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Radial LC-SSFP for Left Atrial Cine Imaging
Jaime Robb1,2, Chenxi Hu1, and Dana Peters1

1Yale University, New Haven, CT, United States, 2University of Glasgow, Glasgow, United Kingdom

Synopsis

Balanced steady-state free precession (b-SSFP) imaging of left atrial cine is severely affected by off-resonance artefacts, particularly in the pulmonary veins. Acquisition and combination of data sets with different phase-cyclings has been shown to remove banding artefacts with a trade-off in scan time. The purpose of this work is to present an approach to banding artefact removal in b-SSFP imaging for left atrial cine. We propose the use of interleaved, undersampled radial projections with LC-SSFP. This method provides increased pulmonary vein conspicuity and image quality with only minor increase in scan time and streaking artefact.

Introduction

Balanced steady state free precession (b-SSFP) is suited to cine imaging, generating high contrast between myocardium and blood using short TRs. b-SSFP displays increased signal-to-noise ratio (SNR) compared with GRE. Although left ventricular cine with b-SSFP is the standard, left atrial cine is currently not possible with b-SSFP due to off-resonance artefacts. Flowing spins in the highly off-resonant lungs enter the left atrium, causing signal voids and severe artefacts.1 Consequently, b-SSFP left atrial cine has poor quality.

b-SSFP artefacts can be reduced by decreasing TR and field strength; however, these modifications are not always practical or successful. Therefore, other methods have been proposed; using alternating TRs to manipulate the signal vs. frequency spectrum (wideband-SSFP),2,3 fast interruption of the steady state (FISS)4,5 to destroy transverse magnetisation and combining data sets with different phase-cyclings (linear combination SSFP (LC-SSFP)).6 LC-SSFP has been employed with innovations,7 in many anatomic regions.7,8,9 Its use in cardiac applications has been limited by the four-fold increase in scan time.

The purpose of this work is to develop improved b-SSFP methods for left atrial cine with an aim to evaluate radial LC-SSFP for cine imaging of the left atrium and pulmonary veins compared with b-SSFP and GRE. To offset the four-fold increase in scan time, we interleaved, four-fold undersampled radial projection sets for each phase-cycling. We hypothesised that this fast approach would be without significant undersampling artefacts, as complementary projections are generated from very similar magnetisation states and artefacts will cancel.

Methods

In this work, four sets of SSFP images were acquired with phase-cycling as described by Vasanwala6 et al. Figure 1 illustrates the acquired data sets and combined signal.

Two phase-cycling orders were studied 0-0, 0-90-180-270, 0-180, 0-270-180-90 (standard order) and 0-0, 0-180, 0-90-180-270, 0-270-180-90 (complementary order, so called because complementary projection sets had the most similar banding patterns).

Simulations: Interleaving of projections, (Figure 1), was investigated to reduce artefact. Using a simulated image, frequency map and the SSFP magnitude and phase vs. frequency relationships, the b-SSFP signal at each pixel for each phase-cycling was calculated to generate radial data and simulate LC-SSFP reconstruction.

A phantom with realistic T1/T2 values was imaged. Six healthy volunteers (age range 27-52 years, four female) were imaged with GRE, radial b-SSFP and four-pass LC-SSFP on a 3.0T MRI scanner (Siemens Trio, Erlangen, Germany) with 5 channel body coil, manually placed shim volume, FOV 360x360mm2, image matrix 160x160, resolution 2x2x8mm, bandwidth 1008Hz/pixel, TR/TE 2.8/1.4ms, flip angle 36-52, slice thickness 8mm. Typically, 48 radial projections were used for each LC-SSFP phase-cycling with total breath-hold time 12 seconds, compared with 9 seconds for conventional fully sampled b-SSFP. The b-SSFP and interleaved undersampled LC-SSFP images were graded for pulmonary vein conspicuity, off-resonance, flow and streaking artefacts by two experienced readers.

