Choroid Plexus Cysts

　　CPCs are iso- to slightly hyperattenuated on nonenhanced CT scans compared with CSF. Peripheral calci?cation is common. The cysts show enhancement that varies from none to striking.

　　Signal intensity on MR images is variable. Most are iso- or hyperintense on precontrast T1-weighted MR images compared with CSF and show rim or nodular contrast enhancement. CPCs are usually hyperintense to CSF on T2-weighted images, especially with long repetition/short echo time sequences. The majority do not become completely hypointense (suppress) on ?uid-attenuated inversion-recovery (FLAIR) images and remain slightly or moderately hyperintense to CSF. Two-thirds show restriction (high signal intensity) on diffusion-weighted images .

　　Differential Diagnosis

　　The major differential diagnosis is ependymal cyst and villous hyperplasia of the choroid plexus. Ependymal cysts do not enhance. Villous hyperplasia is very rare and, when present, enhances strongly and relatively uniformly

　　Enlarged PVSs

　　Prominent PVSs are considered a normal variant. Most appear as smoothly demarcated fluid-filled cysts, typically less than 5 mm in diameter, and often occur in clusters in the basal ganglia or midbrain. They are isointense to CSF at all sequences, including FLAIR. Most show normal signal intensity in the adjacent brain; 25% may have a small rim of slightly increased signal intensity. They do not enhance, cause focal mass effect, or restrict on diffusion-weighted images. In older patients, basal ganglia PVSs sometimes become prominent and sievelike. Occasionally PVSs may become very large and appear bizarre. They are probably caused by the accumulation of interstitial fluid between the penetrating vessels and the pia. If interstitial fluid egress is blocked, fluid accumulates and the PVSs dilate . These lesions cause focal mass effect and occasionally even hydrocephalus. Rarely, so-called giant or tumefactive PVSs may be mistaken for more ominous disease.

　　Ependymal Cysts

　　The best diagnostic clue is a nonenhancing thin-walled CSF-containing cyst of the lateral ventricle .

　　Differential Diagnosis

　　The differential diagnosis for an ependymal cyst includes CPC, arachnoid cyst, neurocysticercosis, and asymmetric ventricles . Part or all of a ventricle (most often the temporal horn, atria of lateral ventricles, or fourth ventricle) may also enlarge if it is “trapped” by neoplasm or infection. CPCs are usually not identical to CSF at all imaging sequences, are typically bilateral, and often enhance. Arachnoid cysts occur in the subarachnoid spaces. Intraventricular neurocysticercosis cysts have a hyperintense rim and scolex on FLAIR images. Large CSF-appearing cysts may occur along the choroid fissure and can be either ependymal or lined with arachnoid .

　　Neuroglial Cysts

　　Imaging

　　The best diagnostic clue to a neuroglial cyst is a nonenhancing CSF-like parenchymal cyst with minimal to no surrounding signal intensity abnormality. The cysts are benign-appearing lesions with smooth, rounded borders. Size is variable.

　　Differential Diagnosis

　　Other lesions that may be mistaken for a neuroglial cyst include an enlarged PVS, infectious cyst, porencephalic cyst, and arachnoid cyst. Enlarged PVSs are typically multiple and cluster around the basal ganglia. Infectious cysts, such as neurocysticercosis, are typically smaller than 1 cm and

　　can partially enhance. Porencephalic cysts communicate with the lateral ventricle and show surrounding gliosis. Arachnoid cysts are typically extraaxial.

　　Pineal Cysts

　　Imaging

　　The best diagnostic clue is unilocular ?uid-?lled mass within the pineal gland. Attenuation or signal intensity varies with cyst content. One-fourth have rim or nodular calcium in the cyst wall on nonenhanced CT scans. Rim or nodular enhancement is also common. On T1-weighted MR images, 55%–60% are slightly hyperintense to CSF. Most do not appear hypointense on FLAIR images, and 60% enhance with use of contrast material.

