4 Presurgical Planning By Images • This chapter provides an overview of common preoperative imaging protocol advices as they relate to different cranial neurosurgical approaches. • CT (computed tomography) scans yield an excellent definition of bone anatomy and are useful to identify highly calcified lesions, but cannot distinguish subtle differences between tissues in the brain. • Conventional MRI (magnetic resonance imaging) sequences (i.e., T2-weighted, FLAIR or fluid-attenuated inversionrecovery, T1-weighted images before and post-contrast administration) have an excellent contrast resolution and bring information about the location, signal intensities, and enhancement features of the lesions. • Beside conventional morphologic evaluation, advanced MRI techniques can improve the specificity of preoperative imaging diagnosis and are useful tools for surgical planning. In this regard, the next sections will propose an integrated approach that includes magnetic resonance angiography (MRA), diffusion MR-based techniques (DWI/DTI: diffusion-weighted imaging/diffusion tensor imaging), functional MRI, perfusion MR imaging, and MR spectroscopy. • Presurgical imaging of lesions in the anterior compartment of the skull base includes meningioma (common), dural metastasis, paranasal sinus tumor invasion, olfactory neuroblastoma or esthesioneuroblastoma (rare), aneurysms, intra-axial frontal tumors. • Pre- and post-contrast MRI (Figs. 4.1A–1F) are the best imaging tool for the diagnosis and characterization of anterior cranial fossa lesions. Tissue signal and enhancement are variable in appearance across different lesions. ◦ In meningiomas, T2-w images well delineate trapped hyperintense cerebrospinal fluid (CSF) clefts (Figs. 4.1A, 4.1C) and vascular flow-voids. The extent of perilesional edema is well demonstrated on FLAIR images (Fig. 4.1F). Meningiomas usually enhance homogeneously and intensely (Figs. 4.1D, 4.1E) on T1-w post-contrast images and usually show a thickening of the adjacent dura (dural tail). ◦ Metastasis and other anterior cranial fossa lesions may differ according to their tissue of origin. • MRA (Fig. 4.1G) demonstrates the mass effect of the lesion on arterial vessels, as well as their possible encasement and narrowing. MRV (magnetic resonance venography) may help to identify dural sinus invasion (superior sagittal sinus). • Non-contrast CT (Figs. 4.1H, 4.1I) may show calcifications and intra-lesional hemorrhages. Bone CT scans of the maxillofacial region are useful to detect hyperostosis (Fig. 4.1I) (typical for meningiomas abutting the dura of the skull base), bone erosion or destruction (Fig. 4.1H) (typical for higher grade or lytic lesions), and enlargement of the paranasal sinuses (pneumosinus dilatans). • Conventional and interventional DSA may be used for detailed delineation of lesion vascular supplies and for preoperative embolization of anterior cranial fossa lesions, which may substantially reduce operative time and blood loss. • Located in the central compartment of the skull base (middle cranial fossa), it includes the sella turcica in the sphenoid bone, which houses the pituitary gland. • Surrounding structures are the sphenoid sinus (inferiorly), the suprasellar cistern, the optic chiasm and the infundibular recess of the third ventricle (superiorly) and the cavernous sinuses (laterally). • Presurgical imaging firstly aims at determining the anatomical sub-location of the lesion: intrasellar, suprasellar, or in the infundibular stalk (alone or in combination). ◦ Intrasellar lesions: Pituitary adenomas, Rathke cleft cyst, craniopharyngioma, neurosarcoidosis (less common). ◦ Suprasellar lesions: Pituitary macroadenoma, meningioma, aneurysm, craniopharyngioma, and astrocytoma; less common suprasellar masses are arachnoid cyst, dermoid cyst, and neurocysticercosis (usually multiple lesions). ◦ Infundibular stalk lesions: Metastasis, lymphoma, pituicytoma, germinoma (more common in children), histiocytosis, hypophysitis, neurosarcoidosis (rare). • Pre- and post-contrast MRI (Figs. 4.2A–4.2F) signal and enhancement varies across different lesions and cyst contents. ◦ In craniopharyngiomas, T2-w images show large hyperintense cystic components, heterogeneous solid components and vascular flow-voids of surrounding arteries (Figs. 4.2A–4.2C). T1-w post-contrast images usually show heterogeneous enhancement of the solid portions of the lesions. Cyst walls of craniopharyngiomas enhance strongly (Fig. 4.2D–4.2F) Fig. 4.1 Anterior cranial fossa meningioma. (A, B, C) Axial and coronal T2-w images show an extra-axial mass arising from the sphenoidal planum and extending into both anterior frontal lobes. The lesion is quite homogeneous, except for its calcified basalpart, which is markedly hypointense (yellow arrow). Peripheral CSF clefts are seen as hyperintense rim between tumor and brain (yellow arrowheads). (D, E) T1-w post-contrast axial and coronal images demonstrates intense enhancement of the lesion, with even more hyperintense “sunburst” of the vessels. (F) Coronal FLAIR image shows markedly hyperintense peritumoral vasogenic edema (asterisks). (G) MRA shows posterior and cranial displacement of both the anterior cerebral arteries, and posterior displacement of both the intracranial carotid arteries (open arrows). (H, I) Bone CT shows enlargement of the olfactory groove bilaterally (dashed arrows) and hyperostosis of the sphenoidal planum and calcification at the basis of the lesion (yellow arrow). ◦ Arachnoid cysts behave like CSF, dermoid cysts appear like fat. ◦ In pituitary adenomas, dynamic T1-w post-contrast images may help to detect slowly enhancing microadenomas. • DTI and MR Tractography (Figs. 4.2G–4.2I) allow to depict the major white matter tracts in the brain. Preoperative DTI tractography may show the location and displacement of the main fiber bundles surrounding the lesions, including the anterior commissure, the fornix, the optic chiasm and tracts. This information may be particularly useful for the resection of large lesions growing toward the third ventricle. • MRA (Fig. 4.2L) demonstrates the vascular displacement or encasement of the arterial vessels surrounding the lesions. • Non-contrast CT shows intra-lesional calcifications. The bone window of CT scans of the maxillofacial region are useful to evaluate the paranasal sinuses before surgery and to detect the bone erosion of sellar floor and sphenoid invasion. • Presurgical imaging of the lateral posterior compartment of the skull base includes cerebello-pontine angle (CPA) cisternal masses (usually involving the temporal bone or infratemporal fossa): most commonly demonstrate vestibular schwannoma (common), meningioma, epidermoid, arachnoid cyst, trigeminal or facial nerve schwannoma and metastasis. • Pre- and post-contrast MRI (Figs. 4.3, 4.4) help to characterize CPA cistern masses. ◦ The MR signal of schwannomas is similar to that of lesions involving other nerves. Vestibular schwannomas, the most common CPA lesions, are intensely enhancing masses with an ‘ice cream on a cone’ appearance (Figs. 4.3A–4.3F). The mass effect on the adjacent cerebellar and brainstem structures is well demonstrated on MR images (Figs. 4.3A–4.3F). DWI MR sequence shows elevated ADC (apparent diffusion coefficient) values with respect to normal brain tissue, reflecting the increased amount of extracellular water in tumor matrix.
4.1 Role of Imaging in Preoperative Planning
4.2 Anterior Cranial Fossa (Fig. 4.1.)
4.2.1 General Features
4.2.2 MR Imaging
4.2.3 CT Imaging
4.2.4 DSA (Digital Subtraction Angiography)
4.3 Sellar and Parasellar Region (Fig. 4.2)
4.3.1 General Features
4.3.2 MR Imaging
4.3.3 CT Imaging
4.4 Posterior Cranial Fossa, Lateral Approaches (Figs. 4.3, 4.4)
4.4.1 General Features
4.4.2 MR Imaging
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