Christina Graf

Robust inversion in fluid-attenuated inversion recovery reduced artifacts in brain MRI that mimic hyperintense white matter lesions.

Purpose: To develop a generic radial sampling scheme that combines the advantages of golden ratio sampling with simplicity of equidistant angular patterns. The irrational angle between consecutive spokes in golden ratio‐based sampling schemes enables a flexible retrospective choice of temporal resolution, while preserving good coverage of k‐space for each individual bin. Nevertheless, irrational increments prohibit precomputation of the point‐spread function (PSF), can lead to numerical problems, and require more complex processing steps. To avoid these problems, a new sampling scheme based on a rational approximation of golden angles (RAGA) is developed.Methods: The theoretical properties of RAGA sampling are mathematically derived. Sidelobe‐to‐peak ratios (SPR) are numerically computed and compared to the corresponding golden ratio sampling schemes. The sampling scheme is implemented in the BART toolbox and in a radial gradient‐echo sequence. Feasibility is shown for quantitative imaging in a phantom and a cardiac scan of a healthy volunteer.Results: RAGA sampling can accurately approximate golden ratio sampling and has almost identical PSF and SPR. In contrast to golden ratio sampling, each frame can be reconstructed with the same equidistant trajectory using different sampling masks, and the angle of each acquired spoke can be encoded as a small index, which simplifies processing of the acquired data.Conclusion: RAGA sampling provides the advantages of golden ratio sampling while simplifying data processing, rendering it a valuable tool for dynamic and quantitative MRI.

Purpose: To employ optimal control for the numerical design of Chemical Exchange Saturation Transfer (CEST) saturation pulses to maximize contrast and stability against inhomogeneities.Theory and Methods: We applied an optimal control framework for the design pulse shapes for CEST saturation pulse trains. The cost functional minimized both the pulse energy and the discrepancy between the corresponding CEST spectrum and the target spectrum based on a continuous radiofrequency (RF) pulse. The optimization is subject to hardware limitations. In measurements on a 7 T preclinical scanner, the optimal control pulses were compared to continuous‐wave and Gaussian saturation methods. We conducted a comparison of the optimal control pulses with Gaussian, block pulse trains, and adiabatic spin‐lock pulses.Results: The optimal control pulse train demonstrated saturation levels comparable to continuous‐wave saturation and surpassed Gaussian saturation by up to 50 % in phantom measurements. In phantom measurements at 3 T the optimized pulses not only showcased the highest CEST contrast, but also the highest stability against field inhomogeneities. In contrast, block pulse saturation resulted in severe artifacts. Dynamic Bloch‐McConnell simulations were employed to identify the source of these artifacts, and underscore the robustness of the optimized pulses.Conclusion: In this work, it was shown that a substantial improvement in pulsed saturation CEST imaging can be achieved by using Optimal Control design principles. It is possible to overcome the sensitivity of saturation to B0 inhomogeneities while achieving CEST contrast close to continuous wave saturation.

Summary: Magnetization transfer spectroscopy relies heavily on the robust determination of relaxation times of nuclei participating in metabolic exchange. Challenges arise due to the use of surface RF coils for transmission (high variation) and the broad resonance band of most X nuclei. These challenges are particularly pronounced when fast mapping methods, such as the dual‐angle method, are employed. Consequently, in this work, we develop resonance offset and robust excitation RF pulses for ³¹P magnetization transfer spectroscopy at 7T through ensemble‐based time‐optimal control. In our approach, we introduce a cost functional for designing robust pulses, incorporating the full Bloch equations as constraints, which are solved using symmetric operator splitting techniques. The optimal control design of the RF pulses developed demonstrates improved accuracy, desired phase properties, and reduced RF power when applied to dual‐angle mapping, thereby improving the precision of exchange‐rate measurements, as demonstrated in a preclinical in vivo study quantifying brain creatine kinase activity.

Summary: Non‐selective inversion pulses find widespread use in MRI applications, where requirements on them are increasingly demanding. With the use of high and ultra‐high field strength systems, robustness to and inhomogeneities, while tackling SAR and hardware limitations, has rapidly become important. In this work, we propose a time‐optimal control framework for the optimization of ‐ and ‐robust inversion pulses. Robustness is addressed by means of ensemble formulations, while allowing inclusion of hardware and energy limitations. The framework is flexible and performs excellently for various optimization goals. The optimization results are analyzed extensively in numerical experiments. Furthermore, they are validated, and compared with adiabatic RF pulses, in various phantom and in vivo measurements on a 3 T MRI system.