TIPS FOR THE RADIOLOGIST

Looking for the 'pea
in a pod' among the

'Swiss cheese'
to diagnose

• •neurocysticercosis

Savvas Andronikou
MB BCh, FCRad(D), FRCR (Lond)

Bruce Mcivor
MBChB

Bruce Smith
MBChB

Department of Paediatric Radiology, Red Cross
War Memorial Children's Hospital, Univeristy of

Cape Town and Institute of Child Health

Usually humans act as the definitive
hosts for the parasite Taenia solium,
but they can occasionally also act as
the intermediate host where the larval
stage (Cysticercus cellulosae) lodges in
many organs of the body. The com-
puted tomography (CT) and magnet-
ic resonance imaging (MRI) appear-
ances in brain tissue vary according to
the stages of evolution of the parasite.
On CT, live larvae (vesicular stage) are
hypodense (cerebrospinal fluid (CSF)
equivalent] round cysts, most com-
monly at the grey/white interface
(Fig. I). The scolex may be visualised
as an eccentric hyperdense nodule on
the inner aspect of the cyst wall- the
'pea in a pod' appearance. On MRI
the cyst contents are isointense to CSF
on all sequences (Fig. 2). MRI is more
sensitive for identifying the scolex,

Fig. 1. Contrast-enhanced axial CT of the brain
demonstrating the typical appearance of neurocys-
ticercosis, with multiple cysts (thin arrows) situated
predominantly at the grey-white interface. The
cysts are in the vesicular stage, as there is no sur-
rounding oedema. The scolices are hyperdense
(thick arrow), owing to the presence of calcareous
corpuscles. This gives rise to the 'pea in a pod'
appearance. Calcified foci are noted in the left
cerebral hemisphere in keeping with the calcified
nodular stage.

which is hyperintense on Tl in the
vesicular stage. When the scolex is
calcified it shows a signal void on all
sequences. When a larva is dying a
granuloma forms, and perilesional
oedema is seen due to the host
response (Fig. 3). The lesion is then
seen as a ring or disc-enhancing lesion
(Fig.4). With multifocal ring lesions,
the brain may have the appearance of
'Swiss cheese' (Fig. 2). The mural nod-
ule (scolex) also enhances with con-

40 SA JOURNAL OF RADIOLOGY • March 2002

Fig. 2. Contrast-enhanced axial MRI scan of the
brain demonstrating 'pea in a pod' (arrow head)
among the swiss cheese. Note the presence of
lesions in different stages - non-enhancing lesions
in the vesicular stage (thin arrow), and discoid/ring-
enhancing lesions in the colloidal vesicular stage
(curved arrow).

Fig. 3. Contrast-enhanced axial CT of the brain
demonstrating single, ring-enhancing granuloma
(thin arrow), with a thick capsule, containing a cal-
cifying scolex (arrow head). There is peri/esional
oedema (curved arrow). These features are in
keeping with a lesion in the granular nodular stage.



TIPS FOR THE RADIOLOGIST

Fig. 4. Contrast-enhanced axial brain CT demon-
strating multiple ring-enhancing lesions (long
arrow), with surrounding oedema. The 'pea in a
pod' appearance of the cyst in the left frontal lobe
(curved arrow) confirms cysticercosis as the aetiol-
ogy. Note also the discoid enhancement (short
arrow) of some of the cysts in the colloidal vesicu-
lar phase.

trast. At this stage (colloidal vesicular)
the cyst content becomes slightly
hyperintense to CSF on Tl weighted
MRI and hyperintense on T2 weight-
ed MRI. The nodule becomes more
visible when gadolinium is used. The
hypointense cyst wall is best seen on
T2 against the hyperintense fluid con-
tent and surrounding oedema.

The next stage of the cyst (granu-
lar nodular stage) involves calcifica-
tion and retraction, with thickening of

the cyst wall (Fig. 3). On CT the cyst
is isodense and the scolex is calcified.
On MRI the cyst is isointense on Tl
and hypo/isointense on T2. When
the larvae are dead, cysts and scolices
are seen as calcified lesions (Fig. I),
and when there are multiple lesions it
is known as the 'starry night' appear-
ance on CT. MRI shows focal areas of
signal void. All the above stages may
be seen in combination or simultane-
ously in one patient (Figs 1,2,4).

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