Synopsis

Challenging imaging characteristics (low SNR, fast bi-exponential decay) have so far limited the application of ²³Na-MRI in clinical environments. This work presents 3D Multi-Echo Radial Imaging of ²³NA (3D-MERINA), a time-efficient acquisition protocol from which multiple parameter maps (sodium-density, T₂*_slow and T₂*_fast, free ²³NA) can be derived in a single acquisition. Phantom and in-vivo measurements were analysed for the evaluation of parameter mapping and the inference of tissue characteristic based on decay behaviour. All parameters were acquired in a single sodium-density-weighted acquisition under relatively low SAR. Future investigation will exploit 3D-MERINA for SQF-SNR enhancement and the detection of MQF contrasts.

Purpose

Methods

Imaging was performed on a research 7T MRI scanner (Siemens Healthcare, Erlangen, Germany) with a transmit/receive dual-tuned ¹H-²³Na head coil (QED, USA). Data was collected with a custom-built radial sequence based on Saff and Kuijaars⁴ algorithm for homogeneous projection placement. TE₀ is based on centre-out half-projections, following echoes are reconstructed from full-projections along the same angle (Figure 1). Reconstruction was performed via re-gridding onto an isotropic 3.1mm resolution grid in Matlab. A Hann-filter was applied for density-compensation; the weighting on full-projection echoes was halved to account for the higher number of sampling points compared to TE₀.

Phantom:

The phantom (Figure 2) consisted of 16 vials (3cm diameter) of varied saline-concentrations (30, 70, 110 and 150mMol/l) and agarose-level (liquid, 1, 3 and 6%). Sodium imaging protocol parameters were: TR=160ms, FA=90°, 10,000 projections, FOV=20cm-iso, TA=26min40s. TE₀ was set to 0.4ms, consecutive TEs depended on the readout duration (T_RO). For T_RO evaluation 10 acquisitions with T_RO=[1,...,10ms] were collected. The dwell time was set to the hardware limit of 5$$$\mu$$$s.

In-vivo:

Human in-vivo data was acquired on a healthy female volunteer. Sodium imaging parameters were kept as above, T_RO was set to 2ms, 31 echoes were recorded. Additionally, a ¹H-FLASH image (TR=11ms, TE=3.06ms, FA=14°, TA=6min12s, 1mm-iso) was acquired for registration to a ¹H-MP2RAGE image (TI=700ms/2700ms, GRAPPA-factor=4, TR=4900ms, TE=2.94ms, TA=6min, FA=5°/6°, 0.9mm-iso, 32-channel head-coil (Nova Medical Inc., Wilmington MA, USA)) for tissue segmentation.

Results

Figure 2 depicts the first 9 echo reconstructions in phantom and in-vivo measurements. TE₀ gives the typically acquired sodium-density-weighted image; subsequent TEs show decay behaviour dependent on the media. The phantom echoes reveal faster decay for more solid solutions; in-vivo, CSF and tissue show different signal decay characteristics (Figure 3). The reconstruction of up to 5 signal echoes show tissue contrast above noise level.

The optimisation of T_RO reveals a trade-off between the number of recorded echoes and the acquired bandwidth/sampling point (higher gradients in shorter T_RO, constant dwell time). Figure 4 displays the signal decay of the mean signal in each vial at all different T_RO. All measurements show very consistent results. However, T_RO=1ms decay shows a stronger noise effect. Fitting results of mono- and bi-exponential decays to the liquid and agar-doped phantoms, respectively, indicate an underestimation of the decay parameter in the T_RO=1ms measurement due to the lower SNR. Hence, T_RO was fixed to 2ms for the in-vivo measurement allowing robust fits while maintaining a high number of echoes. The fitting procedure included an offset to account for noise bias.

The in-vivo decay information was analysed to produce T₂*_fast and T₂*_slow component maps (Figure 5) by applying the same bi-exponential fitting routine. Voxel with a T₂*_slow>30ms were classified as ‘free sodium’ and re-fit by a mono-exponential decay. The ‘free sodium’ mask showed good agreement with the CSF mask segmentation obtained from the MP2RAGE image (bottom row Figure 5). Mean fitting parameters for T₂*_slow and T₂*_fast for segmented brain tissue regions were in agreement with reported values^5,6,7,14.

Discussion and Conclusion

This work demonstrated the application and analysis of 3D Multi-Echo Radial Imaging in ²³NA (3D-MERINA) allowing for a more efficient use of measurement time. The effect of T_RO investigated; at an optimised T_RO of 2ms, the analysis of CSF and brain tissue showed 29 and 5 echo signals above noise level, respectively. Potential SNR enhancement through the use of T₂*-corrected averaging of these additional signals will be investigated in the future.

Phantom and in-vivo measurements revealed additional information in the analysis of later TEs offering the inference of details about the underlying media and, hence, potentially featuring similar contrast as achieved in multi-quantum filtered ²³Na-MRI^8,9, or double-echo bi-exponentially weighted ²³NA imaging^10,11. The analysis of multi-echo signal decay data furthermore provides T₂* maps; through fast 3D radial acquisition T₂*_slow and T₂*_fast components were detected. It should be noted that these additional contrasts were achieved in a standard sodium-density-weighted acquisition time-frame with relatively low SAR due to single 90°-RF-pulses.

Enhanced PSF and SNR are anticipated through the application of density-adapted radial trajectories^12,13.

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 work was also supported by a research collaboration agreement with Siemens Healthcare. The MP2RAGE data was acquired with a prototype sequence provided by Siemens Healthcare.

Jon Cleary is funded by a University of Melbourne McKenzie fellowship.

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