1 RASER MRI: Magnetic Resonance Images formed Spontaneously exploiting Cooperative Nonlinear Interaction Authors: Sören Lehmkuhl 1,2 *, Simon Fleischer 3 , Lars Lohmann 3 , Matthew S. Rosen 4,5 , Eduard Y. Chekmenev 6,7 , Alina Adams 3 , Thomas Theis 2,8,9 *, Stephan Appelt 3,10 * Affiliations: 1 Institute of Microstructure Technology, Karlsruhe Institute of Technology; 76344 Eggenstein-Leopoldshafen, Germany. 2 Department of Chemistry, North Carolina State University; Raleigh, NC 27606, USA. 3 Institute of Technical and Macromolecular Chemistry, RWTH Aachen University; 52056 Aachen, Germany. 4 Massachusetts General Hospital, A. A. Martinos Center for Biomedical Imaging, Boston, MA 02129, USA. 5 Department of Physics, Harvard University; Cambridge, MA 02138, USA. 6 Department of Chemistry, Integrative Biosciences (Ibio), Karmanos Cancer Institute (KCI), Wayne State University Detroit, MI 48202 USA. 7 Russian Academy of Sciences Leninskiy Prospekt 14, Moscow, 119991 Russia. 8 Department of Physics, North Carolina State University Raleigh, NC 27695 USA. 9 Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill & North Carolina State University, Raleigh, NC 27695 USA. 10 Central Institute for Engineering, Electronics and Analytics – Electronic Systems (ZEA-2), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. *Corresponding author. Email: lehmkuhl@kit.edu, ttheis@ncsu.edu, st.appelt@fz-juelich.de Abstract: The spatial resolution of magnetic resonance imaging (MRI) is fundamentally limited by the width of Lorentzian point spread functions (PSF) associated with the exponential decay rate of transverse magnetization (1/T 2 * ). Here we show a different contrast mechanism in MRI by establishing RASER (Radio-frequency Amplification by Stimulated Emission of Radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying (Radio Frequency) RF pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. A small difference in initial population inversion density creates a stronger image contrast than conventional MRI. This contrast is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Both the contrast and the artifacts are demonstrated experimentally and predicted by simulations based on a proposed theory. This theory of RASER MRI is strongly connected to many other distinct fields related to synergetics and non-linear dynamics.