3D first-pass myocardial perfusion stack-of-stars imaging using balanced steady state free precession
Merlin J Fair1,2, Peter D Gatehouse1,2, Liyong Chen3,4, Ricardo Wage2, Edward VR DiBella5, and David N Firmin1,2

1NHLI, Imperial College London, London, United Kingdom, 2NIHR Cardiovascular BRU, Royal Brompton Hospital, London, United Kingdom, 3UC Berkeley, Berkeley, CA, United States, 4Advanced MRI Technologies, Sebastopol, CA, United States, 5UCAIR, University of Utah, Salt Lake City, UT, United States

Synopsis

A method for enabling a balanced steady-state free precession 3D stack-of-stars approach to whole-heart first-pass myocardial perfusion imaging is investigated. Consideration is made of the impact of potential off-resonance effects at 3T and sequence-based modifications to rectify this are examined. Demonstration of the feasibility of this approach is then performed in-vivo.

Introduction

Spoiled gradient-echo (SGRE) and balanced steady-state free precession (bSSFP) are both common sequence readouts utilised after a saturation preparation pulse in myocardial first-pass perfusion (FPP). The refocusing and typically higher flip angles used in bSSFP are well known to increase SNR compared to SGRE, making it particularly suited to FPP. This is important in the case in 3D FPP, where high undersampling rates lose the SNR advantages conferred by 3D acquisition.

Despite this, 3D FPP has so far used only SGRE based readouts1, with the exceptions of specialised examples employing dual-channel transmission2 or continuous acquisition schemes3, both using rectilinear imaging.

For 3D imaging, the slab-selective RF excitation is generally longer than for slice-selection, leading to a slower TR and therefore more off-resonance/stabilisation artefacts in bSSFP, especially at the higher field strengths preferred for FPP.

This work aims to investigate and optimise the performance of bSSFP with 3D radial (‘stack-of-stars’, SOS) acquisition for FPP, including sequence modification aiming to reduce off-resonance artefacts, with first in-vivo demonstrations of bSSFP 3D SOS feasibility.

Methods

A 3D stack-of-stars4 approach was taken (Figure 1), with in-plane diametrical sampling combined with conventional through-plane phase-encoding. Asymmetric readouts (75%) were employed to reduce repetition time, with further partial Fourier applied to the slice direction (75%) to minimise shot duration. Overall shot duration was further reduced by increased undersampling of outer kz partitions, lowering the total number of acquired rays.

Acquisitions were performed with 95 rays at TE/TR: 1.3/2.8ms, SRT (from saturation to central raw-data): 190ms and shot time: 270ms excluding 10 linearly incrementing flip angle startup TRs5. This produced 12-14 usable reconstructed slices, after additional kz zero-padding, with voxel size 2.1x2.1x5.0mm.

Experiments were performed at 3T (Siemens Skyra) with 18-channel anterior and 12-channel posterior arrays. MATLAB (Mathworks) was used to reconstruct the datasets, with non-uniform Fourier transform (NUFFT)6 for early analysis and phantom work, and a temporally constrained reconstruction7 for full reconstructions, with α=0.7 and 50 iterations.

A custom-tailored RF pulse was designed for optimal trade-off between minimal duration, ideal slab select profile (for minimal slab-wraparound affecting edge slices, considering impact of FPP T1 changes on bSSFP slab-profile8) and achievable in-vivo flip angle, derived from transmitter voltages in >200 previous cardiac patients.

Two different trajectory patterns (Figure 1) for the transitions between kz partitions were investigated, comparing 0→ π coverage of each partition against a pattern with a +pi starting angle shift on alternate partitions. The smoother transition between partitions is hypothesised to cause fewer transient bSFFP instabilities exacerbated by off-resonance.

SNR of bSSFP was compared with a SGRE-based variant of the stack-of-stars sequence, using a 100-fold multiple acquisition approach9 in a phantom with T1~180ms and T2~50ms resembling myocardium at peak perfusion.

The finalised sequence was run at rest in 5 volunteers clinically referred for late-enhancement imaging, under ethical approval. Imaging was timed to end-systole for 40 measurements during FPP by antecubital power-injection of 0.1mmol/kg gadobutrol and 25ml saline flush at 5ml/s.

Results

Phantom studies demonstrated bSSFP 3D SOS feasibility with image quality subjectively similar to those produced by the SGRE variant, and had a calculated 44% increase in SNR.

Reduced artefacts were demonstrated with the smooth transition ordering, especially at the larger angular undersampling factors required in-vivo, and this difference became more pronounced with increasing off-resonance (Figure 2).

bSSFP was successfully run in all patients, achieving flip angles of ≥22° without RF pulse clipping (example, Figure 3).

