In the CNS, CT will show low-density areas in the cerebral white matter. The majority are unilateral, but the lesions can be bilateral and there may be localized cortical atrophy in the same areas. The lesions can be scattered in any part of the brain. At the same time there can be one or more small punctate or nodular calcifications, associated with the low-density area. Contrast enhancement shows nodular, linear, irregular or amorphous lesions: a ring appearance may be present (Fig. 7.50 A). These lesions are avascular on contrast angiography. Follow-up CT scans can show that these nodules are moving to different parts of the brain. Over a period of years there can be slight progression or degeneration of the existing lesions. It is likely that the CT findings reflect a longstanding inflammation, with active granulomas and areas of degeneration, probably mirroring the status of the worm.

Magnetic resonance imaging is, however, the most reliable diagnostic modality and is more sensitive than CT in imaging cerebral sparganosis. MRI better demonstrates the difference between the normal and abnormal tissue and can show lesions not identified by CT scanning. In more than half the patients, MRI will show localized cortical atrophy, particularly along the gyri: this may not be so well seen on CT. Another characteristic finding on both CT and MRI is associated ventricular dilatation. This also can be unilateral and is not due to a mass effect.

T1-weighted MRI scans show mildly hypointense areas of cerebral white matter, which on proton and T2-weighted images are hyperintense in the periphery of the lesion with a relatively hypointense central focus (Figs. 7.50 B, 7.51 A). With gadolinium enhancement, the cortical or subcortical lesions are reliably demonstrated as round, nodular, irregular, or conglomerate masses, some being ring-shaped or beaded, two patterns that are better seen by MRI than by CT (Figs. 7.51 B, 7.52). Nevertheless, the multifocal lesions do show on CT as areas of low attenuation, and in many patients there will be small punctate calcifications, which are perhaps due to cortical petechial hemorrhage, that are better seen by CT than by MRI. Many of these will be in the same area as the lesions shown by contrast enhancement, but some will be remote and not related. Although the actual calcification may not be recognized by MRI, T1-weighted and proton density images may show focal hypointense lesions, becoming more hyperintense on T2-weighted images.

The combination of the CT or MRI findings, the ELISA results, and the history of eating raw flesh or using a contaminated poultice will strongly suggest the correct diagnosis of sparganosis. Should the diagnosis be in doubt, the change in the location of the lesions on subsequent scans (CT or MRI) should be sufficient to indicate movement of the worm and establish the probable diagnosis. It is the mobility of the lesions which distinguishes these granulomas from those of other etiology; also, other granulomas do not show the same pattern of enhancement and associated ventricular dilatation.

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Fig. 7.50. (A) Contrast-enhanced CT of the brain of a 21-year-old man from India who presented with generalized tonic-clonic seizures and right homonymous hemianopsia. There is a 5-cm cystic lesion in the left occipital lobe showing ring enhancement. (B) MRI of the same patient shows considerable bright edema surrounding the cystic lesion. The mass was surgically excised and within the cavity were found a small amount of tan exudate and a live undulating white 10 by 1 mm sparganum. Postoperatively, the patient's seizures and visual deficit resolved. (Photographs provided by Dr. M. Holodniy. From Connor DH, Chandler FW, et al (eds): Pathology of Infectious Diseases. Stamford, Conn., Appleton & Lange, 1997).

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Fig. 7.51. A T2-weighted MRI showing a wide area of subcortical hyperintensity in the right frontal region, with some cortical shrinkage. The arrow indicates a hypointense region within the hyperintense area due to a granuloma. B Gadolinium-enhanced MRI showing the tortuous linear, beaded granuloma (arrow) extending down to the periventricular region. There is a surrounding hypointense area of edema or, perhaps, encephalomalacia: there was a similar region in the left frontal cortex. Three live worms were found at surgery. (Courtesy of Dr. W. K. Moon et al and Radiology, 1993)

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Fig. 7.52. (A) Gadolinium-enhanced MRI shows an irregular enhancing lesion with a ring shape and beaded contours (arrow) in the left parietal cortex of a 19-year-old man who had a headache for 7 months. (B,C) At surgery, a live worm (open arrow in B) moved slowly from the brain: it was 10 cm long.(A, C courtesy of Dr. W .K. Moon et al and Radiology, 1993; B from C. L. Carroll, in D. H. Connor (ed): The Pathology of Infectious Diseases. Stamford, Conn., Appleton & Lange, 1997; B, C photographed by Dr. K. H. Chang).

Differential Diagnosis

Sparganosis must be differentiated from all other infectious granulomas, particularly tuberculosis and paragonimiasis. Calcified foci might be mistaken for cysticercosis. However, the location of the low-density areas of extensive degeneration along the white matter bundles, with adjacent ventricular dilatation, the presence of single or multiple nodular lesions, beaded or irregular in outline, and the mobility of the lesions suggest sparganosis rather than tuberculosis or other granulomatous diseases. Metastatic malignant nodules are usually well defined and round and are very unlikely to cause adjacent ventricular dilatation. However, some cases of sparganosis show a mass effect and must be distinguished from a neoplasm: this is somewhat more common within a few months of the start of the infection. The long history and the memory of eating raw flesh, particularly fish, frogs, or snakes, are also significant in making the diagnosis. They provide a warning, once again, that raw flesh is better not eaten, and it is certainly undesirable as a component of a poultice.

Other Tapeworm Infections

Many other species of tapeworm may infect man. The majority have no imaging significance, but radiologists should be aware of the following infections, as they may cause clinical symptoms and important imaging findings in patients who are referred for radiological investigation.