Slice Profile Effects on non-CPMG SS-FSE Acquisitions
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.

Figures

The nCPMG SS-FSE sequence using DIVERSE pulses. As variable rate refocusing pulses such as these can lead to ghosting off-isocenter, careful control of the refocusing modulation was used to ensure no ghosting far off (upwards 15cm) of isocenter.

Signal evolution throughout the course of SS-FSE echo train. (left): Bloch simulation with both time bandwidth pulses. (right): water phantom signal data (signal peak without phase encoding) for each pulse type.

Water phantom images using the CPMG and nCPMG sequences. (first column): standard CPMG acquisitions (second column): nCPMG acquisitions reconstructed using the parallel imaging method (last column): absolute difference between the normalized CPMG and nCPMG images. Each row corresponds to the pulse type used.

In vivo data from a healthy volunteer using the CPMG and nCPMG sequences. (first column): standard CPMG acquisitions (second column): nCPMG acquisitions reconstructed using the parallel imaging method. Each row corresponds to the pulse type used. The red and yellow arrows indicate areas where image sharpness difference between each image is particularly apparent.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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