Discussion

Cardiac shimming was applied before each acquisition using a prescribed, consistent procedure as would be necessary for routine clinical use. In-vivo images show remarkably few bSSFP related artefacts, especially considering high B0 and the slightly long TR for bSSFP in FPP.

Myocardial visibility was impacted by dark transmural regions (arrows on Fig 3) not associated with fixed perfusion defects, requiring further investigation. However, no impact of contrast agent arrival on bSSFP stability was observed (Figure 4).

Future work will focus on decreasing TR, including introduction of asymmetric RF pulses, to further shorten acquisition time. This would additionally reduce the currently long SRT, which could also potentially be reduced with a shorter steady-state startup module.

Overall in-vivo image performance requires further optimisation, currently in progress; however, the potential of bSSFP is shown for the first time in 3D radial SOS FPP.

Conclusions

A radial stack-of-stars bSSFP 3D imaging approach during high-dose fast-injection FPP is demonstrated, with in-vivo imaging not severely disrupted by artefacts even at the high undersampling required to minimise 3D shot duration. Factors essential in artefact reduction were highlighted and investigated.

Acknowledgements

This work was supported by the NIHR Cardiovascular Biomedical Research Unit of Royal Brompton and Harefield NHS Foundation Trust and Imperial College London, UK.

MJF is funded by a British Heart Foundation (BHF) PhD Studentship Grant - FS/13/21/30143.

References

1. Fair MJ, Gatehouse PD, DiBella EVR, Firmin DN. A review of 3D first-pass, whole-heart, myocardial perfusion cardiovascular magnetic resonance, J Cardiovasc Magn Reson. 2015;17(1):68.

2. Jogiya R, Schuster A, Zaman A, et al. Three-dimensional balanced steady state free precession myocardial perfusion cardiovascular magnetic resonance at 3T using dual-source parallel RF transmission: initial experience. J Cardiovasc Magn Reson Med. 2014;16(1):90.

3. Giri S, Xue H, Maiseyeu A, et al. Steady-state first-pass perfusion (SSFPP): A new approach to 3D first-pass myocardial perfusion imaging Magn Reson Med. 2014;71(1):133-144.

4. Chen L, Adluru G, Schabel MC, et al. Myocardial perfusion MRI with an undersampled 3D stack-of-stars sequence. Med Phys. 2012;39(8):5204-5211.

5. Nishimuru DG, Vasanawala S. Analysis and reduction of the transient response in SSFP imaging. In: Proceedings of the 8th Annual Meeting of ISMRM, Denver, 2000. p 301.

6. Fessler JA, Sutton BP. Nonuniform fast Fourier transforms using min-max interpolation. IEEE Trans Signal Process. 2003;51(2):560–74.

7. Adluru G, Awate SP, Tasdizen T, et al. Temporally constrained reconstruction of dynamic cardiac perfusion MRI. Magn Reson Med. 2007;57(6):1027-1036.

8. Coolen BF, Heijman E, Nicolay K, Strijkers GJ. On the use of steady-state signal equations for 2D TrueFISP imaging. Magnetic Resonance Imaging. 2009;27(6):815-822.

9. Reeder SB, Wintersperger BJ, Dietrich O, et al. Practical approaches to the evaluation of signal-to-noise ratio performance with parallel imaging: Application with cardiac imaging and a 32-channel cardiac coil. Magn Reson Med. 2005;54(3):748-754.

Figures

Figure 1: Sequence trajectories. Stack-of-stars approach with 20 asymmetrically sampled rays in the central kz partitions, reducing to 10 then 5 in the outermost partitions. Slice partial Fourier enables shorter shot time and saturation recovery time. Standard ordering (left) collects all rays within each partition rotating from 0 to π. A transition corrected ordering (right) adds + π to the starting angle on alternate partitions for a smoother jump between partitions.

Figure 2: Comparison of undersampled acquisitions using the two different bSSFP SOS trajectory patterns. Increased streaking artefacts appear associated with the angular jump at partition steps (standard ordering), and are reduced by smooth transition. Furthermore, the impact of frequency offsets appears more severe for standard ordering than for smooth transition

Figure 3: 12 slices of a single frame acquired during the first-pass using the smooth-transition 3D bSSFP SOS sequence. Arrows indicate unexplained dark transmural regions appearing in basal slices. Otherwise general image quality is good and relatively free of off-resonance artefacts.

Figure 4: Multiple-frames from a single slice during the first-pass. Image quality remains consistent, even during peak RV blood pool enhancement, when off-resonance effects could be expected to be at their most severe.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3144