Physics of Molecular Imaging Systems

Medical imaging has a key role in the field of diagnostics, interventions, therapy planning and for the assessment of the therapy response. The demand to personalize medical imaging or to improve it with respect to accuracy and cost-effectiveness is continuously growing. In particular, the development of new treatments such as cell-based therapies are requesting for highest sensitivity and precise quantitative imaging. In this respect, magnetic resonance imaging (MRI), which doesn’t require ionizing radiation, offers a huge variety of different contrasts and thus offers a high potential for future clinical applications. In contrast to computed tomography (CT), MRI offers excellent soft-tissue contrast, the measurement of tissue composition, oxygen concentration, pH-values, temperature distributions, dynamics volumes, perfusion and diffusion etc. MRI is based on the nuclear resonance effect, which is mostly the imaging of hydrogen nuclei (protons) in the case of medical applications. The basic idea of MRI to reconstruct three dimensional (3D) tomographic images is based on the fact that the frequency of the nuclei resonance (Larmor-frequency) depends linearly on the outer magnetic field. Hence, an MRI uses strong static (main field), quasi-static (gradient coils for spatial encoding) and radiofrequency (RF) fields to transform the MRI signal into a 3D tomographic image. Beside anatomical medical imaging modalities such as MRI and CT, positron emission tomography (PET) and single photon emission computed tomography (SPECT) offer in-vivo imaging of molecular processes like metabolisms or receptor density. In this way, nuclear imaging modalities offer another view on disease occurrence, progression and therapy response. The basic idea of PET and SPECT is to use radioactively labeled molecules as imaging tracer. In SPECT the tracer emits gamma photons with about 140 – 365 keV, which are detected by use of mechanical collimators. In difference to this mechanical collimation, PET uses the emission of positrons, which annihilate to pairs of gamma photons with 511 keV. As the gamma photons are emitted coincidently in nearly opposite direction, the annihilation can be detected along this “line of response” (LOR). Furthermore, with the measurement of time-of-flight, the location of the annihilation along this LOR canbe measured. Due to the electronic instead of mechanical collimation, the sensitivity of PET is nearly two orders of magnitude higher compared to the sensitivity of SPECT. For both SPECT and PET a large variety of tracer exist, which offer imaging of all kind of molecular processes. A completely new contrast agent based imaging modality has been invented in 2001 by Philips Research: magnetic particle imaging (MPI). MPI uses magnetic nanoparticles (MNP) with a core size of about 20-30 nm as a contrast agent which are magnetically excited. Thus this imaging modality does not involve the use of ionizing radiation. The core idea of MPI is the highly nonlinear response of the MNP to magnetic excitation. Due to this property, the NMP will generate a response that can be well separated from the magnetic excitation field. A particular feature of MPI is that it can be very fast. An acquisition of 45 3D volumes per second has been demonstrated. The department of Physics in Molecular Imaging system (PMI) of the faculty of physics and of medicine is conducting EU, BMBF, and industry-funded research to understand, to optimize, to combine and to invent new methods and technologies for the field of medical imaging. Research topics are along the entire imaging chain, starting from fundamental physics, new detector and processing concepts for PET (in particular digital silicon photomultiplier), new MRI methods and correction technologies, MPI physics and instrumentation, and novel quantitative image reconstruction algorithms for actual and future imaging modalities. A special focus of PMI is the combination of tracer-based methods such as PET and MPI with MRI with the focus on hybrid medical imaging using different modalities simultaneously. In 2012 PMI has jointly developed with partners from academia and industry in Aachen the world first simultaneous PET-MRI system based on digital silicon photomultiplier.

