Multi-Dimensional Reduced Field-of-View Excitation by Integrated RF Pulse and DYNAMITE B0 Field Design
Suryanarayana Umesh Rudrapatna1, Robin de Graaf1, Terrance Nixon1, and Christoph Juchem1

1Yale University, New Haven, CT, United States

Synopsis

Spatially selective multi-dimensional excitation with large flip angles is challenging, as current RF pulse design methods are computationally involved and typically yield low time-bandwidth product (TB < 10) pulses. The difficulty stems from Bloch equation non-linearity and the inflexibility in B0 pattern generation using linear gradients. This study uses the dynamic multi-coil technique (DYNAMITE), that provides unprecedented B0 shaping flexibility, facilitating the use of 1-dimensional Shinnar-Le Roux (SLR) pulses for 2-dimensional excitation. Selective mouse brain excitation was accomplished by adaptively designing the SLR pulse and the underlying B0 fields generated by DYNAMITE. The resultant zoomed MRI achieved more than two-fold acquisition acceleration at < 4% undesired excitation with TB > 12 pulses.

Purpose

Reduced field-of-view (rFOV) imaging can provide vast benefits and flexibility in terms of acquisition time and spatio-temporal coverage, when the region-of-interest (ROI) is smaller than the full FOV (fFOV).1–6 For gradient-echo imaging, the only viable rFOV method at short repetition times (e.g. 3D imaging, EPI) is through selective multi-dimensional (2D/3D) RF excitation (SELEX).7–12,14 Although small tip angle solutions exist,3,7,8 rFOV imaging benefits from large tip angle (LTA) excitation, and many acquisitions even require accurate LTA excitation.13 The methods proposed till date9-12,14 are either approximations,11 or non-unique,10 or computationally involved.10,12 Most methods typically yield excitation pulses with low time-bandwidth product (TB < 10), leaving them vulnerable to B0 inhomogeneity-related signal loss and distortion at high fields. Here, we propose to use the versatile field shaping capability of the dynamic multi-coil technique (DYNAMITE),15–18 in conjunction with 1D-Shinnar-Le Roux (SLR) pulses19 to obtain 2D SELEX. We show that this combination can yield highly selective (TB > 12) 2D excitation.

Methods

We identified mouse brain imaging (fast 3D T1 and T2 mapping13 and EPI) as potential applications for this technique. Previously described DYNAMITE hardware15,16 comprising of 48 coils was used for the studies. The main challenge in the proposed method was to meaningfully link the DYNAMITE field distribution to the 1D SLR pulse to be designed for excitation. Since SLR pulse design inherently links bandwidth (BW), transition width (TW), pass-and stop-band ripples and the pulse TB,19 we used an adaptive pulse design approach. The coil currents that produce the desired B0 field and the corresponding excitation pulse were concurrently optimized to maximize SELEX of the brain.

A transition zone was defined16 around the mouse brain ROI (Fig.1A). The ROI spanned 9x7.5x10 mm in x (phase encode), y (phase encode 2) and z (readout) directions. In the transition zone, varying B0 gradients (100 to 400 Hz/pixel) were prescribed separately (Fig.1A) to identify a starting point for the optimization of SELEX coil currents. For a given field distribution, constrained-least-squares fitting was performed at $$$\pm$$$1 A dynamic range to estimate currents that minimized the errors in the fit to the field distribution. Simultaneously, the fit restricted the fields generated within the ROI to positive values and forced the fields in the region of non-interest (RON) to negative values (Fig.1A). From this partitioned field distribution, the BW in the ROI and TW in the transition zone were used to design a suitable minimum (or linear) phase 90o SLR excitation pulse. The pass- and stop-band ripple were limited to 1%. Bloch-equation simulations provided off-resonance behaviour of the pulse, which was used to assess the objective function $$$\sum_{ROI}sin(flip angle)/\sum_{RON}sin(flip angle)$$$. After identifying suitable starting coil currents for SELEX, the restrictions on the DYNAMITE fields generated in the transition zone was removed and the objective was maximized using a local optimizer. The optimization also yielded the corresponding SLR pulse and the required transmitter frequency offset for SELEX. The process was fully automated, requiring only ROI prescription. Proof-of-principle fast 3D gradient-echo experiments were performed in two euthanized mice at 9.4 T. The DYNAMITE hardware was also used for background B0 shimming, including during excitation.

Results

Successful 2D SELEX could be achieved in the experiments, with TB > 12 SLR pulses.The predicted and measured excitation patterns matched closely (Fig.1C,G). The FOV reduced from 15x15 mm to 9x12 mm in the two phase encoding dimensions. For isotropic acquisitions, this translates to acquisition acceleration of 2.1. The measured rFOV/fFOV signal ratio (Fig.1H) had Mean $$$\pm$$$ SD = 0.987 $$$\pm$$$ 0.069. The ratio in the RON had Mean $$$\pm$$$ SD = 0.0386 $$$\pm$$$ 0.023, indicating near absence of excitation. Images obtained from an accelerated rFOV excitation experiment are shown in Fig.2, and show no signs of aliasing. The coronal and sagittal fFOV and SELEX results are shown in Fig.3.

Discussion

The DYNAMITE approach simplifies the difficult problem of multi-dimensional SLR excitation to the familiar 1D SLR excitation case through its versatile B0 field shaping capability. In combination with background B0 shimming, it is a powerful tool for rFOV excitation, especially due to the high TB achievable. A unique attribute of rodent brain imaging which leads to high TB pulses is that, using DYNAMITE, SELEX of open-sided 2D problems are more amenable than fully enclosed regions. Inclusion of the skin above the skull in the ROI significantly increased the achievable TB, compared to restricting the ROI to the brain. Increasing coil current dynamic range from 1 A to e.g. 5 A is expected to mitigate this, and provide high TB SELEX over fully closed ROIs.

