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Parallel Transmit Optimized Spiral Composite Adiabatic Pulses for 3D Spatial-Spectral Selectivity in Spectroscopy
Xiaoxuan He1, Edward Auerbach1, Michael Garwood1, Xiaoping Wu1, and Gregory J Metzger1

1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Synopsis

In this abstract we demonstrated by simulation and phantom experiments the feasibility of designing composite refocusing pulse for spectroscopy using parallel transmission, featuring inherent 2D spatial and spectral selectivity along with improved immunity to B0 non-uniformity and B1+ heterogeneity while simultaneously providing lipid and water suppression and short echo times of ~18ms.

Introduction

For magnetic resonance spectroscopy, the ability to observe and quantify metabolites of interest is usually compromised by B0 and B1+ inhomogeneity, compromised echo times and SNR for improved spatial selectivity and the necessity to accommodate background signal suppression (i.e. water and lipids) depending on the application. Typical UHF spin echo spectroscopy sequence uses pairs of adiabatic refocusing pulses for high bandwidth spatial localization, along with additional pulses for water and fat suppression. However, this comes at the cost of a long echo times high RF power deposition along with limited flexibility to perform a variety of studies in the brain and body such as relaxometry. In this study, we present a novel composite pulse that uses an adiabatic envelope and parallel transmission (pTx) optimized subpulses that have inherent 2D-spatial and 1D-spectral selectivity. As a proof of concept, we showed an example of designing a composite-spiral refocusing pulse with both 2D-spatial and 1D-spectral selectivity with an intended application to prostate spectroscopy. Both simulation and phantom validations were conducted to evaluate the feasibility and performance.

Methods

- B0/B1+ Field Mapping

For the pTx subpulse design, the B0 and B1+ fields of a prostate spectroscopy phantom were acquired on a Siemens Magnetom 7T scanner (German, Erlangen, Siemens) using an 8-channel transmit and 32-channel receive coil. The per-channel B1+ mapping was calculated using the previously described hybrid method that combines relative B1+ mappings with AFI measurement (Ref 1). The B0 mapping was acquired by a dual-echo GRE sequence. (Figure 1)

- Composite Pulse Design

The pTx sub-pulse was first designed using the spatial domain method (Ref 2), incorporating a mask of the prostate phantom along with the acquired B0 and B1+ field maps. A spiral-out excitation k-space trajectory with a balanced gradient design was used to achieve the desired excitation pattern (Figure 2). The composite pulse was then synthesized by modulating the subpulse with an adiabatic pulse envelope (Ref3). The duration of the sub-pulse, denoted as tp, dictates the periodicity of frequency response of the adiabatic envelope after discretization. For a composite pulse consisting of N subpulses, the time bandwidth product has a relationship of R = BW * N * tp. For prostate spectroscopy, the metabolites of interest range from 3.2ppm (choline) to 2.6ppm (citrate), requiring at least 0.6ppm of passband in the composite pulse. To suppress water (4.65 ppm) and lipid (1.3ppm) at stopband, the subpulse was tailored to have a duration of 1ms, yielding a 1kHz periodicity in the frequency response of the composite pulse when imaging at 7T. To satisfy the passband requirement, an 18-point HS1R9 adiabatic pulse was used as the envelope pulse and gave the composite pulse an approximate bandwidth of 500Hz.

- Phantom Validation of Selectivity

To investigate the robustness of the designed composite pulse against off-resonance, the spatial and spectral profile were calculated for off-resonance ranging from -1,000 Hz to 1,000 Hz in a step size of 10 Hz. The crushed spin echo profile was also calculated to estimate the spatial and spectral profile after spoiling the signals with imperfect refocusing (Figure 3). Customized 3D GRE sequence and spectroscopy sequence were implemented to read the designed pulse and verify the designed spatial and spectral profile. For the spectroscopy, a slice selective excitation pulse was used, yielding a 3D spatial excitation when combined with the designed 2D spatially selective refocusing pulse.