Univ.-Prof. Dr.-Ing.
Volkmar Schulz

Selected publications

Research Papers

  1. Schaart DR, Charbon E, Frach T, and Schulz V. Advances in digital SiPMs and their application in biomedical imaging. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 2016;809:31‑52.
  2. Schug D, Wehner J, Dueppenbecker PM, Weissler B, Gebhardt P, Goldschmidt B, Salomon A, Kiessling F, and Schulz V. PET performance and MRI compatibility evaluation of a digital, ToF-capable PET/MRI insert equipped with clinical scintillators. Physics in Medicine and Biology. 2015;60(18):7045‑67.
  3. Gross-Weege N, Nolte T, and Schulz V. MR image corrections for PET-induced gradient distortions. Physics in Medicine & Biology. 2019;64(2):02.
  4. Mackewn JE, Lerche CW, Weissler B, Sunassee K, de Rosales RTM, Phinikaridou A, Salomon A, Ayres R, Tsoumpas C, Soultanidis GM, Gebhardt P, Schaeffter T, Marsden PK, and Schulz V. PET Performance Evaluation of a Pre-Clinical SiPM-Based MR-Compatible PET Scanner. IEEE Trans Nucl Sci. 2015;62(3):784‑90.
  5. Schug D, Wehner J, Dueppenbecker PM, Weissler B, Gebhardt P, Goldschmidt B, Solf T, Kiessling F, and Schulz V. ToF Performance Evaluation of PET Modules With Digital Silicon Photomultiplier Technology During MR Operation. IEEE Trans Nucl Sci. 2015;62(3):658‑63.
  6. Schug D, Wehner J, Goldschmidt B, Lerche C, Dueppenbecker PM, Hallen P, Weissler B, Gebhardt P, Kiessling F, and Schulz V. Data Processing for a High Resolution Preclinical PET Detector Based on Philips DPC Digital SiPMs. IEEE Trans Nucl Sci. 2015;62(3):669‑78.
  7. Weissler B, Gebhardt P, Lerche CW, Soultanidis GM, Wehner J, Heberling D, and Schulz V. PET/MR Synchronization by Detection of Switching Gradients. IEEE Trans Nucl Sci. 2015;62(3):650‑7.
  8. Gebhardt P, Wehner J, Weissler B, Frach T, Marsden PK, and Schulz V. RESCUE - Reduction of MRI SNR Degradation by Using an MR-Synchronous Low-Interference PET Acquisition Technique. IEEE Trans Nucl Sci. 2015;62(3):634‑43.
  9. Schug D, Weissler B, Gebhardt P, and Schulz V. Crystal Delay and Time Walk Correction Methods for Coincidence Resolving Time Improvements of a Digital-Silicon-Photomultiplier-Based PET/MRI Insert. IEEE Transactions on Radiation and Plasma Medical Sciences. 2017;1(2):178‑90.
  10. Ritzer C, Hallen P, Schug D, and Schulz V. Intercrystal Scatter Rejection for Pixelated PET Detectors. IEEE Transactions on Radiation and Plasma Medical Sciences. 2017;1(2):191‑200.
  11. Gross-Weege N, Schug D, Hallen P, and Schulz V. Maximum likelihood positioning algorithm for high-resolution PET scanners. Medical Physics. 2016;43(6Part1):3049‑61.
  12. Schulz V, Torres-Espallardo I, Renisch S, Hu Z, Ojha N, Börnert P, Perkuhn M, Niendorf T, Schäfer WM, Brockmann H, Krohn T, Buhl A, Günther RW, Mottaghy FM, and Krombach GA. Automatic, three-segment, MR-based attenuation correction for whole-body PET/MR data. Eur J Nucl Med Mol Imaging. 2011;38(1):138‑52.
  13. Salomon A, Goedicke A, Schweizer B, Aach T, and Schulz V. Simultaneous reconstruction of activity and attenuation for PET/MR. IEEE Trans Med Imaging. 2011;30(3):804‑13.
  14. Truhn D, Kiessling F, and Schulz V. Optimized RF shielding techniques for simultaneous PET/MR. Med Phys. 2011;38(7):3995‑4000.
  15. Berker Y, Franke J, Salomon A, Palmowski M, Donker HCW, Temur Y, Mottaghy FM, Kuhl C, Izquierdo-Garcia D, Fayad ZA, Kiessling F, and Schulz V. MRI-based attenuation correction for hybrid PET/MRI systems: a 4-class tissue segmentation technique using a combined ultrashort-echo-time/Dixon MRI sequence. J Nucl Med. 2012;53(5):796‑804. Erratum in: J Nucl Med. 2012 Sep;53(9):1496.
  16. Salomon A, Goldschmidt B, Botnar R, Kiessling F, and Schulz V. A self-normalization reconstruction technique for PET scans using the positron emission data. IEEE Trans Med Imaging. 2012;31(12):2234‑40.
  17. Wehner J, Weissler B, Dueppenbecker PM, Gebhardt P, Goldschmidt B, Schug D, Kiessling F, and Schulz V. MR-compatibility assessment of the first preclinical PET-MRI insert equipped with digital silicon photomultipliers. Phys Med Biol. 2015;60(6):2231‑55.
  18. Wehner J, Weissler B, Dueppenbecker P, Gebhardt P, Schug D, Ruetten W, Kiessling F, and Schulz V. PET/MRI insert using digital SiPMs: Investigation of MR-compatibility. Nucl Instrum Methods Phys Res A. 2014;734(Pt B):116‑121.
  19. Weissler B, Gebhardt P, Dueppenbecker PM, Wehner J, Schug D, Lerche CW, Goldschmidt B, Salomon A, Verel I, Heijman E, Perkuhn M, Heberling D, Botnar RM, Kiessling F, and Schulz V. A Digital Preclinical PET/MRI Insert and Initial Results. IEEE Trans Med Imaging. 2015;34(11):2258‑70.
  20. Goldschmidt B, Schug D, Lerche CW, Salomon A, Gebhardt P, Weissler B, Wehner J, Dueppenbecker PM, Kiessling F, and Schulz V. Software-Based Real-Time Acquisition and Processing of PET Detector Raw Data. IEEE Trans Biomed Eng. 2016;63(2):316‑27.

