Imaging of ependymomas: MRI and CT

Intracranial ependymomas, including both infra- and supratentorial ependymomas, generally demonstrate low T1, high T2, and intermediate-to-high FLAIR signal intensity relative to both gray and white matter (Figs. 1, 2, and 3). These characteristics have been attributed to the high proportion of ependymomas with intracellular myxoid accumulation and cyst formation [9]. As a result, they are often conspicuous on T2-weighted and FLAIR sequences and are also usually visible on unenhanced T1-weighted images (Figs. 1 and 2).

Although ependymomas are generally T1 hypointense and T2 hyperintense relative to brain parenchyma, both T1 and T2 signal intensity are heterogeneous within the tumor. This is particularly true for supratentorial ependymomas, which have a greater propensity than their infratentorial counterparts for extensive cyst formation [2] (Fig. 3). This may be attributable to their later clinical presentation and larger size at the time of discovery. Cystic areas of a lesion may follow fluid signal intensity on both T2-weighted and FLAIR sequences or may remain hyperintense on FLAIR due to proteinaceous fluid contents. Calcifications, which are common in both supratentorial and infratentorial ependymomas (approximately 50%), and old hemorrhage are generally very low in signal intensity on all MRI sequences.

On postgadolinium T1-weighted images, there is usually avid enhancement of the soft tissue components of the tumor, intermixed with poorly enhancing or nonenhancing areas. A minority of lesions demonstrates little or no postgadolinium enhancement despite being composed of mostly soft tissue (Fig. 4).

Diffusion-weighted imaging (DWI) is based on the random movement of water molecules in tissue. Tissues in which water diffusion is reduced demonstrate high DWI signal and low apparent diffusion coefficient (ADC). DWI demonstrates reduced diffusion within the soft tissue components of some ependymomas (Fig. 5), presumably due to high cellularity of some lesions. Diffusivity of ependymomas is generally intermediate between that of pilocytic astrocytomas and primitive neuroectodermal tumors [18]. However, diffusivity values are not completely reliable in making histologic diagnoses.

Perfusion MRI and proton MR spectroscopy are performed in only a limited number of cases, usually to distinguish treatment changes from recurrent tumor, or occasionally when the neoplastic nature of a lesion is in question. Perfusion MRI can be performed using several techniques, including T2*-weighted dynamic susceptibility and fast spoiled gradient-echo techniques, which require bolus administration of gadolinium contrast, and arterial spin labeling, which does not require gadolinium. Perfusion MRI is challenging in infants and young children for several reasons including the difficulty of obtaining proximal intravenous access, the use of small intravenous catheters that limit injection rates, the necessity of using small doses of contrast, and high incidence of motion artifact [3]. When performed, perfusion MRI of ependymoma generally demonstrates markedly elevated cerebral blood volume and, unlike many other glial neoplasms, poor return to baseline that may be attributable to fenestrated blood vessels and an incomplete blood–brain barrier [22] (Figs. 5 and 6).

Proton MR spectroscopy usually demonstrates elevated choline and reduced N-acetylaspartate (NAA) in ependymomas, as in many other brain tumors. Although the use of MR spectroscopy for prospective differentiation of infratentorial ependymoma from medulloblastoma has been investigated, the variability in choline/NAA ratios within each tumor histology results in considerable overlap, and MR spectroscopy is currently used primarily to differentiate tumor recurrence from postradiation or posttreatment change rather than to confirm a histologic diagnosis [3].

MRI is the recommended initial brain imaging modality of choice not only for intracranial tumors but also for any younger child with new neurological signs or symptoms, except in the setting of acute head trauma [3]. However, intracranial ependymoma is sometimes initially diagnosed on head CT. CT demonstrates isodense or mildly hyperdense soft tissue portions of the tumor, calcifications in 50% of lesions, hemorrhage in approximately 10% [21], and often heterogeneous enhancement (Figs. 3a and 5, inset). CT is generally more sensitive for, and will demonstrate a greater number of, calcifications than most MRI sequences, particularly spin echo-type sequences. However, MRI sequences with increased susceptibility-associated signal loss, such as diffusion-weighted and gradient-echo sequences, commonly demonstrate foci of signal void/loss in areas of coarse/gross calcification or hemorrhage (Figs. 3 and 5).

Approximately 70% of intracranial ependymomas are infratentorial, arising from differentiated ependymal cells lining the fourth ventricle and foramina of Luschka. At the time of clinical presentation, infratentorial ependymomas usually fill and distend the fourth ventricle, resulting in hydrocephalus and symptoms of nausea and vomiting in 90% of patients [1, 3]. A highly typical feature of ependymoma is extension through the foramina of Luschka into the cerebellopontine angle (15%) and/or inferiorly through the foramen of Magendie (60%) onto the posterior aspect of the upper cervical cord (Figs. 1, 2, and 4). Although this feature is not entirely pathognomonic, the few medulloblastomas (the main differential diagnosis for infratentorial ependymoma) that extend through the fourth ventricular exit foramina [4] show more bulbous extension rather than thin tongues of tissue through the foramina as in ependymomas [3]. Ependymomas often insinuate around and within structures, encasing vessels and lower cranial nerves, and can cause cranial neuropathies. In addition to the classic appearance of a fourth ventricular mass, infratentorial ependymomas can also present within the cerebellopontine angle, presumably arising from the rests of ependymal cells therein. Ependymomas arising from a location not in the midline may have a worse prognosis than midline lesions, perhaps due to involvement of critical structures such as cranial nerves [3, 12].

Approximately 30% of intracranial ependymomas are supratentorial. Within the pediatric population, supratentorial ependymomas are often seen in older childhood [11]. Unlike posterior fossa ependymomas, most supratentorial ependymomas (70%) are extraventricular in origin, arising within the brain parenchyma of the cerebral hemispheres rather than from ependymal cells lining the ventricular surfaces. Extraventricular ependymomas are thought to originate from the rests of ependymal cells that are retained in the brain parenchyma during embryonic development. They are, however, often located near the ventricular margins and may extend into the ventricular system. In addition, in comparison to posterior fossa tumors, hydrocephalus occurs relatively late (after development of significant subfalcine herniation), by which time the tumors are typically large, with an average size of over 4 cm at presentation (Figs. 3, 5, and 6) [2, 9]. Approximately 30% of supratentorial ependymomas arise in an intraventricular location, generally along the surfaces of the lateral ventricles or along the septum pellucidum. Although the ependymal surface area of the third ventricle is larger than that of the fourth ventricle, third ventricular ependymomas are rare [10, 13].

The World Health Organization classifies ependymal tumors into four types: (1) ependymoma (grade 2), (2) anaplastic ependymoma (grade 3), (3) subependymoma (grade 1), and (4) myxopapillary ependymoma (grade 1). Grade 2 ependymomas are further subdivided into four major histologic subtypes: cellular, papillary, clear-cell, and the rare tanycytic subtype, with these subtypes demonstrating some variation in prognosis [10, 16]. In comparison to the more common grade 2 ependymomas, anaplastic ependymomas have greater incidence of cerebrospinal fluid dissemination at the time of diagnosis and a poorer prognosis (Fig. 7) [3].