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 radioactive 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. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. Truhn D, Kiessling F, and Schulz V. Optimized RF shielding techniques for simultaneous PET/MR. Med Phys. 2011;38(7):3995‑4000.
  14. 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.
  15. 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.
  16. 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.
  17. 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.
  18. 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.
  19. 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

  • Sep 2017: World Molecular Imaging Congress 2017, Philadelphia (WMIC2017)
  • Oct 2017: Nuclear Science Symposium and Medical Imaging Conference 2017, Atlanta (NSS/MIC)

Open Positions

Open topics for Bachelor’s and Master’s theses

Finished Master’s theses

Group members

Dr. rer. medic. Yannick Berker

Yannick is a DAAD P.R.I.M.E. postdoctoral fellow currently located at the Department of Radiology, University of Pennsylvania in Philadelphia, where he continues his works on PET image reconstruction algorithms using scattered coincidences.


Y. Berker

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

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

Dr. Pierre Gebhardt

Pierre (Dipl.-Ing. in Electrical Engineering, Technische Universität Darmstadt) focuses his research on concepts and designs of FPGA-based scalable and modular PET data acquisition architectures suitable for simultaneous PET/MR imaging with support for analogue as well as digital Silicon Photomultiplier (SiPM) detectors. Such a platform will be the enabler for real time FPGA-based PET data processing for novel crystal positioning algorithms, which is a further research domain Pierre is focusing on in his Ph.D. work.


P. Gebhardt

Jan Grahe

Jan (B.Sc. in Physics, RWTH Aachen University, 2014) is working on optical simulations of monolithic scintillators used for PET systems to characterize and optimize the measuring set-up.


J. Grahe

Patrick Hallen

Patrick (M.Sc. in Physics, RWTH Aachen University, 2013) currently works on characterizing the group’s newly developed PET scanner. In addition, he is exploring new PET detector concepts with Monte Carlo simulations.

Photo Patrick Hallen
P. Hallen

Florian Mueller

Florian (B.Sc. in Physics, RWTH Aachen University, 2015) is working on his master thesis in the field of PET detector technologies. Using digital SiPM chips, he wants to evaluate the possibility of employing monolithic scintillator crystals.


F. Mueller

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 (B.Sc. in Medical Physics 2014, Heinrich-Heine-Universität Duesseldorf, 2014) is working on his master thesis in the field of simultaneous MPI-MRI imaging. By means of simulations and initial experiments he wants to show that MR imaging is possible in the constraints of the magnetic field of an MPI system.


D. Pantke

Anne Robens

Anne (B.Sc. in Electrical Engineering, RWTH Aachen University) is working on MRI-compatible Multi-Gigabit Data Link for Digital PET-MRI.


A. Robens

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

Marcel Straub

Marcel (M.Sc. in Physics, RWTH Aachen University, 2012) works on Magnetic Particle Imaging (MPI) detector technology. The focus of his work is the exploration of enhanced magnetic field configurations and the development of reconstruction algorithms.


M. Straub

Tianyu Han

Tianyu (B.Sc. in Physics, Nankai University, China) aims at understanding Magnetic Resonance Fingerprinting and optimizing its performance under different contrast environments, particularly, female breast tissue environments.


T. Han

Nicolas Gross-Weege

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


N. Gross-Weege

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