Rarely, cysts may produce areas of bony erosion in the cranial vault. These are smooth and well defined and may be demonstrated as increased translucency of the skull rather than "cystic" defects. Prior to the advent of CT and MRI, pneumoencephalography was the most reliable examination for neurocysticercosis (Fig. 7.17). The cysts may be located anywhere within the brain and will mimic cerebral tumors. Free-floating cysts may block the ventricular system, causing a symmetrical internal hydrocephalus and dilatation of the aqueduct of Sylvius without deviation. There may be only partial obstruction, which may vary with the position of the patient. Complete obliteration of the aqueduct is rare. In some patients the outline of the cysticercus within the ventricles or aqueduct may be seen. The ventricles may show irregular margins and cysticerci may be seen attached to the ventricular walls. Demonstration of the clear cysts has been described (Filipov's sign) as spherical or ovoid translucent areas, with very clear centers and darker edges and with sharply defined outlines.

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Fig. 7.17. Huge cysticercus cyst in the left cerebral hemisphere protruding into the left lateral ventricle on (A) frontal and (B) lateral views from a 1960s pneumoencephalogram. There is moderate hydrocephalus and a slight shift of the ventricular system towards the right in this U. S. airman, who had visited Mexico previously. (C) Tomographic cuts from the same pneumoencephalogram clearly outline the large mass impinging on the left lateral ventricle. Occasionally cysticerci attain such great size, although much more commonly they are considerably smaller, often measuring about 1 cm. (Courtesy of Dr. Dennis Chalus, Denver).

Cysticerci may be seen anywhere around the periphery of the brain, including the basal cisterns. In a small proportion of patients, there will be basal leptomeningitis, demonstrated as severe hydrocephalus, and there may be signs of cortical atrophy. In some patients, positive contrast ventriculography may be helpful, particularly in posterior fossa cysticercosis. The demonstration of the rounded defects is more reliable utilizing positive contrast media (see Fig. 7.43 B).

Cerebral arteriography has little part to play in the diagnosis of cerebral cysticercosis where CT and MRI are available. It may help in demonstrating the mass effect but is far less accurate in providing the diagnosis than CT and MRI.

Computed tomography and MRI performed before or after administration of intravenous contrast media are the imaging methods of choice in neurocysticercosis, giving reliable information about the location and activity of the cysts. CT will not only demonstrate both calcified and uncalcified cysts, but also free-floating cysts and hydrocephalus in many patients. The findings are characteristic, and the multiplicity of lesions will strongly suggest cysticercosis. However, solitary cysts may occur. MRI with different pulse sequences and with multiplanar capability, aided by intravenous contrast enhancement, is of great value in the diagnosis of neurocysticerosis, particularly in patients with lesions at the base of the brain, temporal lobes, and brain stem, as well as associated vascular insults. However, CT imaging is still a valuable diagnostic tool, especially in detecting small calcified lesions which can be overlooked with MRI.

Classification in Neuroimaging

In the CNS, the cysticercus, by way of the bloodstream, initially invades the subarachnoid space and then the cortical brain, especially at the junction of gray and white matter. On MRI or CT, the lesion may be demonstrated as an ill-defined nodule with minimal enhancement following injection of intravenous contrast medium, indicating the initial inflammatory reaction (Fig. 7.18). Following this, the cysticercus evolves into the vesicular stage, which is the viable stage of the parasite and appears as a low-density cyst on CT with minimal peripheral or no contrast enhancement (Fig. 7.19).

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Fig. 7.18 A,B (left, middle). Early cortical intraparenchymal cysticercal lesions, prior to vesicular stage. CT axial, postcontrast. (A) Enhancing lesion in the right occipitoparietal area (arrow) (B) Enhancing lesions (arrows) in another patient. (Courtesy of Dr. Enrique Palacios, Berwyn).

Fig. 7.19 Cysticercosis, vesicular stage. CT axial, postcontrast. Cystic lesion in the right temporal area (arrow). (Courtesy of Dr. Enrique Palacios, Berwyn).