm, and one reference acquisition without
saturation was also acquired. The frequency offsets of NCO were set to provide
the equivalent values of ∆B0 spanning from −100
to 100 Hz with an increment of 10 Hz.
Data processing
The
Z-spectrum from each individual fat peak was based on the areas under the
corresponding peak in the 1H spectra. Adjacent
fat peaks were analyzed as a group for the Z-spectra analysis, and MTRasym
was calculated to quantify lipid artifacts. Total lipid artifacts from fat
peaks was estimated by adding MTRasym of the fat peaks.
Results
Simulation
Figure 2 shows excitation profiles from the numerical simulation for
WE. The excitation profiles are plotted on the chemical shift scales. A
broad near-zero range exists near the main spectral peak of fat. B0
inhomogeneity leads to a corresponding shift of the excitation profile along
the axis of chemical shift.
In
vivo experiment
Figure 3 shows two types of 1H spectra of the in vivo
experiment from the VOI in spinal cord with PDFF of 57 %. In the Figure 3a, a 1H
spectrum from the conventional PRESS shows the raw spectral composition of the
fat. Figure 3b displays the 1H spectrum
from the reference acquisition
of CEST-mPRESS with ∆B0 spanning from −100 to 100 Hz. Within a limited range
(very roughly |∆B0| < 50 Hz), P3 to P6 are completely suppressed,
while there is no obvious suppression on P1 and P2. Figure 4 shows how the
residual lipid signals from adjacent fat peaks produce
artifacts in the CEST signal, and how these artifacts change with ∆B0.
For ∆B0 = 0 Hz, obvious lipid artifacts only appear at P1. But for ∆B0
= −100 and 100 Hz, Z-spectrum built on P3 and P4, P5 and P6 have significant
contributions to lipid artifacts. Figure 5 shows the lipid artifacts on the
CEST sites of hydroxyl, amine and amide with ∆B0 spanning from −100
to 100 Hz. Slight contamination was seen only when imaging amide sites with ∆B0
spanning from −50 to 50 Hz.Discussion
As shown in Figure 2, WE provide
an effective signal suppression in the chemical-shift
range with a broad bandwidth. Various field inhomogeneity shifts the excitation
profile differently, which results in various residual fat signals. The fat peaks from P3 to P6 can be suppressed simultaneously
with ∆B0 spanning from −50 to 50 Hz. The
residual signals can spread over a wide frequency range.
As shown in Figure 4d-4f, lipid artifacts from P1 have impact
not only on the hydroxyl located at 1 ppm but also on the amine located at 2 ppm.
Therefore, it is no possible to keep the hydroxyl and amine from the lipid
artifacts from P1 in the WE based CEST. Therefore, to remove
lipid artifacts on an CEST site, all the fact peaks near this CEST site should
be suppressed simultaneously. Figure 5 show there are no lipid artifacts on the
amide with ∆B0 spanning from -50 to 50 Hz.
It agrees with the fact that fat peaks
from P3 to P6 are suppressed simultaneously with ∆B0 in this frequency range. Conclusion
In the WE-based CEST
MRI, B0 inhomogeneity is the limiting factor for fat
suppression. Generally, artifact-free
imaging of amides is viable, but imaging of hydroxyl and amine is more
challenging.Acknowledgements
No acknowledgement found.References
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