Extraaxial lesions may also “push away” cranial nerves and adjacent structures.
In some cases, differentiation may be difficult and secondary signs of the extraaxial location (including the identification of a CSF cleft between the lesion and the parenchyma and enlargement of the cisterns) may help in localization. At our institution, temporal bone and CPA imaging is performed with slices of 0.625 mm thickness, which are reconstructed with 0.3 mm interval, with high-quality multiplanar reformatted images obtained from the axially acquired “volume data.” Axial and coronal are the primary planes utilized for imaging evaluation.Įxtraaxial lesions are often readily identified as separate and distinct from brain parenchyma on CT and MRI. The administration of intravenous iodinated contrast enables the identification of enhancing lesions and vessels and may increase lesion conspicuity.Ĭurrently, most CT examinations are performed with multidetector-row CT with submillimeter slice thickness through regions of interest. Fat is substantially lower in density and darker than other soft tissues on CT, and fat-containing lesions are readily identified. Therefore, CT usually has difficulty differentiating fluids from soft tissues without contrast. However, fluid, nerves, and tumors are similar in density. CT provides excellent definition of bone and air-filled spaces. Dynamic contrast-enhanced MR angiography can also be performed as clinically required.ĬT is often performed for the evaluation of nonspecific clinical symptoms related to the CPA and posterior fossa and may initially identify culprit lesions.
Phase-contrast MR angiography is another technique to demonstrate vascular structures without intravenous contrast administration, which enables the quantitative analysis of flow as well as the visualization of arterial and venous structures. If there is suspicion for a vascular lesion as the etiology of a CPA “mass,” two or three-dimensional time-of-flight techniques may be performed for arterial imaging without intravenous contrast administration. Diffusion restriction is seen with these pathologies and this specific sequence can be helpful in differentiation from other conditions (described in greater detail later). High-resolution pre- and postcontrast T1-weighted images through the CPA and IAC are also routinely obtained for complete characterization.ĭiffusion-weighted imaging (DWI) has been utilized for temporal bone and CPA imaging, particularly when abscesses or epidermoid/cholesteatoma are suspected. In addition to conventional MR sequences of the brain, current imaging evaluation of the CPA includes high-resolution heavily T2-weighted imaging, such as constructive interference in steady-state (CISS), driven equilibrium (DRIVE), fast imaging employing steady-state acquisition (FIESTA), with submillimeter slice thickness, allowing evaluation of the cranial nerves and surrounding CSF without intravenous contrast administration. MRI has excellent soft tissue contrast resolution and is the preferred imaging modality for differentiation and characterization of various soft tissues and fluids. This chapter reviews the imaging features of common and less commonly encountered lesions of the CPA. The clinical presentation and symptoms related to CPA lesions depend on the lesion location, size, mass effect, and contact on adjacent structures. These lesions may be congenital masses of the CPA (epidermoid and arachnoid cysts) may originate from the adjacent cerebellum or brainstem (such as gliomas or parenchymal metastases), the vessels in the posterior fossa (arteriovenous malformations or aneurysms), and the adjacent temporal bone (Paget disease) or may be related to systemic leptomeningeal processes (infection or inflammatory processes, such as meningitis or sarcoidosis). Although other intraaxial and extraaxial lesions are less commonly encountered, they are readily identified by imaging. Meningiomas are the second most common lesion in this location. The vast majority of lesions in the CPA are vestibular schwannomas related to cranial nerve VIII, comprising 70%–80% of the lesions in this location. The cranial caudal extent of this region extends from the level of cranial nerve V through the cranial nerve IX-X-XI complex. The cerebellopontine angle (CPA) cistern is a subarachnoid space centered within the posterior cranial fossa at the level of the internal auditory canal and bordered medially and laterally by the cerebellum and petrous temporal bone, respectively.
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