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MR IMAGING OF BRAIN MORPHOLOGY, VASCULARISATION AND
ENCEPHALIZATION IN THE KOALA
JAMIE TAYLOR, FRANK J. RÜHLI, GREG BROWN, CARMEN DE MIGUEL AND MACIEJ
HENNEBERG
THE koala (Phascolarctos cinereus Goldfuss, 1817),
an Australian wildlife icon, is an arboreal marsupial
whose lifestyle requires good motor coordination and
precise three-dimensional orientation. These seem to
require keen senses and a well-developed central
nervous system. The intellectual ability of an animal
is usually assessed by its level of encephalization.
The variously calculated ratio of brain size to body
size has been used in comparative anatomy as an
indicator of encephalization. Evolutionary studies on
encephalization index and brain size / body size
relationships have been widely conducted (Jerison
1973; Henneberg 1990). Endocranial volume is
commonly used in such attempts as a proxy for brain
size because of the obvious fact that only hard tissues
are preserved in paleontological specimens.
Phascolarctos cinereus shows poor
encephalization (Haight and Nelson 1987; Lee and
Martin 1988, De Miguel and Henneberg 1998), in
contrast to the closely related and higher
encephalized wombats (Vombatus ursinus Shaw,
1800; Lasiorhinus latifrons Owen, 1845). The
relationship between the brain volume and
endocranial capacity of P. cinereus is still debated.
Alternative estimations of the actual brain size of P.
cinereus may produce different conclusions
regarding the degree of encephalization. Some
authors (Haight and Nelson 1987), by comparing
average volumes of anatomically fixed brains with
mean cranial capacities, estimated the brain of P.
cinereus to be unusual in occupying only about 60%
of the cranial cavity. In another fresh-brain study (De
Miguel and Henneberg 1998), a ratio of
approximately 75%, which is normal for humans and
other animals, was found. Additionally, the brain in
living animals is well perfused with substantial
amounts of blood. This may explain some
discrepancies in estimation of its relationship to the
endocranial volume.
Anatomical studies of brain parts and their
mutual relations require laborious dissections, which
in case of smaller neural tissue such as in P. cinereus
need microdissection (Kempster and Hirst 2002;
Kempster et al. 2002). Therefore, sophisticated
imaging techniques might be a better method to
examine central neural tissue anatomy of P. cinereus.
Nevertheless, as already pointed out by Kempster and
Hirst (2002), scientific reports, especially
radiological assessments on P. cinereus, are
surprisingly rare (Brown et al. 1984; Carlisle et al.
1989; Mathews et al. 1995) and to our knowledge no
imaging study of P. cinereus brain morphology has
ever been published. Recently, the osseous and soft-
tissue anatomy of the orbit only, including its
vascularisation, has been reported (Kempster and
Hirst 2002; Kempster et al. 2002). Hitherto, the gross
morphology of the koala brain has been just briefly
addressed (Lee and Martin 1988), unlike the wombat
(reviewed in Sanderson and Nelson 1998).
The aim of this study was to visualise the internal
anatomy of the brain in a live P. cinereus by MRI
with a special regard to the brain / endocranial cavity
volume ratio. Furthermore, a possible difference
between brain volumes of live and dead P. cinereus
was assessed. Finally, this study highlights a non-
destructive research on a rare animal species, “sadly
neglected scientifically” (Kempster and Hirst 2002:
288).
A live adult male P. cinereus of about 10 kg was
compared with a previously frozen specimen. The
dead male is part of a larger P. cinereus sample
Taylor J, Rühli FJ, Brown G, De Miguel C and Henneberg M, 2006. MR imaging of brain morphology,
vascularisation and encephalization in the koala. Australian Mammalogy 28: 243-247.
Key words: angiography, brain volume, central nervous system, endocranial volume, marsupial, morphology,
radiography.
J Taylor, FJ Rühli and G Brown, MRI Unit, Royal Adelaide Hospital, North Terrace, Adelaide SA 5000,
Australia. FJ Rühli, C De Miguel and M Henneberg, Department of Anatomical Sciences, University of
Adelaide, Medical School, Adelaide SA 5005, Australia. Current address: FJ Rühli, Institute of Anatomy,
University of Zurich, Winterthurerstr. 190, 8057 Zürich, Switzerland. E-mail: frank.ruhli@anatom.unizh.ch.
