Michael Hintermüller, Tobias Schäffter
Staff: Gouzhi Dong (HU & WIAS)
Associate: Kostas Papafitsoros (WIAS)
01.01.2019 – 31.12.2021
Model-based imaging exploit knowledge of physical models in imaging to enhance the reconstruction quality. This is often challenge in real applications, as the physical models might be not directly available, but hidden in different kinds of data, either experimental data or numerical one. This project aims to develop the concept of model-based imaging methods. The idea is integrating physical models, either dictionary based or learning-informed, into the image reconstruction process. Particularly, the physical model may involve some physical quantities, which are also of interests to estimate their precise values, as the case in quantitative imaging. These problems motivate the research of the this project.
As some methodology guideline, we are interested in the following optimization framework in this project but not limited to:
Here is an upper level objective which might be based on statistical measures, and u denotes some physical quantities. is an element either from an entry of some dictionary, or produced through some machine learning schemes. The lower level objective is usually a matching of fidelity with some regularization of involved. In particular, spatial distributed regularization parameters are of interest and will be investigated under this bilevel optimization framework.
These new mathematical objects then need to be analytically and numerically investigated, including also robust numerical solvers.
Exemplary case study focuses on magnetic resonance imaging. An integrated physics-based models are proposed for quantitatively estimating the tissue parameters, for instance, the T1/T2 relaxation time, the proton spin density. Approaches of using learning-informed physics or dictionary-based physics are under investigation. In particular, we are investigating the following problem in a (partial) differential equation constrained optimization setting:
where denotes some differential equations. The variable typically represents some imaging function one would like to reconstruct out of the detected data , and is some physical quantity which contains quantitative information. The operator is some linear or nonlinear operator, and is a regularization functional.
For instance in the quantitative MRI problem, is the MRI image, could be the Bloch equation, and represents the quantities like relaxation time and , proton spin density , as in the following
Particularly, we have worked on an integrated physics-based approach, where the equation is represented using an explicit map . With this, we study directly the reduced problem:
In practice, this explicit operator may not be available directly, but hidden in numerical or experimental data. Therefore, a dictionary or learning-informed scheme can play the role to replace the underline exact physics. Using this, we come up with the following problem:
Questions like the feasibility of the replacement of using , existence of solutions, regularity, and differentiability of the solution map are important and of interest to study in this project, as well as the error bounds of the solutions, and their numerical analysis, implementations. Related to MRI problem, the dictionary or machine learning techniques are applied to simulate the Bloch map from the numerical solutions for Bloch equations.
In the bilevel optimization framework, a particular line of research which has been continued in this project is a generalization of the idea for automatically selecting the spatially dependent regularization parameters in total generalized variation (TGV) method for imaging and inverse problems.
Techniques using variational methods and also partial differential equation tools have been studied and will continue to be studied in different context.
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