Application of magnetic resonance imaging in zoology

Magnetic resonance imaging (MRI) is a noninvasive imaging technique that today constitutes one of the main pillars of preclinical and clinical imaging. MRI’s capacity to depict soft tissue in whole specimens ex vivo as well as in vivo, achievable voxel resolutions well below (100 μm)³, and the absence of ionizing radiation have resulted in the broad application of this technique both in human diagnostics and studies involving small animal model organisms. Unfortunately, MRI systems are expensive devices and have so far only sporadically been used to resolve questions in zoology and in particular in zoomorphology. However, the results from two recent studies involving systematic scanning of representative species from a vertebrate group (fishes) as well as an invertebrate taxon (sea urchins) suggest that MRI could in fact be used more widely in zoology. Using novel image data derived from representative species of numerous higher metazoan clades in combination with a comprehensive literature survey, we review and evaluate the potential of MRI for systematic taxon scanning. According to our results, numerous animal groups are suitable for systematic MRI scanning, among them various cnidarian and arthropod taxa, brachiopods, various molluscan taxa, echinoderms, as well as all vertebrate clades. However, various phyla in their entirety cannot be considered suitable for this approach mainly due to their small size (e.g., Kinorhyncha) or their unfavorable shape (e.g., Nematomorpha), while other taxa are prone to produce artifacts associated either with their biology (e.g., Echiura) or their anatomy (e.g., Polyplacophora). In order to initiate further uses of MRI in zoology, we outline the principles underlying various applications of this technique such as the use of contrast agents, in vivo MRI, functional MRI, as well as magnetic resonance spectroscopy. Finally, we discuss how future technical developments might shape the use of MRI for the study of zoological specimens.