　　Differential Diagnosis :Pineal cysts are most often mistaken for—and may be indistinguishable from—a benign pineal parenchymal neoplasm called a pineocytoma. Pineocytomas are more likely to have solid components, but it may be impossible to distinguish the two with imaging studies alone. Both benign nonneoplastic pineal cysts and the typical pineocytoma grow extremely slowly, so follow-up scans are often not helpful. CT or MR-guided stereotactic biopsy may be needed for the evaluation and management of symptomatic cases.

　　Differential Diagnosis

　　The most difficult lesion to distinguish from the arachnoid cyst is an epidermoid cyst. Epidermoid cysts can appear nearly identical to CSF on CT scans. On MR images, epidermoid cysts appear isointense to CSF, although close inspection often shows they are not precisely identical in signal intensity to CSF. Arachnoid cysts typically suppress completely on FLAIR images and do not restrict on diffusion-weighted images. Arachnoid cysts displace adjacent arteries and cranial nerves rather than engulf them, as epidermoid cysts often do. Chronic subdural hematoma and porencephalic cyst can also be confused for an arachnoid cyst. Chronic subdural hematomas do not typically show CSF signal intensity on MR images and often have an enhancing membrane. Porencephalic cysts often follow a history of trauma or stroke.

　　Colloid Cysts

　　Imaging

　　The best diagnostic clue to a colloid cyst is its location at the foramen of Monro. The classic colloid cyst appears as a well-delineated hyperattenuated mass on nonenhanced CT scans. On T1-weighted MR images, two-thirds of colloid cysts are hyperintense. The majority are isointense to brain on T2-weighted images. Some demonstrate peripheral rim enhancement. Occasionally, colloid cysts expand rapidly. These colloid cysts typically have a higher water content, which reflects ongoing cyst expansion. Thus, it is hypothesized that potentially the most “dangerous” lesions are hypointense on T1- and hyperintense on T2-weighted images.

　　Differential Diagnosis: The imaging appearance of a colloid cyst is almost pathognomonic. The most common “lesion” mistaken for a colloid cyst is CSF flow artifact (MR pseudocyst) caused by pulsatile turbulent CSF flow around the foramen of Monro. Occasionally, a neurocysticus cyst may occur at the foramen of Monro. Neoplasms such as subependymoma or choroid plexus papilloma that may occur at the foramen of Monro are much less common and typically enhance.

　　Differential Diagnosis

　　The major differential consideration for the epidermoid cyst is an arachnoid cyst. Arachnoid cysts are isointense to CSF at all sequences, including FLAIR. They displace rather than invade structures such as the epidermoid. Finally, arachnoid cysts do not restrict on diffusion-weighted images. Other epidermoid cyst mimics include dermoid cyst, neurocysticercosis, and cystic neoplasm . Dermoid cysts are typically located along the midline and resemble fat, not CSF. Cystic neoplasms often enhance and do not resemble CSF. Neurocysticercosis cysts often enhance and demonstrate surrounding edema or gliosis.

　　Dermoid Cysts

　　Imaging

　　Imaging findings vary, depending on whether the cyst has ruptured. Unruptured cysts have the same imaging characteristics as fat because they contain liquid cholesterol. All are hyperintense on T1-weighted images and do not enhance. The masses have heterogeneous signal intensity on T2-weighted MR images and vary from hypo- to hyperintense . The best diagnostic clue of a ruptured dermoid cyst is fatlike droplets in the subarachnoid cisterns, sulci, and ventricles.

　　Differential Diagnosis :Dermoid cysts may be confused with an epidermoid, craniopharyngioma, teratoma, or lipoma. Epidermoid cysts typically resemble CSF (not fat), lack dermal appendages, and are usually located off midline. Like dermoid cysts, craniopharyngiomas are suprasellar, with a midline location, and demonstrate nodular calcification. However, most cra-

　　niopharyngiomas are strikingly hyperintense on T2-weighted images and enhance strongly. Teratomas may also have a similar location but usually occur in the pineal region. Lipomas demonstrate homogeneous fat attenuation and/or signal intensity and show a chemical shift artifact, which typically does not occur with dermoid cysts.