Composite pulses for RF phase encoded MRI
Somaie Salajeghe1, Paul Babyn2, and Gordon E. Sarty1

1Biomedical Engineering, University of Saskatchewan, Saskatoon, SK, Canada, 2Medical Imaging, University of Saskatchewan, Saskatoon, SK, Canada

Synopsis

RF phase encoded MRI uses spatial RF phase gradients in place of B0 gradients coils to encode information. Using a nonlinear RF phase gradient coil instead of linear one leads to a larger field of view. However, this coil will generate an in-homogeneous B1 field which result in pulse imperfection. To minimize the effect of pulse imperfection, the application of composite pulses is required. Current composite pulses were designed to use in current MRIs (having gradient coils and uniform RF coil), so the feasibility of working composite pulses with an RF encoded coil needs to be checked.

Purpose:

RF phase encoded MRI encodes spatial information through the use of spatially varying $$$B_{1}$$$ phase instead of using traditional $$$B_{0}$$$ gradient coil systems [1]. Designing a linear RF phase gradient coil is challenging and it leads to a limited spatial region after some optimization of coil geometry [2]. A less challenging design and a larger field of view (FOV) is possible using a nonlinear RF phase gradient coil [3]. This coil will generate an in-homogeneous $$$B_{1}$$$ field which leads to a spatially varying flip angle. To minimize the effect of the in-homogeneity of the $$$B_{1}$$$ field on the flip angle, the application of composite pulses is required. Different types of composite pulses have been reported in the literature to compensate for pulse imperfection in conventional MRIs (having gradient coils and uniform RF coils). Here we explore the feasibility of using composite pulses with an RF encoded coil and select the most appropriate composite pulse for compensating pulse imperfection in an RF encoded coil.

Theory/Methods:

RF encoded signals were simulated for a 2D discrete Shepp and Logan mathematical phantom inside an RF encoding coil using Carr-Purcell [4] and Meiboom-Gill [5] sequences with four different composite pulses. Composite pulses are composed of a cluster of RF pulses with different phases and short intervals in between. There are three different types of composite pulses: symmetric, asymmetric and antisymmetric. We applied all the composite pulses reported in the literature and selected the best one for each type of symmetric, asymmetric and antisymmetric according to the minimum root mean squared errors between the reconstructed image and the original phantom.

Carr-Purcell multiple spin echo pulses consist of a $$$90^0$$$ RF excitation pulse from a uniform coil followed by a train of $$$180^0$$$ pulses made through appropriate RF encoding coils and a uniform coil. To encode information in $$$M\times N$$$ dimensions using Carr-Purcell pulses, the RF pulse sequences described below can be used. To encode points in the x and y directions, the following pulse sequences, composed of alternating blocks of $$$180_x $$$, $$$180_y $$$ ($$$180^0$$$ tip angle excitation with the x and y direction RF encoding coils) and $$$180_U$$$ ($$$180^0$$$ tip angle excitation with a uniform coil) pulses, are required: $$90_U – 180_y - [180_U - S – 180_y - S]_{m/2}- [ 180_x - S – 180_U - S] _{N/2}$$ $$90_U – 180_y - [180_U - S – 180_y - S]_{m/2}- [ 180_{-x} - S – 180_U - S]_{ N/2}$$ to obtain $$$N+1$$$ points in the x direction and $$$M+1$$$ points in the y direction.

To use a Meiboom-Gill pulse sequence, a $$$90^0$$$ phase shift was added between the $$$90^0$$$ excitation and the first $$$180^0$$$ refocusing pulse relative to the Carr-Purcell RF pulse sequences for both x and y directions. The four different composite pulses of Levitt [6], Tycko1[7], Tycko2 [8], Wimperis [9] were used in place of a single $$$180^0$$$ tip angle excitation for the RF encoding coils in the Meiboom-Gill RF pulses, see Table (1).

The simulated signals were reconstructed using a constrained least squares method followed with a total variation technique.

Results/Discussion:

Our results indicate that composite pulses can be used to minimize the effect of an in-homogeneous $$$B_1$$$ field on the flip angle produced by an RF encoding coil.

Conclusion:

The composite pulse proposed by Tycko (Tycko2) created images with fewer artifacts and is the best one for decreasing the effects of pulse imperfections among the RF pulses evaluated in this study.

Acknowledgements

This work was support by an NSERC Discovery Grant to GES.

References

[1] J.C. Sharp, S.B King. Magn Reson Med 63:151-161(2010).

[2] Q. Deng et al. Magn Reson Imaging 31:891-899(2013).

[3] S. Salajeghe et al. ISMRM (2015).

[4] Carr H, Purcell E. Phys Rev 1954;94(3):630-8.

[5] Meiboom S, Gill D. Rev Sci Instrum 1958;29(8):688-91.

[6] Levitt MH, Freeman R. J Magn Reson 1979;33(2):473-6.

[7] Tycko R. Phys Rev Lett 1983;51:775-7.

[8] Tycko R, Cho H, Schneider E, Pines a. J Magn Reson 1985;61(1):90-101. [9] Wimperis S. Magn Reson (1969) 1991;93(1):199-206.

Figures

Figure 1: $$$64\times 64$$$pixel reconstruction using different composite RF pulse trains. Noise at an SNR of 50 dB was added to the signals prior to reconstruction. (a) Carr-Purcell. (b) Meiboom-Gill, (c) Levitt, (d) Tycko1, (e) Tycko2, (f) Wimperis.

Table 1: Composite pulses used for the RF encoding $$$180^0$$$ tip angle excitations in Meiboom-Gill RF pulse trains. Here for example $$$90^{0}_{90}$$$, denotes a $$$90^0$$$ tip angle excitation about an axis $$$90^0$$$ from the x axis.



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