Eric Kenneth Gibbons1, John Mark Pauly2, and Adam Bruce Kerr2
1Department of Bioengineering, Stanford University, Stanford, CA, United States, 2Department of Electrical Engineering, Stanford University, Stanford, CA, United States
Synopsis
SS-FSE is a robust method for fast image acquisition in areas where there is significant B0 inhomogeneity. Recent efforts have led to expand the capabilities beyond traditional constraints of SS-FSE meeting the CPMG condition. In this work, we examine the effects of various RF pulse types on the stability of the signal using a quadratic phase modulation as well as propose using a novel DIVERSE pulse.Introduction
SS-FSE
is a robust and fast method to acquire images in regions of $$$B_0$$$ inhomogeneity, unlike an EPI fast imaging sequence which presents
geometric image distortions in such regions. However, in order to
maintain signal throughout it’s long refocusing train, SS-FSE must
generally maintain the CPMG condition [1]. This becomes problematic
for sequences which introduce diffusion weighting after the initial $$$90^\circ$$$ as the excitation phase will be disturbed due to eddy current
effects and motion, which subsequently causes spatially varying
oscillation and rapid decay in the echo signal leading to image
banding and ghosting. Previous efforts [2-3] have been made to
overcome these issues, but each technique suffers from trade-offs. A
full signal can be recovered by the technique in [3] (nCPMG), but
this quadratic phase cycling requires that the refocusing flip angle
is above $$$120^\circ$$$. To achieve a flip angle uniform across a slice, a
more selective refocusing pulse is ideal, but this comes with a
longer pulse duration and ultimately longer ESP, which causes $$$T_2$$$ modulation (i.e., blurring). We examine these effects for pulses
with short as well as broad transition bands through simulation as
well as phantom data on a SS-FSE sequence using nCPMG phase cycling.
We propose a compromise in the form a recently proposed [4] DIVERSE
pulse with a similar slice profile as the higher TBW pulse while
achieving a suitable ESP for SS-FSE.
Methods
A
Bloch simulation was implemented to simulate the response of
100,000 spin isochromats placed over a 7.5cm spacing. This included a
uniform $$$90^\circ$$$ excitation over a 5cm slice but with variable phase to
demonstrate the effects when the excitation varies between being in
phase and in quadrature. Slice profile effects from the refocusing
pulses were included as described in [5]. Here two scenarios were
simulated: (1) a case where the refocusing pulse was a short
hamming-windowed sinc pulse with TBW=1.2 and (2) a TBW=3.5 SLR pulse.
This SS-FSE sequence was implemented on a 1.5T GE scanner.
Resolution phantom images were acquired using a 8ch cardiac coil
using FOV=20cm, 192x128 matrix, TR=1500ms. In vivo images were
acquired using a FOV=48cm. TE depended on ESP, which was dictated by
the pulse type. Three scenarios were tested: (1) a low TBW=1.2
windowed sinc pulse (ESP = 4.1ms) , (2) a TBW=3.5 SLR pulse (ESP =
6.5ms) , and (3) a
TBW=3.5 DIVERSE pulse (ESP = 4.9ms). Images were acquired and
reconstructed using a hybrid ESPIRiT/homodyne reconstruction as
described in [6].
Results
Simulation results in Fig. (2.a) show results from high and low TBW
pulses using this phase cycling scheme. The lower TBW pulses show
higher signal, but suggest less signal stability based on the
apparent signal modulation. Fig. (2.b) shows echo amplitude
variations that corroborates the simulation in terms of signal level
and stability. In all cases there is some signal modulation, which
is manifest in image aliasing when reconstructed. With nCPMG phase
cycling, the high TBW pulse had the most stable signal followed by
the DIVERSE and then low TBW pulse. Fig. (2) shows reconstructed
phantom images. The high TBW pulse performed the best by having the
minimum difference between the CPMG and nCPMG cases. The DIVERSE
pulse shows more aliasing and the low TBW pulse images show even more
aliasing. The low TBW pulse provided the least phase encode blurring
(seen on the left and right edges), while the DIVERSE followed
closely with the high TBW pulse showing much more blurring. Fig. (4)
shows in vivo data from a healthy volunteer. This data also supports
the data from the phantom data where image sharpness (likely from
both sharper slice profile as well as shorter ESP) is best with the
DIVERSE pulse.
Conclusion
We
have simulated and demonstrated in phantom and in vivo studies the effects of
various refocusing pulses on the signal stability and image quality
in nCPMG SS-FSE acquisitions. We have seen that while a high TBW SLR
pulse gives the most stable signal with the least aliased image, the
longer ESP leads to phase encode blurring. A good compromise, then,
is to use a DIVERSE pulse. We postulate that increased signal
instability with the DIVERSE pulse is due to eddy currents and
gradient RF system imperfections, and further work will include
modifying a DIVERSE that is tuned for a nCPMG
application, specifically reducing gradient slew requirements in
order to minimize the impact from eddy currents.
Acknowledgements
The authors would like to thank Patrick Leroux for thoughtful discussion as well as funding from GE Healthcare, NSF DGE-1147470, NIH P41 EB015891, NIH R01 EB009756.References
[1] Carr, Herman Y., and Edward M. Purcell. "Effects of diffusion on free precession in nuclear magnetic resonance experiments." Physical review 94.3 (1954): 630.
[2] Alsop, David C. "Phase insensitive preparation of single-shot RARE: Application to diffusion imaging in humans." Magnetic resonance in medicine 38.4 (1997): 527-533.
[3] Le Roux, Patrick. "Non-CPMG fast spin echo with full signal." Journal of Magnetic Resonance 155.2 (2002): 278-292.
[4] Kerr, A., et al. "Delay-Insensitive Variable-Rate Selective Excitation (DIVERSE)." Proc Intl Soc Mag Reson Med. Vol. 23. 2015.
[5] Pauly, John, et al. "Parameter relations for the Shinnar-Le Roux selective excitation pulse design algorithm [NMR imaging]." Medical Imaging, IEEE Transactions on 10.1 (1991): 53-65.
[6] Gibbons, E., et al. "Body DWI Using nCPMG FSE." Proc Intl Soc Mag Reson Med. Vol. 23. 2015.