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Magnetic Resonance Fingerprinting for Simultaneous T1, T2 and ADC Mapping at 0.55T using Convex-optimized Diffusion Prepared Waveforms1King's College London, London, United Kingdom, 2Intitute for Biological and Medical Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, 3Millennium Institute for Intelligent Healthcare Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, 4MR Research Collaborations, Siemens Healthcare Limited, Camberley, United Kingdom, 5Insititute for Biological and Medical Engineering, Pontificia Universidad Catolica de Chile, Santiago, Chile, 6Electrical Engineering Department, Pontificia Universidad Católica de Chile, Santiago, Chile, 7School of Engineering, Pontificia Universidad Católica de Chile, Santiago, Chile, 8Hans Fischer Senior Fellow Award, Institute for Advanced Study at Technical University of Munich, Munich, Germany
Synopsis
Keywords: Diffusion Acquisition, Data Acquisition
Motivation: Single breath-hold simultaneous T1, T2, and ADC MRF allows comprehensive tissue characterization in a single scan. However, this technique has not been demonstrated at 0.55T.
Goal(s): Investigate the feasibility of T1, T2, and ADC MRF sequence for simultaneous T1, T2, and ADC mapping at 0.55T taking full advantage of the low-field scanner hardware.
Approach: The proposed approach uses a bSSFP radial sequence with varying IR, T2-preparation, and optimized ADC-preparation pulses over 16 heartbeats. Experiments were performed on phantoms and compared with spin-echo references.
Results: T1, T2, and ADC MRF at 0.55T was tested in a phantom, showing excellent agreement with reference values.
Impact: Simultaneous quantification of T1, T2, and ADC is feasible with the proposed MRF sequence in a single breathold, allowing for a more comprehensive tissue characterization through co-registered multiparametric imaging at 0.55T with a low-performance gradient system.
Introduction
Multiparametric T1, T2 and ADC mapping has emerged as a useful clinical tool for comprehensive tissue characterization in multiple clinical applications, including brain, abdominal and prostate imaging, among others. Conventional T1, T2 and ADC maps are acquired in sequential scans. Magnetic Resonance Fingerprinting (MRF) has shown promising results for simultaneous T1 and T2 parametric mapping in a single scan [1] [2], and more recently for simultaneous T1, T2 and ADC mapping. The recent introduction of high-end low-field 0.55T scanners could greatly increase the accessibility and affordability of this type of technique [3]. Furthermore, lower T1 relaxation times, reduced SAR, and fewer B0/B1 inhomogeneities make low-field MRI an attractive alternative for MRF. Brain [4] and liver [5] MRF have been demonstrated at 0.55T for simultaneous T1 and T2. However, the feasibility of T1, T2, and ADC MRF at 0.55T still needs to be proven. Here we demonstrate 2D T1, T2, and ADC MRF at 0.55T and evaluate its performance in phantoms.Methods
The proposed T1, T2, and ADC MRF at 0.55T (Fig. 1) acquires data over 16 heartbeats. This follows our formulation used for T1, T2, and ADC liver MRF at 3T [6], but unlike this implementation, it uses bSSFP radial readouts to boost the SNR at low field and four MLEV refocusing pulses (~4 × 2.2 ms) instead two adiabatic hyperbolic secant pulses (2 × 10 ms) to give more time for the diffusion gradients, as we have less B1 inhomogeneities.All scans were performed on a 0.55T scanner (MAGNETOM Free.Max, Siemens Healthineers, Erlangen; with max. gradient of Gmax = 25 mT/m and slew rate Smax = 40 mT/m/ms) using ECG-triggered acquisitions with an external monitoring system (Maglife Serenity, Schiller). Sequence parameters include TE = 3.9 ms, TR = 7.8 ms, flip angles between 10° − 60°, FOV = 256 × 256 mm2, voxel size of 2×2×10 mm3, and an acquisition time of 16 s. The sequence was implemented using the Pulseq open-source sequence definition [7]. All images were reconstructed using a low-rank reconstruction [8] [9] with HD-PROST regularization [10], and subject-specific dictionaries were simulated by using EPGs [11] [12]
The sequence optimization was performed using Julia [13] and the Pulseq-compatible open-source MRI simulator KomaMRI.jl [14]. We formulated the optimization problem shown in Fig. 2 to get the gradient waveforms using an interior-point solver, IpOpt [15], with JuMP.jl [16]. In contrast to previous works [17] [18], the matrices representing each of the problem’s restrictions and optimization function were obtained using 1D Finite Elements with piece-wise linear basis functions. This enabled us to efficiently solve the problem using a temporal resolution of 16 times greater than the gradient’s raster time, to be linearly interpolated later to the correct raster time. Finally, we to wrote the optimized waveforms into a Pulseq sequence file.
Phantom experiments were performed on the standardized T1MES phantom [19] and home-made diffusion phantoms filled with a fixed concentration of NiCl2 and Agarose to obtain physiological T2 and T2, and varying concentrations of of polyvinylpyrrolidone to generate a range of isotropic diffusion values [20]. T1, T2, and ADC MRF values were compared against their corresponding SpinEcho (SE) reference scans, IR-SE (TIs = [21, 87:187:1300] ms) , SE (TEs = [6.7, 33:17:267] ms), and DWI-SE (b-value (averages) = [20(15), 200(20), 400(25), 600(30), 800(35),1000(35)] s/mm2 ) respectively. All the references were acquired with a resolution of 2×2×10 mm3 and FOV = 256×256 mm2 to match the MRF acquisition.
Results
An example of the optimized diffusion waveform with Maxwell and 0-th order moment compensation can be seen in Fig 3. The resulting waveform satisfies the hardware limits imposed by the optimization problem and was successfully written to a Pulseq sequence to then be run in the scanner.Phantom experiments show good map quality and excellent correlation between the values measured with the proposed T1, T2, and ADC MRF at 0.55T and the reference SE values (Fig. 4), with correlation and r2 close to one.
Conclusion
In this work, we demonstrate the feasibility of T1, T2, and ADC MRF at a field strength of 0.55T with a low-performance gradient system, obtaining accurate parameter values with good precision for the phantom experiments in a short ~16s acquisition. Future work will include further sequence optimization to reduce remaining parameter bias, and the evaluation in healthy volunteers with and without first-order moment compensation.Acknowledgements
The authors acknowledge financial support from: (1) BHF RG/20/1/34802 (2) EPSRC EP/V044087/1 (3) ANID Millennium Institute iHEALTH, ICN2021_004; Fondecyt 1210637, 1210747 and 1210638; Fondequip Mayor EQY210003; Basal Funding, IMPACT, FB210024 and (6) the Technical University of Munich – Institute for Advanced Study, (7) PhD program in Biological and Medical Engineering of the Pontificia Universidad Católica de Chile.References
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