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.
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