Tropical Medicine Central Resource

On CT scanning there is often a large solitary spherical cyst, with a regular smooth border and a cyst-content of fluid or water density similar to cerebrospinal fluid (CSF) (Figs. 3.122, 3.123A). Calcification may be seen on non-contrast CT, even if it is minimal. There is usually no rim enhancement or perifocal edema. With contrast, occasional lesions may show ring enhancement, suggesting inflammation. Infection (including TB) or abscess will have to be excluded, which can be difficult. Multiple cerebral hydatid cysts show the same characteristics, and are often quite different in size.

The differential diagnosis of an uncomplicated cerebral hydatid includes porencephalic cyst, arachnoid cyst, and cystic tumors. Arachnoid cysts are, however, not spherical and porencephalic cysts usually communicate with the lateral ventricles; a closed porencephalic cyst may present a differential problem on CT scanning. The CT density of cystic tumors differs from that of CSF, unlike hydatid cysts.

MRI studies of intracerebral cysts show a low signal intensity of the cyst wall, as found in liver and other hydatids. MRI is also more sensitive than CT in detecting perifocal edema. If present, a high intensity halo or rim around the cyst on T1-weighted images (the rim is hypointense on T2 images), with cystic contents on T1- and T2-weighted images similar to CSF (Fig. 3.123), constitutes a very significant combination for the diagnosis of cerebral hydatid disease.

Ventriculography is very rarely practiced today and seldom provides exact diagnostic information for hydatid disease. The cyst will show as a smooth space-occupying lesion, often causing quite marked ventricular displacement and, sometimes, hydrocephalus. Prior to 1975, pneumoencephalography would demonstrate similar findings.

Fig. 3.122 Large left parietal hydatid cyst as seen on (A) coronal and (B) axial views of a CT scan. The cyst appears multilobulated and septated due to the presence of daughter cysts. The cyst contents have the same fluid density as the CSF. There is displacement of the falx and the ventricular system across the midline to the right with marked compression and displacement of the posterior horn of the left lateral ventricle.

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Fig. 3.123 An 11-year- old Saudi Arabian boy admitted for suspected brain tumor. Medical imaging revealed a huge, simple unilocular Type I hydatid cyst of the brain. (A) Axial CT scan of a right parietal spherical hydatid, showing displacement of the ventricular system to the left. (B) MRI T1-weighted (spin echo, 950/20) axial image and (C) coronal image (600/20) show that the signal intensity of cyst contents is identical to that of CSF. (D) MRI T2-weighted (spin echo, 2000/80) coronal image and (E) (spin echo, 2000/40) axial image of the right parietal cyst reveals a peripheral rim of low signal intensity. (F) Intraoperative photograph of typical pearly white brain hydatid. (Courtesy of Drs. Coates and von Sinner and AJNR, 1990).