Manuscript received 1 February 2006; accepted 17 November 2006.
AUSTRALIAN MAMMALOGY
244
of which details have been published earlier (De
Miguel and Henneberg 1998). The specimen was
frozen fresh and not fixed with any chemicals.
Both subjects underwent MRI examination
(Siemens Magnetom Vision, Erlangen, Germany) at
the Royal Adelaide Hospital, Adelaide, Australia.
The standard CP extremity coil and Numaris VB31B
software was used. In the live animal cranial T2 Fast
Spin Echo (FSE) series and cervical Magnetic
Resonance Angiography was performed. The dead
animal was examined with the same T2 sagittal
sequence as well as a double echo FSE series and an
isotropic 3D T1 weighted gradient echo sequence
(MP-RAGE). Maximum intensity projections and
volume measurements were obtained with the
standard Numaris functions. The volumes of the
brain and cranial vault were estimated by measuring
the areas seen on the T2 sagittal images, and
multiplying by the slice separation, which is the slice
thickness plus gap.
The MRI scans, with a maximal resolution of
0.5 mm, show all major structures in the brain of P.
cinereus. We show and describe here a parasagittal
section lying approximately 1 mm latral to
midsagittal plane (Fig. 1). Myelin presents as a dark
colour on MRI while fluid is light coloured. The
darkness of images of various parts of the brain thus
corresponds to their degree of myelinisation.
The brain is not flexed. The nearly 90 degrees
flexure occurs only at the junction of medulla
oblongata with the spinal cord. This is due to the lack
of cranial base flexion that is replaced by flexion of
the first free vertebral segment, the atlas is wedge-
shaped.
The cerebral cortex is relatively light while
deeper parts of the hemisphere are progressively
darker. The surface of the hemisphere is smooth,
without gyrification. The lateral ventricle is located
caudally. Posteriorly it is separated from the large
cistern overlying the tectctum of the midbrain by a
thin layer of tissue forming caudal part of the
hemisphere. The anterior commisure is the darkest
spot on the image. Nerves leaving the brain stem
appear as dark spots.
The olfactory bulb is large lying far rostral to the
frontal pole of the hemisphere. The olfactory tract
progressively narrows postero-inferiorly leaving
superiorly a space for a cistern in front of the
hemisphere. Upon entering under the frontal pole of
the hemisphere, olfactory tract fibres seem to spread
under and around the anterior commisure. Below the
anterior commisure a dark thin, slightly sloping
anteriorly line represents optic chiasma.
The round thalamus is well visible while the
hypothalamus is lighter in colour, probably due to the
parasagittal location of the slice that mixes it with
parts of the lumen of the third ventricle. The
interventricular foramen is clearly visible between
the anterior commisure and the thalamus.
The midbrain is clearly delineated with
prominent anterior and posterior colliculi of the
tectum, wide aqueduct and thick tegmentum. Below
the tegmentum nerve roots are visible in the
interpeduncular fossa. The pons is small in relation to
thick medulla oblongata. Numerous nerve roots lie
under the entire length of the medulla. The vermis of
cerebellum is large, well delineated with clear pattern
of the arbor vitae in its four lobes. Caudal to the
small fourth ventricle the spinal canal is well-visible.
Overall, the P. cinereus brain is small with small
smooth hemispheres, large cerebellum and medulla
surrounded by voluminous cisterns. Frontal and
parietal bones forming the skull vault are thick, with
a narrow layer of diploё between thick external and
internal laminae. With the MRI resolution applied, it
is, however, impossible to describe brain anatomy in
more detail.
Estimated cranial volume for the live animal was
32.27 ml and for the dead specimen 26.73 ml. The
olfactory bulb size - 0.83 ml for the dead and 1.02 ml
for the live specimen - contributes approximately 3%
(3.09% and 3.17%, respectively) to the total cranial
volume. The measured brain volume was 23.36 ml
(87.41% of total cranial volume) for the dead and
25.77 ml (79.86%) for the live specimen. The ratio of
total brain volume, including olfactory bulb, to the
endocranial volume is 83.03% for the live (total brain
volume 26.80 ml) and 90.49% for the dead animal
(total brain volume 24.19 ml). These differences fall
within normal range of individual variation of P.
cinereus previously studied (De Miguel and
Henneberg 1998).
