Felix Zimmermann

Felix Zimmermann

Physicist and physician. PhD research on physics-informed quantitative MRI at PTB and TU Berlin.

Berlin, Germany

I’m a physicist and physician working on quantitative MRI, inverse problems and machine learning at PTB and TU Berlin.

I combine physical models with learned methods to reconstruct images and tissue parameters, and build open-source tools for working with real scanner data. Before MRI, I worked on X-ray fluorescence and diffraction imaging at Stanford/SLAC and SACLA.

Projects and research software

MRpro

Open MR reconstruction in PyTorch

I lead the development of MRpro: a shared framework for model-based reconstruction, learned methods and quantitative MRI, with common data structures and differentiable operators. The MRpro team received second prize for relaxometry in the 2024 ISMRM QMRI Study Group Challenge.

PhD research

Physics-Informed Machine Learning for Robust Quantitative MRI

Research at PTB and TU Berlin combining MR acquisition and signal models with learned priors. My work spans differentiable optimization, quantitative parameter estimation, uncertainty and low-field imaging.

PINQI network schematic connecting an acquisition model with learned regularization
PINQI combines a physical model and learned regularization in one network.

PINQI

Learning with the physics of quantitative MRI

End-to-end quantitative reconstruction through unrolled optimization, differentiable solvers and learned regularization. The signal and acquisition models are part of the network.

T₁ maps and error maps comparing PINQI with other reconstruction methods
T₁ maps and reconstruction errors from the PINQI project.

Augment to Augment

Ultra-low-field MRI enhancement

Diverse augmentations and auxiliary learning tasks for enhancing 64 mT brain scans with limited paired training data. My ULF-EnC submission placed fourth on the final MICCAI 2025 test leaderboard.

B₁⁺ prediction

Transmit-field mapping from localizers

Learning spatial transmit-field maps from localizer images, with training and evaluation code for whole-brain B₁⁺ prediction at ultra-high field.

s2wasabi

Semi-supervised T₁, B₀ and B₁ mapping

Spatially regularized parameter estimation from WASABIT! data, combining synthetic pretraining with supervised and self-supervised fine-tuning. Includes trained models and a Pulseq sequence. Presented orally at ISMRM 2023 and recognized with a Magna Cum Laude award.

CMRxRecon / NoSENSE

Cardiac reconstruction without explicit sensitivity maps

Learned, unrolled reconstruction for the CMRxRecon 2023 challenge, developed with Andreas Kofler. NoSENSE works without a separate coil-sensitivity-map estimation step.

Learning regularization maps

Data-adapted regularization for reconstruction

Collaborative work on learning spatially and temporally varying regularization parameters, pairing neural networks with variational reconstruction and algorithm unrolling.

Siemens-to-ISMRMRD converter with a file drop area and metadata editor

siemens2mrd.app

Raw-data conversion, directly in the browser

Convert Siemens Twix measurements to ISMRMRD, inspect and edit headers, and merge metadata. Conversion runs locally in the browser; measurement data is not uploaded to a server.

PyPulseq browser editor showing the default gradient-echo sequence code

PyPulseq editor

Write and run MRI sequences in the browser

A browser workspace for PyPulseq code using Pyodide. Run Python, inspect plots and download generated .seq files without a local Python setup.

MRui reconstruction interface showing a reconstructed image and processing controls

MRui

A web interface for MRtwo reconstructions

Upload data, select an algorithm and inspect results through a web interface. Background jobs handle reconstruction work and keep the interface responsive.

MRpro server

Bring reconstruction to Siemens Open Recon

An Open Recon server using MRpro for scanner-side reconstruction, with configurable processing and a scanner interface.

PyPulseq (contributions)

Contributions to open MRI sequence design

Contributions and sequence-development work with the community PyPulseq package for vendor-neutral MRI pulse sequences in Python. It provides the foundation for my sequence experiments and browser-based editor.

torchnd

N-dimensional operations for PyTorch

Convolution, padding, cropping, interpolation and adjoints with flexible dimension layouts, including complex tensors where supported.

ImplicitOptimLayer

Optimization as a network layer

A compact example of differentiating through nonlinear optimization and using regression inside a neural network. An early building block for work such as PINQI.

epgtorch

Differentiable MR signal simulation

Extended phase graph simulations for fast spin echo and magnetic resonance fingerprinting, implemented with batched PyTorch operations and autograd.

torch_mri_utils

Practical building blocks for MRI research

A collection of PyTorch tools including adjoint grid sampling, separable convolutions, coil-sensitivity estimation and sliding-window views.

IDI

Incoherent diffraction imaging

Software for simulating, reconstructing and analyzing X-ray fluorescence data, developed alongside experimental imaging work at Stanford/SLAC and SACLA.

Simulated particle and three-dimensional reconstruction from X-ray diffraction

Bachelor’s thesis

X-ray diffraction imaging

GPU-accelerated simulations of X-ray scattering from three-dimensional particles, comparing iterative reconstruction and in-flight holography.

