Abstract

Postmortem imaging, known as virtopsy, is conducted with sectional imaging modalities and may give invaluable initial or additional information to the conventional necropsy. Multidetector computed tomography (MDCT), in particular, allows production of a vast quantity of volumetric data rapidly, permits high-resolution visualization, and creates isotropic voxel data for reliable two-/three-dimensional image reformation. However, primary axial images obtained from MDCT may limit accurate diagnosis or pathologies documentation. Direct coronal or sagittal orientation of head, flippers, and fluke could only be obtained by means of positioning, with these parts dissembled from the intact body trunk. MDCT image reformation techniques generate images in a plane or orientation different from the prospective one, which permits visualization of anatomical details and relationships that would be difficult to evaluate using axial reconstructions alone.^1-4

This study aimed to identify various MDCT image reformation techniques applied on virtopsy to facilitate the death and life history investigation of stranded cetaceans found in the Hong Kong waters.

Using a 16-row MDCT scanner Alexion^TM (Toshiba Medical Systems, Tochigi, Japan), 56 stranded cetaceans found in the Hong Kong waters were examined, and findings were compared by subsequent necropsy. TeraRecon Aquarius workstation (San Mateo, California) and Toshiba Alexion^TM build-in software (Toshiba Medical Systems, Tochigi, Japan) were used in conjunction to reconstruct projection data into volumetric data set, allowing the application of image reformation techniques for death and life history investigation.

Results indicated that slice and volumetric visualization are the most useful on stranded cetaceans. Multiplanar reconstruction (MPR) shows other planes (sagittal, oblique, curved, or coronal), which are not acquired directly during the acquisition in axial plane. It is useful to assess any intact anatomical structure/pathology in any required plane, and it is even possible to obtain a curved plane for visualization of the anatomical details of vessels. Maximum intensity projection (MIP) emphasizes the highly intense and narrow structures in the carcass, which is useful to detect metallic foreign bodies, calcifications in vessels and tracheobronchial tree, and small lung nodules. Shaded surface display (SSD) creates three-dimensional images of surface information of various tissues (parenchyma, bone, airways, and vessels) of the carcass. With successive interactive steps of exclusion/inclusion of these different tissue types and resizing/trimming of the region of interest, surfaces that would otherwise be very difficult to visualize can be visualized. It is particularly helpful in studying articular surface fracture lines, which often remain hidden behind adjacent bone surfaces. Direct volume rendering (DVR) generates a three-dimensional volume in a two-dimensional platform, which increases the three-dimensional spatial understanding of the anatomical structure/pathology when the observer rotates the volume structure and facilitates the search of optimal view/set of views to answer a pathological question. Cutting away in DVR allows the observer to cut into a structure to see its interior, which helps to remove portions of structures that may be obscuring other areas of interest. Transfer function (TF) in DVR controls the opacity, brightness, and colour of selected volume, which allows the observer to selectively reveal structures that would otherwise be obscured in virtopsy.

To conclude, MDCT image reformation techniques could be applied on virtopsy to identify foreign bodies, pathology, and condition of internal structures in carcass. Operator should make use of the volumetric data set to reform new images with the aforementioned techniques, which allows assessments of anatomical structures and pathologies from different perspectives in death and life history investigation of stranded cetaceans.

Acknowledgements

This project was financially supported by the Hong Kong Research Grants Council [Grant number: UGC/FDS17/M07/14]. The authors would like to thank the Agriculture, Fisheries and Conservation Department of the Hong Kong SAR Government for the continuous support in this project. Sincere appreciation is also extended to veterinarians, staff, and volunteers from Ocean Park Hong Kong, Ocean Park Conservation Foundation Hong Kong, and Tung Wah College for paying great effort on the stranding response and necropsy in this project. Special gratitude is owed to technicians from Hong Kong Veterinary Imaging Center for operating the CT and MRI for this research. Wholehearted thanks to TeraRecon, Inc. for the friendly loan Aquarius workstation to the project and Ms Carus Yu for her guidance in workstation operation.

* Presenting author
+ Student presenter

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