The resolution of the MR images is sufficient to
visualise the major anatomical structures of the P.
cinereus brain (Fig. 1). Our finding of an absence of
cerebral lobe folding is consistent with earlier reports
on P. cinereus brain morphology (Lee and Martin
1988, De Miguel and Henneberg 1998). The large
volume of the live nasal mucosa (Fig. 2) is related to
the reliance on smell in P. cinereus, rather than on
vision, for primary sensory input.
The visualisation of the cranial blood vessels by
MRI (Fig. 2) is far superior to what can be achieved
by dissection only. Surprisingly, the blood flow
distribution out of the common carotid artery was
different from, for example, the situation in humans.
The majority of the blood in P. cinereus seems to be
directed to the branches of the external carotid. This
supplies large nasal and auricular vessels, while the
internal carotid is much smaller and vertebral arteries
TAYLOR ET AL.: KOALA BRAIN REVEALED BY MRI 245
245
Fig. 1. Mid-sagittal MR-image (unlabelled and labelled) of a P. cinereus brain showing the major brain parts.
AUSTRALIAN MAMMALOGY
246
Fig. 2. MR-angiography of P. cinereus skull base (left) and corresponding schematic drawing of blood vessels (right).
the smallest. The small relative size of the brain in P.
cinereus seems to be responsible for this
disproportion in the size of external and internal
carotid arteries and their branches. Extensive supply
for the external ears and large nasal cavity is
probably selected to thermoregulatory need of this
animal. Living high in the trees it is exposed to
extremes of temperatures-very high when directly
exposed to summer sun and very low, even freezing
during long winter nights in Victoria and costal South
Australia. With limited resolution of the MRI scan it
was difficult to visualize clearly the arterial circle of
the brain and to distinguish its parts from cavernous
sinus. It seems that basilar artery is selectively long
and has a sinous course. The parotid glands are large
and highly perfused by branches of the external
carotid artery. Also, maxillary sinus is apparently
well perfused for possible cooling.
The particular vascular situation of P. cinereus
has already been highlighted with respect to the orbit,
which only receives vascular supply by the internal
carotid artery (Kempster and Hirst 2002; Kempster et
al. 2002), unlike in eutherians. Our scans suggest
possible anastomosis outside the orbit. Furthermore,
Kempster et al. (2002) also emphasise that P.
cinereus has a peculiar vascularisation of the central
nervous system of pairs of arteries and veins, with the
arterioles and venules joining in loops instead of the
capillary anastomosis as in placental species. This,
however, is beyond the level of MRI resolution.
The ratio of total brain volume - including
olfactory bulb - to the endocranial volume is
remarkably larger than previously reported (Haight
and Nelson 1987) and closer though still larger than
what De Miguel and Henneberg (1998) found. It may
be a result of studying a live animal and a fresh dead
one, rather than chemically preserved brains. Data
presented here fit better into known encephalization
data for other marsupials and eutherians (Tobias
1971; Martin 1990). Nevertheless, there is no doubt
that marsupials show low encephalization in
comparison with eutherians (Martin 1990; De Miguel
and Henneberg 1998).
Our MRI study indicates a substantial size
difference between the living P. cinereus brain and
the ones of cadaver specimens, which again
highlights the importance of in vivo morphological
studies. Hitherto encephalization quotients were
routinely calculated either from endocranial volumes
or from volumes/weights of cadaveric brains. Use of
MRI determined brain volumes should provide more
accurate assessment of encephalization in the future.
The use of MRI allows fast and easy observation of
the koalas’ internal brain structures, their sizes and
the distribution of their peculiar blood flow. More
animals should be scanned to ascertain ranges of
variability.
ACKNOWLEDGEMENTS
We would like to thank H Sonderegger, Institute of
Anatomy, University of Zurich for technical help in
preparing Figs 1 and 2.
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