Leaflet map of Tanzania with population-cluster filters and location information

Webmap Tanzania

Planning rural electrification

An interactive Leaflet map of population clusters and geodata for electrification planning in Tanzania, built with open-source tools and open data.

TeXlive.js

LaTeX in the browser

TeXlive compiled to JavaScript with Emscripten, based on the work of manuels. Write and compile LaTeX without installing a local TeX distribution.

MSF

Master stability function and cluster synchronization

A nonlinear-dynamics project analyzing the stability of cluster synchronization in complex networks.

Publications

  1. 2026
  2. Physics-informed denoising method for image reconstruction in quantitative low-field MRI

    C. Redshaw Kranich, C. Prieto, C. Kolbitsch & F. F. Zimmermann

    arXiv:2609.12966

  3. Cross-Site Generalization of CNN-Based B₁⁺ Mapping in UHF MRI

    K. Hadjikiriakos, F. Krüger, F. F. Zimmermann et al.

    NMR in Biomedicine

  4. Physics-Informed Deep Learning for Shear Wave Speed Estimation in MR Elastography

    S. Martin, J. Schattenfroh, P. Schuenke, F. F. Zimmermann et al.

    IEEE Transactions on Biomedical Engineering

  5. 2025

    Fluorescence intensity correlations enable 3D imaging without sample rotations

    R. G. Radloff, F. F. Zimmermann et al.

    arXiv:2510.24386

  6. Non-linear enhancement of ultrafast X-ray diffraction through transient resonances

    S. Kuschel, P. J. Ho, A. Al Haddad, F. F. Zimmermann et al.

    Nature Communications

  7. Deep learning-based whole-brain B₁⁺ mapping at 7 T

    F. Krueger et al., including F. F. Zimmermann

    Magnetic Resonance in Medicine

  8. Robust Myocardial Perfusion MRI Quantification With DeepFermi

    S. Brahma, A. Kofler, F. F. Zimmermann et al.

    IEEE Transactions on Biomedical Engineering

  9. Metrology for artificial intelligence in medicine

    H. Rabus et al., including F. F. Zimmermann

    Measurement: Sensors

  10. Zero-Shot Self-Supervised Motion Corrected Cardiac T₁ Mapping

    M. Guastini, F. F. Zimmermann, J. Schulz-Menger, C. Kolbitsch & A. Kofler

    ISMRM, #0119

  11. Higher Fields, Deeper Insights: DL-based B₁⁺ prediction at 10.5 T

    K. Hadjikiriakos, F. F. Zimmermann, F. Krüger et al.

    ISMRM, #4252

  12. Enhancement of scattering and absorption cross-sections by transient resonances in copper and iron

    R. G. Radloff, N. Muthreich, F. F. Zimmermann et al.

    SPIE, X-Ray Free-Electron Lasers VII PC135360Z

  13. 2024

    PINQI: An End-to-End Physics-Informed Approach to Learned Quantitative MRI Reconstruction

    F. F. Zimmermann, C. Kolbitsch, P. Schuenke & A. Kofler

    IEEE Transactions on Computational Imaging

    arXiv
  14. Machine Learning for Quantitative Magnetic Resonance Image Reconstruction

    A. Kofler, F. F. Zimmermann & K. Papafitsoros

    Quantification of Biophysical Parameters in Medical Imaging

    arXiv
  15. NoSENSE: Learned unrolled cardiac MRI reconstruction without explicit sensitivity maps

    F. F. Zimmermann & A. Kofler

    STACOM at MICCAI 2023, Springer, pp. 454–466

    arXiv
  16. Long-term lipoprotein apheresis reduces cardiovascular events in high-risk patients with isolated lipoprotein(a) elevation

    F. Schumann, U. Kassner, D. Spira, F. F. Zimmermann et al.

    Journal of Clinical Lipidology

  17. Neural Implicit Quantitative Imaging

    F. F. Zimmermann, S. Hufnagel, P. Schuenke, A. Kofler & C. Kolbitsch

    ISMRM, #0659, oral presentation

  18. 2023
  19. Semi-supervised learning for spatially regularized quantitative MRI reconstruction — application to simultaneous T₁, B₀ and B₁ mapping

    F. F. Zimmermann, A. Kofler, C. Kolbitsch & P. Schuenke

    ISMRM, #1166, oral presentation, Magna Cum Laude award

  20. Towards Incoherent X-ray Imaging with High Resolution

    F. F. Zimmermann, S. Kuschel, M. Ware et al.

    SPring-8/SACLA Research Report 11(5), pp. 387–389

  21. Ultrafast 3D X-Ray Fluorescence Imaging

    R. G. Radloff et al., including F. F. Zimmermann

    APS DAMOP, X11.005

  22. 2022

    Physics-Informed Deep Learning for Image Distortion Correction from B₀-inhomogeneities in Low-Field MRI

    D. Schote, L. Winter, C. Kolbitsch, F. F. Zimmermann et al.

    ISMRM, #1816

  23. 2021
  24. 2020

    Potential benefits of electronic damage in X-ray imaging

    S. Kuschel et al., including F. F. Zimmermann

    APS DAMOP, S04.007

  25. 2018