Description of the koala (Phascolarctos cinereus) brain based on magnetic resonance images

Australia's native fauna is unique through millions of years of isolated evolution resulting in a significant divergence of marsupial brain anatomy, and thus neural pathways, from eutherian mammals. This study provides a morphometric description of a mature female koala (Phascolarctos cinereus) through segmentation of high-resolution magnetic resonance images. As the availability of brain specimens of the endangered koala is scarce, brain segmentation has been performed on a single specimen.

The brain of a naturally deceased koala was removed and fixed in 10% neutral buffered formaldehyde (pH 7.0). The MRI was acquired on a 9.4 T Bruker Biospec MR system with Paravision 7.0 software, using a 3D multigradient echo sequence, with TR/TE = 70 msec/ (8 echoes 4-32 msec), a field of view = 7.0 × 6.4 × 4.0 cm³. AMIRA post image processing software was used to create three-dimensional volume reconstruction of different brain structures, allowing for the calculation of their volumes.

The koala brain is lissencephalic and has a volume of 15720.98 mm³. It was found that the olfactory bulbs of the koala were 418 mm³ or 2.66% of the total brain volume (tbv). The cerebral hemispheres were 9336.92 mm³ (59.39% of the tbv). The interhemispheric connections were 148 mm³ (0.94% of tbv). The combined volume of the anterior commissure and fasciculus aberrans, which is specific to marsupials, has also been labeled and segmented. The ventricular system was 172 mm³ (1.09% of tbv). The brainstem was 1874 mm³ (11.92% of tbv). The cerebellum was 2304.69 mm³ (14.66% of tbv).

This study can be used as a basis for clinical imaging of koalas and as baseline data for future research into brain development of koalas and other marsupials.