Fast T2 Mapping with Improved Accuracy Using Undersampled Spin-echo MRI and Model-based Reconstructions with a Generating Function

A model-based reconstruction technique for accelerated T2 mapping with improved accuracy is proposed using undersampled Cartesian spin-echo MRI data. The technique employs an advanced signal model for T2 relaxation that accounts for contributions from indirect echoes in a train of multiple spin echoes. An iterative solution of the nonlinear inverse reconstruction problem directly estimates spin-density and T2 maps from undersampled raw data. The algorithm is validated for simulated data as well as phantom and human brain MRI at 3 T. The performance of the advanced model is compared to conventional pixel-based fitting of echo-time images from fully sampled data. The proposed method yields more accurate T2 values than the mono-exponential model and allows for undersampling factors of at least 6. Although limitations are observed for very long T2 relaxation times, respective reconstruction problems may be overcome by a gradient dampening approach. The analytical gradient of the utilized cost function is included as Appendix.Comment: 10 pages, 7 figure

Real-time magnetic resonance imaging: Radial gradient-echo sequences with nonlinear inverse reconstruction.

Objective The aim of this study is to evaluate a real-time magnetic resonance imaging (MRI) method that not only promises high spatiotemporal resolution but also practical robustness in a wide range of scientific and clinical applications. Materials and Methods The proposed method relies on highly undersampled gradient-echo sequences with radial encoding schemes. The serial image reconstruction process solves the true mathematical task that emerges as a nonlinear inverse problem with the complex image and all coil sensitivity maps as unknowns. Extensions to model-based reconstructions for quantitative parametric mapping further increase the number of unknowns, for example, by adding parameters for phase-contrast flow or T1 relaxation. In all cases, an iterative numerical solution that minimizes a respective cost function is achieved with use of the iteratively regularized Gauss-Newton method. Convergence is supported by regularization, for example, to the preceding frame, whereas temporal fidelity is ensured by downsizing the regularization strength in comparison to the data consistency term in each iterative step. Practical implementations of highly parallelized algorithms are realized on a computer with multiple graphical processing units. It is "invisibly" integrated into a commercial 3-T MRI system to allow for conventional usage and to provide online reconstruction, display, and storage of regular DICOM image series. Results Depending on the application, the proposed method offers serial imaging, that is, the recording of MRI movies, with variable spatial resolution and up to 100 frames per second (fps)-corresponding to 10 milliseconds image acquisition times. For example, movements of the temporomandibular joint during opening and closing of the mouth are visualized with use of simultaneous dual-slice movies of both joints at 2 x 10 fps (50 milliseconds per frame). Cardiac function may be studied at 30 to 50 fps (33.3 to 20 milliseconds), whereas articulation processes typically require 50 fps (20 milliseconds) or orthogonal dual-slice acquisitions at 2 x 25 fps (20 milliseconds). Methodological extensions to model-based reconstructions achieve improved quantitative mapping of flow velocities and T1 relaxation times in a variety of clinical scenarios. Conclusions Real-time gradient-echo MRI with extreme radial undersampling and nonlinear inverse reconstruction allows for direct monitoring of arbitrary physiological processes and body functions. In many cases, pertinent applications offer hitherto impossible clinical studies (eg, of high-resolution swallowing dynamics) or bear the potential to replace existing MRI procedures (eg, electrocardiogram-gated cardiac examinations). As a consequence, many novel opportunities will require a change of paradigm in MRI-based radiology. At this stage, extended clinical trials are needed

Refinement of the crystal structure of praseodymium orthoscandate, PrScO3

O3PrSc, Prima (no. 62), a = 5.780(1) Å, b = 8.025(2) Å, c = 5.608(1) Å, V= 260.1 Å3, Z = 4, R gr(F) = 0.025, wRref(F2) = 0.060, T= 298 K. © by Oldenbourg Wissenschaftsverlag, München

Crystal structure of distrontium praseodym gallium pentaoxide, Sr2PrGaO5

GaO5PrSr2, tetragonal, I4/mcm (No. 140), a = 6.8441(2) Å, c = 11.2534(4) Å, V = 527.1 Å3, Z = 4, R(P) = 0.035, wR(P) = 0.052, R(I) = 0.038, T = 295 K. © by Oldenbourg Wissenschaftsverlag

Refinement of the crystal structure of praseodymium orthoscandate, PrScO3

O3PrSc, Prima (no. 62), a = 5.780(1) Å, b = 8.025(2) Å, c = 5.608(1) Å, V= 260.1 Å3, Z = 4, R gr(F) = 0.025, wRref(F2) = 0.060, T= 298 K. © by Oldenbourg Wissenschaftsverlag, München

Crystal structure of praseodym gallate, Pr4Ga2O9

Ga2O9Pr4, monoclinic, P121/c1 (No. 14), a = 7.8256(4) Å, b = 11.0322(5) Å, c = 11.4959(7) Å, β = 109.187(3)°, V = 937.4 Å3, Z = 4, R(P) = 0.026, wR(P) = 0.034, R(I)= 0.033, T = 295 K