Reviews / Perspectives

  1. Keereman V, Mollet P, Berker Y, Schulz V, and Vandenberghe S. Challenges and current methods for attenuation correction in PET/MR. MAGMA. 2013;26(1):81‑98.

Upcoming conferences

  • Mar 2019: International Workshop on Magnetic Particle Imaging 2019, New York (IWMPI)
  • Apr 2019: Conference on PET/MR and SPECT/MR 2019, Munich (PSMR)
  • Oct 2019: Nuclear Science Symposium and Medical Imaging Conference 2019, Manchester(NSS/MIC)

Open Positions

Open topics for Bachelor’s and Master’s theses

Finished Master’s theses

Finished Bachelor’s theses

Group members


Dr.-Ing. Thomas Dey

Thomas (Dr.-Ing. in Electrical Engineering, RWTH Aachen University, 2015) works on data acquisition and processing software for hybrid PET/MR imaging systems. Furthermore, he develops new image reconstruction algorithms and evaluates high performance computing platforms for these tasks.

T. Dey

Dr. rer. nat. Ronja Hetzel

Ronja (Dr. rer. nat. in Physics, RWTH Aachen University, 2017) works in our PET group. She focuses on optical simulations with Geant4 and analyses different crystal designs.

R. Hetzel

Dr. Sebastian Reinartz

Sebastian (Board-certified Radiologist, Dr. in Medicine, RWTH Aachen, 2009) works as a senior physician in the Department of Diagnostic and Interventional Radiology (Uniklinik RWTH Aachen) since 2014. Besides his clinical research on cardiac CT and EIT (Electrical impedance tomography) he develops MPI-devices and evaluates MPI capabilities in a preclinical environment as preclinical MPI group leader.

S. Reinartz

Dr. rer. nat. David Schug

David (Dr. rer. nat. in Physics, RWTH Aachen University, 2015) is leading the PET physics group which works on PET detector technologies for digital PET/MR and evaluates the capabilities of the digital SiPM chip and develops novel signal processing algorithms. Further research topics are monolithic and complex crystal architectures.

