Fast 3D Acquisition for Quantitative Mapping and Synthetic Contrasts Using MIRACLE and trueCISS
Tom Hilbert1,2,3, Damien Nguyen4,5, Jean-Philippe Thiran2,3, Gunnar Krueger2,3,6, Oliver Bieri4,5, and Tobias Kober1,2,3

1Advanced Clinical Imaging Technology (HC CMEA SUI DI BM PI), Siemens Healthcare AG, Lausanne, Switzerland, 2Department of Radiology, University Hospital (CHUV), Lausanne, Switzerland, 3LTS5, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland, 4Radiological Physics, Department of Radiology, University of Basel Hospital, Basel, Switzerland, 5Department of Biomedical Engineering, University of Basel, Basel, Switzerland, 6Siemens Medical Solutions USA, Inc., Boston, MA, United States

Synopsis

Quantitative imaging promises to be a good biomarker of disease but requires long acquisition times, especially when 3D acquisition techniques are used. Here we propose to use a highly undersampled 3D phase-cycled balanced steady-state free-precession sequence in combination with the reconstruction methods MIRACLE and trueCISS, providing quantitative maps (T2,T1,M0). Additionally, synthetic contrasts can be generated using the previously calculated quantitative maps and signal models, exemplarily shown for bSSFP, T2- and T1-weighted contrast. In summary, the proposed method provides a set of quantitative maps and various conventional contrasts using a single 3:31min acquisition.

Introduction

The quantitative assessment of physical tissue properties (qMRI) has great potential for providing disease biomarkers since it facilitates intra- and inter-subject comparison1. Various acquisition techniques have been proposed to enable its application in clinical practice. However, many of them are 2D methods; most of the 3D qMRI techniques are still very time-consuming and 2D imaging imposes various limitations such as reduced resolution and SNR as well as anisotropic voxel size. Here, we suggest to combine the recently proposed 3D method “Motion-Insensitive Rapid Configuration Relaxometry” (MIRACLE)2 with a highly undersampled 3D phase-cycled balanced steady-state free-precession (bSSFP) sequence termed trueCISS3 enabling the acquisition of high-resolution quantitative T1 and T2 maps in less than 4min. Additionally, we use the estimated quantitative maps to synthesize various conventional MRI contrasts as a visual benchmark.

Materials & Methods

After obtaining written consent, a prototype trueCISS sequence was used to acquire an 8-fold undersampled, 3D phase-cycled bSSFP k-space from one healthy volunteer at 3T (MAGNETOM Prisma, Siemens Healthcare, Germany) using a 20-channel head/neck coil (resolution 1x1x1.3mm3, TR/TE 6.58ms/3.29ms, flip-angle α=15°, 16 phase-cycles with ɸ=0°, 22.5°, 45°, …, 337.5°, acq. matrix 256x256x128, TA 3:31min).

First, a sparse iterative reconstruction was used to reconstruct each phase-cycled image by enforcing data consistency with the acquired data and sparsity in the wavelet-domain4. Following the MIRACLE algorithm2, the complex phase-cycled 3D images were then discrete-Fourier-transformed along the phase dimension yielding 16 SSFP configurations or “modes”5. Subsequently, the lowest-order modes can be used to derive T1 and T2 using a golden section search in each voxel as proposed for TESS relaxometry6.

To extract more information, the trueCISS algorithm3 was applied on the phase-cycled bSSFP images as well. To that end, the bSSFP signal model was fitted onto the magnitude of the phase-cycled images and is defined as follows:

$$$M=M_{0}\mid\frac{2\sin\alpha\cos(\frac{\phi-\triangle\phi}{2})}{1+\cos\alpha+\cos(\phi-\triangle\phi)+(4\Lambda-\cos(\phi-\triangle\phi)^{2})\sin(\frac{\alpha}{2})^{2}}\mid$$$

yielding quantitative maps of the equilibrium magnetization M0, the relaxation time ratio Λ=T1/T2 and the local phase offset due to field inhomogeneities ∆ɸ.

The quantitative maps provided by these two algorithms in conjunction with the forward signal models of different contrast mechanism can be used to generate synthetic images simulating various conventional MRI techniques. Here, we exemplarily used them to generate T2-weighted “Rapid Acquisition with Relaxation Enhancement” (RARE)7 and T1-weighted “Magnetization‐Prepared Rapid Gradient‐Echo” (MP-RAGE)8 contrasts alongside with a trueCISS image (i.e. an on-resonant bSSFP image) and a maxCISS image (i.e. a bSSFP image with ideal flip angle depending on the relaxation time ratio Λ of a tissue ). A workflow diagram of this prototype algorithm is illustrated in Figure 1.

