Synopsis

Multiband accelerated cine SSFP of the heart is demonstrated using blipped gradients for controlled aliasing of simultaneous slices. The benefit of using this method over the more complex phase cycling RF multiband pulse sets is that the frequency response, and thus banding artefacts, remain unchanged. Here we demonstrate up to MB4 acceleration of full short-axis stacks, sufficient to cover the left ventricle within a single breath-hold.

Background

Multiband (MB) acceleration offers the key benefit over in-plane sub-sampling strategies of maintaining the SNR through sample averaging¹. Reconstructed images still suffer from a g-factor penalty, but this can be greatly reduced by combining with controlled aliasing patterns (CAIPIRINHA)².

Previously, CAIPIRINHA-SSFP has been demonstrated with RF phase modulation used to shift slices^3,4. A drawback is that this also shifts the SSFP frequency response, leading to a shift in banding-artefacts making it challenging for cardiac applications. Blipped multiband using gradients to apply a spatially localised phase shift between simultaneous slices can also be applied to balanced SSFP⁵. Gradient blipping is invisible to the frequency response so that standard RF phase alternation can be combined with the most beneficial aliasing pattern for g-factor, ultimately freeing up the use of higher MB factors.

Here we demonstrate blipped multiband cine SSFP at 3T, with up to MB=4 providing good results, allowing for full left ventricle coverage within a single breath-hold.

Methods

Blipped multiband SSFP was implemented on a 3T Philips Achieva system. Pilot data was collected on 6 healthy adult volunteers with a 32-channel cardiac coil.

Retrospective-gated SSFP cine stacks were collected using standard single-band (SB) RF pulses in all subjects alongside MB=2,3,4 accelerated acquisitions with varying shift patterns. In all cases resolution was 2x2x8mm, 12 short-axis slices, 30 cardiac phases, half-Fourier=0.63, FA=40°, TE/TR=2/4ms, but varying phase-encode (fold over) dimensions and heart rates resulted in different breath-hold requirements (typically 6 breath-holds for a short axis stack - see figure 1), MB4 stacks were all collected within a single extended breath-hold (range: 27-33 seconds).

Images were reconstructed offline utilising ReconFrame (GyroTools) and custom MB-unfolding based on an iterative SENSE approach for joint estimation of sensitivity profiles and unfolded data⁶.

Results and Discussion

Comparative data from a single subject using SB, MB2, MB3, and MB4 accelerated stacks is shown in Figure 1. Reconstructed images reveal good SNR and blood-myocardium contrast for all MB factors, with banding artefact locations unchanged from the single band reference. Fewer breath-holds are required for higher MB factors, which reduces subject stress and promotes greater consistency in the through slice direction. In this example, integrated breath-hold times were 114 seconds for SB reducing geometrically to 28 seconds for MB4.

Figure 2 demonstrates the advantage of using gradient blip shifts over RF modulation, where the SSFP response also shifts. Here a simple 90° phase shift was applied to alternate MB2 pulses resulting in the upper slice being positioned at the centre of the pass-band, while the second slice is at the band-edge (Δf=1/TR). The second slice in this case is unusable and also leaks into the first slice, whereas in the blipped gradient both slices are comparable to the single-band reference, just avoiding banding in the blood pool.

In practice, alternative phase-cycling patterns to shift slices in opposite directions should be used to avoid placing slices directly in the stop-band, but ultimately there is always a trade-off between effective pass-band width and optimal shift for g-factor⁷. This is fully mitigated by using gradient blipping instead.

MB4 acceleration allowed for a full acquisition in a single (extended) breath-hold. Some residual unfold artefacts are observed. These are influenced by the double oblique short-axis imaging plane being suboptimal for coil encoding with the receiver array. Improved reconstruction techniques and dedicated reference scans may be required to improve these.

Peak B1 and SAR limits are important constraining factors for implementation of MB in SSFP, particularly at 3T. Further developments in MB pulse design could help reduce peak B1, however SAR may remain a challenge. Application of the same method at 1.5T may therefore be less constrained and could lead to larger potential gains in acceleration.

Conclusion

This study demonstrates blipped multiband SSFP cine imaging of the heart. Results suggest it could provide a valuable additional acceleration method to more conventional in plane and k-t acceleration techniques, without the associated loss of SNR.

The use of gradient blips to encode slice shifts, rather than RF phase cycling, retains the full SSFP frequency response minimising banding artefacts at 3T. MB4 allowed for full ventricular coverage in a single breath-hold. The acceleration could equally well be deployed to increase resolution, thus potentially allowing a more isotropic protocol that could save time by eliminating complex piloting to achieve doubly oblique views.

Acknowledgements

References

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