Magnetic resonance imaging and computed tomography as tools for the investigation of sperm whale (Physeter macrocephalus) teeth and eye

Abstract

Background: Scanning techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) are useful tools in veterinary and human medicine. Here we demonstrate the usefulness of these techniques in the study of the anatomy of wild marine mammals as part of a necropsy. MRI and CT scans of sperm whale teeth (n = 4) were performed. The methods were compared and further compared to current standard methods for evaluation of tooth layering. For MRI a zero echo time sequence was used, as previously done for imaging of intact human teeth. For CT two different clinical scanners were used.

Results: The three scanners did not provide sufficient information to allow age estimation, but both MRI and CT provided anatomical information about the tooth cortex and medulla without the need for sectioning the teeth. MRI scanning was also employed for visualizing the vascularization of an intact eye from one of the stranded sperm whale.

Conclusions: Clearly, MRI was useful for investigation of the retinal vasculation, but optimum results would require well-preserved tissue. It was not possible to estimate age based on CT scans of tooth growth lines. Further research is needed to clarify the usability of MRI and CT as tools for marine mammal research when samples need to remain intact or when a spatial (three dimensional) arrangement of features needs to be determined.

Keywords: Computer tomography; Eyes; Imaging techniques; Magnetic resonance; Sperm whales; Teeth.

Fig. 1

The two whale teeth (tooth a MCE1652 and tooth b MCE1654) used for evaluation of the use of ZTE MRI in whale age estimation

Fig. 2

CT images of both teeth (tooth A and B) using Somatom Definition Flash and the scan- and reconstruction parameters in the CT method details. Coarse inner structures such as small inner holes and different densities of enamel and marrow are clearly visualized. Parts of the V-shaped structures can be seen

Fig. 3

CT image of tooth B using Biograph 64 and the scan- and reconstruction parameters in the CT method details. The slice is similar (but not exactly the same) as in Fig. 2. This image has smaller slice thickness and the filter was not as smooth as in Fig. 2. Thus, this image has more noise but higher spatial resolution, which improves visualization of the fine V-shaped pattern. Still only parts of the V-shaped structures can be visualized and the total number of layers cannot be determined from these images

Fig. 4

Examples of ZTE images in tooth A (see Fig. 1) along various axis (axial and sagittal). Axial at top left. Three sagittal slice planes are shown. Note that layering is visible in all examples, but it is not possible to determinate the exact numbers

Fig. 5

Examples of ZTE images in tooth B (see Fig. 1) along various axis (axial and sagittal). Axial at top left. Three sagittal slice planes are shown. Here, the layered structure is visible as well as the border between marrow and enamel

Fig. 6

Left pig eye scan showing optic nerve, lens, lens suspension, iris and cornea. Right whale eye scan showing optic nerve, lens, and lack of internal structures in the eye. Deterioration of the cornea is evident in whale eye

Fig. 7

Left an example of the visualization of the eye vasculature (elongated dark structures in the central bright area) of the whale. Right the vascular components isolated by manual segmentation over 12 image planes