It is our pleasure to present this special issue about electron paramagnetic resonance imaging (EPRI) and its applications to biomedical sciences on the 80th anniversary of the discovery of EPR. First introduced in 1944 by Evgeniy Konstantinovich Zavoisky (Fig. 1), EPR has emerged as a powerful tool for investigating paramagnetic species. EPR detects unpaired electron spins subjected to a constant uniform magnetic field by manipulating the spins using radio-frequency electromagnetic radiation. For the past eight decades, EPR has significantly contributed to our understanding of biological systems, materials, and processes.

Celebrating 80th anniversary of EPR with this historical picture of its inventor, Dr. E. Zavoisky

More recently, EPRI has emerged as an attractive modality to image the tissue microenvironment. Both continuous wave and pulse modes are commonly used. The pulse mode is specifically used to image the partial pressure of oxygen (pO2), which is more widely known as pEPROI (pulse EPR oxygen imaging). Being a magnetic resonance technique, pEPROI bears similarities to conventional MRI. For example, commonly, pEPROI employs inversion recovery and radial acquisition methods, resulting in 3D images with ~ 500 μm spatial resolution in vivo. Yet, EPRI has major differences compared to MRI. First and foremost, because the magnetic moment of electrons is 658 times higher compared to protons, the magnetic fields used in EPRI are much lower, in a typical range of 9 mT to 42 mT. Another notable difference is that EPRI almost always needs an exogenous contrast agent due to the absence of EPR-sensitive radicals in the body with sufficient concentration and long relaxation times (in the 1-10 μs range) that can be imaged using EPRI. Importantly, these “long” EPRI relaxation times are much shorter than proton relaxation times that are typically in the 0.1–1 s range. Therefore, EPR imaging and spectroscopy must address very broad line widths in the MHz range. As a result, the timing of EPRI pulse sequences and the image reconstruction methods are different compared to commonly used MRI methods. An impressive set of publications in this special issue focuses on the development of imaging technology and new instrumentation, as well as new EPR probes and new chemistry.

There is an urgent need to explore the emerging trends in EPRI, due to its unique and distinct characteristics to probe biomarkers of the tissue microenvironment, such as pO2, pH, and redox potential, that are not easy to evaluate through other modalities. In particular, a number of publications in this special issue highlight the imaging of pO2 for applications in cancer, tissue engineering, and type I diabetes. This need to spotlight these trends prompted us to initiate the call for this special issue about EPRI in molecular imaging and biology. Interestingly, researchers worldwide have enthusiastically responded to this call to submit original research, as well as reviews that explain important concepts and highlight future trends in this rapidly developing imaging field.

We thank Dr. Jason Lewis, Editor-in-Chief, for his leadership and unwavering support from the idea formation to the release of this special issue. We also thank our reviewers, who spent countless hours evaluating manuscripts and provided constructive critiques that improved the quality of this special issue. We also thank the Museum of Kazan State University, for Dr. Zavoisky's picture. Finally, we thank Ms. Sabine Ben Ghechir of the Springer Nature Group for all the editorial support.

We hope you will enjoy this special issue about EPRI and its application to molecular imaging and biology.