Acknowledgements

This research was supported by NIH grants P30-NS052519, R01-EB000473 and R01-EB014861.

References

1. Alley MT, Pauly JM, Sommer FG, and Pelc NJ. Angiographic imaging with 2D RF pulses. Magnetic Resonance in Medicine, 1997, 37(2):260–267.

2. Yang GZ, Gatehouse PD, Keegan J, Mohiaddin RH, and Firmin DN. Three-dimensional coronary MR angiography using zonal echo planar imaging. Magnetic Resonance in Medicine, 1998, 39(5):833–842.

3. Rieseberg S, Frahm J, and Finsterbusch J. Two-dimensional spatially-selective RF excitation pulses in echo-planar imaging. Magnetic Resonance in Medicine, 2002, 47(6):1186–1193.

4. Zhao L, Madore B, and Panych LP. Reduced field-of-view MRI with two-dimensional spatially-selective RF excitation and UNFOLD. Magnetic Resonance in Medicine, 2005, 53(5):1118–1125.

5. Glaser KJ, Felmlee JP, and Ehman RL. Rapid MR elastography using selective excitations. Magnetic Resonance in Medicine, 2006, 55(6):1381–1389.

6. Saritas EU, Cunningham CH, Lee JH, Han ET, and Nishimura DG. DWI of the spinal cord with reduced FOV single-shot EPI. Magnetic Resonance in Medicine, 2008, 60(2):468–473.

7. Pauly J, Nishimura D, and Macovski A. A k-space analysis of small-tip-angle excitation. Journal of Magnetic Resonance (1969), 1989, 81(1):43 – 56.

8. Yip Cy, Fessler JA, and Noll DC. Iterative RF pulse design for multidimensional, small-tip-angle selective excitation. Magnetic Resonance in Medicine, 2005, 54(4):908–917.

9. Pauly J, Nishimura D, and Macovski A. A linear class of large-tip-angle selective excitation pulses.Journal of Magnetic Resonance (1969), 1989, 82(3):571 – 587.

10. Xu D, King KF, Zhu Y, McKinnon GC, and Liang ZP. Designing multichannel, multidimensional,arbitrary flip angle RF pulses using an optimal control approach. Magnetic Resonance in Medicine,2008, 59(3):547–560.

11. Grissom W, Xu D, Kerr A, Fessler J, and Noll D. Fast Large-Tip-Angle multidimensional and parallel RF pulse design in MRI. Medical Imaging, IEEE Transactions on, Oct 2009, 28(10):1548–1559.

12. Grissom WA, McKinnon GC, and Vogel MW. Nonuniform and multidimensional Shinnar-Le RouxRF pulse design method. Magnetic Resonance in Medicine, 2012, 68(3):690–702.

13. Deoni SC, Rutt BK, and Peters TM. Rapid combined T 1 and T 2 mapping using gradient recalled acquisition in the steady state. Magnetic Resonance in Medicine, 2003, 49(3):515–526.

14. Ma C and Liang ZP. Design of multidimensional Shinnar–Le Roux radio frequency pulses. Magnetic Resonance in Medicine, 2015, 73(2):633–645.

15. Juchem C, Brown PB, Nixon TW, McIntyre S, Rothman DL, and de Graaf RA. Multicoil shimming of the mouse brain. Magnetic Resonance in Medicine, 2011, 66(3):893–900.

16. Juchem C, Nixon TW, Brown PB, McIntyre S, Rothman DL, and de Graaf RA. Spatial selection through multi-coil magnetic field shaping. In Proceedings of the International Society of MRM,number 19, page 385, 2011.

17. Juchem C, Green D, and de Graaf RA. Multi-coil magnetic field modeling. J Magn Reson, 2013,236:95 – 104.

18. Juchem C, Nahhass OM, Nixon TW, and de Graaf RA. Multi-slice MRI with the dynamic multi-coil technique. NMR in Biomedicine, 2015, 28(11):1526–1534.

19. Pauly J, Le Roux P, Nishimura D, and Macovski A. Parameter relations for the Shinnar-Le Roux selective excitation pulse design algorithm [NMR imaging]. Medical Imaging, IEEE Transactions on,Mar 1991, 10(1):53–65.

Figures

Figure 1: A: Planning of 2D SELEX overlaid on anatomy (B). ROI (RON) denote region-of-interest (no-interest). B0 distribution in transition zone determines DYNAMITE currents, C: Predicted SELEX, D:Predicted rFOV/fFOV signal ratio, E: Background B0 inhomogeneity, F: Predicted DYNAMITE field for SELEX (full range=4200 Hz), G: Experimental SELEX (3D-gradient-echo), H: Measured rFOV/fFOV signal ratio.

Figure 2: Example rFOV 3D-gradient-echo acquisition (12x9x10 mm) instead of fFOV (15x15x40 mm), showing excitation across different z-locations. No visible aliasing was detected. Sequence parameters: TR: 20ms, TE: 5ms, Averages: 8, Acquisition matrix: 64x64x64, RF pulse: 3.37ms SLR minimum phase (designed for 90o, actual flip $$$\approx$$$ 20o), TB: 13.5, offset: 2050Hz, BW: 3.61kHz.

Figure 3: Images corresponding to sagittal fFOV slice (A) and rFOV slice (B) and coronal fFOV slice(C) and rFOV slice (D) obtained from 3D gradient-echo acquisitions in one of the mice. Apart from 2D-SELEX, result B demonstrates possible FOV reduction in z-direction as well.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
1010