3D Imaging using MRI of a Non‐mineralized Complex Organism: The Jawless Sea Lamprey ( Petromyzon marinus ) as a Model

The three‐dimensional visualization of complex organisms lacking mineralized tissues has been challenging because (1) histology maintains cohesiveness of the tissues, but hinders a 3D conceptualization; (2) microdissection provides a three‐dimensional view, but is destructive; and (3) x‐ray CT imaging provides three‐dimensionality, but requires contrast staining that results in significant shrinkage due to the absence of mineralized tissue. MRI, however, is a non‐destructive method that provides soft‐tissue contrast without shrinkage. Jawless vertebrates, such as lampreys, are good models for organisms without mineralized tissues, yet are characterized by a complex connective tissue skeleton that provides a framework for their segmentally arranged musculature (myomeres). Furthermore, as the largest lamprey species, adult Sea Lampreys (ca. 75 cm long and 5 cm in diameter) are amenable to standard MRI scanning. We acquired high resolution MRI scans of a Sea Lamprey specimen by using an 8‐channel wrist coil and 3‐Tesla MRI scanner. Both 2D (horizontal and sagittal) and 3D scans were acquired as T1 and T2 weighted without moving the specimen in order to contrast tissues with high fat content (T1) and high water content (T2). For a field of view of 101 cm 2 , a slice thickness of 1.2 mm, and an acquisition matrix of 288 × 288, the voxel size was 0.451 mm × 0.271 mm × 1.2 mm. 3D scans take more time, but minimize gaps between slices. The MRI data were visualized by using the imaging software Avizo ® . In our MRI data, certain connective tissue structures, such as myosepta, were not identifiable in their entirety in either T1 or T2 scans, but parts of them could be identified in one or the other. Therefore, segmentation (marking as a separate object) of a myoseptum was performed in T1 and the available information was augmented by toggling to T2, or vice versa . Using this approach, we were able to show how the sheet‐like myosepta curve and twist dorso‐ventrally and cranio‐caudally within the trunk. In combination with microdissection, the configuration of the visualized myosepta provides a framework for diagramming the length and orientation of the myomeric muscle fibers. The visualization‐of the connective tissue and muscle structures as separate objects is necessary for 3D animation as a step towards modeling lamprey locomotion. Support or Funding Information LSU Foundation to D.G. Homberger