Results & Discussion

An axial and sagittal slice of the different quantitative maps M0, T2 and T1 is shown in Figure 2. The quantitative T2 values within regions of interest (ROIs) correspond to what was reported in literature for this measurement method9, with white matter ~53ms and grey matter ~88ms. However, the estimate of T1 values is affected by B1-field inhomogeneities resulting in longer values in the centre of the brain. As it has been already discussed for the fully sampled MIRACLE algorithm, the bias in the T1 values can be corrected using an additional B1-field acquisition. However, it appears that T1 values are systematically underestimated potentially due to asymmetries in the bSSFP profile10.

Currently MIRACLE and trueCISS are applied separately onto the sparse reconstructed phase-cycled images. However, both fitting procedures incorporate relaxation, i.e. T1 and T2 for MIRACLE and Λ=T1/T2 in trueCISS. In the future, both algorithms should be combined by using mutual variables within a single optimization to improve the fitting procedures in order to increase the robustness and quality of the estimates.

The synthetically generated contrasts of the same axial and sagittal slices are shown in Figure 3. All simulated images resemble the contrast of the conventional sequence, their clinical usefulness, however, still has to be validated.

Conclusion

We propose to combine two recently reported techniques: trueCISS and MIRACLE (in a novel undersampled version) to obtain high-resolution three-dimensional quantitative maps of M0, T1 and T2 in clinically feasible times (3:30min in the presented example). The obtained quantitative values were in the expected range in different brain regions and allowed for an application of different forward signal models, yielding simulated RARE and MPRAGE contrasts. The possibility of obtaining high-resolution qMRI images with isotropic voxel size may increase the clinical value of those techniques thanks to the improved conspicuity of small pathologies. Further work should focus on evaluating the clinical usefulness of qMRI as well as synthetic contrasts; furthermore, databases with quantitative values of healthy subjects should be built up, enabling the identification of abnormal tissue from a single-time-point scan.

Acknowledgements

No acknowledgement found.

References

1Margaret Cheng, Hai-Ling, et al. "Practical medical applications of quantitative MR relaxometry." Journal of Magnetic Resonance Imaging 36.4 (2012): 805-824.

2Nguyen, Damien, et al. "MIRACLE: Motion-Insensitive RApid Configuration ReLaxomEtry." Proc. Intl. Soc. Mag. Reson. Med.. Toronto, Canada. 2015.

3Hilbert, Tom, et al. "TrueCISS: Genuine bSSFP Signal Reconstruction from Undersampled Multiple-Acquisition SSFP Using Model-Based Iterative Non-Linear Inversion." Proc. Intl. Soc. Mag. Reson. Med.. Toronto, Canada. 2015.

4Lustig, Michael, David Donoho, and John M. Pauly. "Sparse MRI: The application of compressed sensing for rapid MR imaging." Magnetic resonance in medicine 58.6 (2007): 1182-1195.

5Zur, Y., M. L. Wood, and L. J. Neuringer. "Motion-insensitive, steady-state free precession imaging." Magnetic resonance in medicine 16.3 (1990): 444-459.

6Heule, Rahel, Carl Ganter, and Oliver Bieri. "Triple echo steady-state (TESS) relaxometry." Magnetic Resonance in Medicine 71.1 (2014): 230-237.

7Hennig, J., A. Nauerth, and H. Friedburg. "RARE imaging: a fast imaging method for clinical MR." Magnetic Resonance in Medicine 3.6 (1986): 823-833.

8Mugler, John P., and James R. Brookeman. "Three-dimensional magnetization-prepared rapid gradient-echo imaging (3D MP RAGE)." Magnetic Resonance in Medicine 15.1 (1990): 152-157.

9Deoni, Sean CL, Terry M. Peters, and Brian K. Rutt. "High-resolution T1 and T2 mapping of the brain in a clinically acceptable time with DESPOT1 and DESPOT2." Magnetic resonance in medicine 53.1 (2005): 237-241.

10Miller, Karla L. "Asymmetries of the balanced SSFP profile. Part I: Theory and observation." Magnetic Resonance in Medicine 63.2 (2010): 385-395.

Figures

Figure 1: A flowchart of the proposed algorithm that shows how to achieve quantitative maps and synthetic contrasts based on a highly undersampled phase-cycled bSSFP 3D sequence.

Figure 2: Exemplary M0, T2 and T1 maps of a sagittal and axial slice in a healthy volunteer estimated using MIRACLE and trueCISS.

Figure 3: Synthetic contrasts in a sagittal and axial slice of the healthy volunteer.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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