Abstract

In the course of the pioneering virtopsy-driven stranding response program in the HK waters, virtopsy using postmortem computed tomography (PMCT) and postmortem magnetic resonance imaging (PMMRI) have been implemented to provide supplementary or complementary information for conventional necropsy. Stranded carcasses found in the HK waters are often badly degraded by weathering and decomposition, which induce challenges to veterinarians due to the considerable organ alternations hindering documentation of postmortem findings.^1,2 The brain is susceptible to rapid autolysis and putrefaction due to its high water content, leading to its discolouration, softening and liquefaction at an early stage of decomposition.³ In conventional necropsy, the opening of the cranial cavity to explore the brain and the dissection of other accessory cranial structures are complex and time-consuming, and in case of decomposed brain, it often defied a specific diagnosis. Veterinarians may overlook these parts during the necropsy and consequently may miss abnormalities in these important areas.

Limited valuable sample analysis can be undertaken in decomposed brains, hindering full differential diagnosis. Studies on in vivo and in vitro PMMRI neuroimaging have been mainly focused on neuroanatomy and brain morphology in freshly deceased cetaceans⁴ or preserved specimens.⁵ To the best of our knowledge, the literature is devoid of any reference on the potential value of PMMRI examination of decomposed brains. This project aims to evaluate and present the possible benefits of PMMR neuroimaging in vivo in decomposed cetacean carcasses in comparison to PMCT.

A total of 18 carcasses (10 females, 7 males, 1 unknown sex; calf to adult; 77 cm to 250 cm in length) of 3 cetacean species; i.e., finless porpoise (Neophocaena phocaenoides), Indo-Pacific bottlenose dolphin (Tursiops aduncus) and Indo-Pacific humpbacked dolphin (Sousa chinensis) were included in the study. The carcass code ranged from 2–3. Inclusion criteria were carcasses with external and internal signs of advanced decomposition.

PMCT brain examinations (n=17) were performed on a 16-slice multi-detector row Toshiba Alexion^TM CT system (n=3) or Somatom Definition 64 slices Dual Energy CT system (n=14), while PMMRI brain examinations (n=15, sequences: T1-weighted (T1w), T2-weighted (T2w)) were performed on a Siemens MRI Magneton Trio 3.0 Tesla unit (n=14), a 0.25 Tesla Esaote Vet MRI Grande scanner (n=2) or a Philips 3.0 Tesla Achieva scanner (n=1). Intracranial gas accumulations, brain midline/symmetry, the superior sagittal sinus, anatomical brain structures (ventricles, gray and white matter junction, basal ganglia, brain stem, and cerebellum), visible brain pathologies were assessed in terms of being recognizable and/or allowing diagnosis.⁶

In PMCT examinations, intracranial gas accumulation could visualize and allowed diagnosis in 16 cases (94%). In ventricles, only 1 case (6%) allowed diagnostic evaluation, 5 cases (29%) could partially recognize the structure and 4 cases (24%) can show complete recognition. Brain stem and cerebellum could be recognized in 1 case (6%). Brain midline and symmetry, superior sagittal sinus, gray and white matter junction and basal ganglia were unable to recognize in all the cases.

In PMMRI examinations, brain midline and symmetry were assessable and allowed diagnostic evaluation in all cases. Intracranial gas accumulation was recognized in 16 cases (94%). Ventricles, basal ganglia and cerebellum allowed diagnostic evaluation in T2w in all cases, but only 10 cases (59%), 6 cases (35%) and 11 cases (65%) allowed diagnostic evaluation in T1w, respectively. Similarly, superior sagittal sinus in 15 cases (88%) and brain stem in 16 cases (94%) allowed diagnostic evaluation and anatomy recognition.

PMMRI is a modality that is complementary to PMCT, which is far superior in depicting brain anatomy and parenchymal pathologies in fresh cetacean carcass. PMMRI of decomposed brain still offered distinct superior soft tissue contrast, allowing morphological assessment of decomposed brain. PMMRI brain should be considered to be incorporated in death investigation of decomposed cetacean carcasses.

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 Institute of Hydrobiology, Chinese Academy of Sciences and 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, Institute of Hydrobiology, Chinese Academy of Sciences, and Tung Wah College for paying great effort on the stranding response and necropsy in this project. Special gratitude is owed to technicians from Zhong Nan Hospital of Wuhan University and Hong Kong Veterinary Imaging Center for operating the CT and MRI for this research.

* Presenting author

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