Marco Barbieri1, Akshay S. Chaudhari1,2, Catherine J. Moran1, Garry E. Gold1,3, Brian A. Hargreaves1,3,4, and Feliks Kogan1
1Department of Radiology, Stanford University, Stanford, CA, United States, 2Department of Biomedical Data Science, Stanford University, Stanford, CA, United States, 3Department of Bioengineering, Stanford University, Stanford, CA, United States, 4Department of Electrical Engineering, Stanford University, Stanford, CA, United States
Synopsis
Quantitative
T2 mapping is a valuable tool for studying OA changes. qDESS is a
rapid sequence that provides accurate T2 measurements and SNR-efficient morphological
imaging. B0 mapping is an auxiliary scan acquired to correct field
inhomogeneity-induced errors using techniques such as WASSR and 2-GRE. This work proposes a method for B0 mapping
that exploits the phase difference between the two echoes acquired with qDESS.
The experiments with phantom and in-vivo simultaneous bilateral knee
acquisitions showed that the B0 maps obtained with the qDESS method were
in good agreement with those obtained using the WASSR method and the 2-GRE
method.
Introduction
Quantitative
T2 mapping is a valuable tool for assessing macromolecular changes
in collagenous tissues and studying osteoarthritis (OA) progression1,2.
The quantitative double-echo in steady-state (qDESS) pulse sequence provides
accurate T2 measurements and SNR-efficient morphological imaging in five
minutes or less and is becoming more widely utilized in knee OA research8.
However, acquiring accurate T2
measurements can require long scan times, and often multiple sequences. For example, B0 mapping is an auxiliary scan acquired to correct field inhomogeneity-induced
errors in qMRI4,5, using techniques such as the off-resonance
saturation-based Water Saturation Shift Referencing6 (WASSR) and two
Gradient‐Recalled-Echo7 (2-GRE).
This work proposes a method for B0 mapping that
exploits the phase difference between the two echoes acquired with a qDESS
sequence9. This may allow implementing B0 correction for qDESS T2
mapping without acquiring an additional scan with an additional benefit of
co-registration of the T2 and B0 maps. The method was validated with phantom
and in-vivo simultaneous bilateral knee acquisitions by comparing the B0 maps
obtained using the proposed method with those obtained using the WASSR method
and a standard 2-GRE method.Theory
For the first (S1) and second (S2) qDESS echoes
acquired at echo times TE1 and TE2, respectively, ΔB0
(in Hz) can be computed according to eq.1. $$ \Delta B_{0} = \frac{angle(S_{1} \cdot S_{2}^{*})}{2 \pi \cdot 2TE_{1}} $$
Where the numerator is the phase difference, Δφ, between S2
and S1 and $$$ angle(S_{1}\cdot S_{2}^{*}) $$$ is the Hermitian inner
product10 of the two echoes. When ΔB0 varies more than the $$$ \left( -\frac{1}{4TE_{1}},\frac{1}{4TE_{1}} \right) $$$ range across the volume, Δφ is affected by phase wraps
and requires unwrapping for accurate B0 mapping.Methods
To
verify Eq. 1, qDESS Bloch simulations varying the ΔB0 experienced by the spin ensembles for different
combinations of TE1 and TE2 were simulated.
To validate the qDESS B0 measurement method, a phantom and a healthy
subject were scanned using qDESS, WASSR, and fast multi-echo GRE (FGRE)
sequences. MRI acquisitions were performed on a 3T SIGNA Premier scanner (GE Healthcare, Milwaukee, WI, USA) using 16-channel flexible
phased-array, receive-only coils (NeoCoil, Pewaukee, WI, USA). For the healthy
subject, a simultaneous bilateral knee acquisition was used11. The
first two echoes of the FGRE acquisition were used to implement the 2-GRE
method. All sequence parameters are summarized in Fig.1. The coil-combined phase
difference between the second and the first echo was obtained using the sum Hermitian inner
product method12. WASSR and FGRE volumes were registered
to the first echo of qDESS.
For the in-vivo data, B0 mapping was
assessed in the femoral cartilage (FC) and the pipeline is summarized in Fig.
2. For both knees, after coil-combination, the FC was segmented using DOSMA13,
and the 3D segmented phase map was unwrapped with PRELUDE14. qDESS B0
maps were then computed according to eq. 1 and projected onto a 2D space for
visualization15.
The B0 maps obtained with the qDESS and the
2-GRE methods were compared against those obtained with the WASSR method,
considered as reference. The comparison was quantitatively assessed exploiting
pixel-wise difference maps and Bland-Altman
(BA) analysis. Lin's concordance coefficient (ρc) and the coefficient
of variation (%CV) between methods were also evaluated.Results
Bloch simulations (Fig. 3) showed that
only TE1 defined the slope of the linear relationship between TE1 and ΔB0. Whit TE1=5ms, the slope did not change for both TE2=15 and 25ms.
The phantom
B0 maps obtained with the WASSR, qDESS, and 2-GRE methods and BA plots with the
WASSR reference are shown in Fig. 4. Overall, qDESS had better agreement with
WASSR (ρc=0.98, Mean difference=-1Hz) than 2-GRE WASSR (ρc=0.90, Mean
difference=9.4Hz). For the
in-vivo acquisition, both the qDESS and 2-GRE methods produced B0 maps that
were in good agreement with those obtained with WASSR as highlighted by the 2D
projected B0 maps and the BA plots(Fig. 5). For the left knee, ρc was equal to 0.94 and 0.92 with a mean difference(MD) of -5.5 Hz and 4.4 Hz for
qDESS and 2-GRE, respectively. For the right knee, B0 values obtained using
qDESS had an MD of -11.9 Hz, whereas, with the 2-GRE method,
MD was equal to 3.3 Hz. Discussion
The proposed method for
measuring B0 inhomogeneities from a qDESS acquisition provided B0 maps that
were in good agreement with those obtained using WASSR both in phantom and
in-vivo. The agreement between qDESS and WASSR was comparable to that of a
standard 2-GRE method. Errors
in qDESS B0 measurements may be due to errors introduced by the unwrapping
algorithm. When phase
unwrapping was not necessary, there was an almost perfect agreement between
qDESS and WASSR. The echo time TE1 defines the wrap free bandwidth
as shown with the Bloch simulation. Conclusion
A
method for accurately measuring B0 field inhomogeneity with qDESS was proposed
and validated in phantom and in-vivo bilateral knee acquisitions against
standard B0 mapping methods. The method may allow for the implementation of a B0 correction method for qDESS T2
mapping using an inherently co-registered B0 map without additional
scan time. More generally, the method may help shorten knee imaging protocols
that require an auxiliary B0 map by exploiting a qDESS acquisition that also
provides T2 measurements and high-quality morphological imaging.Acknowledgements
This work was supported by GE Healthcare and NIH grants
R01-AR077604, R01-EB002524, R01-AR074492, K24-AR062068, and R00-EB022634.References
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