Introduction

The goal of this work is to 1) assess the feasibility of performing MR Fingerprinting (MRF) in the liver on a 0.55T scanner; 2) compare T₁ and T₂ measurements made with MRF in the liver at 0.55T to values previously reported using clinically standard techniques; and 3) examine the feasibility of water-fat separation using rosette MRF at low field. MRF has been shown to enable efficient and accurate quantification of relaxation times in a variety of organs at 1.5 and 3 T^1–4, and in the brain at 0.55T⁵. While low-field MRI systems may offer efficient imaging due to shorter T₁, longer T₂/T₂*, potentially improved field homogeneity, and lower cost compared to clinical 1.5T and 3T scanners, the lower SNR inherent at lower field may pose challenges for MRF. While the MRF acquisition can be lengthened to combat the lower SNR at lower field strengths in organs such as the brain, this option is not possible in the liver, where data must be collected in a breathhold. Additionally, the much smaller chemical shift of fat at 0.55T compared to 1.5T and 3T makes it challenging to achieve the robust fat suppression required for quantitative mapping in the liver. In this work, the potential of rosette MRF, which has been demonstrated for fat-water separation in cardiac applications⁶, is explored for the quantification of T₁ and T₂ in the liver at 0.55 T.

Methods

Two MRF sequences using spiral and rosette trajectories, respectively, were implemented on a 0.55T scanner (Siemens Free.Max, Erlangen, Germany). The lower maximal gradient amplitude and slew rate of this scanner were accommodated (15mT/m and 40mT/m/ms). The spiral trajectory (Figure1a) has a readout duration of 11.9ms with FOV 300x300mm² and matrix size 192x192. The rosette trajectory (Figure1b) was designed for the same FOV and matrix size, as well as fat signal suppression at 0.55T with a readout duration of 9.87ms.
For both spiral and rosette MRF, 1000 time points were acquired using a FISP readout following an inversion pulse with inversion time of 21ms ². The flip angles were sinusoidally ramped up to 70 degrees² with fixed TR/TE of 15ms/1.6ms for spiral MRF and 13ms/1.6ms for rosette MRF. The total acquisition time was 15s/13s during a breath-hold for spiral/rosette MRF. In-plane resolution was 1.56x1.56mm², and slice thickness was 5mm for all phantom and in vivo experiments. The MRF dictionary incorporating slice profile imperfection correction was generated using Bloch equation simulations⁷. A low-rank reconstruction with l₁-TV regularization was used to generate the final T₁ and T₂ maps and proton density images⁸. For rosette MRF, as described in previous work⁶, fat images were generated by demodulating the acquired data at the fat frequency; a B₀ map was generated by dividing the data at two echoes; and B₀ correction was performed to generate the final water T₁ and T₂ maps and water and fat proton density images.
The accuracy of these low-field MRF methods for T₁ and T₂ quantification was validated in the ISMRM/NIST phantom. The means and standard deviations of the T₁ and T₂ values in the T₂ layer within physiological range measured using MRF were compared with gold standard values acquired using inversion recovery spin-echo and single-echo spin-echo methods. Six healthy subjects were scanned after written informed consent in this IRB-approved study. A transverse slice in the liver was acquired using the proposed spiral and rosette MRF sequences. ROIs in the liver were drawn manually, and the mean and standard deviation in T₁ and T₂ values were calculated.

Results

In the ISMRM/NIST phantom, T₁ and T₂ measurements using spiral and rosette MRF were in good agreement with the reference values (Figure 2). In the liver, the averaged T₁ and T₂ values measured by spiral MRF were 369.8±18.3 ms and 42.2±5.2 ms. The averaged T₁ and T₂ values measured by rosette MRF were 372.2±19.5 ms and 47.6±2.9 ms. Representative T₁ and T₂ maps, and proton density images in a healthy subject are shown in Figure 3.

Discussion

Both spiral and rosette MRF are feasible at 0.55T using a low-rank reconstruction, and both achieved good image quality in the liver in a single ~15s breath-hold. Rosette MRF enables fat suppression in the water T₁ and T₂ maps, and proton density image. Furthermore, a fat image could also be generated from the same dataset with no penalty in acquisition time. T₁ values measured by spiral MRF and rosette MRF are comparable, and both are higher than literature values (339ms measured by MOLLI)⁹. This trend is consistent with previous findings in cardiac MRF at 1.5T and 3T compared with MOLLI measurements⁶. MRF T₂ measurements are lower than literature values (66ms measured by T₂prep-bSSFP) ⁹, which is also consistent with previous findings in MRF measurements of the liver at 3T⁴. Rosette MRF generated higher T₂ values than spiral MRF. Previous work has also reported similar findings in the heart at 1.5T and 3T⁶.

Conclusions

This study shows that it is feasible to measure T₁ and T₂ simultaneously in the liver using MRF at 0.55T within the acquisition time of 15s. In addition, water-fat separation can be achieved along with T₁ and T₂ quantification with no time penalty using rosette MRF.

Acknowledgements

This work is supported by NSF/CBET 1553441, NIH/NHLBI R01HL094557, and Siemens Healthineers (Erlangen, Germany).

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