Imaging in Brain Ependymoma 

Ependymoma is a central nervous system (CNS) neoplasm composed of glial cells that have differentiated along ependymal lines. These lesions occur most commonly in the ependymal lining of the ventricles, but they also arise in the filum terminale and the central spinal canal.^{[1, 2, 3, 4]} See the image below.

Preferred Examination

Radiologic imaging plays a role in both the diagnostic workup and treatment of patients with ependymoma; imaging is essential to assess for response to therapy and recurrence. Patients with CNS symptoms routinely undergo cross-sectional imaging. Computed tomography (CT) scanning is often the modality used initially to evaluate for intracranial hemorrhage, mass, or mass effect. A general limitation of CT is radiation exposure. Additionally, the use of iodinated contrast material may sometimes be associated with nausea, vomiting, and rare anaphylactoid reactions. Limitations of CT with respect to ependymoma include imprecise anatomic detail.

If a tumor is suspected, magnetic resonance imaging (MRI) is the next study performed. In fact, MRI is the chief modality used in the study of ependymomas. MRI better characterizes CNS tumors, and findings often lead to a presumptive diagnosis. CT scanning is a useful adjunct. Before the development of cross-sectional and multiplanar imaging, angiography and pneumoencephalography were used to localize brain masses and characterize tumor vascularity.

General limitations of MRI include its cost and the need for patient cooperation. Patient motion is a cause of considerable artifact. Many patients, especially children and patients with claustrophobia, require sedation. Another general limitation is the incompatibility of MRI with numerous foreign and/or medically implanted objects, such as pacemakers. Finally, MRI is of limited benefit in the evaluation of cortical bone and the detection of calcium.

Ultrasonography, nuclear medicine studies, angiography, and radiography are of no benefit in the workup of ependymoma.

The final diagnosis of ependymoma, as with most CNS neoplasms, is achieved with tissue sampling; however, when correlated with demographic and clinical features, MRI and CT scan findings can be strongly suggestive of ependymoma.

Radiographic findings are included only for historical interest. A study by Barone and Elvidge demonstrated that in 45 pathology-proven cases of ependymoma, intracranial calcifications were present in 6 patients.^[5] The pineal gland was calcified in 4 patients, and the pineal gland was displaced from the midline in 2 patients. Separation of the sutures occurred in 12 patients. In the 43 patients in whom ventriculography was performed, 41 demonstrated hydrocephalus with identification of the site of obstruction.^[5]

MRI has supplanted CT scanning as the diagnostic modality of choice in the workup and follow-up observation of intracranial neoplasms, including ependymoma. The most appropriate role for MRI in the treatment of ependymoma is in the detection of tumor and direction of its resection and/or irradiation. MRI is used to monitor ongoing treatment and to survey for recurrence. Although the MRI findings can be of great help in narrowing the differential diagnosis of brain tumors, final diagnosis is achieved through histologic sampling.

Solid portions of ependymoma are typically isointense to hypointense relative to white matter on short recovery time/echo time (TR/TE) T1-weighted images. The tumor is hyperintense to white matter on long TR/TE T2-weighted images. As many as 50% of ependymomas demonstrate signal heterogeneity, which may indicate calcification, necrosis, methemoglobin, hemosiderin, or tumor vascularity.^{[6, 7, 8]} For example, hyperintense foci on both T1- and T2-weighted images suggest methemoglobin in subacute hemorrhage of 1-4 weeks in age, whereas hypointense foci on both T1- and T2-weighted images suggest hemosiderin, calcium, or necrosis. See the images below.

Punctate calcific foci are difficult to diagnose prospectively but are present in as many as 45% of ependymomas.^{[8, 9]} See the following images.

Cystic changes result in high signal intensity on T2-weighted MRIs, as shown in the images below.

Signal heterogeneity is a feature useful in distinguishing ependymoma from the more homogeneous medulloblastoma. Calcification and hemorrhagic foci are more typical of ependymoma than medulloblastoma. Additionally, ependymomas are more apt to extend through the foramina of Luschka and Magendie, hence the term plastic ependymoma (see the following images). Similarly, choroid plexus papilloma is more homogeneous than ependymoma and lacks the typical irregular margins and surrounding edema of ependymoma.

Enhancement with gadolinium is useful in differentiating tumor from adjacent vasogenic edema and normal brain parenchyma. Without intravenous contrast enhancement, T2-weighted images are more reliable in differentiating tumor margins than are T1-weighted images.^[9]

Some reports describe ependymomas that cause displacement of the vein of the lateral recess of the fourth ventricle on cerebral arteriography.^[9] This vein normally courses from the transverse and lateral supratonsillar veins along the anterior and lateral aspect of the superior pole of the cerebellar tonsil. It then courses lateral to the cerebellopontine angle, over the brachium pontis, to join the petrosal vein. Ependymoma expanding the fourth ventricle and its lateral recesses can displace this vein posteriorly and laterally.

Supratentorial ependymomas can differ in appearance from intraventricular ependymomas. Supratentorial ependymomas are more commonly located in the brain parenchyma than infratentorial ependymomas, which are often intraventricular. Swartz and colleagues reported that 83% of supratentorial ependymomas were located in the parenchyma.^[10] Supratentorial ependymomas tend to be larger than infratentorial ependymomas, with 94% being larger than 4 cm in one study.^[11] In addition, supratentorial extraventricular ependymomas are often extraventricular and more often have a cystic component, with or without a mural nodule. In these cases, the differential diagnosis includes ganglioglioma, pleomorphic xanthoastrocytoma, and pilocytic astrocytoma. In the posterior fossa, medulloblastoma and cerebellar astrocytoma can mimic the appearance of an ependymoma.^[5]

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy.

NSF/NFD has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see FDA Information on Gadolinium-Based Contrast Agents or Medscape.

The role of ultrasonography in the evaluation of ependymoma is limited. Fetal ultrasonography and pediatric transcranial sonography are used primarily as screening tools for other pathologic conditions but can detect hydrocephalus reliably. A study by Han and colleagues demonstrated that 6 of 1528 infants undergoing transcranial ultrasonography had a pathologically proven brain neoplasm.^[9] One patient had ependymoma. Ultrasonography demonstrated a solid echoic fourth ventricular mass with localized, well-defined, anechoic cystic areas.^[9] These findings are not sensitive or specific for ependymoma.