Synopsis

The simultaneous multi-slice (SMS) imaging technique CAIPIRINHA has proven to be highly efficient for extending the slice coverage in 2D imaging. When accelerating balanced steady-state free-precession (bSSFP) sequences with SMS-CAIPIRINHA, modulating k-space by means of slice-specific RF phase cycles leads to undesired slice-specific shifts of the bSSFP pass-band structure. Gradient-controlled local Larmor adjustment (GC-LOLA) removes this drawback. By means of slice gradient unbalancing, the Larmor frequency is made slice position dependent, which allows compensating for the pass-band shifts and stabilizes CAIPIRINHA-accelerated bSSFP imaging with respect to B0 field inhomogeneity.

Target Audience

Introduction

Balanced steady-state free-precession (bSSFP) imaging has become the standard acquisition technique for applications such as cardiac MRI by providing an intrinsically high signal-to-noise ratio (SNR), a unique contrast, short acquisition times and robustness in the presence of flow and motion. Accelerating bSSFP with the simultaneous multi-slice (SMS) imaging technique CAIPIRINHA¹ has been shown to be highly SNR efficient for extending the slice coverage in 2D imaging², and two distinct methods, namely RF phase cycling based encoding² and blipped gradient encoding³ can be employed. The first provides each slice with an individual RF phase cycle, which conserves the sequence’s robustness to flow and motion, but leads to a higher susceptibility to B0 field inhomogeneity. The second, by contrast, employs balanced slice gradient blipping⁴ to realize the combination. This, however, involves toggling the gradients every TR, potentially increasing the sensitivity to eddy currents, particularly for small inter-slice distances.

In this work, we propose a new method, called gradient-controlled local Larmor adjustment (GC-LOLA) that eliminates the drawback of the first approach and stabilizes CAIPIRINHA accelerated bSSFP imaging with respect to B0 field inhomogeneities.

Theory

SMS-CAIPIRINHA excites multiple slices at the same time and shifts them with respect to each other in the FOV by providing each slice with a specific phase modulation in k-space. When combining SMS-CAIPIRINHA with bSSFP, this phase modulation has to be generated while maintaining the bSSFP steady state within each simultaneously excited slice. To achieve this, the RF phase cycling based encoding technique (2) uses an individual RF phase cycle in each slice. The phase of the j^th RF pulse in slice s is given by:

$$ \psi_{S}(j) =\phi_{S} \cdot j + C_{S} $$

with $$$\phi_{S}$$$ being the RF phase increment and $$$C_{S}$$$ denoting an arbitrary slice-specific constant. With respect to standard bSSFP, an undesired slice-specific shift $$$\Delta \Theta_{S} = (\phi_{S}-\pi)$$$ of the pass-band structure along the off-resonance axis is introduced, effectively reducing the overall off-resonance robustness depending on the phase cycling employed (Fig. 1a).

Gradient-controlled local Larmor adjustment (GC-LOLA) eliminates this drawback and removes the bSSFP pass-band shifting by:

1. slightly unbalancing the slice-select gradient so that the Larmor frequency difference between the individual slices compensates for the pass-band shifting (Fig. 1b), and
2. modifying the RF phase cycles homogeneously across all slices to align the centers of the pass-bands to resonance.

A simple two-slice example is given in Fig. 1. The two slices S₁ and S₂ located at z₁ and z₂ are acquired using the RF phase cycles with $$$\phi_{1,2} = \pm \pi/2$$$. The off-resonance bandwidth is effectively reduced by a factor of two (Fig. 1a). To correct for the pass-band shift, the slice gradient is unbalanced by the momentum

$$ \Delta M = \frac{\phi_{1}-\phi_{2}}{\gamma(z_{1}-z_{2})} $$

which might be evenly distributed on the slice pre- and rephaser. The remaining residual pass-band shifts $$$\Delta \Theta_{1}^{*}$$$ and $$$\Delta \Theta_{2}^{*}$$$ may be corrected for by subtracting

$$\phi_{G} = \frac{\phi_{2}z_{1}-\phi_{1}z_{2}}{z_{1}-z_{2}}$$

from $$$\phi_{1}$$$ and $$$\phi_{2}$$$ (Fig. 1b).

Methods

Results

Discussion

Acknowledgements

References

1. Breuer FA, Blaimer M, Heidemann RM, et al. Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging. Magnetic Resonance in Medicine 2005;53:684–69.
2. Stäb D, Ritter CO, Breuer FA, et al. CAIPIRINHA accelerated SSFP imaging. Magn. Reson. Med. 2011;65:157–164.
3. Duerk J, Griswold M, Dara K. Multi-slice Blipped TrueFISP-CAIPIRINHA. United States Patent Application US 9,086,468 (2015).
4. Setsompop K, Gagoski BA, Polimeni JR, et al. Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty. Magn. Reson. Med. 2012;67:1210–1224.
5. Stäb D, Speier P, et al. Restating MS-CAIPIRINHA as an In-Plane Acceleration Problem: an Efficient Method for Integrating High Coverage Cardiac Perfusion MRI Into Clinical Workflow. Proc. Intl. Soc. Magn. Reson. Med 23 (2015), #2686.