B0 maps were acquired from GRE images at TEs of 3.6 and 5ms, using methods described by Hu1 and Bernstein.10 Radial cine were reconstructed offline in MATLAB (version R2017a; Mathworks Inc., Natick Massachusetts, USA), using a gridding toolbox10 and the four phase-cycled complex images were combined to generate the LC-SSFP image.

Results

Figure 2 shows simulation results. Interleaved radial LC-SSFP generates less artefacts than non-interleaved LC-SSFP and eliminates banding.

In phantoms, LC-SSFP removed banding artefacts, compared to b-SSFP (see Figure 3). Interleaved LC-SSFP demonstrated fewer artefacts and higher signal-to artefact and noise ratio, a 30% increase compared to non-interleaved LC-SSFP. Complementary order outperformed standard order providing a further 30% increase in SNR.

Figure 4 shows LC-SSFP provided significantly improved visualisation of the left atrium and pulmonary veins compared to b-SSFP in volunteers.

Figure 5 displays results of image quality comparisons. Even in this small study, LC-SSFP outperformed b-SSFP in pulmonary vein conspicuity and number of veins visualised, with a minor increase in scan time. Streak artefact was significantly increased but was mild.

Discussion

b-SSFP images display consistent signal nulling within the pulmonary veins, intruding into the left atrium during high flow in atrial filling. LC-SSFP provides superior visualisation of the pulmonary veins and left atrium, continuity at the ostia and uniformity of signal throughout the left atrium at all phases of the cardiac cycle compared to b-SSFP. Use of interleaved undersampled radial projections minimises LC-SSFP acquisition time without significant streaking artefacts and improves clinical feasibility for cardiac cine.

Conclusion

LC-SSFP, applied to the left atrium, significantly reduces SSFP banding artefacts and improves image quality. Scan time was only minimally increased with the interleaved, undersampled radial approach. Use of this technique will enable improved cine imaging, especially for the left atrium, with important applications to non-contrast 3D MRA.

Acknowledgements

No acknowledgement found.

References

  1. Hu, P., Stoeck, C.T., Smink, J., Peters, D.C., Ngo, L., Goddu, B., Kissinger, K.V., Goepfert, L.A., Chan, J., Hauser, T.H. and Rofsky, N.M., 2010. Noncontrast SSFP pulmonary vein magnetic resonance angiography: Impact of off‐resonance and flow. Journal of Magnetic Resonance Imaging, 32(5), pp.1255-1261.
  2. Lee, H.L., Pohost, G.M. and Nayak, K.S., 2006. Gated and real-time wideband SSFP cardiac imaging at 3t. In Proceedings of the 14th Annual Meeting of ISMRM (p. 143).
  3. Nayak, K.S., Lee, H.L., Hargreaves, B.A. and Hu, B.S., 2007. Wideband SSFP: alternating repetition time balanced steady state free precession with increased band spacing. Magnetic Resonance in Medicine, 58(5), pp.931-938.
  4. Koktzoglou, I. and Edelman, R.R., 2018. Radial fast interrupted steady‐state (FISS) magnetic resonance imaging. Magnetic resonance in medicine, 79(4), pp.2077-2086.
  5. Edelman, R.R., Serhal, A., Pursnani, A., Pang, J. and Koktzoglou, I., 2018. Cardiovascular cine imaging and flow evaluation using Fast Interrupted Steady-State (FISS) magnetic resonance. Journal of Cardiovascular Magnetic Resonance, 20(1), p.12.
  6. Vasanawala, S.S., Pauly, J.M. and Nishimura, D.G., 2000. Linear combination steady‐state free precession MRI. Magnetic Resonance in Medicine: An Official Journal of the International Society for Magnetic Resonance in Medicine, 43(1), pp.82-90.
  7. Benkert, T., Ehses, P., Blaimer, M., Jakob, P.M. and Breuer, F.A., 2015. Dynamically phase‐cycled radial balanced SSFP imaging for efficient banding removal. Magnetic resonance in medicine, 73(1), pp.182-194.
  8. Bultman, E.M., Klaers, J., Johnson, K.M., Fraçois, C.J., Schiebler, M.L., Reeder, S.B. and Block, W.F., 2014. Non-contrast enhanced 3D SSFP MRA of the renal allograft vasculature: a comparison between radial linear combination and Cartesian inflow-weighted acquisitions. Magnetic resonance imaging, 32(2), pp 190-195.
  9. Çukur, T., Bangerter, N.K. and Nishimura, D.G., 2007. Enhanced spectral shaping in steady‐state free precession imaging. Magnetic resonance in medicine, 58(6), pp.1216-1223.
  10. Bernstein, M.A., Grgic, M., Brosnan, T.J. and Pelc, N.J., 1994. Reconstructions of phase contrast, phased array multicoil data. Magnetic resonance in medicine, 32(3), pp.330-334.
  11. Hargreaves, B., Beatty, P., 2004. Gridding Functions (http://mrsrl.stanford.edu/~brian/gridding/), mrsl.standford.edu. Retrieved July 2018.
  12. Deimling, M. and Heid, O., 1994, August. Magnetization prepared true FISP imaging. In Proceedings of the 2nd Annual Meeting of ISMRM, San Francisco (p. 495).
  13. Roeloffs, V., Rosenzweig, S., Holme, H.C.M., Uecker, M. and Frahm, J., 2018. Frequency-modulated SSFP with radial sampling and subspace reconstruction: A time-efficient alternative to phase-cycled bSFFP. arXiv preprint arXiv:1803.06274.

