Optimized noise filters for improved contrast in MRI
Final Report Abstract
The main goal of this project was to develop and implement optimized experimental schemes for magnetic resonance imaging (MRI) experiments. This includes pulse sequences that modulate the interaction between the nuclear spins and their environment such that the information content of the resulting images is maximized. For additional optimisation, we also adapted the subsequent data processing steps. Depending on the specific diagnostic requirements, this allows us to optimally suppress the effects of decoherence and noise in structured media characteristic of living tissue or to measure and analyze the noise spectra, and to extract diagnostic information contained in the noise. For both goals, the optimal modulation schemes must be robust against experimental imperfections that are typically encountered in a clinical setting and the SAR values must be kept at acceptable levels. We could show that optimally designed field modulation schemes result in substantial improvements in signal-to-noise ratios and they generate images that contain more direct information, e.g. on molecular diffusion processes in the tissues. Starting from basic quantum mechanical principles, we were able to derive ultimate limits on the precision of such measurements and find the parameters required to attain those limits. The sequences were designed with the help of optimal control theory, building on a substantial amount of expertise acquired during our work on quantum information processing. Most of the experimental work was performed on our microimaging systems, while a few test experiments were performed on clinical scanners in other labs. Putting the focus on our own instruments resulted in significantly more measurement time and avoided many of the complications associated with clinical scanners. In addition, these instruments provide the most direct comparison between theory and experiment. The pulse sequences designed here can be applied to different physical systems. Our focus was on two types of microscopic properties for generating contrast: the coupling network of the spins with each other and with the molecular system, which allows one to identify different biological markers, and the diffusion of water molecules, which explores the compartments that structure biological tissues and cells. Depending on the length- and time-scale, the diffusion process tends to become structured, often anisotropic and the effective diffusion constants become time-dependent. The possibilities of targeting different parts of these complex processes are therefore virtually unlimited, and we our present work therefore may only have scratched the surface. One approach consists in refocusing the unwanted interactions, e.g. by some type of echo experiment, which effectively invert the system-environment interaction. While this is relatively straightforward for interactions between the qubits and their environment, the general situation is that the environment itself evolves in time and the interaction between system and environment therefore fluctuates. This must be taken into account in the design of the refocusing operations. Several of these approaches had been implemented already before the start of this project but was extended and adapted to the current goals. Much of the work was performed in collaboration with partners in Germany, Russia, Israel and Argentina. In summary, the main goals of this project have been achieved. Many of the long-term goals of this project were taken up by the EU project PATHOS3 , which started in 2019 and will run until 2024. It includes, in addition to the TU Dortmund group, partners from Italy and Israel.
Publications
- 'Optimized multiple-quantum filter for robust selective excitation of metabolite signals’, J. Magn. Reson. 243, 8-16 (2014)
Mirjam Holbach, Jörg Lambert, and Dieter Suter
(See online at https://doi.org/10.1016/j.jmr.2014.03.007) - 'Optimized selective lactate excitation with a refocused multiple-quantum filter’, J. Magn. Reson. 255, 34-38 (2015)
Mirjam Holbach, Jörg Lambert, Sören Johst, Mark E. Ladd, and Dieter Suter
(See online at https://doi.org/10.1016/j.jmr.2015.03.004) - 'Anisotropic diffusion phantoms based on microcapillaries’, J. Magn. Reson. 279, 1-10 (2017)
S. Vellmer, D. Edelhoff, D. Suter, and I. I. Maximov
(See online at https://doi.org/10.1016/j.jmr.2017.04.002) - 'Comparative analysis of isotropic diffusion weighted imaging sequences’, J. Magn. Reson. 275, 137-147 (2017)
S. Vellmer, R. Stirnberg, D. Edelhoff, D. Suter, T. Stöcker, and I. I. Maximov
(See online at https://doi.org/10.1016/j.jmr.2016.12.011) - 'Validation of DWI pre-processing procedures for reliable differentiation between human brain gliomas’, Z. Med. Physik 28, 14-24 (2018)
S. Vellmer, A. S. Tonoyan, D. Suter, I. N. Pronin, and I. I. Maximov
(See online at https://doi.org/10.1016/j.zemedi.2017.04.005) - 'Precision Limits of Tissue Microstructure Characterization by Magnetic Resonance Imaging’, Phys. Rev. Applied 14, 024088 (2020)
A. Zwick, D. Suter, G. Kurizki, and G. A. Alvarez
(See online at https://doi.org/10.1103/PhysRevApplied.15.014045)
