Synopsis

The fast signal decay in sodium MRI makes high quality image acquisition challenging. Here we enhance image quality by improving encoding time (t_enc) efficiency using a special case of ramped hybrid encoding , zero-gradient-under-RF-excitation RHE (zG_RF-RHE). This provides 1) gradient-free-excitation for high flip angle, non-selective excitation profiles necessitated by low signal in sodium MRI and 2) gradient ramping during deadtime for optimised t_enc, to reduce T2 decay influence during acquisition. The performance of zG_RF-RHE is demonstrated in simulations, phantom and in vivo experiments and improves image quality (SNR and T2 blurring). It is applicable to any centre-out trajectory design.

Introduction

Sodium MRI offers promise as a clinical imaging modality, but currently provides comparatively low image quality resulting from low MR-sensitivity and fast biexponential MR-signal decay of sodium¹. The latter necessitates image acquisition with dedicated sequences enabling short encoding time (t_enc). Typically applied UTE sequences are, however, limited due to finite gradient slew rates, and zTE t_enc improvements are not readily achievable in sodium MRI due to B1 limitations^2-4. Finite transmit/receive switching time (deadtime) introduces an additional hardware constraint on fast sampling onset (~40-100μs on clinical systems).

This work proposes a special case of ramped hybrid encoding (RHE)⁵ for an optimisation of t_enc under low sodium MR-signal conditions. Zero-gradient-excitation (zG_RF)-RHE (Figure 1) uses hybrid encoding and provides 1) gradient-free excitation for high flip angle, non-selective excitation profiles and 2) gradient ramping during deadtime for t_enc optimisation to reduce T2 signal decay influence during acquisition.

Methods

Acquisition

Imaging was conducted on a research 7T MRI scanner (Siemens Healthcare, Erlangen, Germany) with a transmit/receive dual-tuned ¹H-²³Na head coil (QED, USA) using a custom-built 3D-radial sequence. Simulations and experiments in phantoms and in vivo brain were performed with standard UTE and zG_RF–RHE sequence setups. Simulation and imaging parameters were: FA=90°, TE=0.3ms, TR=160ms, resolution=(3.1mm)³, FOV=20cm, T_RO=2-8ms, 8000 projections. Central k-space in zG_RF-RHE was acquired through single point measurements at the minimum achievable TE=0.3ms.

Phantom

Vials with varying diameter (28, 14 and 6mm) and 3%-agar saline concentrations (30 and 45mM) were submerged in distilled water. Experiments at T_RO=2,4,6 and 8ms were repeated 8 times for statistical SNR analysis.

In vivo

Human in vivo acquisitions were performed on a healthy female volunteer with T_RO=2ms - TA_UTE=21min20sec, TA_zGRF-RHE=21min24sec. Additionally, a proton-based FLASH image was measured with TR=11ms, TE=3.06ms, FA=14°, resolution=(1mm)³, TA=6min12sec.

Reconstruction

Images were reconstructed offline via Kaiser-Bessel re-gridding in MATLAB with an oversampling factor of 1.5. Radial and single point measurements were combined for zG_RF-RHE reconstruction. An iterative density compensation was applied⁶. A Hann-filter was applied to radial rawdata to reduce Gibbs ringing.

Results

Figure 2a illustrates the modulation transfer function (MTF) and t_enc for T_RO=2ms and T_RO=8ms under biexponential decay (T2_short=1.5ms, T2_long=20ms; 0.6 and 0.4 signal fractions, respectively). Figure 2c depicts the required t_enc across k-space. For any T_RO, zG_RF-RHE traverses through k-space quicker than UTE thanks to gradient pre-ramping leading to better MTF characteristics and a shortened sampling duration. Single point measurements in zG_RF-RHE fixate t_enc in k-space center. Given the smaller gradients for longer T_RO the central k-space gap is smaller.