Figures

Simulations of standard order phase-cycling. The frequency spectra for each phase-cycling is shown alongside the radial trajectories acquired. The resultant LC-SSFP signal has fluctuations of less than 0.01 across frequencies -500 to 500Hz. Displaying the combined trajectories illustrates how the undersampling artefacts can be compensated for using interleaving. Variables used were, TR = 3ms, T1 = 1500ms, T2 = 200ms, flip angle = 60, 3000 excitations and 4 projections per phase-cycling.

Simulation of streaking artefact in the left atrium a) Fully sampled (128 radial projections) b-SSFP, with the inset showing the simulated frequency map b) Undersampled (32 radial projections) 0-0 phase-cycling with banding artefact in left atrium c) Combined LC-SSFP projections without interleaving and d) with interleaving, both use standard order phase-cycling. Note that the interleaved image has dramatically reduced artefacts.

Phantom study showing the effect of LC-SSFP on removal of banding artefacts. a) Radial b-SSFP image of walnut oil (large bottle) and agar phantoms, banding artefact is visible in all bottles. b) Radial undersampled image with 0-0 phase-cycling highlighting the different phase-cycling dependent banding artefacts, c) Radial LC-SSFP image with interleaving and complementary phase-cycling order, the removal of banding is apparent.

a) B0 map, created using phase difference from two GRE acquisitions with ΔTE of 1.4ms , shows approximately 200Hz off resonance in the pulmonary veins. Note the phase wrapping at the ostia. Comparison of mid diastole images for single volunteer; b) Cartesian GRE with labelled left atrium (LA), right inferior pulmonary vein (RIPV) and left inferior pulmonary vein (LIPV) c) Radial b-SSFP with severe off-resonance artefacts in the pulmonary veins and flow artefact in the left atrium and d) LC-SSFP with 64 projections per phase-cycling.

Comparison of fully sampled radial b-SSFP (a, c, e, g) with radial LC-SSFP (b, d, f, h) in four volunteers. LC-SSFP improves uniformity in the left atrium. Pulmonary veins (PVs) have improved visualisation. i) Results of image grading for all six volunteers. Artefacts were graded: 1 = severe, 2 = moderate, 3 = mild, 4 = visible and 5 = normal/barely visible, the remaining criteria used: 1 = non-diagnostic, 2 = poor, 3 = moderate, 4 = good and 5 = excellent and maximum number of PVs was 2. The p values were calculated using a Wilcox signed rank test.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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