Expansion of the IAC may be seen in up to 70% to 90% of vestibular schwannomas ( Fig. Schwannomas are typically hyperintense on T2W imaging, and if cystic degeneration is present more focal areas of increased signal will be seen. These lesions enhance intensely after gadolinium infusion, and the degree of homogeneous enhancement depends on the amount of cystic change ( Fig. However, T1 hyperintensity may be seen if there has been hemorrhage into the schwannoma or if regions of cystic degeneration contain proteinaceous material. Typically, schwannomas are slightly hypointense to isointense on T1-weighted (T1W) imaging. As the lesions become larger they may undergo cystic change, and this alters their signal characteristics. The appearance of schwannomas on MRI, like CT, may vary with the tumor size. Administration of a contrast agent is a necessary part of the examination because smaller lesions may be missed if contrast enhancement is not utilized. The utilization of thin-section (3 mm) MRI is beneficial in evaluation of most lesions of the CPA. Thin-section (1.5 mm) imaging utilizing bone algorithm assists in visualizing the expansion of the IAC as well as any small calcifications within the lesion. Calcification is very rare within schwannomas and, if seen, should raise the suspicion of a meningioma. 8 On postcontrast imaging schwannomas demonstrate enhancement. In fact, most, if not all, schwannomas larger than 2.5 cm become heterogeneous due to cystic and necrotic components. Larger lesions may have a heterogeneous appearance due to cystic degeneration. 6 In addition, the intracanalicular lesions may “dumbbell” into the vestibule or cochlea, which is better appreciated on MRI. Expansion of the IAC may be seen in up to 70% to 90%.
This is referred to by some as the “ice cream cone” pattern. As the lesion enlarges, it extends outward into the CPA and presents as a rounded mass that is either hypoattenuating or isodense to brain. In the case of vestibular schwannomas a small lesion arising along the intracanalicular portion of the nerve will be contained entirely within the internal auditory canal (IAC) and may not be readily visible by CT. Smaller schwannomas may not be readily visible on CT. The imaging characteristics of schwannomas on CT depend in part on the size of the lesion. When CT is utilized, thin-section (1.5 mm) imaging should be performed. Locating a point of origin along with knowledge of a lesion’s morphology, CT density, MR intensity, and reaction of adjacent structures can help narrow the differential diagnosis.įor the most part, MRI has replaced CT in the evaluation of CPA angle masses. Exophytic brain stem neoplasms or ventricular tumors can also involve the CPA. Lesions, such as chondromatous tumors, chordomas, paragangliomas, and endolymphatic sac tumors, involve the CPA by extension from surrounding structures such as the skull base and petrous apex. Embryologic remnants may also be found, including lipomas, epidermoid cysts, and neurenteric cysts. These lesions arise within the cerebellopontine cistern or from related structures and include arachnoid cysts, nonacoustic schwannomas, and meningeal lesions. 3 However, a variety of other lesions also make their home in the CPA region. Indeed, meningiomas and schwannomas comprise approximately 90% of neoplasms in this region. 1, 2 The two most common lesions are the meningioma and the vestibular schwannoma. Neoplasms occurring in this region make up 6% to 10% of all intracranial tumors in adults and only 1% of lesions in children. The presenting symptoms of CPA lesions are usually not related to the histology of the lesion but to the nerves and other structures that the lesion affects. The Cranial nerves V through VIII pass through the upper portion of the cistern, whereas the cranial nerves IX through XI pass through the lower portion.
The cerebellopontine angle (CPA) cistern is a cerebrospinal fluid (CSF)–filled space whose boundaries are made up by the pons and cerebellum medially, the petrous portion of the temporal bone laterally, and the tentorium superiorly.
0 kommentar(er)