Figure 2b shows simulated profile reconstructions for UTE and zG_RF-RHE at T_RO=2ms and T_RO=8ms. Quicker sampling results in stronger signal in zG_RF-RHE. The finite difference illustration in Figure 2d emphasises sharper edges due to reduced T2 blurring in zG_RF-RHE, particularly at shorter T_RO.

Figure 3a depicts the phantom setup, UTE and zG_RF-RHE images at T_RO=2ms are shown in Figure 3c. Contour lines from normalised images illustrate sharper edges between vials in zG_RF-RHE measurements and a better delineation of small details (red arrows). The mean line profile across the central slice of 14mm phantoms in T_RO=2ms and T_RO=8ms experiments is plotted in Figure 3b. At the same readout bandwidth, zG_RF-RHE achieves higher signal due to improved t_enc. Figure 3d depicts the mean edge sharpness in 14mm phantoms. Edges were measured between the central voxel of the vial and background voxel to either side. Edge definition improves with shorter T_RO. Across all T_RO, zG_RF-RHE shows mean edge enhancements for both sodium concentrations.

SNR results calculated from a central region in 28mm vials are displayed in Figure 4. An average SNR increase of up to 4.8% was observed for zG_RF-RHE across all T_RO with significant gain at longer T_RO. Concomitant improvements in contrast-to-noise ratio (CNR) between 30 and 45mM vials were found.

Figure 5a depicts cross sections in in vivo experiments. zG_RF-RHE shows better detail around the brain stem. A FLASH-based CSF mask was registered to UTE and zG_RF-RHE images using FSL. Figure 5b shows three consecutive slices of CSF segmentation through the ventricles with contour masks of normalised sodium images superimposed. zG_RF-RHE measurements reveal a better delineation of the lateral ventricles.

Discussion and Conclusion

Acknowledgements

We acknowledge the facilities, and the scientific and technical assistance of the Australian National Imaging Facility at the Melbourne Brain Centre Imaging Unit. The Australian National Imaging Facility is funded by the Australian Government NCRIS program.

The work was also supported by a research collaboration agreement with Siemens Healthcare.

Jon Cleary is funded by a University of Melbourne McKenzie fellowship. Brad Moffat is supported by the Australian National Imaging Facility (NIF).

References

1. Thulborn KR. Quantitative sodium MR imaging: A review of its evolving role in medicine. Neuroimage 2016.

2. Balcom BJ, Macgregor RP, Beyea SD, Green DP, Armstrong RL, Bremner TW. Single-Point Ramped Imaging with T1 Enhancement (SPRITE). J Magn Reson A 1996;123(1):131-134.

3. Romanzetti S, Halse M, Kaffanke J, Zilles K, Balcom BJ, Shah NJ. A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine. J Magn Reson 2006;179(1):64-72.

4. Heid O, Deimling, M. Rapid Single Point (RASP) Imaging. 1995; Nice, France. p 684.

5. Jang H, Wiens CN, McMillan AB. Ramped hybrid encoding for improved ultrashort echo time imaging. Magn Reson Med 2016;76(3):814-825.

6. Zwart NR, Johnson KO, Pipe JG. Efficient sample density estimation by combining gridding and an optimized kernel. Magn Reson Med 2012;67(3):701-710.

7. Boada FE, Gillen JS, Shen GX, Chang SY, Thulborn KR. Fast three dimensional sodium imaging. Magn Reson Med 1997;37(5):706-715.

8. Lu A, Atkinson IC, Claiborne TC, Damen FC, Thulborn KR. Quantitative sodium imaging with a flexible twisted projection pulse sequence. Magn Reson Med 2010;63(6):1583-1593.

9. Nagel AM, Laun FB, Weber MA, Matthies C, Semmler W, Schad LR. Sodium MRI using a density-adapted 3D radial acquisition technique. Magn Reson Med 2009;62(6):1565-1573.

10. Riemer F, Solanky BS, Stehning C, Clemence M, Wheeler-Kingshott CA, Golay X. Sodium ((23)Na) ultra-short echo time imaging in the human brain using a 3D-Cones trajectory. MAGMA 2014;27(1):35-46.