Hyun-Soo Lee1, Seung Hong Choi2, and Sung-Hong Park1
1Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology, Daejeon, Korea, Republic of, 2Department of Radiology, Seoul National University College of Medicine, Seoul, Korea, Republic of
Synopsis
The quality of
balanced steady-state free precession is vulnerable to eddy-currents and
transient oscillations. However, the conventional centric phase-encoding (PE)
scheme makes these artifacts severe, thus needs additional compensation
strategies. In this study, we propose an improved PE scheme where k-space is
encoded from center to periphery in a group-wise manner (PE-grouping). This
reduces related artifacts by preventing big jumps in k-space along PE
direction. Also proposed were various averaging strategies that could further
eliminate the residual artifacts by averaging two full images acquired not only
with the PE-grouping, but also with the conventional centric and pairing schemes.Introduction
For physiological MR imaging (i.e., diffusion, perfusion,
magnetization transfer imaging), balanced steady-state free precession (bSSFP)
is frequently used due to its excellent signal-to-noise ratio (SNR) and short
scan time. For data acquisition, a centric phase-encoding (PE) order is
preferred to a linear order in order to maximize prepared magnetization
contrast. However, the conventional centric PE scheme induces severe signal
oscillations due to non-constant eddy-currents. In this study, we propose (i) a
pseudo-centric PE-grouping (PE-grouping) which is a hybrid of the conventional
centric and linear orders for optimal contrast and minimal
signal fluctuations. Also, we propose (ii) four
averaging strategies which further reduce the residual eddy-current and transient
oscillation artifacts by averaging two full images so that oppositely-oscillating
signals of two images can be canceled out. The proposed strategies markedly
removed eddy-current and transient oscillation artifacts for both phantom and
in vivo while maintaining the SNR and temporal resolution/scan of the
conventional centric scheme, which is better than the pairing [1] and
comparable to existing double averaging scheme (dAVE) [2].
Theory
In principle, eddy-current-induced dephasing of
transverse magnetization is proportional to the amplitude of gradients but with
opposite polarity [3]. With the conventional centric PE scheme, the amplitude
and polarity of PE gradients change every repetition time (TR), which induces
time-varying eddy-current dephasing resulting in severe spatial phase offsets. Since
bSSFP is robust to constant or slowly-changing spatial phase errors [4], we
propose PE-grouping in which PE lines with a certain number N are grouped and the
PE-groups are linearly encoded from k-space center to edge (Fig. 1d). This method
is expected to reduce eddy-current-induced signal oscillations because the
number of jumps in k-space are reduced. Also, the signal oscillations including
transient oscillation can be canceled out by averaging two signals that have opposite
oscillation patterns. The
phases of oscillations can be adjusted by changing RF phases of PE line of
bSSFP (i.e., phase cycling angle). Four average schemes can be differentiated by the way two images have
opposite RF phases for the same PE lines: “dummy average” for different number
of dummy scans, “backward average” for opposite PE order within groups, “up-down
average” for different start point of encoding as upper or lower-half of
k-space, and “up-down-backward” as the combination of preceding two average
schemes.
Data Acquisition
All experiments were performed on a Siemens 3T Trio
system (Siemens Medical Solution, Erlangen, Germany) on a doped water phantom
(measured =290/230
ms) and a volunteer. The half alpha preparation was performed with no dummy
scans and measurements were performed five times repeatedly for each scan, in
order to investigate the differences between the transient and the steady states.
A delay time of 10 s was applied before each different acquisition (not
between the five dynamic scans), considering longitudinal relaxation time.
Results
In Fig. 2a, the oscillation peak of the
conventional centric scheme was reduced with PE-grouping of both N=3 and 4,
which was lower than that of the pairing (N=2). Up-down average further reduced
the peaks, even lower than dAVE. Also, the number of peaks increased with N of
PE-grouping, while their amplitudes decreased (Fig. 2b-c). The two signals from
the conventional centric scheme with zero (black line) and one (gray line)
dummy scans oscillated in opposite phases to each other; thus the transient oscillations
were markedly reduced after complex averaging (Fig. 3). For phantom, the
artifacts of the conventional centric scheme (indicated by arrowheads) were reduced
with PE-grouping (N=4) for both inner and outer parts of the phantom, while the
background artifact increased with the pairing; furthermore, the four average
schemes clearly removed artifacts comparable to dAVE (Fig. 4, top). In vivo
results (Fig.4, bottom) confirmed that the proposed strategies clearly removed
artifacts compared to the existing methods (N=1 and N=2) for both transient
(top) and steady-states (bottom).
Discussion and Conclusion
The PE-grouping considerably reduced eddy-current-induced signal
oscillations and proposed average schemes further eliminated artifacts
including transient oscillations, which is beyond or comparable to existing
compensation schemes. For the choice of N with PE-grouping,
there are some tradeoffs between stability and contrast. Signal oscillations
are reduced with greater N independent of off-resonance condition (Fig. 2c),
while there would be bigger heterogeneity in contrast for physiological imaging
due to the prolonged delay time between PE-groups near k-space center. However,
proposed average schemes can reduce artifacts for small N, where the
heterogeneity is negligible. Therefore, the proposed schemes are
expected to improve the image quality of physiological MRI with the bSSFP
readout because of reduced artifacts from eddy-currents and transient
oscillations, smaller number of dummy scans, and equal temporal resolution.
Acknowledgements
No acknowledgement found.References
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Med 2005; 54:965-975. 3. Ahn, Magn Reson Med 1991; 17(1):149-163. 4.
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