Computed Tomography
Findings
Techniques and findings
In a retrospective study, CT scans of 35 patients with intracranial cryptococcal infection were reviewed.16 Findings were unremarkable in 43% of the patients. Positive findings included diffuse atrophy (34%), cryptococcomas (11%), hydrocephalus (9%), and diffuse cerebral edema (3%).
Cryptococcomas are round, hypoattenuating or isoattenuating lesions that occur less commonly as gelatinous pseudocysts (rather than as granulomas or abscesses).
In immunologically intact hosts, the cryptococci usually induce a chronic granulomatous reaction. On CT scans, cerebral cryptococcal granulomas appear as hypoattenuating or isoattenuating lesions, with or without enhancement.
Accuracy
- CT findings are often nonspecific for CNS cryptococcosis.
Imaging pearls
- Intracranial mass lesions occur frequently in patients with AIDS. The most common mass lesions detected by CT or MRI are abscesses from toxoplasmosis, although lymphoma and less common infectious processes, such as cryptococcomas, must also be considered.
Magnetic Resonance Imaging
Findings
Techniques and findings
Cryptococcal meningitis is the most common CNS manifestation of cryptococcosis; the disease often has an insidious course (see Image below and Image 1 in Multimedia Section). In cases of AIDS-related cryptococcal meningitis, leptomeningeal enhancement is not always seen on CT scans or MRIs.13,15,17
Axial contrast-enhanced T1-weighted magnetic resonance image shows diffuse, gyriform leptomeningeal enhancement. Enhancing lesions in the left basal ganglia, left temporal lobe, and left occipital lobe correspond to intraparenchymal cryptococcosis.
The cryptococcal organism is surrounded by a polysaccharide capsule, which may protect it from the host inflammatory response even in immunocompetent patients. In response to the attack on the organisms by the host's immune system, the cryptococci produce a mucoid material. In a study by Arnder et al, 3 patients were diagnosed with cryptococcal meningitis; in 2 of the patients, thick enhancing subarachnoid spaces were seen on postcontrast MRIs.18 Diagnoses were confirmed at autopsy. In the postmortem MRI and pathologic examinations of the third patient, these areas of enhancement corresponded to the abundant mucoid material secreted by the organisms.
Cryptococcal organisms spread from the basal cisterns through the Virchow-Robin spaces to the basal ganglia (see Images below and Images 1-2 in Multimedia Section), internal capsule, thalamus, and brainstem.19 The perivascular spaces may be enlarged as a result of the production of voluminous mucoid material. MRI is more sensitive than CT in demonstrating abnormalities such as dilated perivascular spaces. These manifest on T2-weighted MRIs as punctate, hyperintense, round or oval lesions that are usually smaller than 3 mm. Enlarged perivascular spaces are not always a consequence of cryptococcosis; they may be the result of age-related changes or HIV-related atrophy.
Axial contrast-enhanced T1-weighted magnetic resonance image shows diffuse, gyriform leptomeningeal enhancement. Enhancing lesions in the left basal ganglia, left temporal lobe, and left occipital lobe correspond to intraparenchymal cryptococcosis.
T1-weighted magnetic resonance image demonstrates dilated perivascular spaces in the bilateral basal ganglia.
Generalized atrophy is common in patients with AIDS; however, the observation of punctate hyperintensities is suggestive of cryptococcal disease, especially if other signs of diffuse atrophy (eg, ventricular and sulcal enlargement) are not present or if the patient has clinical signs or symptoms of meningitis.
By definition, cryptococcomas represent a collection of organisms, inflammatory cells, and gelatinous mucoid material in the brain parenchyma. Cryptococcomas may develop when organisms have extended directly from perivascular spaces into the parenchyma (see Images below and Images 3-5 in Multimedia Section) or, possibly, when they have invaded the parenchyma from other meningeal or ependymal surfaces.
T2-weighted axial magnetic resonance image demonstrates linear and punctate hyperintensities in the basal ganglia; these represent dilated perivascular spaces caused by small cryptococcomas. Cryptococcomas vary in size from several millimeters to several centimeters.
Axial T2-weighted magnetic resonance image shows clustered hyperintensities in the left caudate; these are consistent with enlarged Virchow-Robin spaces caused by small cryptococcomas.
T2-weighted axial magnetic resonance image demonstrates hyperintense cryptococcomas in the midbrain.
Cryptococcomas are hyperintense on T2-weighted images. Although contrast enhancement of cryptococcomas is uncommon in cases of AIDS-related CNS cryptococcosis,15 it has been reported.18 The cryptococcal organism is surrounded by a polysaccharide capsule, which may protect it from the host inflammatory response, even in immunocompetent patients.
In immunologically intact hosts, the organisms usually induce a chronic granulomatous reaction. On MRI, the most common findings are punctate masses that demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, without surrounding edema.
Choroid plexitis of the brain is a pathologic presentation of cryptococcosis. MRI demonstrates unilateral or bilateral enlargement and dense enhancement of the choroid plexus in the lateral and fourth ventricles.20 These findings occur in association with clinical findings of leptomeningitis. The lesion may appear as an enhancing, intraventricular mass.
Unilateral cystic dilatation of the temporal horn of the lateral ventricle has also been described.20 Cho et al believed this dilatation may be the result of entrapment of the temporal horn by inflamed choroid plexus, as well as extensive edema around the ipsilateral ventricle.20
Accuracy
- Although MRI is useful as part of the initial investigation protocol for patients with suspected cryptococcal meningitis, serial imaging probably has a minimal role in monitoring response to therapy. However, magnetic resonance spectroscopy may detect changes in the metabolites that are related to inflammatory activity.
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References
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Mathews VP, Alo PL, Glass JD, Kumar AJ, McArthur JC. AIDS-related CNS cryptococcosis: radiologic-pathologic correlation. AJNR Am J Neuroradiol. Sep-Oct 1992;13(5):1477-86. [Medline].
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Kamezawa T, Shimozuru T, Niiro M, Nagata S, Kuratsu J. MRI of a cerebral cryptococcal granuloma. Neuroradiology. Jun 2000;42(6):441-3. [Medline].
Arnder L, Castillo M, Heinz ER, et al. Unusual pattern of enhancement in cryptococcal meningitis: in vivo findings with postmortem correlation. J Comput Assist Tomogr. Nov-Dec 1996;20(6):1023-6. [Medline].
Wehn SM, Heinz ER, Burger PC, Boyko OB. Dilated Virchow-Robin spaces in cryptococcal meningitis associated with AIDS: CT and MR findings. J Comput Assist Tomogr. Sep-Oct 1989;13(5):756-62. [Medline].
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Further Reading
Related eMedicine topics:
Cryptococcosis (from Infectious Diseases)
HIV-1 Associated Opportunistic Infections: CNS Cryptococcosis
Related Medscape topics:
Radiology CME and News
Specialty Site Radiology
Specialty Site Infectious Diseases
Keywords
cryptococcosis of the central nervous system, Cryptococcus neoformans, C neoformans, cryptococcal meningitis, cryptococcal meningoencephalitis, AIDS-related cryptococcal meningitis, AIDS-related CNS cryptococcosis, cryptococcomas, gelatinous pseudocysts, Toxoplasma gondii, T gondii, inhaled yeast, CNS yeast infection