D. Schug

Dr.-Ing. Björn Weißler

Björn (Dipl.-Ing. in Electrical Engineering, RWTH Aachen University, 2005) works on the system architecture of the pre-clinical PET/MR inserts and coordinates the respective development projects. His research is specialized on the design and realisation of MRI-compatible hardware. Additionally, he writes the control software for the inserts.

B. Weissler

PhD Students

Jochen Franke

Jochen (M.Sc. in Medical Engineering, RWTH Aachen University, 2010, Dipl.-Ing. (FH), in Engineering Physics, University of Applied Sciences Münster, 2008) compiles his Ph.D. work externally at Bruker BioSpin MRI GmbH. There, he places a fully integrated preclinical MPI-MRI hybrid system suitable for small animals into operation. In this feasibility research project (BMBF FKZ 13N11088) he focuses on all aspects of device integration, system optimization as well as evaluation and implementation of technical innovations.

J. Franke

Jan Grahe

Jan (M.Sc. in Physics, RWTH Aachen University, 2017) promotes quantitative imaging in positron emission tomography by developing new dynamic imaging concepts.

J. Grahe

Tianyu Han

Tianyu (M.Sc. in Physics, RWTH Aachen University, 2018; B.Sc. in Physics, Nankai University, Tianjin, 2014) currently works on the development of a machine learning (deep learning) based approach to synthesize and analyze breast MR images. Within his project, the work mainly focuses on quantitative MR image reconstruction and tumor segmentation on low-dose contrast agent MR images.

T. Han

Karl Krüger

Karl (M.Sc. in Electrical Engineering, RWTH Aachen University, 2018) works on FPGA-based interference reduction techniques for PET/MR systems.

K. Krüger

Florian Mueller

Florian (M.Sc. in Physics, RWTH Aachen University, 2017) is working in the field of PET detector technologies and evaluates the possibility of employing monolithic and complex scintillator crystals.

F. Mueller

Vanessa Nadig

Vanessa (M.Sc. in Physics RWTH, 2019) is characterizing a novel analog ASIC model for time-of-flight PET applications.

V. Nadig

Teresa Nolte

Teresa (M.Sc. in Physics, RWTH Aachen University, 2015) works on the implementation of MR fingerprinting, a novel concept which aims for quantitative MR imaging.

T. Nolte

Dennis Pantke

Dennis (M.Sc. in Biomedical Engineering, RWTH Aachen University, 2017; B.Sc. in Medical Physics, Heinrich-Heine-Universitaet Duesseldorf, 2014) currently works on the instrumentation of a novel Magnetic Particle Imaging (MPI) Scanner. Within this project, his work comprises the design and implementation of system components as coil topologies, electronic circuits and control mechanisms.

D. Pantke

Nicolas Gross-Weege

Nicolas (M.Sc. in Physics, RWTH Aachen University, 2014) works on Nuclear Magnetic Resonance (NMR) field probes.

N. Gross-Weege

Laiyin Yin

Laiyin (M.Sc. in Electrical Engineering, RWTH Aachen University, 2016) works on the development of a shared-volume PET/MRI insert for a 7T MRI system.

L. Yin

Master’s Students

Federica Demattè

Federica (B.Sc. in Physics RWTH, 2017) works on the characterization of detector stacks for the PET ring of a breast PET/MR insert.

F. Demattè

Katrin Mueller

Katrin (B.Sc. in Electrical Engineering, RWTH Aachen University, 2017) works on the implementation of an algorithm for steering a medical device (e.g. catheter) in the non-static magnetic field of a preclinical Magnetic Particle Imaging (MPI) Scanner.

K. Mueller

Student researchers

Franziska Schrank

Franziska (B.Sc. in Physics RWTH, 2018) works on the evaluation of RF shielding materials for PET/MR inserts.

F. Schrank