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Model-based reconstruction for simultaneous multi-slice myocardial T1 mapping using single-shot inversion-recovery radial FLASH1Department of Interventional and Diagnostic Radiology of the University Medical Center Göttingen, Göttingen, Germany, 2DZHK (German Centre for Cardiovascular Research), partner site Göttingen, Göttingen, Germany
Synopsis
Fast multi-slice myocardial T1 mapping is of great interest in clinical cardiovascular magnetic resonance (CMR) imaging. This work extends a simultaneous-multi-slice (SMS) model-based reconstruction method for 3-slice myocardial T1 mapping using single-shot Inversion-recovery radial FLASH. Initial results on experimental phantom and one healthy volunteer have demonstrated simultaneous 3-slice myocardial T1 mapping (1.6 x 1.6 x 8 mm3 ) might be feasible within a single inversion recovery of 4 s. More clinical validations of the proposed method will be explored in future studies.
Introduction
Quantitative myocardial T1 mapping is of increasing interest in clinical cardiovascular magnetic resonance (CMR) imaging [1,2]. While conventional MOLLI-based methods use 2 to 3 inversions for data acquisition [3], recent developments in non-Cartesian sampling [4-6] in combination with advanced reconstructions such as compressed sensing [6], model-based reconstructions [7-10] have enabled single-slice myocardial T1 mapping within a single inversion-recovery (IR). To further enable T1 mapping with multiple sections within a single inversion, this work extends a simultaneous-multi-slice (SMS) model-based reconstruction method for high-resolution 3-slice myocardial T1 mapping using single-shot IR radial FLASH. Validations of the proposed method have been performed on an experimental phantom and one healthy subject.Methods
We employ a triggered IR SMS radial FLASH scheme for data acquisition: After a non-selective inversion, radial SMS data is continuously acquired using a golden-angle FLASH readout with the rotated golden-angle used in the partition dimension [11, 12]. To eliminate effects from systolic motion, only data from the diastolic phase is retrospectively selected for T1 estimation. Similar to [11], let $$$p, q$$$ define the partition index and the slice index and let $$$Q$$$ be the total number of partitions/slices. The estimation of parameter maps and coil sensitivity maps from all slices is formulated as a single nonlinear inverse problem [12]:$$\hat{x} = \text{argmin}_{x\in D} \|F(x) -\widetilde{Y} \|_{2}^{2} + \alpha \sum_{q=1}^{Q}R(x^{q}_{p}) + \beta \sum_{q=1}^{Q}U(x^{q}_{c})$$
with $$$F$$$ a nonlinear operator which maps all unknowns x from all slices to the measured undersampled SMS data $$$\widetilde{Y}$$$. $$$x^{q}_{p}$$$ contains steady-state signal $$$M_{ss}$$$, equilibrium signal $$$M_{0}$$$ and effective relaxation rate $$$R_{1}^{*}$$$ of the $$$q$$$th slice, while $$$x^{q}_{c}$$$ is a set of the corresponding coil sensitivity maps. $$$R(\cdot)$$$ defines the joint $$$\ell_{1}$$$-Wavelet regularization in the parameter dimension [8], $$$U({\cdot})$$$ represents the Sobolev norm enforcing the smoothness of coil sensitivity maps. $$$\alpha$$$ and $$$\beta$$$ are the regularization parameters for parameter maps and coil sensitivity maps, respectively. $$$D$$$ is a convex set ensuring the effective relaxation rate $$$R_{1}^{*}$$$ to be nonnegative [8]. The above nonlinear inverse problem is solved by the IRGNM-FISTA algorithm [8].
Data acquisition was performed on a Magnetom Skyra 3T (Siemens Healthineers, Erlangen, Germany). Phantom studies were conducted with a 20-channel head/neck coil, whereas combined thorax and spine coils with 26 channels were utilized for in-vivo scans. The acquisition parameters for phantom were: FOV: 192 x 192 mm$$$^{2}$$$, matrix size: 192 x 192, slice thickness 5 mm (gap 15 mm), TR/TE = 4.10/2.58 ms, bandwidth 630 Hz/pixel. Parameters for the in-vivo measurements (female, age 27, heart rates 55 bpm) were: FOV: 256 x 256 mm$$$^{2}$$$, matrix size: 160 x 160, slice thickness 8 mm (gap 20 mm), TR/TE = 2.90/1.79 ms, bandwidth 850 Hz/pixel. All single-shot measurements were acquired within 4 seconds using the nominal flip angle $$$6^{\circ}$$$. In-vivo experiments were performed during a brief breathhold. All data processing was done offline and implemented in BART [13].
Results
Figure 1 shows the estimated SMS T1 maps (center-slice) of an experimental phantom for simulated heart rates between 40 and 100 bpm. Visual inspection reveals good agreement for all heart rates and T1 values when compared to a conventional IR spin-echo method. These findings are confirmed by quantitative ROI-analyses summarized in Table 1. Figure 2 presents 3-slice myocardial T1 maps acquired using single-shot IR SMS radial FLASH and reconstructed by the proposed model-based algorithm. All 3-slice T1 maps are visually in good agreement with the corresponding single-slice ones, which is further confirmed in the ROI-analyzed septal T1 values in Table 2. Further, figure 3 demonstrates four synthetic T1-weighted images for all 3 slices, which shows a distinct contrast between myocardium and blood pool.Discussion & Conclusion
This work extends IR SMS radial FLASH sequence and sparsity-constrained model-based reconstructions [12] into single-shot multi-slice myocardial T1 mapping. Initial results on phantom and one healthy volunteer have demonstrated simultaneous 3-slice myocardial T1 mapping might be feasible within a single inversion recovery of 4 s. The main limitation of the present work is the limited number of subjects. More clinical validations of the proposed method will be explored in future studies.Acknowledgements
No acknowledgement found.References
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