Results

Both the simulation and phantom experiment showed that the spatial and spectral profiles of the designed composite pulse closely matched the desired pattern (figure). The simulated crushed spin echo profile had an approximately 260Hz of 95% passband (Figure 3), with stopband for water and fat approximately 320 Hz. Inherent water and fat suppression (Figure 4) of the designed pulse were confirmed in the phantom experiment.

Discussion

To our best knowledge, this is the first demonstration of designing spectral and spatial selective pulse using parallel transmission, with an example of composite refocusing pulse for prostate spectroscopy. This novel design approach enabled field following excitation and extended selectivity to the spectral domain. As a preliminary result, there is still great potential for improvement such as by gradient optimization and adiabatic pulse design. The proposed method could also be applied to design the excitation pulse which will enable even more flexibility and potentially improved performance for other useful applications in the brain and body.

Conclusion

We have demonstrated the feasibility of designing a spatial and spectral selective composite refocusing pulse using pTx optimized spiral sub-pulses, with an intended application to prostate spectroscopy. The tailored composite adapts to the measured B0 and B1+ fields, yielding improved robustness against field non-uniformities, along with potentially great flexibility in different applications.

Acknowledgements

Supported by: NCI R01 CA155268, NIBIB P41 EB015894.

References

Wu, X., Auerbach, E. J., Vu, A. T., Moeller, S., Van de Moortele, P. F., Yacoub, E., & Uğurbil, K. (2019). Human Connectome Project-style resting-state functional MRI at 7 Tesla using radiofrequency parallel transmission. NeuroImage, 184, 396-408.

Grissom, W., Yip, C. Y., Zhang, Z., Stenger, V. A., Fessler, J. A., & Noll, D. C. (2006). Spatial domain method for the design of RF pulses in multicoil parallel excitation. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine, 56(3), 620-629.

Garwood, M., & Uğurbil, K. (2018). RF pulse methods for use with surface coils: Frequency-modulated pulses and parallel transmission. Journal of Magnetic Resonance, 291, 84-93.

Figures

Figure 1. Prostate phantom setup (a) and the measured B0 and B1+ field maps (b & c) for pTx pulse design. The phantom consisted of a an out solution of doppedsaline inside of which was a 3D printed prostate phantom with rough dimensions of 4 x 6 x 5 cm3. The inner volume of the prostate phantom was filled with a solution to approximate the cation and metabolite concentrations in the human prostate (blue arrow) and the secondary outer compartment was fill with oil to represent peri-prostatic lipids (yellow arrow).

Figure 2. Tailored spiral subpulse (a) using pTx and parent pulse (b) used in the study. The subpulse was tailored to be 1ms in duration, with a spiral-out k-space trajectory with maximal gradient amplitude 14mT/m and slew rate 171T/m/s. For the parent pulse, a 38-point HS1R9 adiabatic pulse was used, with the amplitude and phase shown in the figure.

Figure 3. The resulting composite pulse (a) had a total duration of 18ms. The spatial profile (b) and crushed spin echo profile (c) were calculated by Bloch simulation. Note that the pulse has a sharp spatial selectivity profile, and an approximately 260Hz width of 95% passband, which could be even improved by using an adiabatic pulse with higher R-value at the expense of longer duration. However, with more transmit channels, the pulse duration could be shortened.

Figure 4. (a) A gradient echo imaging showing the 2D spatially selective spiral profile. (b) The planning of an axial 24x24 CSI grid on a sagittal scout through the prostate phantom. (c) Spatially resolved spectra showing localization of signal to the designed region shown in (a). (d) A zoomed in version of (c). (e) The spectral selectivity of the composite pulse shows excellent suppression of both water and fat. No additional water or lipid suppression pulses were used. (f) The spectra from the prostate phantom demonstrates high quality spectra with little J-coupling as expected with its short echo time (18ms).

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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