Feng Xu1,2, Wenbo Li1,2, Dapeng Liu1,2, Dan Zhu3, Kelly Myers1, Michael Shär1, and Qin Qin1,2
1The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, School of Medicine, Baltimore, MD, United States, 2F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States, 3Biomedical Engineering, Johns Hopkins University, School of Medicine, Baltimore, MD, United States
Synopsis
Chemical shift-selective fat saturation (CHESS)
is the most commonly used fat suppression technique for clinical MRI. Conventional Gaussian-shaped pulses are sensitive to B1 inhomogeneity
and their wide transitional band can be affected by B0 off-resonance. Uniform
fat saturation across a large field of
view (FOV) is especially challenging for body and breast MRI at 3T. Here, we
designed a novel frequency-selective RF pulse
based on the optimal control theory with robustness to a targeted wide range of B0/B1 conditions.
Its superior performance than the regular ones was demonstrated using T1-weighted
sequences with whole-breast coverage.
Introduction
Chemical shift-selective fat saturation (CHESS) 1-3 can be added to any pulse sequence and thus is the
most commonly used fat suppression technique for clinical MRI. Typically, a frequency-selective RF pulse is applied with the
main fat peaks (around 3.5 ppm) in the saturation band and water frequency (0 ppm)
in the pass band. However, conventional Gaussian-shaped
pulses are sensitive to B1 inhomogeneity and their wide transitional band
suffers from B0 off-resonance. Uniform fat saturation across a large field of view (FOV) is especially
challenging for body and breast MRI at 3T 4,5. In
this work, we will demonstrate a novel design
of fat-sat pulse with robustness
to B0/B1 inhomogeneity and compare it with the regular one for coverage of
whole breast on normal volunteers.Methods
An optimized composite (OCP) fat-sat pulse was designed through
numerical optimization using the optimal control theory6. Previously, we demonstrated
a superior performance of OCP 10° excitation pulse in our angiography sequence 7. The desired range of immunity to the B0/B1 offset incurred in the breast
at 3T is: B0 = ±200Hz and B1+ scale =
±0.3. An optimal control routine in MatPulse7,8 was
utilized to generate a 12ms 90° OCP pulse with 400 components of block-shaped
subpulses. Numerical simulations of the Bloch equations were performed using
Matlab to examine the responses of the longitudinal magnetizations (Mz)
following the OCP pulse under various B0/B1 conditions.
Experiments were performed on a 3T Philips
Ingenia scanner using a 16-channel breast coil for reception. Five healthy female
volunteers (44±15 yo) were enrolled with informed consent and scanned with
prone and feet-first position. B0 maps were obtained by a standard gradient
echo sequences with two echo times and B1 maps were measured with the DREAM
method9, both using vendor-provided
fully automated shimming methods that take into account geometries of individual breasts.
Fat suppression using the SPIR (spectral
presaturation with inversion recovery) module was compared between the OCP 90°
pulse and two Sinc-Gaussian (SG) pulses with 90° and 105° respectively. The fat-sat
pulse was immediately followed by a spoiling gradient and then the T1-weighted multi-shot
turbo field echo (TFE) acquisition. The axial slab was acquired with left-right phase-encoding
and following parameters: FOV=200x366x120mm3, acquired
resolution=1x1x2mm3, reconstructed resolution=0.85x0.85x1mm3,
TR/TE=3.5/1.8ms, flip angle=7°, low-high profile
ordering,
turbo field factor=70, shot number=91, TFE acquisition window = 245ms, TRshot=500ms,
and total
scan duration
was 0.8min.
Six-point 3D gradient echo DIXON was used to delineate water, fat masks based
on a threshold of 80% tissue content. Quantitative analysis of fat, water
signals were averaged from the fat, water masks respectively.Results and Discussion
The regular SG 90° pulse
shape and its simulated Mz responses over the B0 off-resonance (from -600Hz to
200Hz) at B1+ scale = 1 or from 0.7 to 1.3 are shown in Figures 1a,b,c, respectively.
The waveform of the complexed OCP 90° pulse and its Mz responses over various
B0/B1 variations are displayed in Figures 1d,e,f correspondingly. Compared to
SG 90° fat saturation, OCP 90° pulse offers half of the transition band (95%-5%:
90Hz vs. 185Hz) and much less sensitivity to B1 inhomogeneity.
The measured B0 off-resonance
and B1+ scale maps of five different slice locations across the breasts of five
subjects are shown in Figures 2a,b, respectively. A combined 2D histogram displays
the B0 and B1+ distribution from all slices of all five subjects (Figure 2c). The
region in red lines is the range of B0 off-resonance within ±200Hz and B1+
scale error within ±0.3, which contains 98% of all scattered data.
The performance of OCP
90° fat sat in comparison with regular SG pulses are illustrated in three
subjects with three orthogonal orientations, respectively (Figure 3). All
subjects manifested partial unsaturated fat signal at different locations
(arrowheads) using either SG 90° or SG 105°, indicating their sensitivity to B1
inhomogeneities. In contrast, fat signal appeared homogeneously suppressed using
OCP 90° as expected. The last column of DIXON images were used as reference. Note
that fat signal is partially recovered during the 245ms TFE acquisition through
a blurring effect and this resulted in less ideal fat suppression.
Within the
80%-threshold fat mask, the ratios of the mean and standard deviation (STD) of residual
fat signal level after different fat-sat pulses relative to the averaged fat
signal intensity without fat saturation of each subject are exhibited in Figures
4a,b, respectively. The contrast ratios of water to fat signal after different
fat-sat pulses of each subject are shown in Figure 4c. Table 1 lists the group
results of each metric. OCP 90° yields 27% and 31% lower fat signal (mean) with
27% less variation (STD) compared to SG 105° and 90° (P=0.01), and 25% higher
water-fat contrast than SG 90° (P=0.02).Conclusion
A new frequency-selective
fat saturation OCP pulse was designed based on optimal control theory for a
targeted wide range of B0 off-resonance and B1+ scales. Examined at 3T with a whole-breast
coverage, this OCP pulse was demonstrated with a significantly more robust fat suppression
compared to conventional Gaussian shaped pulses.
One downside is that OCP90°
utilizes maximum B1 power for a longer time, which results in higher SAR.Acknowledgements
No acknowledgement found.References
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